In as far as he does discover healthy maturational changes at work in his body and personality, changes that he realizes to be wonderful and priceless, he experiences the poignant accompanying realization that there is no one there to welcome these changes and to share his joy. The parents, if sufficiently free from anxiety to recognize such changes at all, have a tendency to accept them as evidence that their child is rejecting then by growing functionally. Also to be noted, in this connexion, is their lack of trust in him, their lack of assurance that he is elementally good and can be trusted to maturational bases of a good healthy adult. Instead they are alert to find, and warn him against, manifestations in him that can be construed as evidence that he is on a predestined, downward path into an adulthood of criminality, insanity, more at best ineptitude for living.
Moreover, he emergences change not as something within his own power to wield, for the benefit of himself and others but as something imposed from without. This is due not only to structures that the parents place upon his autonomy, but also to the process of increasing repression of his emotions and life as, such that when this latter manifest themselves, they do so in a projected expressive style, for being uncontrollable changed, inflicted upon him from the surrounding world? We see extreme examples of this mechanism later on. In the full-blown schizophrenic person who experiences sexual feelings not as such but as electric shocks sent into him from the outside world, and who experiences anger not as an emerging emotion directorially fittingly as in a way up from within, but a massive and sudden blow coming somehow from the outer world. In fewer extreme instances, in the life of the yet-to-become-schizophrenic youth, he finds repeatedly that when he reaches out to another person, the other suddenly undergoes a change in demeanour, from friendliness to antagonism, in reaction to an unwitting manifestation of the youths’ unconscious hostility. The youth himself, if unable to recognize his own hostility, can only be left feeling increased helplessness in face of an unpredictably changeable world of people.
The final incident that occurs before his admission to the hospital, giving him still further reason for anxiety as for change, is his experience of the psychotic symptoms as an overwhelming anxiety-laden and mysterious change. His own anxiety about this frightened away by the seismic disturbance and horror of the members of his family who finds hi ‘changed’ by what they see as an unmitigated catastrophe, a nervous or mental ‘breakdown’. Although the therapist can come to see, in retrospect, a potential positive element via this occurrence - namely, the emergence of onetime-repressed insights concerning the true state of affairs involving the patient and his family, none of those participants can integrate so radically changed a picture at that time. Over the preceding years the family members could not tolerate their child’s seeing himself and them with the eyes of a normally maturing offspring, and when repressed percepts emerge from repression in him, neither they nor he possesses the requisite ego-strength to accept them as badly needed changes in his picture of himself and of them. Instead, the tumult of depressed percepts foes into the formation of such psychotic phenomena as misidentifications, hallucinations, and delusions in which neither he nor the member of his family can discern the links to reality that we, upon investigation in individual psychotherapy with him, can find in these psychotic phenomena - links, that is, to the state of affairs that has really held sway in the family. Paretically, it should be marked and noted that the psychotic episode often occurs in such ac way as to leave the patient especially fearful of sudden change, for in many instances the de-repressed material emerges suddenly and leads him to damage, in the short space of a few hours or even moments, his life situation so grievously that repair can be affected only very slowly and painfully, over many subsequent months of treatment in the confines of a hospital.
It should be conveyed, in that the regression of the thought-processes, which occurs as one of the features of the developing schizophrenia, results in an experience of the world so kaleidoscopic as to make up still another reason for the individual’s anxiety concerning change. That is, as much as he has lost thee capacity to grasp the essentials of a given whole - to the extent that he has regressed to what Goldstein (1946) terms the ‘concrete attitude’ - he experiences any change, even if it is only in an insignificant (by mature standards) detail of that which he perceives, as a metamorphosis that leaves him with no sense of continuity between the present perception and that immediately preceding. This thought disorder, various aspects of which have been described also by Angyal (1946), Kasanin (1946), Zucker (1958), and others, is compared by Werner with the modes of thought that are found in members of so-called primitive cultures (and in healthy children of our own culture): . . . in the primitive mentality, particulars often as self-subsisting things that do not necessarily become synthized into larger entities. . . . The natives of the Kilimanjaro region do not have a word for the whole mountain range that they inhabit, only words for its peaks. . . . The same is reported of the aborigines of East Australia. From each twist and turn of a river has a name, but the language does not permit of a single all-embracing differentiation for the whole river. . . . [He] quotes Radin (1927) as saying that for the primitive man: 'A mountain is not thought of as a unified whole. It is a continually changing entity’ . . . [and, Radin continues, such a man lives in a world that is] ‘dynamic and ever-changing . . . Since he sees the same objects changing in their appearance from day to day, the primitive man regards this phenomenon as definitely depriving them of immutability and self-subsistence’ (Werner 1957).
Langer (1942) has called the symbolic-making function ‘one of man’s primary activities, like eating, looking, or moving about. It is the fundamental process of his mind’, she says, as she terms the need of symbolization ‘a primary need in man, which other creatures probably do not have’. Kubie (1953) terms the symbolizing capacity ‘the unique hallmark of man . . . capacities’, and he states that it is in impairment of this capacity to symbolize that all adult psychopathology essentially consists.
As for schizophrenia, we find that since 1911 this disease was described by Bleuler (1911) as involving an impairment of the thinking capacities, and in the thirty years many psychologists and psychiatrists, including Vigotsky (1934) Hanfmann and Kasanin (1942) Goldstein (1946) Norman Cameron (1946) Benjamin (1946) Beck (1946) von Domarus (1946) and Angtal (1946) - to mention but a few - has described various aspects of this thinking disorder. These writers, agreeing that one aspect of the disorder consists in over -concreteness or literalness of thought, have variously described the schizophrenic as unable to think in figurative (including metaphorical) terms, or in abstractions, or in consensually validated concepts and symbols, mor in categorical generalizations. Bateson (1956) described the schizophrenic as using metaphor, but unlabelled metaphor.
Werner (1940) has understood this most accurately matter of regression to a primitive level of thinking, comparable with the found in children and in members of so-called primitive cultures, a level of thinking in which there is a lack of differentiation between the concrete and the metaphorical. Thus we might say that just as the schizophrenic is unable to think in effective, consensually validated metaphor, as too as he is unable to think in terms that are genuinely concrete, free from an animistic forbear of a so-called metaphorical overlay.
The defensive function of the dedifferentiation that in so characterized of schizophrenic experience, and one find that this fragmentation o experience, justly lends itself to the repression of various motions that are too intense, and in particular too complex, for the weak ego to endure, which must be faced as one becomes aware of change as involving continuity rather than total discontinuity.
That is, the deeply schizophrenic patient who, when her beloved therapist makes a unkind or stupid remark, experiences him now for being a different person from the one who was there a moment ago - who experiences that a Bad Therapist has replaced the Good Therapist - is by that spared the complex feeling of disillusionment and hurt, the complex mixture of love and anger and contempt that a healthier patient would feel then. Similarly, if she experiences it in tomorrow’s session - or even later in the same session - that another good therapist has now come on the scene. The bad therapist is now totally gone, she will feel none of the guilt and self-reproach that a healthier patient would feel at finding that this therapist, whom she has just now been hated or despising, is after all a person capable of genuine kindness. Likewise, when she experiences a therapist’s departure on vacation for being a total deletion of him from her awareness, this bit of discontinuity, or fragmentation, in her subjective experience spars her from feeling the complex mixture of longing, grief, separation-anxiety, rejection, rage and so on, which a less ill patient feels toward a therapist who is absent but of whose existence he continues to be only too keenly aware.
Finally, such repressed emotions as hostility and lust may readily be seen, as these feelings not easy to hear expressed, as, for instance, the woman, who, at the beginning of her therapy, had been encased for years I flint lock paranoid defenses, become able to express her despair by saying that 'If I had something to get well for, it would make a difference,' her grief, by saying, 'The reason I am afraid to be close to people is because I feel so much like crying': Her loneliness, by expressing a wish that she would turn an insect into a person, so then she would have a friend. Her helplessness in face of her ambivalence by saying, to her efforts to communicate with other persons, 'I feel just like a little child, at the edge of the Atlantic or Pacific Ocean, trying to build a castle - right next to the water. Something just starts to be gasped [by the other person], and then bang! It has gone - another wave. As joining the mainstream of fellow human beings.
In the compliant charge of bringing forward three hypotheses are to be shown, they're errelated or portray in words as their interconnectivity, are as (1) in the course of a successful psychoanalysis, the analyst goes through a phase of reacting to, and eventually relinquishing, the patient as his oedipal love-object, (2) in normal personality development, the parent reciprocates the child's oedipal love with greater intensity than we have recognized before, and (3) in such normal developments, the passing of the Oedipus complex is at least important a phase in ego-development as in superego-development.
While doing psycho-analysis, time and again patients who have progressed to, or very far toward, a thorough going analysis to cure, become aware of experiential romantic and erotic desires and fantasies. Such fantasizing and emotions have appeared in a usual but of late in the course of treatment, have been preset not briefly but usually for several months, and have subsided only after having experienced a variety of feelings - frustration, separation anxiety, grief and so forth - entirely akin to those that attended as the resolution of an Oedipus complex late in the personal analysis.
Psycho-analysis literature is, in the main. Such as to make one feel more, rather than less, troubled at finding in oneself such feelings toward one's patient. As Lucia Tower (1956) has recently noted, . . . Virtually every writer on the subject of countertransference . . . states unequivocally that no form of erotic reaction to a patient is to be tolerated . . .
Still, in recent years, many writers, such as P. Heimann (1950), M. B. Cohen (1952) and E. Weigert (1952, 1954), have emphasized how much the analyst can learn about the patient from noticing his own feelings, of whatever sort, in the analytic relationship. Weigert (1952), defining countertransference as emphatic identification with the analysand, has stated that . . . 'In terminal phases of analyses the resolution of countertransference goes hand in hand with the resolution of transference.'
Respectfully, these additional passages are shown in view of countertransference, in the special sense in which defines the analyst for being innate, inevitable ingredients in the psycho-analytic relationship, in particular, the feelings of loss that the analyst experiences with the termination of the analysis. However, case in point, that the particular variety of countertransference with which are under approach is concerned that of the analyst's reacting as a loving and protective parent to the analysand, reacted too as an infant: There are plausible reasons why in the last phase it is especially difficult to achieve and maintain analytic frankness. The end of analysis is an experience of loss that mobilizes all the resistances in the transference (and in the counter-transference too), for a final struggle. . . . Recently, Adelaide Johnson (1951) described the terminal conflict of analysis as fully reliving the Oedipus conflict in which the quest for the genitally gratifying parent is poignantly expressed and the intense grief, anxiety and wrath of its definitive loss are fully reactivated. . . . Unless the patient dares to be exposed to such an ultimate frustration he may cling to the tacit permission that his relation to the analyst will remain his refuge from the hardships of his libidinal cravings to an aim-inhibited, tender attachment to the analyst as an idealized parent, he can get past the conflicts of genital temptation and frustration.
. . . . The resolution of the counter-transference permits the analyst to be emotionally freer and spontaneous with the patient, and this is an additional indication of the approaching end of an analysis.
. . . . When the analyst observes that he can be unrestrained with the patient, when he no longer weighs his words to maintain as cautious objectivity, this empathic countertransference and the transference of the patient are in a process of resolution. The analyst can treat the analysand on terms of equality; he is no longer needed as an auxiliary superego, an unrealistic deity in the clouds of detached neutrality. These are signs that the patient's labour of mourning for infantile attachments nears completion.
In stressing the point, which before an analysis can properly bring to an end, the analyst must have experienced a resolution of his countertransference to the patient for being a deep beloved, and desired, figure not only on this infantile level that Weigert has emphasized valuably, but also on an oedipal-genital level. Weigeret's paper, which helped to formulate the views that are set down, that is, as expressing the total point that a successful psycho-analysis involves the analyst's deeply felt relinquishment of the patient both as a cherished infant, and for being a fellow adult who is responded to at the level of genital love?
The paper by L. E. Tower (1956) comes similarly close to the view that, unlike Weigert, limits the term counter-transference to those phenomena that are transferences of the analyst to the patient. It is much more striking, therefore, that she finds even this classification defined countertransference to be innate to the analytic process: . . . . That there is inevitably, naturally, and often desirable, many countertransference developments in every analysis (some evanescent - some sustained), which is a counterpart of the transference phenomena. Interactions (or transactions) between the transference of the patient and the countertransference of the analyst, going on at unconscious levels, may be - or perhaps are always - of vital significance for the outcome of the treatment. . . .
. . . . Virtually every writer on the subject of countertransference. States unequivocally that no form of erotic reaction to a patient is to be tolerated. This would suggest that temptations in this area are great, and perhaps ubiquitous. This is the one subject about which almost every author is very certain to state his position. Other 'counter-transference' manifestations are not routinely condemned. Therefore, it must be to assume that erotic responses to some extent trouble nearly every analyst. This is an interesting phenomenon and one that call for investigation; nearly all physicians, when they gain enough confidence in their analysts, report erotic feelings and imply toward their patients, but usually do so with a good deal of fear and conflict. . . .
Of our tending purposes, we are to pay close attention to the libidinal resources that are of our applicative theory, in that large amounts of resulting available libido are necessary to tolerate the heavy task of many intensive analyses. While, we deride almost every detectable libidinal investment made by an analyst in a patient . . . various forms of erotic fantasy and erotic countertransference phenomena of a fantasy and of an affective character are in some experiential ubiquitous and presumably normal. Which lead to suspect that in many - perhaps every - intensive analytic treatment there develops something like countertransference structures (perhaps even a 'neurosis') which are essential and inevitable counterparts of the transference neurosis. These countertransference structures may be large or small in their quantitative aspects, but in the total picture they may be of considerable significance for the outcome of the treatment. They function in the manner of a catalytic agent in the treatment process. Their understanding by the analyst may be as important to the final working through of the transference neurosis as is the analyst's intellectual understanding of the transference neurosis itself, perhaps because they are, so to speak, the vehicle for the analyst's emotional understanding of the transference neurosis. Both transference neurosis and countertransference structure seem intimately bound together in a living process and both must be considered continually in the work that is the psychoanalysis. . . .
. . . . Seemingly questionable, is any thorough working through a deep transference neurosis, in the strictest sense, which does not involve some form of emotional upheaval in which both patient and analysts are involved. In other words, there are both a transference neurosis and a corresponding Countertransference 'neurosis' (no matter how small and temporary) which are both analyzed in the treatment situation, with eventual feelings of a new orientation by both one another toward any other but themselves.
Freud, in his description of the Oedipus complex (1900, 1921, 1923), tended largely to give us a picture of the child as having an innate, self-determined tendency to experience, under the conditions of a normal home, feelings of passionate love toward the parent of the opposite sex; we get little hints, from his writings, that in this regard the child enters a mutual relatedness of passionate love with that parent, a relatedness in which the parent's feelings may be of much the same quality and intensity as those in the child (although this relatedness must be very important in the life of the developing child than it is in the life of the mature adult, with his much stronger, more highly differentiated ego and with his having behind him the experience of a successfully resolved oedipal experience during his own maturation).
Nevertheless, in the earliest of his publications concerning the Oedipus complex, namely The Interpretation of Dreams (1900), Freud makes a fuller acknowledgements of the parent's participation in the oedipal phase of the child's life than does in any of his later writings on the subject'. . . a child's sexual wishes - if in their embryonic stage they deserve to be so described - awaken very early. . . . A girl's first affection is for her father and boy's first childish desires are for his mother. Accordingly, the father becomes a disturbing rival to the boy and the mother to the girl. The parents too give evidence as a rule of sexual partiality: A natural predilection usually sees to it that a man tends to spoil his little daughters, while his wife takes her sons' part; though both of them, where their judgement is not disturbed by the magic of sex, keep a strict eye upon their children's education. The child is very well aware of this patriality and turns against that one of his parents who is opposed to showing it. Being loved by an adult does not merely bring a child the satisfaction of a special need; it also means that he will get what he wants in every other respect as well. Thus, he will be following his own sexual instinct and while giving fresh strength to the inclination shown by his parents if his choice between them falls in with theirs (1900).
Theodor Reik, in his accounts of his coming to sense something of the depths of possessiveness, jealousy, fury at rivals, and anxiety in the face of impending loss, in himself regarding his two daughters, conveys a much more adequate picture of the emotions that genuinely grip the parent in the oedipal relationship than is conveyed by Freud's sketchy account, as Reik's deeply moving descriptions occupy a chapter in his Listening with the Third Ear (1949), written at the time when his daughters were twelve and six years of age; and a chapter in his The Secret Self (1952), when the oldest daughter was now seventeen.
Returning to a further consideration of the therapist's oedipal-love responses to the patient, it seems that these response flows from four different sources. In actual practice the responses from these four tributaries are probably so commingled in the therapists that it is difficult of impossible fully to distinguish one kind from another; the important thing is that he is maximally open to the recognition of these feelings in himself, no matter what their origin, for he can probably discern, in as far as is possible, from where they flow they signify, therefore, concerning the patient's analysis.
First among these four sources may be mentioned the analyst's feeling-responses to the patient's transference. This, when, as the analysis progresses and the patient enter an experiencing of oedipal love, ongoing, jealousy y, frustration and loss as for the analyst as a parent in the transference, the analyst will experience to at least some degree, response's reciprocally th those of the patient-responses, that is, such for being present within the parent in questions, during the patient's childhood and adolescence, which the parent presumably was not ably to recognize freely and accept within himself. Some writers apply the term 'counter-transference' to such analyst-responese to the patient's transference, unlike others some do not do so.
The second source consists in the countertransference in the classical sense in which this term is most often used: The analyst's responding to the patient about transference-feelings carried over from a figure out of the analyst 's own earlier years, without awareness that his response springs predominantly from this early-life, rather than being based mainly upon the reality of the patient analyst-patient relationship. It is this source, of course, which we wish to reduce to a minimum, by means of thoroughgoing personal analysis and ever-continuing subsequent alertness for indications that our work with a patient has come up against, in us, unanalyzed emotional residues from our past. This source is so very important, in fact, as to make the writing of such a paper as a somewhat precarious venture. Must expect that some readers will charge him with trying to portray, as natural and necessary to the annalistic process generally, certain analyst-responese that in actuality is purely the result of an unworked-through? Oedipus' complex in himself, which are dangerously out of place in his own work with patients that have no place in the well-analysed analyst's experience with his patient.
It can only be surmised that although this source may play an insignificant role in the responses of a well-analysed analyst who has conducted many analyses through to completion - to an intensified inclusion as a thoroughgoing resolution of the patient's Oedipus complex - it is probably to be found, in some measure, in every analyst. This is, it seems that the nature and conflictual feeling-experience in this regard - a fostering of his deepest love toward the fellow human being with whom she participates in such prolonged and deeply personal work, and a simultaneous, unceasing, and rigorous taboo against his behavioural expression of any of the romantic or erotic components of his love - as to require almost any analyst's tending to relegate the deepest intensities of these conflictual feelings to his own unconscious mind, much as were the deepest intensities of his oedipal strivings toward a similar beloved, and similarly unobtainable and rigorously tabooed, parent in particular, and in the hope of the remaining in the analyst's unconscious. That is hoping that this will help analysts - in particular, to a lesser extent-experienced analyst - whereas to some readers awareness, and by that diminution, of this countertransference feeling, as justly dealing with other kinds of countertransference feelings, by such as those wrote by P. Heumann (1950, M. B., Cohen (19520 and E. Weigert (1952?)
A third source is to be found in the appeal that the gratifyingly improving patient makes to the narcissistic residue in the analyst's personality, the Pygmalion in him. He tends to fall in love with this beautifully developing patient, regarded at this narcissistic level as his own creation, just as Pygmalion fell in love with the beautiful statu e of Galatea that he had sculptured. This source, like the second one that we can expect to holds little sways in the well-analysed practitioner of long experience, but it, too, is probably never absent of great experience and professional standing, than we may like to think. Particularly in articles and books that describe the author's new technique or theoretical concept as an outgrowth of the work with a particular patient, or a very few patients, do we see this source very prominently present in many instances.
The fourth source, based on the genuine reality of the analyst-patient situation, consists in the circumstance that nearly becomes, per se, a likeable, admirable and insightfully speaking lovable, human being from whom the analyst will soon become separated. If he is not himself a psychiatrist, the analyst may very likely never see him again. Even if he is a professional colleague, the relationship with him will become in many respects far more superficial, far less intimate, than it has been. This real and unavoidable circumstance of the closing analytic work tends powerfully to arouse within the analyst feelings of painfully frustrated love that deserve to be compared with the feelings of ungratifiable love that both child and parent experience in the oedipal phase of the child's development. Feelings from this source cannot properly be called countertransference. They may flow from the reality of the present circumstances but they may be difficult or impossible e to distinguish fully from countertransference.
There are, then four essentially powerful sources having to promote of the tendency toward the feelings of deep love with romantic and erotic overtones, and with accompanying feelings of jealousy, anxiety, frustration-rage, separation-anxiety, and grief, in the analyst about the patient. These feelings come to him, like all feelings, without tags showing from where they have come, and only if he is open and accepting to their emergence into his awareness does he have a chance to set about finding out their origin and thus their significance in his work with the patient.
Finally, with which the considerations have been presented so far, a few remarks concerning the passing of the Oedipus complex in normal development and in a successful psycho-analysis.
In the Ego and the Id (1923) we find italicized a passage in which Freud stresses that the oedipus phase results in the formation of the superego; we find that he stresses the patient's opposition to ther child's oedipal swosh, and lastly, we see this resultant suprerego to be predominantly a severe and forbidding one: The broad general outcome of the sexual phase dominated by the Oedipus complex may, therefore, be taken to be the forming of a precipitating in the ego . . . This modification of the ego
. . . comforts the other contents of the ego as an ego ideal or super-ego.
. . . . The child's parents, and especially his father, were perceived as the obstacle to verbalizations of his Oedipus wishes, so his infantile ego fortified itself for the carrying out of the repression by building this obstacle within itself. It borrowed the strength to do this, so to seek, from the father, and this loan was an extraordinarily nonentous act. The super-ego retains the character of the father, while the more powerful the Oedipus complex was and the more rapid succumbed to repression (under the influence of authority, religious teachings, schooling and reading), this strictly will be the domination of the super-ego over the ego later on - as conscience or perhaps of an unconscious sense of guilt. . . .
The subject dealt within the subjective matter through which generative pre-oedipal origins are to be found of the superego, on which has been dealt by M. Klein (1955). E. Jacobson (1954) and others, also apart from that subject, a regard for Freud's above-quoted description as more applicable to the child who later becomes neurotic or psychotic, than to the 'normal'; child. Since we can assume that there is virtually a wholly complimentary neurotic difficulty, we may then have in assuming that Freud's formation holds true to some degree in every instance. Still, to the extent that a child's relationships with his parents are healthy, he finds the strength to accept the unrealizibilityy of his oedipal strivings, not mainly through the identification with the forbidding rival-parent, but mainly, as an alternative, the ego-strengthening experiences of finding the beloved parent reciprocate his love - responds to him, that is, for being a worthwhile and loveable individual, for being, a conceivably desirable love-partner - and renounces him only with an accompanying sense of loss on the parent's own part. The renunciation, again, something that is mutual experience for the chid and parent, and is made in deference to a recognizedly greater limiting realty, a reality that includes not only the taboo maintained by the rival-parent, but also the love of the oedipal desired parent toward his or her spouse - a love that undeterred the child's birth and a love to which, in a sense, he owes his very existence?
Out of such an oedipal situation the child emerges, with no matter how deep and painful sense of loss at the recognition that he can never displace the rival-parent and posses the beloved on e in a romantic-and-erotic relationship, in a state differently from the ego-diminished, superego-domination state that Freud described. This child that his love, however unrealized, is reciprocated. Strengthened, too, out of the realization, which his relationship with the beloved parent has helped him to achieve, that he lives in a wold in which any individual's strivings are encompassed by a reality much larger than he: Freud, when he stressed that the oedipal phase normally results mainly in the formations of a forbidding superego, and if it is resulting mainly in enchantments of the ego's ability to test both inner and outer reality.
All experiences with both neurotic and psychotic patients had shown that, in every individual instance, in as far as the oedipal phase was entered the course of their past elements, it led to ego impairment rather than ego functioning as primarily because the beloved parent had to repress his or her reciprocal desire for the child, chiefly through the mechanism of unconscious denial of the child's importance to the parent. More often than not, in these instancies, that suggested that the parent would unwittingly act out his or her repressed desires in the unduly seductive behaviour toward the child; yet whenever the parents come close to the recognition of such desires within him, he would unpredictably start reacting to the child as unlovable - undesirable.
With many of these parents, appears that, primarily because of the parent's own unresolved Oedipus complex, his marriage proved too unsatisfying, and his emotional relationship to his own culture too tenuous, for him to dare to recognize the strength of his reciprocal feelings toward his child during the latter's oedipal phase of development. The child is reacting too as a little mother or father transference-figure to the parent, a transference-figure toward whom the parent's repressed oedipal love feelings are directed. If the parent had achieved the inner reassurance of a deep and enduring love toward his wife, and a deeply felt relatedness with his culture including the incest taboos to which his culture adheres, he would have been able to participate in as deeply felt, but minimally acted out, relationship with the chid in a way that fostered the healthy resolutions of the child's Oedipus complex. Instead, what usually happens in such instances, in that the child's Oedipus complex remains unresolved because the child stubbornly - and naturally - refuses to accept defeat within these particular family circumstances, whereas the acceptance of oedipal defeat is tantamount to the acceptance of irrevocable personal worthlessness and unlovability.
It seems much clearer, then this former child, now neurotic or psychotic adult, requires from us for the successful resolution to his unresolved Oedipus complex: Not such a repression of desire, acted-out seductiveness, and denial of his own worth as he met in the relationship with his parent, but a maximal awareness on our part of the reciprocal feelings while we develop in response to his oedipal strivings. Our main job remains always, of course, to further the analysis of his transference, but what might be described seems to be the optimal feeling background in the analyst for such analytic work.
Formidably, when applied not to a moderate degree found in the background of the neurotic person but invested with all the weight of actual biological attributes, have much ado with the person's unconscious refusal to relinquish, in adolescence and young adulthood, his or her fantasied infantile omnipotence in exchange for a sexual identity of - in these-described terms - a 'man' or a 'woman'. It would be like having to accept only certain dispensations as well as salvageable sights, if ony to see the whole fabric ruined into the bargin. A person cannot deeply accept an adult sexual identity until he has been able to find that this identity can express all the feeling-potentialities of his comparatively boundless infancy. This implies that he has become able to blend, for example, his infantile - dependent needs into his more adult erotic strivings, than regard these as mutually exclusive in the way that the mother of the future patient or the persons infant frighteningly feels that her lust has been placed in her mothering. Another difficult facet of this situation resides in a patient's youngful conviction, based on his intrafamiliar experiences, which he can win parental love only if he can become or, perhaps, at an unconscious level remain - a girl; accepting her sexuality as a woman is equated with the abandonment of the hope of being loved.
Concerning the warped experiences their persons have and with the oedipal phase of development, calls to our attention of two features. First, the child whose parents are more narcissistic than truly object-related in faced with the basically hopeless challenge of trying to compete with the mother's own narcissistic love for herself, and with the father's similar love for himself, than being presented with a competitive challenge involving separate, flesh-and-blood human beings. Secondly, concerning warped oedipal experiences, in, as far as the parents succeeded in achieving object-relatedness, this has often become only weakly established as a genital level, so that it remains much more prominently at the mother-infant level of ego-development. Thus, the mother, for example, is much more able to love her infant son than her adult husband, and the oedipal competition between husband and son are in terms of who can better become, or remain, the infant whom the mother is capable of loving. When the infant becomes chronologically a young man, having learned that one wins a woman not through genial assertiveness but through regression, he is apt to shy away from entering into true adult genitality, and is tempted to settle for what amounts to 'regressive victory' in the oedipal struggle
We write much about the analyst’s or therapist’s being able to identify or empathize with the patient for helping in the resolution of the neurotic or psychotic difficulties. Such writings always portray a merely transitory identification, an empathic sensing of the patient’s conflicts, an identification that is of essentially communicative value only. However, it should be seen that we inevitably identify with the patient another fashion also, we identify with the healthy elements in him, in a way that entails enduing, constructive additions to our own personality. Patients - above all schizophrenic patients - need and welcome our acknowledgement, simply and undemonstratively, that they have contributed, and are contributing, in some such significant way, to our existence.
Increasing maturity involves increasing ability not merely to embrace change in the world around one, but to realize that one is oneself in a constant state of change. By contrast, the recovering, maturing patiently becomes less and less dependent upon any such sharply delineated, static self-image or even a constellation of such images, the answer to the question, 'Who are you?' is almost as small, solid, and well defined as a stone, but is a larger, fluid, richly-laden, and sniffingly outlined as an ocean? As the individual becomes well, he comes to realize that, as Henri Bergson (1944) puts it, 'reality is a perpetual growth, a creation pursued without end. . . . A perpetual becoming,' and to the extent that he can actively welcome change and let it become part of him, he comes to know that - again in Bergson’s phrase - 'to exist is to change, to change is too mature, to mature is to go on creating oneself endlessly.'
Philosophical issues about ‘perception’ tend to be issues specifically about ‘sense-perception’. In England (and the same is true of comparable terms in many other languages) the term ‘perception’ has a wider connotation than anything that has to do with the senses and sense-organs, though it generally involves the idea of what may imply, if only in a metaphorical sense, a point of view. Thus it is now increasingly common for news-commentators, for example, to speak of events, even though those people have not been witnesses of them. In one sense, however, there is nothing new about this, in seventeenth-and-eighteenth-century philosophical usage, words for perception were used with a much wider coverage than sense-perception alone. It is, however, sense-perception that has typically raised the largest and most obvious philosophical problems.
Such problems may be said to fall into two categories. These are, for the epistemological problems about the role of sense-perception in connection with the acquisition and possession of knowledge of the world around us. These problems - does perception give us knowledge of the so-called ‘external world’, and to what extent? - have become dominant in epistemology since Descartes because of his invocation of the method of doubt, although they undoubtedly existed in philosophers’ minds in one way or another before that. In early and middle twentieth-century Anglo-Saxon philosophy such problems centred on the question whether there are firm data provided by the senses - so-called sense-data - and if so what is the relation of such sense-data to so-called material objects. Such problems are not essentially problems for the philosophy of mind, although certain answers to questions about perception which undoubtedly belong to the philosophy of mind can certainly add to epistemological differences. If perception is assimilated, for example, to sensation, there is an obvious temptation to think that in perception we are restricted, at any rate initially, to the contents of our own minds.
The second category of problems about perception - those that fall directly under the heading of the philosophy of mind - are thus in a sense prior to the problems that exercised many empiricist in the first half of this century. They are problems about how perception is to be construed and how it relates to a number of other aspects of the mind’s functioning - sensations, concepts and other things involved in our understanding of things, beliefs and judgements, and the imagination, our action in relation to the world around us, and the causal processes involved in the physics, biology and psychology of perception. Some of the last were central to the considerations that Aristotle raised about perception in his ‘De Anima’.
It is obvious enough that sense-perception involves some kind of stimulation of sense organs by stimuli that are themselves the product of physical processes which are biological in character are then initiated. Moreover, only if the organism in which this takes place is adapted to such excitation, for which the stimulation can perception ensue. Aristotle had something to say about such matters, but it was evident to him that such an account was insufficient to explain what perception itself is. It might be thought that the most obvious thing is missing in such an account is some reference to consciousness. But while it may be the case that perception can take place only in creatures that have consciousness in some sense, it is not clear that every case of perception directly involves consciousness. There is such a thing as unconscious perception and psychologists have recently drawn attention to the phenomenon which is described as ‘blind-sight’ - an ability, generally manifested in patients with certain kinds of brain-damage, to discriminate sources of light, even when the people concerned have no consciousness of the lights and think that they are guessing about them. It is important, then, not to confuse the plausible claim that perception can take place only in conscious beings with the less plausible claim that perception always involves consciousness of objects. A similar point may apply to the relation of perception to some of the other items exposed to concept-possession.
Consciousness may possibly be the most challenging and persuasive source of problems in the whole of philosophy. Our own consciousness seems to be the most basic fact confronting us, yet it is almost impossible to say what consciousness is. Is mine like yours? Is ours like that of animals? Might machines come to have consciousness? Is it possible for there to be disembodied consciousness? Whatever complex biological and neural processes go on back-stage, it is my consciousness that provides the theatre where my experiences and thoughts have their existence, where my desires are felt and where my intentions are formed. But then how am I to conceive that ‘I’, or ‘self’ that is the spectator, or at any rate the owner of this theatre? There problems together make up what is sometimes called ‘the hard problem’ of consciousness. One of the difficulties in thinking about consciousness is that the problems seem not to be scientific ones. Gottfried Wilhelm Leibniz (1646-1716) remarked that if we could construct a field or machine, per se, and find to its expansive area, we still would not be able to find consciousness, so that consciousness resides in simple subjects, not complex ones. Even if we are convinced that consciousness somehow emerges from the complexity of brain functioning, we may still feel baffled about the way the emergence takes place, or why it takes place in just the way it does.
The nature of conscious experience has been the largest single obstacle to physicalism, behaviourism and functionalism in the philosophy of mind: These are all views that according to their opponents, can only be believed by feigning permanent anaesthesia. But many philosophers are convinced that we can divide and conquer: We may make progress not by thinking of one ‘hard’ problem, but by breaking the subject up into different skills and recognizing that rather than a single self or observer we would do better to think of a relatively undirected whirl of cerebral activity, with no inner theatre, no inner lights, and above all no inner spectator.
Til most recently it has been thought that in the study of how nerve cells, or neurons, receives and transmits information. Two types of phenomena are involved in processing nerve signals: Electrical and chemical. Electrical events propagate a signal within a neuron, and chemical processes transmit the signal from one neuron to another neuron or to a muscle cell.
A neuron is a long cell that has a thick central area containing the nucleus, it also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites receive impulses from other neurons. (The exceptions are sensory neurons, such as those that transmit information about temperature or touch, in which the signal is generated by specialized receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the tip of the axon the signal is chemically transmitted to an adjacent neuron or muscle cell.
Like all other cells, neurons contain charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they can produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes called membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory information or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negative too positively. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process. Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; These points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
When the electrical signal reaches the tip of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neurotransmitters, which are released into the microscopic space between neurons (the synaptic cleft). The neurotransmitter attaches on the surface of the adjacent neuron. This stimulus causes the adjacent cell to depolarize and propagate an action potential of its own. The duration of a stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and the reuptake by the neuron that produced them. Formerly, each neuron was thought to make only one transmitter, but recent studies have shown that some cells make two or more.
During the early 1900s, in examining the workings of the nervous system, physiologists were beginning to explore the idea that the transmission of nerve impulses takes place, in part, via chemical means. Loewi decided to explore this idea. During a stay in London in 1903, he met Sir Dale, who was also interested in the chemical transmission of nerve impulses. However, for Loewi, Dale, and all the other researchers pursuing a chemical transmitter of nerve impulses, years of effort produced no solid evidence. In 1921 Loewi suspended two frogs' hearts in solution, one with a major nerve removed. Removing fluid from the heart that still contained the nerve, and injecting the fluid into the nerveless heart, Loewi observed that the second heart behaved as if the missing nerve were present. The nerves, he concluded, do not act directly on the heart - it is the action of chemicals, freed by the stimulation of nerves, that causes increases in heart rate and other functional changes. In 1926 Loewi and his colleagues identified one of the chemicals in his experiment as acetylcholine. This was indisputably a neurotransmitter - a chemical that serves to transmit nerve impulses in the involuntary nervous system.
We acknowledge the neurotransmitters are inherently made by chemically induced neurons, or nerve cells. Neurons send out neurotransmitters as chemical signals to activate or inhibit the function of neighboring cells.
Within the central nervous system, which consists of the brain and the spinal cord, neurotransmitters pass from neuron to neuron. In the peripheral nervous system, which is made up of the nerves that run from the central nervous system to the rest of the body, the chemical signals pass between a neuron and an adjacent muscle or gland cells.
Chemical compounds - belonging to three chemical families - are widely recognized as neurotransmitters. In addition, certain other body chemicals, including adenosine, histamine, enkephalins, endorphins, and epinephrine, have neurotransmitterlike properties. Experts believe that there are many more neurotransmitters yet undiscovered.
The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Among the amine neurotransmitters are acetylcholine, norepinephrine, dopamine, and serotonin. Acetylcholine is the most widely used neurotransmitter in the body, and neurons that leave the central nervous system (for example, those running to skeletal muscle) use acetylcholine as their neurotransmitter; neurons that run to the heart, blood vessels, and other organs may use acetylcholine or norepinephrine. Dopamine is involved in the movement of muscles, and it controls the secretion of the pituitary hormone prolactin, which triggers milk production in nursing mothers.
The second neurotransmitter family is composed of amino acids, organic compounds containing both an amino group (NH2) and a carboxylic acid group (COOH). Amino acids that serve as neurotransmitters include glycine, glutamic and aspartic acids, and gamma-amino butyric acid (GABA). Glutamic acid and GABA are the most abundant neurotransmitters within the central nervous system, and especially in the cerebral cortex, which is largely responsible for such higher brain functions as thought and interpreting sensations.
The third neurotransmitter family is composed of peptides, which are compounds that contain at least two, and sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly understood, but scientists know that the peptide neurotransmitter called substance P influences the sensation of pain.
Overall, each neuron uses only a single compound as its neurotransmitter. However, some neurons outside the central nervous system can release both an amine and a peptide neurotransmitter.
Neurotransmitters are manufactured from precursor compounds like amino acids, glucose, and the dietary amine-called choline. Neurons modify the structure of these precursor compounds in a series of reactions with enzymes. Neurotransmitters that comes from amino acids include serotonin, for which it is derived from tryptophan. Dopamine and norepinephrine, under which are derived from tyrosine, and glycine, which is derived from threonine. Among the neurotransmitters made from glucose are glutamate, aspartate, and GABA. The choline serves as the precursor for acetylcholine
Neurotransmitters are released into a microscopic gap, called a synapse, that separates the transmitting neuron from the cell receiving the chemical signal. The cell that generates the signal is called the presynaptic cell, while the receiving cell is termed the postsynaptic cell.
After their release into the synapse, neurotransmitters combine chemically with highly specific protein molecules, termed receptors, embedded in the surface membranes of the postsynaptic cell. When this combination occurs, the voltage, or electrical force, of the postsynaptic cell is either increased (excited) or decreased (inhibited).
When a neuron is in its resting state, its voltage is about -70 millivolts. An excitatory neurotransmitter alters the membrane of the postsynaptic neuron, making it possible for ions (electrically charged molecules) to move back and forth across the neuron’s membranes. This flow of ions makes the neuron’s voltage rise toward zero. If enough excitatory receptors have been activated, the postsynaptic neuron responds by firing, generating a nerve impulse that causes its own neurotransmitter to be released into the next synapse. An inhibitory neurotransmitter causes different ions to pass back and forth across the postsynaptic neuron’s membrane, lowering the nerve cell’s voltage to -80 or -90 millivolts. The drop in voltage makes it less likely that the postsynaptic cell will fire.
If the postsynaptic cell is a muscle cell rather than a neuron, an excitatory neurotransmitter will cause the muscle to contract. If the postsynaptic cell is a gland cell, an excitatory neurotransmitter will cause the cell to secrete its contents.
While most neurotransmitters interact with their receptors to create new electrical nerve impulses that energize or inhibit the adjoining cell, some neurotransmitter interactions do not generate or suppress nerve impulses. Instead, they interact with a second type of receptor that changes the internal chemistry of the postsynaptic cell by either causing or blocking the formation of chemicals called second messenger molecules. These second messengers regulate the postsynaptic cell’s biochemical processes and enable it to conduct the maintenance necessary to continue synthesizing neurotransmitters and conducting nerve impulses. Examples of second messengers, which are formed and entirely contained within the postsynaptic cell, include cyclic adenosine monophosphate, diacylglycerol, and inositol phosphates.
Once neurotransmitters have been secreted into synapses and have passed on their chemical signals, the presynaptic neuron clears the synapse of neurotransmitter molecules. For example, acetylcholine is broken down by the enzyme acetylcholinesterase into choline and acetate. Neurotransmitters like dopamine, serotonin, and GABA is removed by a physical process called reuptake. In reuptake, a protein in the presynaptic membrane acts as a sort of sponge, causing the neurotransmitters to reenter the presynaptic neuron, where they can be broken down by enzymes or repackaged for reuse.
Neurotransmitters are known to be involved in many disorders, including Alzheimer’s disease. Victims of Alzheimer’s disease suffer from loss of intellectual capacity, disintegration of personality, mental confusion, hallucinations, and aggressive - even violent - behavior. These symptoms are the result of progressive degeneration in many types of neurons in the brain. Forgetfulness, one of the earliest symptoms of Alzheimer’s disease, is partly caused by the destruction of neurons that normally release the neurotransmitter acetylcholine. Medications that increase brain levels of acetylcholine have helped restore short-term memory and reduce mood swings in some Alzheimer’s patients.
Neurotransmitters also play a role in Parkinson disease, which slowly attacks the nervous system, causing symptoms that worsen over time. Fatigue, mental confusion, a masklike facial expression, stooping posture, shuffling gait, and problems with eating and speaking are among the difficulties suffered by Parkinson victims. These symptoms have been partly linked to the deterioration and eventual death of neurons that run from the base of the brain to the basal ganglia, a collection of nerve cells that manufacture the neurotransmitter dopamine. The reasons why such neurons die are yet to be understood, but the related symptoms can be alleviated. L-dopa, or levodopa, widely used to treat Parkinson disease, acts as a supplementary precursor for dopamine. It causes the surviving neurons in the basal ganglia to increase their production of dopamine, by that compensating to some extent for the disabled neurons.
Many other effective drugs have been shown to act by influencing neurotransmitter behavior. Some drugs work by interfering with the interactions between neurotransmitters and intestinal receptors. For example, belladonna decreases intestinal cramps in such disorders as irritable bowel syndrome by blocking acetylcholine from combining with receptors. This process reduces nerve signals to the bowel wall, which prevents painful spasms.
Other drugs block the reuptake process. One well-known example is the drug fluoxetine (Prozac), which blocks the reuptake of serotonin. Serotonin then remains in the synapse for a longer time, and its ability to act as a signal is prolonged, which contributes to the relief of depression and the control of obsessive-compulsive behaviors.
Dopamine, chemical known as a neurotransmitter essential to the functioning of the central nervous system. During neurotransmission, dopamine is transferred from one nerve cell, or neuron, to another, playing a key role in brain function and human behavior.
Dopamine forms from a precursor molecule called dopa, which is manufactured in the liver from the amino acid tyrosine. Dopa is then transported by the circulatory system to neurons in the brain, where the conversion to dopamine takes place.
Dopamine is a versatile neurotransmitter. Among its many functions, it plays a major role in two activities of the central nervous system: one that helps control movement, and a second that are strongly associated with emotion-based behaviors.
The pathway involved in movement control is called the nigrostriatal pathway. Dopamine is released by neurons that originate from an area of the brain called the substantia nigra and connect to the part of the brain known as the corpora striata, an area known to be important in controlling the musculoskeletal system.
The second brain pathway in which dopamine plays a major role is called the mesocorticolimbic pathway. Neurons in an area of the brain called the ventral tegmentalarea transmits dopamine to other neurons connected to various parts of the limbic system, which is responsible for regulating emotion, motivation, behavior, the sense of smell, and variously autonomic, or involuntary, functions like heartbeat and breathing. A growing body of evidence suggests that dopamine be involved in several major brain disorders. Narcolepsy, a disorder characterized by brief, recurring episodes of sudden, deep sleep, is associated with abnormally high levels of both dopamine and a second neurotransmitter, acetylcholine. Huntington’s chorea, an inherited, fatal illness in which neurons in the base of the brain are progressively destroyed, is also linked to an excess of dopamine.
Commonly known as shaking palsy, Parkinson disease is another brain disorder in which dopamine is involved. Besides tremors of the limbs, Parkinson patients suffer from muscular rigidity, which leads to difficulties in walking, writing, and speaking. This disorder results from the degeneration and death of neurons in the nigrostriatal pathway, resulting in low levels of dopamine. The symptoms of Parkinson disease can be reduced by treatment with a drug called levodopa, or L-dopa, which converts to dopamine in the brain.
Schizophrenia is a psychiatric disorder characterized by loss of contact with reality and major changes in personality. Schizophrenics have normal levels of dopamine in the brain, but because they are highly sensitive to this neurotransmitter, these normal levels of dopamine triggers unusual behaviors. Drugs such as thorazine that blocks the action of dopamine have been found to decrease the symptoms of schizophrenia.
Studies suggest that people who are addicted to alcohol and other drugs like, cocaine and nicotine have less dopamine in the mesocorticolimbic pathway. These drugs appear to increase dopamine levels, resulting in the pleasurable feelings associated with the drugs.
Serotonin, neurotransmitter, or chemical that transmits messages across the synapses, or gaps, between adjacent cells. Among its many functions, serotonin is released from blood cells called platelets to activate blood vessel constriction and blood clotting. In the gastrointestinal tract, serotonin inhibits gastric acid production and stimulates muscle contraction in the intestinal wall. Its functions in the central nervous system and effects on human behavior - including mood, memory, and appetite control - have been the subject of a great deal of research. This intensive study of serotonin has revealed important knowledge about the serotonin-related cause and treatment of many illnesses.
Serotonin is produced in the brain from the amino acid tryptophan, which is derived from foods high in protein, such as meat and dairy products. Tryptophan is transported to the brain, where it is broken down by enzymes to produce serotonin. During neurotransmission, serotonin is transferred from one nerve cell, or neuron, to another, triggering an electrical impulse that stimulates or inhibits cell activity as needed. Serotonin is then reabsorbed by the first neuron, in a process known as reuptake, where it is recycled and used again or converted into an inactive chemical form and excreted.
While the complete picture of serotonin’s function in the body is still being investigated, many disorders are known to be associated with an imbalance of serotonin in the brain. Drugs that manipulate serotonin levels have been used to alleviate the symptoms of serotonin imbalances. Some of these drugs, known as selective serotonin reuptake inhibitors (SSRIs), block or inhibit the reuptake of serotonin into neurons, enabling serotonin to remain active in the synapses for a longer period. These medications are used to treat such psychiatric disorders as depression; Obsessive-compulsive disorder, in which repetitive and disturbing thoughts trigger bizarre, ritualistic behaviors, and impulsive aggressive behaviors. Fluoxetine (more commonly known by the brand name Prozac), is a widely prescribed SSRI used to treat depression, and more recently, obsessive-compulsive disorder.
Drugs that affect serotonin levels may prove beneficial in the treatment of nonpsychiatric disorders as well, including diabetic neuropathy (degeneration of nerves outside the central nervous system in diabetics) and premenstrual syndrome. Recently the serotonin-releasing agent dexfenfluramine has been approved for patients who are 30 percent or more over their ideal body weight. By preventing serotonin reuptake, dexfenfluramine promotes satiety, or fullness, after eating less food.
Other drugs serve as agonists that react with neurons to produce effects similar to those of serotonin. Serotonin agonists have been used to treat migraine headaches, in which low levels of serotonin cause arteries in the brain to swell, resulting in a headache. Sumatriptan is an agonist drug that mimics the effects of serotonin in the brain, constricting blood vessels and alleviating pain.
Drugs known as antagonists bind with neurons to prevent serotonin neurotransmission. Some antagonists have been found effective in treating the nausea that typically accompanies radiation and chemotherapy in cancer treatment. Antagonists are also being tested to treat high blood pressure and other cardiovascular disorders by blocking serotonin’s ability to constrict blood vessels. Other antagonists may produce an effect on learning and memory in age-associated memory impairment.
The Synapse is the signal conveying everything that human beings sense and think, and every motion they make, follows nerve pathways in the human body as waves of ions (atoms or groups of atoms that carries electric charges). Australian physiologist Sir John Eccles discovered many intricacies of this electrochemical signaling process, particularly the pivotal step in which a signal is conveyed from one nerve cell to another. He shared the 1963 Nobel Prize in physiology or medicine for this work, which he described in a 1965 Scientific American article.
How does one nerve cell transmit the nerve impulse to another cell? Electron microscopy and other methods show that it does so by means of special extensions that deliver a squirt of transmitter substance
The human brain is the most highly organized form of matter known, and in complexity the brains of the other higher animals are not greatly inferior. For certain purposes regarding the brain for being analogous to a machine is expedient. Even if it is so regarded, however, it is a machine of a totally different kind from those made by man. In trying to understand the workings of his own brain man meets his highest challenge. Nothing is given; There are no operating diagrams, no maker's instructions.
The first step in trying to understand the brain is to examine its structure to discover the components from which it is built and how they are related to each another. After that one can attempt to understand the mode of operation of the simplest components. These two modes of investigation - the morphological and the physiological - have now become complementary. In studying the nervous system with today's sensitive electrical device, however, finding physiological events that cannot be correlated with any known anatomical structure is all too easy. Conversely, the electron microscope reveals many structural details whose physiological significance is obscure or unknown.
At the close of the past century the Spanish anatomist Santiago Ramón Cajal showed how all parts of the nervous system are built up of individual nerve cells of many different shapes and sizes. Like other cells, each nerve cell has a nucleus and the surrounding cytoplasm. Its outer surface consists of many fine branches - the dendrites - that receive nerve impulses from other nerve cells, and one relatively long branch - the axon - that transmits nerve impulses. Near its end the axon divides into branches that end at the dendrites or bodies of other nerve cells. The axon can be as short as a fraction of a millimeter or if a meter, depending on its place and function. It has many properties of an electric cable and is uniquely specialized to conduct the brief electrical waves called nerve impulses. In very thin axons these impulses travel at less than one meter per second; In others, for example in the large axons of the nerve cells that activate muscles, they travel as fast as 100 meters per second.
The electrical impulse that travels along the axon ceases abruptly when it comes to the point where the axon's terminal fibers contact another nerve cell. These junction points were given the name ‘synapses’ by Sir Charles Sherrington, who laid the foundations of what is sometimes called synaptology. If the nerve impulse is to continue beyond the synapse, it must be regenerated afresh on the other side. As recently as 15 years ago some physiologists held that transmission at the synapse was predominantly, if not exclusively, an electrical phenomenon. Now, however, there is abundant evidence that transmission is made by the release of specific chemical substances that trigger a regeneration of the impulse. In fact, the first strong evidence showing that some transmitter substance act across the synapse was provided more than 40 years ago by Sir Henry Dale and Otto Loewi.
It has been estimated that the human central nervous system, which of course includes the spinal cord and the brain itself, consists of about 10 billion (1010) nerve cells. With rare exceptions each nerve cell receives information directly as impulses from many other nerve cells - often hundreds - and transmits information to a like number. Depending on its threshold of response, a given nerve cell may fire an impulse when stimulated by only a few incoming fibers or it may not fire until stimulated by many incoming fibers. It has long been known that this threshold can be raised or lowered by various factors. Moreover, it was supposed some 60 years ago that some incoming fibers must inhibit the firing of the receiving cell rather than excite it. The conjecture was subsequently confirmed, and the mechanism of the inhibitory effect has now been clarified. This mechanism and its equally fundamental counterpart - nerve-cell excitation - are of its topic.
In the levels of anatomy there are some clues to show how the fine axon terminals impinging on a nerve cell can make the cell regenerate a nerve impulse of its own nerve cell and its dendrites are covered by fine branches of nerve fibers that end in knob-like structures. These structures are the synapses.
The electron microscope has revealed structural details of synapses that fit in nicely with the view that a chemical transmitter is involved in nerve transmission. Enclosed in the synaptic knob are many vesicles, or tiny sacs, which appear to contain the transmitter substances that induce synaptic transmission. Between the synaptic knob and the synaptic membrane of the adjoining nerve cell is a remarkably uniform space of about 20 millimicrons that is termed the synaptic cleft. Many of the synaptic vesicles are concentrated adjacent to this cleft; It seems plausible that the transmitter substance is discharged from the nearest vesicles into the cleft, where it can act on the adjacent cell membrane. This hypothesis is supported by the discovery that the transmitter is released in packets of a few thousand molecules.
The study of synaptic transmission was revolutionized in 1951 by the introduction of delicate techniques for recording electrically from the interior of single nerve cells. This is done by inserting into the nerve cell an extremely fine glass pipette with a diameter of .5 microns - about a fifty-thousandth of an inch. The pipette is filled with an electrically conducting salt solution such as concentrated potassium chloride. If the pipette is carefully inserted and held rigidly in place, the cell membrane appears to seal quickly around the glass, thus preventing the flow of a short-circuiting current through the puncture in the cell membrane. Impaled in this fashion, nerve cells can function normally for hours. Although there is no way of observing the cells during the insertion of the pipette, the insertion can be guided by using as clues the electric signals that the pipette picks up when close to active nerve cells.
At the John Curtin School of Medical Research in Canberra first employed this technique, choosing to study the large nerve cells called motoneurons, which lie in the spinal cord whose function is to activate muscles. This was a fortunate choice: Intracellular investigations with motoneurons are easier and more rewarding than those with any other kind of mammalian nerve cell.
Finding that when the nerve cell responds to the chemical synaptic transmitter, the response depends in part on characteristic features of ionic composition that are also concerned with the transmission of impulses in the cell and along its axon. When the nerve cell is at rest, its physiological makeup resembles that of most other cells in that the water solution inside the cell is quite different in composition from the solution in which the cell is bathed. The nerve cell can exploit this difference between external and internal composition and use it in quite different ways for generating an electrical impulse and for synaptic transmission.
The composition of the external solution is well established because the solution is essentially the same as blood from which cells and proteins have been removed. The composition of the internal solution is known only approximately. Indirect evidence suggests that the concentrations of sodium and chloride ions outside the cell are respectively some 10 and 14 times higher than the concentrations inside the cell. In contrast, the concentration of potassium ions inside the cell is about 30 times higher than the concentration outside.
How can one account for this remarkable state of affairs? Part of the explanation is that inside the cell is negatively charged with the respect of the cell about 70 millivolts. Since like charges repel each other, this internal negative charge tends to drive chloride ions (Cl-) outward through the cell membrane and, at the same time, to impede their inward movement. In fact, a potential difference of 70 millivolts is just sufficient to maintain the observed disparity in the concentration of chloride ions inside the cell and outside it; Chloride ions diffuse inward and outward at equal rates. A drop of 70 millivolts across the membrane therefore defines the ‘equilibrium potential’ for chloride ions.
To obtain a concentration of potassium ions (K) that is 30 times higher inside the cell than outside would require that the interior of the cell membrane be about 90 millivolts negative with respect to the exterior. Since the actual interior is only 70 millivolts negative, it falls short of the equilibrium potential for potassium ions by 20 millivolts. Evidently the thirtyfold concentration can be achieved and maintained only if there is some auxiliary mechanism for ‘pumping’ potassium ions into the cell at a rate equal to their spontaneous net outward diffusion.
The pumping mechanisms have fewer, but more difficult tasks of pumping sodium ions (Na) out of the cell against a potential gradient of 130 millivolts. This figure is obtained by adding the 70 millivolts of internal negative charge to the equilibrium potential for sodium ions, which is 60 millivolts of internal positive charge. If it were not for this postulated pump, the concentration of sodium ions inside and outside the cell would be almost the reverse of what is observed.
In their classic studies of nerve-impulse transmission in the giant axon of the squid, A. L. Hodgkin, A. F. Huxley and Bernhard Katz of Britain proved that the propagation of the impulse coincides with abrupt changes in the permeability of the axon membrane. When a nerve impulse has been triggered in some way, what can be described as a gate opens and lets sodium ions pour into the axon during the advance of the impulse, making the interior of the axon locally positive. The process is self-reinforcing in that the flow of some sodium ions through the membrane opens the gate further and makes it easier for others to follow. The sharp reversal of the internal polarity of the membrane makes up the nerve impulse, which moves like a wave until it has traveled the length of the axon. In the wake of the impulse the sodium gate closes and a potassium gate opens, by that restoring the normal polarity of the membrane within a millisecond or less.
With this understanding of the nerve impulse in hand, one is ready to follow the electrical events at the excitatory synapse. One might guess that if the nerve impulse results from an abrupt inflow of sodium ions and a rapid change in the electrical polarity of the axon's interior, something similar must happen at the body and dendrites of the nerve cell in order to generate the impulse in the first place. Indeed, the function of the excitatory synaptic terminals on the cell body and its dendrites is to depolarize the interior of the cell membrane essentially by permitting an inflow of sodium ions. When the depolarization reaches a threshold value, a nerve impulse is triggered.
As a simple instance of this phenomenon we have recorded the depolarization that occurs in a single motoneuron activated directly by the large nerve fibers that enter the spinal cord from special stretch-receptors known as annulospiral endings. These receptors in turn are found in the same muscle that is activated by the motoneuron under study. Thus the whole system forms a typical reflex arc, such as the arc responsible for the patellar reflex, or ‘knee jerk.’
To conduct the experiment we anesthetize an animal (most often a cat) and free by dissection a muscle nerves that contains these large nerve fibers. By applying a mild electric shock to the exposed nerve one can produce a single impulse in each of the fibers; Since the impulses travel to the spinal cord almost synchronously, they are referred to collectively as a volley. The number of impulses contained in the volley can be reduced by reducing the stimulation applied to the nerve. The volley strength is measured at a point just outside the spinal cord and is displayed on an oscilloscope. About half a millisecond after detection of a volley there is a wavelike change in the voltage inside the motoneuron that has received the volley. The change is detected by a microelectrode inserted in the motoneuron and is displayed on another oscilloscope.
What we find is that the negative voltage inside the cell becomes progressively fewer negative as more of the fibers impinging on the cell are stimulated to fire. This observed depolarization is in fact a simple summation of the depolarizations produced by each individual synapse. When the depolarization of the interior of the motoneuron reaches a critical point, a ‘spike’ suddenly appears on the second oscilloscope, showing that a nerve impulse has been generated. During the spike the voltage inside the cell changes from about 70 millivolts negative to as much as 30 millivolts positive. The spike regularly appears when the depolarization, or reduction of membrane potential, reaches a critical level, which is usually between 10 and 18 millivolts. The only effect of a further strengthening of the synaptic stimulus is to shorten the time needed for the motoneuron to reach the firing threshold. The depolarizing potentials produced in the cell membrane by excitatory synapses are called excitatory postsynaptic potentials, or EPSP's.
Through one barrel of a double-barreled microelectrode one can apply a background current to change the resting potential of the interior of the cell membrane, either increasing it or decreasing it. When the potential is made more negative, the EPSP rises more steeply to an earlier peak. When the potential is made less negative, the EPSP rises more slowly to a lower peak. Finally, when the charge inside the cell is reversed so as to be positive with respect to the exterior, the excitatory synapses give rise to an EPSP that is actually the reverse of the normal one.
These observations support the hypothesis that excitatory synapses produce what amounts virtually to a short circuit in the synaptic membrane potential. When this occurs, the membrane no longer acts as a barrier to the passage of ions but lets them flow through in response to the differing electric potential on the two sides of the membrane. In other words, the ions are momentarily allowed to travel freely down their electrochemical gradients, which means that the sodium ions flow into the cell and, to a lesser degree, potassium ions flow out. It is this net flow of positive ions that creates the excitatory postsynaptic potential. The flow of negative ions, such as the chloride ion, is apparently not involved. By artificially altering the potential inside the cell one can establish that there is no flow of ions, and therefore no EPSP, when the voltage drop across the membrane is zero.
How is the synaptic membrane converted from a strong ionic barrier into an ion-permeable state? It is currently accepted that the agency of conversion is the chemical transmitter substance contained in the vesicles inside the synaptic knob. When a nerve impulse reaches the synaptic knob, some of the vesicles are caused to eject the transmitter substance into the synaptic cleft. The molecules of the substance would take only a few microseconds to diffuse across the cleft and become attached to specific receptor sites on the surface membrane of the adjacent nerve cell.
Presumably the receptor sites are associated with fine channels in the membrane that are opened in some way by the attachment of the transmitter-substance molecules to the receptor sites. With the channels thus opened, sodium and potassium ions flow through the membrane thousands of times more readily than they normally do, by that producing the intense ionic flux that depolarizes the cell membrane and produces the EPSP. In many synapses the current flows strongly for only about a millisecond before the transmitter substance is eliminated from the synaptic cleft, either by diffusion into the surrounding regions or as a result of being destroyed by enzymes. The latter process is known to occur when the transmitter substance is acetylcholine, which is destroyed by the enzyme acetylcholinesterase.
The substantiation of this general picture of synaptic transmission requires the solution of many fundamental problems. Since we do not know the specific transmitter substance for the vast majority of synapses in the nervous system, we do not know whether there are many different substances or only a few. The only one identified with reasonable certainty in the mammalian central nervous system is acetylcholine. We know practically nothing about the mechanism by which a presynaptic nerve impulse causes the transmitter substance to be injected into the synaptic cleft. Nor do we know how the synaptic vesicles not immediately next to the synaptic cleft follow to moved up to the firing line to replace the emptied vesicles. It is supposed that the vesicles contain the enzyme systems needed to recharge themselves. The entire process must be swift and efficient: The total amount of transmitter substance in synaptic terminals is enough for only a few minutes of synaptic activity at normal operating rates. There are also knotty problems to be solved on the other side of the synaptic cleft. What, for example, is the nature of the receptor sites? How are the ionic channels in the membrane opened?
The second type of synapse that has been identified in the nervous system. These are the synapses that can inhibit the firing of a nerve cell even though it may be receiving a volley of excitatory impulses. When inhibitory synapses are examined in the electron microscope, they look very much like excitatory synapses. (There are probably some subtle differences, but they need not concern us here.) Microelectrode recordings of the activity of single motoneurons and other nerve cells have now shown that the inhibitory postsynaptic potential (IPSP) is virtually a mirror image of the EPSP. Moreover, individual inhibitory synapses, like excitatory synapses, have a cumulative effect. The chief difference is simply that the IPSP makes the cell's internal voltage more negative than it is normally, which is in a direction opposite to that needed for generating a spike discharge.
By driving the internal voltage of a nerve cell in the negative direction inhibitory synapses oppose the action of excitatory synapses, which of course drive it in the positive direction. So if the potential inside a resting cell is 70 millivolts negative, a strong volley of inhibitory impulses can drive the potential to 75 or 80 millivolts depreciating count. One can easily see that if the potential is made more negative in this way the excitatory synapses find it more difficult to raise the internal voltage to the threshold point for the generation of a spike. Thus, the nerve cell responds to the algebraic sum of the internal voltage changes produced by excitatory and inhibitory synapses.
If, as in the experiment described earlier, the internal membrane potential is altered by the flow of an electric current through one barrel of a double-barreled microelectrode, one can observe the effect of such changes on the inhibitory postsynaptic potential. When the internal potential is made less negative, the inhibitory postsynaptic potential is deepened. Conversely, when the potential is made more negative, the IPSP diminishes; it finally reverses when the internal potential is driven below minus 80 millivolts.
One can therefore assume that inhibitory synapse’s share with excitatory synapses the ability to change the ionic permeability of the synaptic membrane. The difference is that inhibitory synapses enable ions to flow freely down an electrochemical gradient that has an equilibrium point at minus 80 millivolts rather than at zero, as is the case for excitatory synapses. This effect could be achieved by the outward flow of positively charged ions such as potassium or the inward flow of negatively charged ions such as chloride, or by a combination of negative and positive ionic flows such that the interior reaches equilibrium at minus 80 millivolts.
If the concentration of chloride ions within the cell is increased as much as three times, the inhibitory postsynaptic potential reverses and acts as a depolarizing current; that is, it resembles an excitatory potential. On the other hand, if the cell is heavily injected with sulfate ions, which are also negatively charged, there is no such reversal. This simple test shows that under the influence of the inhibitory transmitter substance, which is still unidentified, the subsynaptic membrane becomes permeable momentarily to chloride ions but not to sulfate ions. During the generation of the IPSP the outflow of chloride ions is so rapid that it more than outweighs the flow of other ions that generate the normal inhibitory potential.
The effect of injecting motoneurons with more than 30 kinds of negatively lunged ions. With one exception the hydrated ions (ions bound to water) to which the cell membrane is permeable under the influence of the inhibitory transmitter substance are smaller than the hydrated ions to which the membrane is impermeable. The exception is the formate ion (HCO2-), which may have an ellipsoidal shape and so be able to pass through membrane pores that block smaller spherical ions.
Apart from the formate ion all the ions to which the membrane is permeable have a diameter not greater than 1.14 times the diameter of the potassium ion; That is, they are less than 2.9 angstrom units in diameter. Comparable investigations in other laboratories have found the same permeability effects, including the exceptional behavior of the formate ion, in fishes, toads and snails. It might be that the ionic mechanism responsible for synaptic inhibition is the same throughout the animal kingdom.
The significance of these and other studies is that they strongly suggest that the inhibitory transmitter substance open the membrane to the flow of potassium ions but not to sodium ions. It is known that the sodium ion is somewhat larger than any of the negatively charged ions, including the formate ion, that are able to pass through the membrane during synaptic inhibition. Testing the effectiveness of potassium ions by injecting excess amounts into the cell is not possible, however, because the excess is immediately diluted by an osmotic flow of water into the cell.
The concentration of potassium ions inside the nerve cell is about 30 times greater than the concentration outside, and to maintain this large difference in concentration without the help of some metabolic pumps inside of the membrane would have to be charged 90 millivolts negative with respect to the exterior. This implies that if the membrane were suddenly made porous to potassium ions, the resulting outflow of ions would make the inside potential of the membrane even more negative than it is in the resting state, and that is just what happens during synaptic inhibition. The membrane must not simultaneously become porous to sodium ions, because they exist in much higher concentration outside the cell than inside and their rapid inflow would more than compensate for the potassium outflow. In fact, the fundamental difference between synaptic excitation and synaptic inhibition is that the membrane freely passes sodium ions in response to the former and largely excludes the passage of sodium ions in response to the latter.
This fine discrimination between ions that are not very different in size must be explained by any hypothesis of synaptic action. It is most unlikely that the channels through the membrane are created afresh and accurately maintained for a thousandth of a second every time a burst of transmitter substance is released into the synaptic cleft. It is more likely that channels of at least two different sizes are built directly into the membrane structure. In some way the excitatory transmitter substance would selectively unplug the larger channels and permit the free inflow of sodium ions. Potassium ions would simultaneously flow out and thus would tend to counteract the large potential change that would be produced by the massive sodium inflow. The inhibitory transmitter substance would selectively unplug the smaller channels that are large enough to pass potassium and chloride ions but not sodium ions.
To explain certain types of inhibition other features must be added to this hypothesis of synaptic transmission. In the simple hypothesis chloride and potassium ions can flow freely through pores of all inhibitory synapses. It has been shown, however, that the inhibition of the contraction of heart muscle by the vagus nerve is due almost exclusively to potassium-ion flow. On the other hand, in the muscles of crustaceans and in nerve cells in the snail's brain synaptic inhibition is due largely to the flow of chloride ions. This selective permeability could be explained if there were fixed charges along the walls of the channels. If such charges were negative, they would repel negatively charged ions and prevent their passage; if they were positive, they would similarly prevent the passage of positively charged ions. One can now suggest that the channels opened by the excitatory transmitter are negatively charged and so do not permit the passage of the negatively charged chloride ion, even though it is small enough to move through the channel freely.
One might wonder if a given nerve cell can have excitatory synaptic action at some of its axon terminals and inhibitory action at others. The answer is no. Two different kinds of nerve cells are needed, one for each type of transmission and synaptic transmitter substance. This can readily be shown by the effect of strychnine and tetanus toxins in the spinal cord; They specifically prevent inhibitory synaptic action and leave excitatory action unaltered. As a result the synaptic excitation of nerve cells is uncontrolled and convulsions result. The special types of cells responsible for inhibitory synaptic action are now being recognized in many parts of the central nervous system.
This account of communication between nerve cells is necessarily oversimplified, yet it shows that some significant advances are being made at the level of individual components of the nervous system. By selecting the most favorable situations we have been able to throw light on some details of nerve-cell behavior. We can be encouraged by these limited successes. Nevertheless, the task of understanding in a comprehensive way how the human brain operates staggers its own imagination.
Our brain begins with its portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions - including love, hate, fear, anger, elation, and sadness - are controlled by the brain. It also receives and interprets the countless signals that are sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.
The human brain has three major structural components: the large dome-shaped cerebrum, the smaller somewhat spherical cerebellum, and the brainstem. Prominent in the brainstem are the medulla oblongata and the thalamus - between the medulla and the cerebrum. The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus act as a relay center for electrical impulses traveling to and from the cerebral cortex.
The adult human brain is a 1.3-kg. (3-lb.) Mass of pinkish-gray jellylike tissue made up of approximately 100 billion nerve cells or neurons: The Neuroglia (supporting-tissue) cells, and vascular (blood-carrying) and other tissues.
Between the brain and the cranium - the part of the skull that directly covers the brain - are three protective membranes, or meninges. The outermost membrane, the dura mater, is the toughest and thickest. Below the dura mater is a middle membrane, called the arachnoid layer. The innermost membrane, the pia mater, consists mainly of small blood vessels and follows the contours of the surface of the brain.
A clear liquid, the cerebrospinal fluid, bathes the entire brain and fills a series of four cavities, called ventricles, near the center of the brain. The cerebrospinal fluid protects the internal portion of the brain from varying pressures and transports chemical substances within the nervous system.
From the outside, the brain appears as three associatively distinct but connected parts, the cerebrum (the Latin word for brain) - two large, almost symmetrical hemispheres; the cerebellum ('little brain') - two smaller hemispheres located at the back of the cerebrum; and the brain stem - a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. Two other major parts of the brain, the thalamus and the hypothalamus, lie in the midline above the brain stem underneath the cerebellum.
The brain and the spinal cord together make up the central nervous system, which communicates with the rest of the body through the peripheral nervous system. The peripheral nervous system consists of 12 pairs of cranial nerves extending from the cerebrum and brain stem; a system of other nerves branching throughout the body from the spinal cord, and the autonomic nervous system, which regulates vital functions is not very consciously of its own control, such as the activity of the heart muscle, smooth muscle (involuntary muscle found in the skin, blood vessels, and internal organs), and glands.
Many motor and sensory functions have been ‘mapped’ to specific areas of the cerebral cortex, some of which are indicated here. In general, these areas exist in both hemispheres of the cerebrum, each serving the opposite side of the body. Fewer defined are the areas of association, located mainly in the frontal cortex, operatives in functions of thought and emotion and responsible for linking input from different senses. The areas of language are an exception: Both Wernicke’s area, concerned with the comprehension of spoken language, and Broca’s area, governing the production of speech, have been pinpointed on the cortex.
Most high-level brain functions take place in the cerebrum. Its two large hemispheres make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortices - roughly, 1.5 m2 (16 ft2) in an adult - to fit within the cranium. The pattern of these convolutions is similar, although not identical, in all humans.
The two cerebral hemispheres are partially separated from each other by a deep fold known as the longitudinal fissure. Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest of which is the corpus callosum.
Several major sulci divides the cortex into distinguishable regions. The central sulcus, or Rolandic fissure, runs from the middle of the top of each hemisphere downward, forwards, and toward another major sulcus, the lateral (side), or Sylvian, sulcus. These and other sulci and gyri divide the cerebrum into five lobes: The frontal, parietal, temporal, and occipital lobes and the insula.
Although the cerebrum is symmetrical in structure, with two lobes emerging from the brain stem and matching motor and sensory areas in each, certain intellectual functions are restricted to one hemisphere. A person’s dominant hemisphere is usually occupied with language and logical operations, while the other hemisphere controls emotion and artistic and spatial skills. In nearly all right-handed and many left-handed people, the left hemisphere is dominant.
The frontal lobe is the largest of the five and consists of all the cortices in front of the central sulcus. Broca's area, a part of the cortex related to speech, is located in the frontal lobe. The parietal lobe consists of the cortex behind the central sulcus to some sulcus near the back of the cerebrum known as the parieto-occipital sulcus. The parieto-occipital sulcus, in turn, forms the front border of the occipital lobe, which are the rearmost part of the cerebrum. The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is located in the temporal lobe. The insula lies deep within the folds of the lateral sulcus.
The cerebrum receives information from all the sense organs and sends motor commands (signals that results in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, parallel strips of cerebral cortex just in back of the central sulcus, receives input from the sense organs.
Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortices are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.
The cerebellum coordinates body movements. Located at the lower back of the brain beneath the occipital lobes, the cerebellum is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum, the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem - the midbrain, the pons, and the medulla oblongata.
The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.
The limbic system is a group of brain structures that play a role in emotion, memory, and motivation. For example, electrical stimulation of the amygdala in laboratory animals can provoke fear, anger, and aggression. The hypothalamus regulates hunger, thirst, sleep, body temperature, sexual drive, and other functions.
The thalamus and the hypothalamus lie underneath the cerebrum and connect it to the brain stem. The thalamus consist of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus are the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.
The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.
The brain stem, shown here in colored cross section, is the lowest part of the brain. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rates, as well as reflexes like eye movement and vomiting. The brain stem has three main parts: the medulla, pons, and midbrain. A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.
The brain stem is revolutionarily the most primitive part of the brain and is responsible for sustaining the basic functions of life, such as breathing and blood pressure. It includes three main structures lying between and below the two cerebral hemispheres - the midbrain, pons, and medulla oblongata.
The topmost structure of the brain stem is the midbrain. It contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers - pathways carrying sensory (input) information and motor (output) command. Relays and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei, called the inferior colliculus, controls auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.
Continuous with and below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.
The long, stalklike lowermost portion of the brain stem is called the medulla oblongata. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body, and the right half of the brain with the left half of the body.
Running up the brain stem from the medulla oblongata through the pons and the midbrain is a netlike formation of nuclei known as the reticular formation. The reticular formation controls respiration, cardiovascular function, digestion, levels of alertness, and patterns of sleep. It also determines which parts of the constant flow of sensory information into the body are received by the cerebrum.
There are two main types of brain cells, neurons and neuroglia. Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, and a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m. (3.3 ft.) in length. Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.
Neuroglial cells are twice as numerous as neurons and account for half of the brain's weight. Neuroglia (from glia, Greek for 'glue') provides structural support to the neurons. Neuroglial cells also form myelin, guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.
Twelve pairs of cranial nerves arise symmetrically from the base of the brain and are numbered, from front to back, in the order in which they arise. They connect mainly with structures of the head and neck, such as the eyes, ears, nose, mouth, tongue, and throat. Some are motor nerves, controlling muscle movement; some are sensory nerves, conveying information from the sense organs; and others contain fibers for both sensory and motor impulses. The first and second pairs of cranial nerves - the olfactory (smell) nerves and the optic (vision) nerve - carry sensory information from the nose and eyes, respectively, to the undersurface of the cerebral hemispheres. The other ten pairs of cranial nerves originate in or end in the brain stem.
The brain functions by complex neuronal, or nerve cell, circuits. Communication between neurons is both electrical and chemical and always travels from the dendrites of a neuron, through its soma, and out its axon to the dendrites of another neuron.
Dendrites of one neuron receive signals from the axons of other neurons through chemicals known as neurotransmitters. The neurotransmitters set off electrical charges in the dendrites, which then carry the signals electrochemically to the soma. The soma integrates the information, which is then transmitted electrochemically down the axon to its tip.
At the tip of the axon, small, bubble-like structures called vesicles’ release neurotransmitters that carries the signal across the synapse, or gap, between two neurons. There are many types of neurotransmitters, including norepinephrine, dopamine, and serotonin. Neurotransmitters can be excitatory (that is, they excite an electrochemical response in the dendrite receptors) or inhibitory (they block the response of the dendrite receptors).
One neuron may communicate with thousands of other neurons, and many thousands of neurons are involved with even the simplest behavior. It is believed that these connections and their efficiency can be modified, or altered, by experience.
Scientists have used two primary approaches to studying how the brain works. One approach is to study brain function after parts of the brain have been damaged. Functions that disappear or that is no longer normal after injury to specific regions of the brain can often be associated with the damaged areas. The second approach is to study the response of the brain to direct stimulation or to stimulation of various sense organs.
Neurons are grouped by function into collections of cells called nuclei. These nuclei are connected to form sensory, motor, and other systems. Scientists can study the function of somatosensory (pain and touch), motor, olfactory, visual, auditory, language, and other systems by measuring the physiological (physical and chemical) change that occur in the brain when these senses are activated. For example, electroencephalography (EEG) measures the electrical activity of specific groups of neurons through electrodes attached to the surface of the skull. Electrodes incorporate directly into the brain can give readings of individual neurons. Changes in blood flow, glucose (sugar), or oxygen consumption in groups of active cells can also be mapped.
Although the brain appears symmetrical, how it functions is not. Each hemisphere is specializing and dominates the other in certain functions. Research has shown that hemispheric dominance is related to whether a person is predominantly right-handed or left-handed. In most right-handed people, the left hemisphere processes arithmetic, language, and speech. The right hemisphere interprets music, complex imagery, and spatial relationships and recognizes and expresses emotion. In left-handed people, the pattern of brain organization is more variable.
Hemispheric specialization has traditionally been studied in people who have sustained damage to the connections between the two hemispheres, as may occur with a stroke, an interruption of blood flow to an area of the brain that causes the death of nerve cells in that area. The division of functions between the two hemispheres has also been studied in people who have had to have the connection between the two hemispheres surgically cut in order to control severe epilepsy, a neurological disease characterized by convulsions and loss of consciousness.
The visual system of humans is one of the most advanced sensory systems in the body. More information is conveyed visually than by any other means. In addition to the structures of the eye itself, several cortical regions - collectively called a primary visual and visual associative cortex - as well as the midbrain are involved in the visual system. Conscious processing of visual input occurs in the primary visual cortex, but reflexive - that is, immediate and unconscious - responses occur at the superior colliculus in the midbrain. Associative cortical regions - specialized regions that can associate, or integrate, multiple inputs - in the parietal and frontal lobes along with parts of the temporal lobe are also involved in the processing of visual information and the establishment of visual memories.
Language involves specialized cortical regions in a complex interaction that allows the brain to comprehend and communicate abstract ideas. The motor cortex initiates impulses that travel through the brain stem to produce audible sounds. Neighboring regions of motor cortices, called the supplemental motor cortex, are involved in sequencing and coordinating sounds. Broca's area of the frontal lobe is responsible for the sequencing of language elements for output. The comprehension of language is dependent upon Wernicke's area of the temporal lobe. Other cortical circuits connect these areas.
Memory is usually considered a diffusely stored associative process - that is, it puts together information from many different sources. Although research has failed to identify specific sites in the brain as locations of individual memories, certain brain areas are critical for memory to function. Immediate recall - the ability to repeat short series of words or numbers immediately after hearing them - is thought to be located in the auditory associative cortex. Short-term memory - the ability to retain a limited amount of information for up to an hour - is located in the deep temporal lobe. Long-term memory probably involves exchanges between the medial temporal lobe, various cortical regions, and the midbrain.
The autonomic nervous system regulates the life support systems of the body reflexively - that is, without conscious direction. It automatically controls the muscles of the heart, digestive system, and lungs; Certain glands, and homeostasis - that is, the equilibrium of the internal environment of the body. The autonomic nervous system itself is controlled by nerve centers in the spinal cord and brain stem and is fine-tuned by regions higher in the brain, such as the midbrain and cortex. Reactions such as blushing indicate that cognitive, or thinking, centers of the brain are also involved in autonomic responses.
The brain is guarded by several highly developed protective mechanisms. The bony cranium, the surrounding meninges, and the cerebrospinal fluid all contribute to the mechanical protection of the brain. In addition, a filtration system called the blood-brain barrier protects the brain from exposure to potentially harmful substances carried in the bloodstream.
Brain disorders have a wide range of causes, including head injury, stroke, bacterial diseases, complex chemical imbalances, and changes associated with aging.
Head injury can initiate a cascade of damaging events. After a blow to the head, a person may be stunned or may become unconscious for a moment. This injury, called - concussion, - usually leaves no permanent damage. If the blow is more severe and hemorrhage (excessive bleeding) and swelling occurs, however, severe headache, dizziness, paralysis, a convulsion, or temporary blindness may result, depending on the area of the brain affected. Damage to the cerebrum can also result in profound personality changes.
Damage to Broca's area in the frontal lobe causes difficulty in speaking and writing, a problem known as Broca's aphasia. Injury to Wernicke's area in the left temporal lobe results in an inability to comprehend spoken language, called Wernicke's aphasia.
An injury or disturbance to a part of the hypothalamus may cause a variety of different symptoms, such as loss of appetite with an extreme drop in body weight, increase in appetite leading to obesity; Extraordinary thirst with excessive urination (diabetes insipidus), failure in body-temperature control, resulting in either low temperature (hypothermia) or high temperature (fever), excessive emotionality, and uncontrolled anger or aggression. If the relationship between the hypothalamus and the pituitary gland is damaged, other vital bodily functions may be disturbed, such as sexual function, metabolism, and cardiovascular activity.
Injury to the brain stem is even more serious because it houses the nerve centers that control breathing and heart action. Damage to the medulla oblongata usually results in immediate death.
A stroke is damage to the brain due to an interruption in blood flow. The interruption may be caused by a blood clot, constriction of a blood vessel, or rupture of a vessel accompanied by bleeding. A pouchlike expansion of the wall of a blood vessel, called an aneurysm, may weaken and burst, for example, because of high blood pressure.
Sufficient quantities of glucose and oxygen, transported through the bloodstream, are needed to keep nerve cells alive. When the blood supply to a small part of the brain is interrupted, the cells in that area die and the function of the area is lost. A massive stroke can cause a one-sided paralysis (hemiplegia) and sensory loss on the side of the body opposite the hemisphere damaged by the stroke.
Some brain diseases, such as multiple sclerosis and Parkinson disease, are progressive, becoming worse over time. Multiple sclerosis damages the myelin sheath around axons in the brain and spinal cord. As a result, the affected axons cannot transmit nerve impulses properly. Parkinson disease destroys the cells of the substantia nigra in the midbrain, resulting in a deficiency in the neurotransmitter dopamine that affects motor functions.
Cerebral palsy is a broad term for brain damage sustained close to birth that permanently affects motor function. The damage may take place either in the developing fetus, during birth, or just after birth and is the result of the faulty development or breaking down of motor pathways. Cerebral palsy is nonprogressive - that is, it does not worsen with time.
A bacterial infection in the cerebrum or in the coverings of the brain, swelling of the brain, or an abnormal growth of healthy brain tissue can all cause an increase in intracranial pressure and result in serious damage to the brain.
Scientists are finding that certain brain chemical imbalances are associated with mental disorders such as schizophrenia and depression. Such findings have changed scientific understanding of mental health and have resulted in new treatments that chemically correct these imbalances.
During childhood development, the brain is particularly susceptible to damage because of the rapid growth and reorganization of nerve connections. Problems that originate in the immature brain can appear as epilepsy or other brain-function problems in adulthood.
Several neurological problems are common in aging. Alzheimer's disease damages many areas of the brain, including the frontal, temporal, and parietal lobes. The brain tissue of people with Alzheimer's disease shows characteristic patterns of damaged neurons, known as plaques and tangles. Alzheimer's disease produces progressive dementia, characterized by symptoms such as failing attention and memory, loss of mathematical ability, irritability, and poor orientation in space and time.
A magnetic resonance imaging (MRI) scan of the human brain reveals the contours of one of the brain’s hemispheres. The gyri, or ridges, appear in red, while the sulci, or valleys, are shown in blue. Each person has slightly different patterns of gyri and sulci, which reflect individual differences in brain development.
Several commonly used diagnostic methods give images of the brain without invading the skull. Some portray anatomy - that is, the structure of the brain - whereas others measure brain function. Two or more methods may be used to complement each other, together providing a more complete picture than would be possible by one method alone.
Magnetic resonance imaging (MRI), introduced in the early 1980s, beams high-frequency radio waves into the brain in a highly magnetized field that causes the protons that form the nuclei of hydrogen atoms in the brain to reemit the radio waves. The reemitted radio waves are analyzed by computer to create thin cross-sectional images of the brain. MRI provides the most detailed images of the brain and is safer than imaging methods that use X-rays. However, MRI is a lengthy process and also cannot be used with people who have pacemakers or metal implants, both of which are adversely affected by the magnetic field.
Computed tomography (CT), also known as CT scans, developed in the early 1970s. This imaging method X-rays the brain from many different angles, feeding the information into a computer that produces a series of cross-sectional images. CT is particularly useful for diagnosing blood clots and brain tumors. It is a much quicker process than magnetic resonance imaging and is therefore advantageous in certain situations - for example, with people who are extremely ill.
This positron emission tomography (PET) scans of the brain shows the activity of brain cells in the resting state and during three types of auditory stimulation. PET uses radioactive substances introduced within the brain to measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. This imaging method collects data from many different angles, feeding the information into a computer that produces a series of cross-sectional images.
Changes in brain function due to brain disorders can be visualized in several ways. Magnetic resonance spectroscopy measures the concentration of specific chemical compounds in the brain that may change during specific behaviors. Functional magnetic resonance imaging (fMRI) maps changes in oxygen concentration that correspond to nerve cell activity.
Positron emission tomography (PET), developed in the mid-1970s, uses computed tomography to visualize radioactive tracers, radioactive substances are introduced into the brain intravenously or by inhalation. PET can measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. Single photon emission computed tomography (SPECT), developed in the 1950s and 1960s, used radioactive tracers to visualize the circulation and volume of blood in the brain.
Brain-imaging studies have provided new insights into sensory, motor, language, and memory processes, as well as brain disorders such as epilepsy, cerebrovascular disease; Alzheimer's, Parkinson, and Huntington's diseases, and various mental disorders, such as schizophrenia.
Although all vertebrate brains share the same basic three-part structure, the development of their constituent parts varies across the evolutionary scale. In fish, the cerebrum is dwarfed by the rest of the brain and serves mostly to process input from the senses. In reptiles and amphibians, the cerebrum is proportionally larger and begins to connect and form conclusions about this input. Birds have well-developed optic lobes, making the cerebrum even larger. Among mammals, the cerebrum dominates the brain. It is most developed among primates, in whom cognitive ability is the highest.
In lower vertebrates, such as fish and reptiles, the brain is often tubular and bears a striking resemblance to the early embryonic stages of the brains of more highly evolved animals. In all vertebrates, the brain is divided into three regions: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). These three regions further sub-divide into different structures, systems, nuclei, and layers.
The more highly evolved the animal, the more complex is the brain structure. Human beings have the most complex brains of all animals. Evolutionary forces have also resulted in a progressive increase in the size of the brain. In vertebrates lower than mammals, the brain is small. In meat-eating animals, particularly primates, the brain increases dramatically in size.
The cerebrum and cerebellum of higher mammals are highly convoluted in order to fit the most gray matter surface within the confines of the cranium. Such highly convoluted brains are called gyrencephalic. Many lower mammals have a smooth, or lissencephalic (smooth head), cortical surfaces.
There is also evidence of evolutionary adaption of the brain. For example, many birds depend on an advanced visual system to identify food at great distances while in flight. Consequently, their optic lobes and cerebellum are well developed, giving them keen sight and outstanding motor coordination in flight. Rodents, on the other hand, as nocturnal animals, do not have a well-developed visual system. Instead, they rely more heavily on other sensory systems, such as a highly-developed sense of smell and facial whiskers.
Recent research in brain function suggests that there may be sexual differences in both brain anatomy and brain function. One study indicated that men and women may use their brains differently while thinking. Researchers used functional magnetic resonance imaging to observe which parts of the brain were activated as groups of men and women tried to determine whether sets of nonsense words rhymed. Men used only Broca's area in this task, whereas women used Broca's area plus an area on the right side of the brain.
The Cell, in [biology] is the most basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of the trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.
The word cell refers to several types of organisms. Cells such as paramecia, dinoflagellates, diatoms, and spirochetes are self-maintaining organisms; cells such as lymphocytes, erythrocytes, muscle cells, nerve cells, cardiac muscle, and chromoplasts are more specializing cells that are a part of higher multicellular organisms. Nonetheless, of its size or whether the cell is a complete organism or just part of an organism, all cells have certain structural components in common. All cells have some type of outer cell boundary that permits some materials to leave and enter the cell and a cell interior composed of a water-rich, fluid material called cytoplasm that contains hereditary material in the form of deoxyribonucleic acid (DNA).
Cells vary considerably in size. The smallest cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm. (0.000004 in.) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair. Among the largest cells are the nerve cells that run down a giraffe’s neck; these cells can exceed 3 m. (9.7 ft.) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm. (0.00003 in.) to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present an array of shapes. Some, such as the bacterium Escherichia coli, resemble rods. The paramecium, a type of protozoan, is a slipper shaped. The amoeba, another protozoan, has an irregular form that changes shape as it moves around. Plant cells typically resemble boxes or cubes. In humans, the outermost layers of skin cells are flat, while muscle cells are long and thin. Some nerve cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored to the cell’s job. For example, flat skin cells pack tightly into a layer that protects the underlying tissues from invasions by bacteria. Long, thin muscle cells’ contract readily to move bones. The numerous extensions from a nerve cell enable it to connect to several other nerve cells in order to send and receive messages rapidly and efficiently.
By itself, each cell is a model of independence and self-containment. Like some miniature, walled city in perpetual rush hour, the cell constantly bustles with traffic, shuttling essential molecules from place to place to carry out the business of living. Despite their individuality, however, cells also display a remarkable ability to join, communicate, and coordinate with other cells. The human body, for example, consists of an estimated 20 to 30 trillion cells. Dozens of different kinds of cells are organized into specialized groups called tissues. Tendons and bones, for example, are composed of connective tissue, whereas skin and mucous membranes are built from epithelial tissue. Different tissue types are assembled into organs, which are structures specialized to perform particular functions. Examples of organs include the heart, stomach, and brain. Organs, in turn, are organized into systems such as the circulatory, digestive, or nervous systems. All together, these assembled organ systems form the human body.
The components of cells are molecules, nonliving structures formed by the union of atoms. Small molecules serve as building blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and also participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membrane-bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid (RNA), works with DNA to build the thousands of proteins the cell needs.
Cells fall into one of two categories: Prokaryotic or eukaryotic, in a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell’s interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain numerous compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell’s command center and information library. The term prokaryote comes from Greek words that mean ‘before the nucleus’ or ‘prenucleus,’ while eukaryote means ‘a true nucleus.’
Bacteria’s cells typically are surrounded by a rigid, protective cell wall. The cell membrane, also called the plasma membrane, regulates passage of materials into and out of the cytoplasm, the semi-fluid that fill the cell. The DNA, located in the nucleoid region, contains the genetic information for the cell. Ribosomes carry out protein synthesis. Many bacteria contain some pilus (plural pili), a structure that extends out of the cell to transfer DNA to another bacterium. The flagellum, found in numerous species, is used for the locomotion. Some bacteria contain a plasmid, a small chromosomes with extra genes. Others have a capsule, a sticky substance external to the cell wall that protects bacteria from attack by white blood cells. Mesosomes were formerly thought to be structures with unknown functions, but now are known to be artifacts created when cells are prepared for viewing with electron microscopes.
Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm. (0.000004 to 0.0001 in.) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rod-like, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.
Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell’s contents from its surrounding fluids.
While small molecules such as water, oxygen, and carbon dioxide diffuse freely across the plasma membrane, the passage of many larger molecules, including amino acids (the building blocks’ of proteins) and sugars, is carefully regulated. Specialized transport proteins accomplish this task. The transport proteins span the plasma membrane, forming an intricate system of pumps and channels through which traffic is conducted. Some substances swirling in the fluid around the cell can enter it only if they bind to and are escorted in by specific transport proteins. In this way, the cell fine-tunes its internal environment.
The plasma membrane encloses the cytoplasm, the semifluid that fill the cell. Composed of about 65 percent water, the cytoplasm is packed with up to a billion molecules per cell, a rich storehouse that includes enzymes and dissolved nutrients, such as sugars and amino acids. The water provides a favorable environment for the thousands of biochemical reactions that take place in the cell.
Within the cytoplasm of all prokaryote is deoxyribonucleic acid (DNA), a complex molecule in the form of a double helix, a shape similar to a spiral staircase. The DNA is about 1,000 times the length of the cell, and to fit inside, it repeatedly twists and folds to form a compact structure called a chromosome. The chromosome in prokaryote is circular, and is located in a region of the cell called the nucleoid. Often, smaller chromosomes called plasmids are located in the cytoplasm. The DNA is divided into units called genes, just like a long train is divided into separate cars. Depending on the species, the DNA contains several hundred or even thousands of genes. Typically, one gene contains coded instructions for building all or part of a single protein. Enzymes, which are specialized proteins, determine virtually all the biochemical reactions that support and sustain the cell.
Also, immersed in the cytoplasm are the only organelles in prokaryotic cells. Tiny bead-like structures called ribosomes. These are the cell’s protein factories. Following the instructions encoded in the DNA, ribosomes churn out proteins by the hundreds every minute, providing needed enzymes, the replacements for worn-out transport proteins, or other proteins required by the cell.
While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in their larger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitats include polar ice, deserts, and hydrothermal vents - deep regions of the ocean under great pressure where hot water geysers erupt from cracks in the ocean floor.
An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus directs activities of the cell and carries genetic information from generation to generation. The mitochondria generates energy for the cell. Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm.
Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell’s outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.
The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for one major difference: Eukaryotic cells house a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are located.
The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including medications such as tranquilizers and sleeping pills, to freely cross the membrane. Proteins embedded in the plasma membrane carry out a variety of functions, including transport of large water soluble molecules such as sugars and certain amino acids. Glycoproteins, proteins bonded to carbohydrates, serve in part to identify the cell as belonging to a unique organism, enabling the immune system to detect foreign cells, such as invading bacteria, which carry different glycoproteins. Cholesterol molecules in the plasma membrane act as stabilizers that limit the movement of the two slippery phospholipids layer, which slide back and forth in the membrane. Tiny gaps in the membrane enable small molecules such as oxygen to diffuse readily into and out of the cell. Since cells constantly use up oxygen, decreasing its concentration within the cell, the higher concentration of oxygen outside the cell causes a net flow of oxygen into the cell. The steady stream of oxygen into the cell enables it to carry out aerobic respiration continually, a process that provides the cell with the energy needed to carry out its functions.
The nucleus is the largest organelle in an animal cell. It contains numerous strands of DNA, the length of each strand being many times the diameter of the cell. Unlike the circular prokaryotic DNA, long sectors of eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over onto itself several times, forming a rod-shaped chromosome.
The nucleus is surrounded by a double-layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. In each nuclear pore, molecular signals flash back and forth as often as ten times per second. For example, a signal to activate a specific gene comes into the nucleus and instructions for production of the necessary protein go out to the cytoplasm.
The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, and the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.
Attached to the nuclear membrane is an elongated membranous sac called the endoplasmic reticulum. This organelle tunnels through the cytoplasm, folding back and forth on itself to form a series of membranous stacks. Endoplasmic reticulums take two forms: Rough and smooth. A rough endoplasmic reticulum (RER) is so called because it appears bumpy under a microscope. The bumps are actually thousands of ribosomes attached to the membrane’s surface. The ribosomes in eukaryotic cells have the same function as those in prokaryotic cells - protein synthesis - but they differ slightly in structure. Eukaryote ribosomes bound to the endoplasmic reticulum help assemble proteins that typically are exported from the cell. The ribosomes work with other molecules to link amino acids to partially completed proteins. These incomplete proteins then travel to the inner chamber of the endoplasmic reticulum, where chemical modifications, such as the addition of a sugar, are carried out. Chemical modifications of lipids are also carried out in the endoplasmic reticulum.
The endoplasmic reticulum and its bound ribosomes are particularly dense in cells that produce many proteins for export, such as the white blood cells of the immune system, which produce and secrete antibodies. Some ribosomes that manufacture proteins are not attached to the endoplasmic reticulum. These so-called free ribosomes are dispersed in the cytoplasm and typically make proteins - many of them enzymes - that remain in the cell.
The second form of an endoplasmic reticulum, the smooth endoplasmic reticulum (SER), lacks ribosomes and has an even surface. Within the winding channels of the smooth endoplasmic reticulum are the enzymes needed for the construction of molecules such as carbohydrates and lipids. The smooth endoplasmic reticulum is prominent in liver cells, where it also serves to detoxify substances such as alcohol, drugs, and other poisons.
Proteins are transported from free and bound ribosomes to the Golgi apparatus, an organelle that resembles a stack of deflated balloons. It is packed with enzymes that complete the processing of proteins. These enzymes add sulfur or phosphorus atoms to certain regions of the protein, for example, or chop off tiny pieces from the ends of the proteins. The completed protein then leaves the Golgi apparatus for its final destination inside or outside the cell. During its assembly on the ribosome, each protein has acquired a group of from 4 to 100 amino acids called a signal. The signal works as a molecular shipping label to direct the protein to its proper location.
Lysosomes are small, often spherical organelles that function as the cell’s recycling center and garbage disposal. Powerful digestive enzymes concentrated in the lysosome break down worn-out organelles and ship their building blocks to the cytoplasm where they are used to construct new organelles. Lysosomes also dismantle and recycle proteins, lipids, and other molecules.
The mitochondria is the powerhouse of the cell. Within these long, slender organelles, which can appear oval or bean shaped under the electron microscope, enzymes convert the sugar glucose and other nutrients into adenosine triphosphate (ATP). This molecule, in turn, serves as an energy battery for countless cellular processes, including the shuttling of substances across the plasma membrane, the building and transport of proteins and lipids, the recycling of molecules and organelles, and the dividing of cells. Muscle and liver cells are particularly active and require dozens and sometimes up to hundreds mitochondria per cell to meet their energy needs. Mitochondria is unusual in that they contain their own DNA in the form of a prokaryote-like circular chromosome; Have their own ribosomes, which resemble prokaryotic ribosomes, and divide independently of the cell.
Unlike the tiny prokaryotic cell, the relatively large eukaryotic cell requires structural support. The cytoskeleton, a dynamic network of protein tubes, filaments, and fibers, crisscrosses the cytoplasm, anchoring the organelles in place and providing shape and structure to the cell. Many components of the cytoskeleton are assembled and disassembled by the cell as needed. During cell division, for example, a special structure called a spindle is built to move chromosomes around. After cell division, the spindle, no longer needed, is dismantled. Some components of the cytoskeleton serve as microscopic tracks along which proteins and other molecules travel like miniature trains. Recent research suggests that the cytoskeleton also may be a mechanical communication structure that converses with the nucleus to help organize events in the cell.
Plant cells have all the components of animal cells and boast several added features, including chromoplasts, a central vacuole, and a cell wall. Chromoplasts convert light energy - typically from the Sun - into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chromoplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosomes, which manufacture the proteins that the chloroplasts typically need.
The central vacuole of a mature plant cell typically takes up most of the room in the cell. The vacuole, a membranous bag, crowds the cytoplasm and organelles to the edges of the cell. The central vacuole stores water, salts, sugars, proteins, and other nutrients. In addition, it stores the blue, red, and purple pigments that give certain flowers their colors. The central vacuole also contains plant wastes that taste bitter to certain insects, thus discouraging the insects from feasting on the plant.
In plant cells, a sturdy cell wall surrounds and protects the plasma membrane. Its pores enable materials to pass freely into and out of the cell. The strength of the wall also enables a cell to absorb water into the central vacuole and swell without bursting. The resulting pressure in the cells provides plants with rigidity and support for stems, leaves, and flowers. Without sufficient water pressure, the cells collapse and the plant wilts.
To stay alive, cells must be able to carry out a variety of functions. Some cells must be able to move, and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment.
Although many forms of bacteria are not capable of independent movement, species such as the Salmonella bacterium pictured here can move by means of fine threadlike projections called flagella. The arrangement of flagella across the surface of the bacterium differs from species to species; they can be present at the ends of the bacterium or all across the body surface. Forward movement is accomplished either by a tumbling motion or in a forward manner without tumbling.
Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms, such as the euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellums work by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.
Movement in eukaryotes is also accomplished with cilia, short, hairlike proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.
Still other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia, or false feet, which actually are placed in front of the cell, rather like extended arms. They then drag the trailing end of their cytoplasm up to the pseudopodia. A cell using amoeboid motion would lose a race to a euglena or paramecium. But while it is slow, amoeboid motion is strong enough to move cells against a current, enabling water-dwelling organisms to pursue and devour prey, for example, or white blood cells roaming the blood stream to stalk and engulf a bacterium or virus.
An amoeba, a single-celled organism lacking internal organs, is shown approaching a much smaller paramecium, which it begins to engulf with large outflowings of its cytoplasm, called pseudopodia. Once the paramecium is completely engulfed, a primitive digestive cavity, called a vacuole, forms around it. In the vacuole, acids break the paramecium down into chemicals that the amoeba can diffuse back into its cytoplasm for nourishment.
All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, are also capable of reaching out and grabbing food. They used a process known as endocytosis, in which the plasma membrane surrounds and engulfed the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.
Cells require energy for a variety of functions, including moving, building up and breaking down molecules, and transporting substances across the plasma membrane. Nutrients contain energy, but cells must convert the energy locked in nutrients to another form - specifically, the ATP molecule, the cell’s energy battery - before it is useful. In single-celled eukaryotic organisms, such as the paramecium, and in multicellular eukaryotic organisms, such as plants, animals, and fungi, mitochondria is responsible for this task. The interior of each mitochondrion consists of an inner membrane that is folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP; thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.
Some prokaryotic organisms also carry out aerobic respiration. They lack mitochondria, however, and carry out aerobic respiration in the cytoplasm with the help of enzymes sequestered there. Many prokaryote species live in environments where there is little or no oxygen, environments such as mud, stagnant ponds, or within the intestines of animals. Some of these organisms produce ATP without oxygen in a process known as anaerobic respiration, where sulfur or other substances take the place of oxygen. Still other prokaryotes, and yeast, a single-celled eukaryote, build ATP without oxygen in a process known as fermentation.
Almost all organisms rely on the sugar glucose to produce ATP. Glucose is made by the process of photosynthesis, in which light energy is transformed to the chemical energy of glucose. Animals and fungi cannot carry out photosynthesis and depend on plants and other photosynthetic organisms for this task. In plants, as we have seen, photosynthesis takes place in organelles called chloroplasts. Chloroplasts contain numerous internal compartments called thylakoids where enzymes aid in the energy conversion process. A single leaf cell contains 40 to 50 chloroplasts. With sufficient sunlight, one large tree is capable of producing upwards of two tons of sugar in a single day. Photosynthesis in prokaryotic organisms - typically aquatic bacteria - is carried out with enzymes clustered in plasma membrane folds called chromatophores. Aquatic bacteria produce the food consumed by tiny organisms living in ponds, rivers, lakes, and seas.
A typical cell must have on hand, about. 30,000 proteins at any-one time. Many of these proteins are enzymes needed to construct the major molecules used by cells - carbohydrates, lipids, proteins, and nucleic acids - nor to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure - the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.
Before a protein can be made, however, the molecular directions to build, it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.
Once in the cytoplasm, the messenger RNA molecule links up with a ribosome. The ribosome moves along the messenger RNA like a monorail car along a track, stimulating another form of RNA - transfer RNA - to gather and link the necessary amino acids, pooled in the cytoplasm, to form the specific protein, or section of protein. The protein is modified as necessary by the endoplasmic reticulum and Golgi apparatus before embarking on its mission. Cells teem with activity as they forge the numerous, diverse proteins that are indispensable for life. For a more detailed discussion about protein synthesis, When there are a hundred or more cells, they formed a hollow ball of cells, called a blastula, surrounding a fluid-filled cavity. Later divisions produce three layers of cells - endoderm (inner), mesoderm (middle), and ectoderm (outer) - from which the principal features of the animal will differentiate.
Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: Binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell divisions occur every second in order to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research indicates that even brain cell, once thought to be incapable of mitosis, undergo cell division in the part of the brain associated with memory.
In a landmark intersection of science and fiction, cloning leapt from the world’s imagination to its front page in February 1997. It arrived in the innocent form of a sheep named Dolly: The first exact genetic duplicate of an adult mammal due to genetic engineering. Scottish scientists had created Dolly from deoxyribonucleic acid (DNA) - the basic unit of heredity - taken from a single adult sheep cell. The accomplishment threw open the door to profoundly ethical as well as scientific controversy over the potential uses and abuses of cloning. ‘However the debate is resolved,’ wrote Los Angeles Times science reporter Thomas H. Maugh II, ‘the genie is irretrievably out of the bottle.’
The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals - including, of course, human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs, that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell resulting from the union, called a zygote, contains the full number of chromosomes.
The story of how cells evolved remains an open and actively investigated question in science. The combined expertise of physicists, geologists, chemists, and evolutionary biologists has been required to shed light on the evolution of cells from the nonliving matter of early Earth. The planet formed about 4.5 billion years ago, and for millions of years, violent volcanic eruptions blasted substances such as carbon dioxide, nitrogen, water, and other small molecules into the air. These small molecules, bombarded by ultraviolet radiation and lightning from intense storms, collided to form the stable chemical bonds of larger molecules, such as amino acids and nucleotides - the building blocks of proteins and nucleic acids. Experiments indicate that these larger molecules form spontaneously under laboratory conditions that simulate the probable early environment of Earth.
Scientists speculate that rain may have carried these molecules into lakes to create a primordial soup - the breeding ground for the assembly of proteins, the nucleic acid RNA, and lipids. Some scientists postulate that these more complex molecules formed in hydrothermal vents rather than in lakes. Other scientists propose that these key substances may have reached Earth on meteorites from outer space. Regardless of the origin or environment, however, scientists do agree that proteins, nucleic acids, and lipids provided the raw materials for the first cells. In the laboratory, scientists have observed lipid molecules joining to form spheres that resemble a cell’s plasma membrane. As a result of these observations, scientists postulate that millions of years of molecular collisions resulted in lipid spheres enclosing RNA, the simplest molecule capable of self-replication. These primitive aggregations would have been the ancestors of the first prokaryotic cells.
Fossil studies indicate that Cyanobacteria, bacteria capable of photosynthesis, were among the earliest bacteria to evolve, an estimated 3.4 billion to 3.5 billion years ago. In the environment of the early Earth, there were no oxygen, and cyanobacteria probably used fermentation to produce ATP. Over the eons, cyanobacteria performed photosynthesis, which produces oxygen as a byproduct; The result was the gradual accumulation of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen in aerobic respiration, a more efficient ATP-producing process than fermentation. Some molecular studies of the evolution of genes in archaebacteria suggest that these organisms may have evolved in the hot waters of hydrothermal vents or hot springs slightly earlier than cyanobacteria, around 3.5 billion years ago. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.
Eukaryotic cells may have evolved from primitive prokaryotes about 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell’s plasma membrane to expand and fold. These folds, ultimately, may have given rise to separate compartments within the cell - the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells situated with mitochondria - the ancestors of animals - or with both mitochondria and chloroplasts - the ancestors of plants - evolved.
The first observations of cells were made in 1665 by English scientist Robert Hooke, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork. Noting the rows of tiny boxes that made up the dead wood’s tissue, Hooke coined the term cell because the boxes reminded him of the small cells occupied by monks in a monastery. While Hooke was the first to observe and describe cells, he did not comprehend their significance. At about the same time, the Dutch maker of microscopes Antoni van Leeuwenhoek pioneered the invention of one of the best microscopes of the time. Using his invention, Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva, one-celled organisms cavorting in pond water, and sperm swimming in semen. Two centuries passed, however, before scientists grasped the true importance of cells.
Many advances have been made in microscope technology. This article from the 1994 Collier’s Year Book begins with the microscope most young students are familiar with and tracks the breakthroughs in the development of new types of microscopes - including those that use ultrasonic imaging and those that ‘feel’ an object’s surface.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. Working together, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between plant and animal cells. In 1839 they proposed the revolutionary idea that all living things are made up of cells. Their theory gave rise to modern biology: a whole new way of seeing and investigating the natural world.
By the late 1800s, as light microscopes improved still further, scientists were able to observe chromosomes within the cell. Their research was aided by new techniques for staining parts of the cell, which made possible the first detailed observations of cell division, including observations of the differences between mitosis and meiosis in the 1880s. In the first few decades of the 20th century, many scientists focused on the behavior of chromosomes during cell division. At that time, it was generally held that mitochondria transmitted the hereditary information. By 1920, however, scientists determined that chromosomes carry genes and that genes transmit hereditary information from generation to generation.
During this period, scientists began to understand some of the chemical processes in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge is an instrument that spins cells or other substances in test tubes at high speeds, which causes the heavier parts of the substance to fall to the bottom of the test tube. This instrument enabled scientists to separate the relatively abundant and heavy mitochondria from the rest of the cell and study their chemical reactions. By the late 1940s, scientists were able to explain the role of mitochondria in the cell. Using refined techniques with the ultracentrifuge, scientists subsequently isolated the smaller organelles and gained an understanding of their functions.
The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.
While some scientists were studying the functions of cells, others were examining details of their structure. They were aided by a crucial technological development in the 1940s, the invention of the electron microscope, which uses high-energy electrons instead of light waves to view specimens. New generations of electron microscopes have provided resolution, or the differentiation of separate objects, thousands of times more powerful than that available in light microscopes. This powerful resolution revealed organelles such as the endoplasmic reticulum, lysosomes, the Golgi apparatus, and the cytoskeleton. The scientific fields of cell structure and function continue to complement each other as scientists explore the enormous complexity of cells.
The discovery of the structure of DNA in 1953 by American biochemist James D. Watson and British biophysicist Francis Crick ushered in the era of molecular biology. Today, investigation inside the world of cells - of genes and proteins at the molecular level - constitutes one of the largest and fastest moving areas in all of science. One particularly active field in recent years has been the investigation of cell signaling, the process by which molecular messages find their way into the cell via a series of complex protein pathways in the cell.
Another busy area in cell biology concerns programmed cell death, or apoptosis. Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. This also seems to be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. If this fails to occur, the cell may divide and give rise to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumor. This unregulated growth by rogue cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. The study of apoptosis is one avenue that scientists explore in an effort to understand how cells become cancerous.
Scientists are also discovering exciting aspects of the physical forces within cells. Cells employ a form of architecture called tensegrity, which enables them to withstand battering by a variety of mechanical stresses, such as the pressure of blood flowing around cells or the movement of organelles within the cell. Tensegrity stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton and other cell components. Tensegrity also may explain how a change in the cytoskeleton, where certain enzymes are anchored, initiates biochemical reactions within the cell, and can even influence the action of genes. The mechanical rules of tensegrity may also account for the assembly of molecules into the first cells. Such new insights - made some 300 years after the tiny universe of cells was first glimpsed - show that cells continue to yield fascinating new worlds of discovery.
The Nervous System signifies of those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
The reception of stimuli is the function of special sensory cells. The conducting elements of the nervous system are cells called neurons; these may be capable of only slow and generalized activity, or they may be highly efficient and rapidly conducting units. The specific response of the neuron—the nerve impulse - and the capacities of the cell to be stimulated make this cell a receiving and transmitting unit capable of transferring information from one part of the body to another.
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension, it is especially important in the transmission of nerve impulses from the region of the cell body to other cells.
Although all many-celled animals have some kind of nervous system, the complexity of its organization varies considerably among different animal types. In simple animals such as jellyfish, the nerve cells form a network capable of mediating only a relatively stereotyped response. In more complex animals, such as shellfish, insects, and spiders, the nervous system is more complicated. The cell bodies of neurons are organized in clusters called ganglia. These clusters are interconnected by the neuronal processes to form a ganglionated chain. Such chains are found in all vertebrates, in which they represent a special part of the nervous system, related especially to the regulation of the activities of the heart, the glands, and the involuntary Vertebrate animals have a bony spine and skull in which the central part of the nervous system is housed; The peripheral part extends throughout the remainder of the body. That part of the nervous system located in the skull is referred to as the brain that found in the spine is called the spinal cord. The brain and the spinal cord are continuous through an opening in the base of the skull; Both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain.
In the skin are cells of several types called receptors; each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the central nervous system and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a two-neuron arc that begins and ends in the periphery. Many of the actions of the nervous system can be explained on the basis of such reflex arcs, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.
The cranial nerves connect to the brain by passing through openings in the skull, or cranium. Nerves associated with the spinal cord pass through openings in the vertebral column and are called spinal nerves. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called afferent, and the latter are called efferent. Afferent impulses are referred to as sensory; efferent impulses are referred to as either somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements.
The cranial and spinal nerves are paired; The number in humans are 12 and 31, respectively. Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the tenth cranial nerve, called the vagus. In addition to supplying structures in the neck, the vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste is mediated by the second, eighth, and seventh cranial nerves, respectively. Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing. Spinal nerves, after they exit from the vertebrae, are distributed in a band-like fashion to regions of the trunk and to the limbs. They interconnect extensively, thereby forming the brachial plexus, which runs to the upper extremities, and the lumbar plexus, which passes to the lower limbs.
Among the motor’s fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
Consideration of disorders of the nervous system is the province of neurology; Psychiatry deals with behavioral disturbances of a functional nature. The division between these two medical specialties cannot be sharply defined, because neurological disorders often manifest both organic and mental symptoms.
Diseases of the nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumors, and they involve either nerve cells or their supporting elements. Vascular disorders, such as cerebral hemorrhage or other forms of a stroke, are among the most common causes of paralysis and other neurologic complications. Some diseases exhibit peculiar geographic and age distribution. In temperate zones, multiple sclerosis is a common degenerative disease of the nervous system, but it is rare in the Tropics.
The nervous system is subject to infection by a great variety of bacteria, parasites, and viruses. For example, meningitis, or infection of the meninges investing the brain and spinal cord, can be caused by many different agents. On the other hand, one specific virus causes rabies. Some viruses causing neurological ills effect only certain parts of the nervous system. For example, the virus causing poliomyelitis commonly affects the spinal cord, as Viruses manufacturing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part affected. Myelitis is an inflammation of the spinal cord; Neuritis is an inflammation of a nerve. It may be caused not only by infection but also by poisoning, alcoholism, or injury. Tumors originating in the nervous system usually are composed of meningeal tissue or neuroglia (supporting tissue) cells, depending on the specific part of the nervous system affected, but other types of a tumor may metastasize to or invade the nervous system. In certain disorders of the nervous system, such as neuralgia, migraine, and epilepsy, no evidence may exist of organic damage. Another disorder, cerebral palsy, is associated with birth defects.
Pain, is an unpleasant sensory or emotional experience caused by real or potential injury or damage to the body or described in terms of such damage. Scientists believe that pain evolved in the animal kingdom as a valuable three-part warning system. First, it warns of injury. Second, pain protects against further injury by causing a reflexive withdrawal from the source of injury. Finally, pain leads to a period of reduced activity, enabling injuries to heal more efficiently.
Pain is difficult to measure in humans because it has an emotional, or psychological component as well as a physical component. Some people express extreme discomfort from relatively small injuries, while others show little or no pain even after suffering severe injury. Sometimes pain is present even though no injury is apparent at all, or pain lingers long after an injury appears to have healed.
The signals that warn the body of tissue damage are transmitted through the nervous system. In this system, the basic unit is the nerve cell or neuron. A nerve cell is composed of three parts: a central cell body, a single major branching fiber called an axon, and a series of smaller branching fibers known as dendrites. Each nerve cell meets other nerve cells at certain points on the axons and dendrites, forming a dense network of interconnected nerve fibers that transmit sensory information about touch, pressure, or warmth, as well as pain.
Sensory information is transmitted from the different parts of the body to the brain via the spinal cord, which is a complex set of nerves that extend from the brain down along the back, protected by the bones of the spine. About as wide as a finger, the spinal cord is like a cable packed with many bundles of wires. The bundles are nerve pathways for transmitting information. But the spinal cord is more than just a message transmitter, it is also an extension of the brain. It contains neurons that process incoming sensory information, and generates messages to be sent back down to cells in other parts of the body.
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to ‘fire,’ or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Information being transmitted between and within the brain and spinal cord travels through the nervous system using both chemical and electrical mechanisms. A message-carrying impulse travels from one end of a nerve cell to another by means of an electric signal. When the electric signal reaches the terminal end of a nerve cell, a gap called a synapse prevents the electric signal from crossing to the next cell. The electric signal triggers the cell to release chemicals called neurotransmitters, which float across the synapse to the neighboring nerve cell. These neurotransmitters fit into specialized receptors found on the adjacent nerve cell, much as a key fits into a lock, generating an electric impulse in the neighboring cell. This new impulse travels to the end of the long cell, in turn triggering the release of neurotransmitters to carry the message across the next synapse. Not all neurotransmitters initiate a message in a neighboring nerve cell. Some specialize in preventing neighboring cells from generating an electrical signal, while others function as helpers, facilitating the message's journey to the brain.
While most of the sensory nerves in the skin and other body tissues have special structures covering their nerve endings, those nerves that signal injury have free nerve endings. These simple nerve endings specialize in detecting noxious stimuli - a catchall term for injury-causing stimuli such as intense heat, extreme pressure, or sharp pricks or cuts. The nerve endings that detect pain are called nociceptors, and the process of transmitting pain signals when harmful stimulation occurs is called nociception. Several million nociceptors are interlaced through the tissues and organs of the body.
When a person experiences an injury, such as a stubbed toe, specialized cells called nociceptors sense potential tissue damage (1) and send an electric signal, called an impulse, to the spinal cord via a sensory nerve (2). A specialized region of the spinal cord known as the dorsal horn (3) processes the pain signal, immediately sending another impulse back down the leg via a motor nerve (4). This causes the muscles in the leg to contract and pull the toe away from the source of injury (6). At the same time, the dorsal horn sends another impulse up the spinal cord to the brain. During this trip, the impulse travels between nerve cells. When the impulse reaches a nerve ending (7), the nerve released chemical messengers, called neurotransmitters, which carry the message to the adjacent nerve. When the impulse reaches the brain (8), it is analyzed and processed as an unpleasant physical and emotional sensation.
An injury triggers pain signals in two types of nociceptors, one with large, insulated axons known as A-delta fibers and one with small, uninsulated axons known as C fibers. The large A-delta fibers conduct signals quickly, and the smaller C fibers transmit information slowly. The difference in the functions of these two fibers becomes obvious to a person who stubs a toe. At first the injured person is aware of a sharp, flashing pain at the point of injury. Generated by the A-delta fibers, this short-lived pain intrudes upon the thoughts and perceptions occurring in the brain. Just as this first pain subsides, a second pain begins that is vague, throbbing, and persistent. This sensation is derived from the C fibers.
Pain information from the A-delta and C fibers travels through the spinal cord to the brain. When it receives the pain message, the spinal cord generates impulses that travel back down to muscles, which lead to a reflexive contraction that pulls the body away from the source of injury. Other reflexes may affect skin temperature, blood flow, sweating, and other changes.
While this reflex action is underway, the pain message continues up the spinal cord to relay centers in the brain. The sensory information is routed to many other parts of the brain, including the cortex, where thinking processes occur
The Adrenal Gland is the vital endocrine gland that secretes hormones into the bloodstream, situated, in humans, on top of the upper end of each kidney. The two parts of the gland - the inner portion, or medulla, and the outer portion, or the cortex - are like separate organs: They are composed of different types of tissue and perform different functions. The adrenal medulla, composed of chromaffin cells secretes the hormone epinephrine, also called adrenaline, in response to stimulation of the sympathetic nervous system at times of stress. The medulla also secretes the hormone norepinephrine, which plays a role in maintaining normal blood circulation. The hormones of the medulla are called catecholamines. Unlike the adrenal cortex, the medulla can be removed without endangering the life of an individual.
The adrenal outer layer, or cortex, secretes about 30 steroid hormones, but only a few are secreted in significant amounts. Aldosterone, one of the most important hormones, regulates the balance of salt and water in the body. Cortisone and hydrocortisone are necessary to regulate fat, carbohydrate, and protein metabolism. Adrenal sex steroids have a minor influence on the reproductive system. Modified steroids, now produced synthetically, are superior to naturally secreted steroids for treatment of Addison's disease and other disorders.
Adrenocorticotropic Hormone is also known as corticotropin, hormones secreted by the anterior part of the pituitary gland. The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layers) of the adrenal gland. One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolisms. ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis. ACTH is a complex protein molecule containing 39 amino acids. Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increased in response to stress, disease, and decreased blood pressure.
The Pituitary Gland is the master endocrine gland in vertebrate animals. The hormones secreted by the pituitary stimulate and control the functioning of almost all the other endocrine glands in the body. Pituitary hormones also promote growth and control the water balance of the body.
The pituitary is a small bean-shaped, reddish-gray organ located in the saddle-shaped depression (sella turcica) in the floor of the skull (the sphenoid bone) and attached to the base of the brain by a stalk; it is located near the hypothalamus. The pituitary has two lobes - the anterior lobe, or adenohypophysis, and the posterior lobe, or neurohypophysis - which differ in structure and function. The anterior lobe is derived embryologically from the roof of the pharynx and is composed of groups of epithelial cells separated by blood channels; the posterior lobe is derived from the base of the brain and is composed of nervous connective tissue and nerve-like secreting cells. The area between the anterior and posterior lobes of the pituitary is called the intermediate lobe; it has the same embryological origin as the anterior lobe.
Concentrated chemical substances, or hormones, which control 10 to 12 functions in the body, have been obtained as extracts from the anterior pituitary glands of cattle, sheep, and swine. Eight hormones have been isolated, purified, and identified; All of them are peptides, that is, they are composed of amino acids. A growth hormone (GH), or the somatotropic hormone (STH), is essential for normal skeletal growth and is neutralized during adolescence by the gonadal sex hormones. Thyroid-stimulating hormones (TSH) control the normal functioning of the thyroid gland, and the adrenocorticotropic hormone (ACTH) controls the activity of the cortex of the adrenal glands and takes part in the stress reaction. Prolactin, also called lactogenic, luteotropic, or mammotropic hormone, initiates milk secretion in the mammary gland after the mammary tissues have been prepared during pregnancy by the secretion of other pituitary and sex hormones. The two gonadotropic hormones are follicle-stimulating hormones (FSH) and a luteinizing hormone (LH). Follicle-stimulating hormones stimulates the formation of the Graafian follicle in the female ovary and the development of spermatozoa in the male. The luteinizing hormone stimulates the formation of ovarian hormones after ovulation and initiates lactation in the female, in the male, it stimulates the tissues of the testes to elaborate testosterone. In 1975 scientists identified the pituitary peptide endorphin, which acts in experimental animals as a natural pain reliever in times of stress. Endorphin and ACTH are made as parts of a single large protein, which subsequently splits. This may be the body's mechanism for coordinating the physiological activities of two stress-induced hormones. The same large prohormone that contains ACTH and endorphin also contains short peptides called melanocyte-stimulating hormones. These substances are analogous to the hormone that regulates pigmentation in fish and amphibians, but in humans they have no known function.
Research has shown that the hormonal activity of the anterior lobe is controlled by chemical messengers sent from the hypothalamus through tiny blood vessels to the anterior lobe. In the 1950s, the British neurologist Geoffrey Harris discovered that cutting the blood supply from the hypothalamus to the pituitary impaired the function of the pituitary. In 1964, chemical agents called releasing factors were found in the hypothalamus; These substances, it was learned, affect the secretion of growth hormones, a thyroid-stimulating hormone called thyrotropin, and the gonadotropic hormones involving the testes and ovaries. In 1969 the American endocrinologist Roger Guillemin and colleagues isolated and characterized thyrotropin-releasing factors, which stimulates the secretion of thyroid-stimulating hormones from the pituitary. In the next few years his group and that of the American physiologist Andrew Victor Schally isolated the luteinizing hormone-releasing factor, which stimulates secretion of both LH and FSH, and somatostatin, which inhibits release of growth hormones. For this work, which proved that the brain and the endocrine system are linked, they shared the Nobel Prize in physiology or medicine in 1977. Human somatostatin was one of the first substances to be grown in bacteria by recombinant DNA.
The presence of the releasing factors in the hypothalamus helped to explain the action of the female sex hormones, estrogen and progesterone, and their synthetic versions contained in oral contraceptives, or birth-control pills. During a woman's normal monthly cycle, several hormonal changes are needed for the ovary to produce an egg cell for possible fertilization. When the estrogen level in the body declines, the follicle-releasing factor (FRF) flows to the pituitary and stimulates the secretion of the follicle-stimulating hormone. Through a similar feedback principle, the declining level of progesterone causes a release of luteal-releasing factors (LRF), which stimulates secretion of the luteinizing hormone. The ripening follicle in the ovary then produces estrogen, and the high level of that hormone influences the hypothalamus to shut down temporarily the production of FSH. Increased progesterone feedback to the hypothalamus shuts down LH production by the pituitary. The daily doses of synthetic estrogen and progesterone in oral contraceptives, or injections of the actual hormones, inhibit the normal reproductive activity of the ovaries by mimicking the effect of these hormones on the hypothalamus.
In lower vertebrates this part of the pituitary secretes melanocyte-stimulating hormones, which brings about skin-color changes. In humans, it is present only for a short time early in life and during pregnancy, and is not known to have any function.
Two hormones are secreted by the posterior lobe. One of these is the antidiuretic hormone (ADH), vasopressin. Vasopressin stimulates the kidney tubules to absorb water from the filtered plasma that passes through the kidneys and thus controls the amount of urine secreted by the kidneys. The other posterior pituitary hormone is oxytocin, which causes the contraction of the smooth muscles in the uterus, intestines, and blood arterioles. Oxytocin stimulates the contractions of the uterine muscles during the final stage of pregnancy to stimulate the expulsion of the fetus, and it also stimulates the ejection, or let-down, of milk from the mammary gland following pregnancy. Synthesized in 1953, oxytocin was the first pituitary hormone to be produced artificially. Vasopressin was synthesized in 1956.
Pituitary functioning may be disturbed by such conditions as tumors, blood poisoning, blood clots, and certain infectious diseases. Conditions resulting from a decrease in anterior-lobe secretion include dwarfism, acromicria, Simmonds's disease, and Fröhlich's syndrome. The dwarfism occurs when anterior pituitary deficiencies occur during childhood; acromicria, in which the bones of the extremities are small and delicate, results when the deficiency occurs after puberty. Simmonds's disease, which is caused by extensive damage to the anterior pituitary, is characterized by premature aging, loss of hair and teeth, anemia, and emaciation; it can be fatal. Fröhlich's syndrome, also called adiposogenital dystrophy, is caused by both anterior pituitary deficiency and a lesion of the posterior lobe or hypothalamus. The result is obesity, dwarfism, and retarded sexual development. Glands under the influence of anterior pituitary hormones are also affected by anterior pituitary deficiency.
Over secretion of one of the anterior pituitary hormones, somatotropin, results in a progressive chronic disease called acromegaly, which is characterized by enlargement of some parts of the body. Posterior-lobe deficiency results in diabetes insipidus.
Tissue
Tissue, - group of associated, similarly structured cells that perform specialized functions for the survival of the organism. Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm. Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups.
These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epitheliums are made up of tightly packed cells with little intercellular substance between them.
About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and is pushed toward the tissue surface, where they die and are shed. Another is a simple columnar epithelium, which lines the digestive system from the stomach to the anus; Simple columnar epithelium cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium-for examples, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures.
These tissues, which support and hold parts of the body together, comprises the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone. In contrast to an epithelium, the cells of these tissues are widely separated from one another, with a large amount of intercellular substance between them. The cells of fibrous tissue, found throughout the body, connect to one another by an irregular network of strands, forming a soft, cushiony layer that also supports blood vessels, nerves, and other organs. Adipose tissue has a similar function, except that its fibroblasts also contain store fat. Elastic tissue, found in ligaments, the trachea, and the arterial walls, stretches and contracts again with each pulse beat. In the human embryo, the fibroblast cells that originally secreted collagen for the formation of fibrous tissue later change to secrete a different form of protein called chondrion, for the formation of cartilage, some cartilage later becomes calcified by the action of osteoblast to form bones. Blood and lymph are also often considered connective tissues.
Tissues, which contract and relax, comprise the striated, smooth, and cardiac muscles. Striated muscles, also called skeletal or voluntary muscles, include those that are activated by the somatic, or voluntary, nervous system. They are joined together without cell walls and have several nuclei. The smooth, or involuntary muscles, which are activated by the autonomic nervous system, are found in the internal organs and consist of simple sheets of cells. Cardiac muscles, which have characteristics of both striated and smooth muscles, are joined together in a vast network. These highly complex groups of cells, called ganglia, transfer information from one part of the body to another. Each neuron, or nerve cell, consists of a cell body with branching dendrites and one long fiber, or axons. The dendrites connect one neuron to another; The axon transmits impulses to an organ or collects impulses from a sensory organ.
Crossing a Synapse
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to fire or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Reflex
Reflex, in physiology, is the involuntary response to a stimulus by the animal organism. In its simplest form, it consisted of the stimulation of an afferent nerve through a sense organ, or receptor, followed by transmission of the stimulus, usually through a nerve center, to an efferent motor nerve, resulting in action of a muscle or gland, called the effector. In most reflex action, however, the stimulus passes through one or more intermediate nerve cells, which modify and direct its action, sometimes to the extent of involving the muscular activity of the entire organism. For example, a painful stimulus applied to the hand causes a reflex withdrawal of the hand, which involves contraction of the flexor group of muscles and reflexation of the opposing extensor group; if the stimulus is strong, the coordinating nerve cells pass it to the arm muscles and also to the muscles of the trunk and legs, the result being a jump that removes not only the arm, but the entire person from the vicinity of the painful stimulus.
The system of coordinating nerve cells is such that several different kinds of stimuli may produce the same result. For example, the stimulus produced by the sight of food and that caused by the smell of food travel different afferent pathways, but both have a common final path that stimulates the salivary glands to secretion. The final common path may also be activated through associated nerve tracts by a stimulus that ordinarily is not directly connected with the response. This type of reflex was named conditioned reflex by its discoverer, the Russian physiologist Ivan Pavlov, about 1904. Pavlov found that sounding a bell every time a dog was about to be given food eventually caused a reflex flow of saliva, which later persisted even when no food was produced. Elaborations of this habituative type of reflex are regarded by some physiologists and psychologists as an important basis for many behaviors, both voluntary and involuntary.
The normal pathways of many reflexes are generally known, and the presence, absence, or exaggerations of the normal physical responses to certain stimuli are symptoms used by neurologists to determine the condition of the neural pathways involved. A familiar reflex commonly tested by physicians is the patellar reflex, in which an involuntary jerk of the knee is evoked by lightly striking the tendon of the patella, or kneecap, indicating the efficiency of certain nerve tracts in the spinal cord.
Like all other cells, neurons contain charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they are able to produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes referred to as membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory input or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negative to positive. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process.
Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; These points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
The nervous system has two divisions: The somatic, which allow voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: The sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.
Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; Measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
The human brain has three major structural components: The large dome-shaped cerebrum, the smaller somewhat spherical cerebellum, and the brainstem. Prominent in the brainstem is the medulla oblongata (the egg-shaped enlargement at the center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus act as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.
Movement may occur also in direct response to an outside stimulus, thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do of those special receptors concerned with sight, hearing, smell, and taste.
Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles is usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.
In 1946 Axelrod joined the laboratory of American pharmacologist Bernard Brodie at Goldwater Memorial Hospital in New York. The pair conducted research on pain-relieving drugs called analgesics. They identified a pain-relieving chemical known as acetaminophen. This drug was later developed and marketed by the drug company Johnson & Johnson under the brand-name Tylenol.
In 1949 Axelrod took a position at the National Heart Institute, a branch of the National Institutes of Health (NIH). Their Axelrod studied how the body processes certain drugs that cause behavioral changes, including amphetamines, ephedrine, and mescaline. He identified a group of enzymes that help these drugs break down in the body. These enzymes, called cytochrome-P450 monoxygenases, have been studied extensively by other scientists, particularly in cancer research.
Realizing that career advancement in the sciences requires a doctoral degree, in 1954 Axelrod took a leave of absence from his job at the National Heart Institute to attend The George Washington University. He earned his doctorate in pharmacology in 1955. That same year he was named chief of pharmacology at the National Institute of Mental Health (NIMH) another branch of NIH.
At NIMH, Joseph Axelrod began research on neurotransmitters. A nerve cell releases a neurotransmitter to spur a neighboring cell into action. In the 1950s most scientists believed that a neurotransmitter became inactive once it stimulated a neighboring cell. But Axelrod’s research found that the neurotransmitter returns to the first nerve cell, in a process known as reuptake, where it is broken down by enzymes or repackaged for reuse. This research led to the creation of a number of drugs that prevent the reuptake process, enabling a neurotransmitter to remain active for a longer period of time.
Axelrod’s research revolutionized the understanding of many mental-health disorders, including depression, anxiety, and schizophrenia. Prior to his research, psychiatry focused on the relationship of life experiences to mental health problems. But Axelrod's research proved that mental-health disorders were often the result of complicated brain chemistry. His research spurred the development of new drugs that advanced the treatment of mental-health conditions. Among these are selective serotonin reuptake inhibitors, including the antidepressants fluoxetine, sold under the brand name Prozac, sertraline(Zoloft) and paroxetine (Paxil).
The study of the biochemistry of memory is another exciting scientific enterprise, but one that can only be touched upon here. Scientists estimate that an adult human brain contains about 100 billion neurons. Each of these is connected to hundreds or thousands of other neurons, forming trillions of neural connections. Neurons communicate by chemical messengers called neurotransmitters. An electrical signal travels along the neuron, triggering the release of neurotransmitters at the synapse, the small gap between neurons. The neurotransmitters travel across the synapse and act on the next neuron by binding with protein molecules called receptors. Most scientists believe that memories are somehow stored among the brain's trillions of synapses, rather than in the neurons themselves.
Scientists who study the biochemistry of learning and memory often focus on the marine snail Aplysia because its simple nervous system allows them to study the effects of various stimuli on specific synapses. A change in the snail's behavior due to learning can be correlated with a change at the level of the synapse. One exciting scientific frontier is discovering the changes in neurotransmitters that occur at the level of the synapse.
Other researchers have implicated glucose, a sugar and insulin(a hormone secreted by the pancreas) as important to learning and memory. Humans and other animals given these substances show an improved capacity to learn and remember. Typically, when animals or humans ingest glucose, the pancreas responds by increasing insulin production, so it is difficult to determine which substance contributes to improved performance. Some studies in humans that have systematically varied the amount of glucose and insulin in the blood have shown that insulin may be the more important of the two substances for learning.
Scientists also have examined the influence of genes on learning and memory. In one study, scientists bred strains of mice with extra copies of a gene that helps build a protein called N-methyl-D-aspartate, or NMDA. This protein acts as a receptor for certain neurotransmitters. The genetically altered mice outperformed normal mice on a variety of tests of learning and memory. In addition, other studies have found that chemically blocking NMDA receptor impairs learning in laboratory rats. Future discoveries from genetic and biochemical studies may lead to treatments for memory deficits from Alzheimer's disease and other conditions that affect memory.
Alzheimer's Disease, progressive brain disorders that causes a gradual and irreversible decline in memory, language skills, perception of time and space, and, eventually, the ability to care for oneself. First described by German psychiatrist Alois Alzheimer in 1906, Alzheimer's disease was initially thought to be a rare condition affecting only young people, and was referred to as prehensile dementia. Today late-onset Alzheimer's disease is recognized as the most common cause of the loss of mental function in those aged 65 and over. Alzheimer's in people in their 30s, 40s, and 50s, called early-onset Alzheimer's disease, occurs less frequently, accountings for less than 10 percent of the estimated 4 million Alzheimer's cases in the United States.
Although Alzheimer's disease is not a normal part of the aging process, the risk of developing the disease increases as people grow older. About 10 percent of the United States population over the age of 65 is affected by Alzheimer's disease, and nearly 50 percent of those over age 85 may have the disease.
Alzheimer's disease takes a devastating toll, not only on the patients, but also on those who love and care for them. Some patients experience immense fear and frustration as they struggle with once commonplace tasks and slowly lose their independence. Family, friends, and especially those who provide daily care suffer immeasurable pain and stress as they witness Alzheimer's disease slowly take their loved one from them.
The onset of Alzheimer's disease is usually very gradual. In the early stages, Alzheimer's patients have relatively mild problems learning new information and remembering where they have left common objects, such as keys or a wallet. In time, they begin to have trouble recollecting recent events and finding the right words to express themselves. As the disease progresses, patients may have difficulty remembering what day or month it is, or finding their way around familiar surroundings. They may develop a tendency to wander off and then be unable to find their way back. Patients often become irritable or withdrawn as they struggle with fear and frustration when once commonplace tasks become unfamiliar and intimidating. Behavioral changes may become more pronounced as patients become paranoid or delusional and unable to engage in normal conversation.
Eventually Alzheimer's patients become completely incapacitated and unable to take care of their most basic life functions, such as eating and using the bathroom. Alzheimer's patients may live many years with the disease, usually dying from other disorders that may develop, such as pneumonia. Typically the time from initial diagnosis until death is seven to ten years, but this is quite variable and can range from three to twenty years, depending on the age of the onset, other medical conditions present, and the care patients receive.
The brains of patients with Alzheimer's have distinctive formations - abnormally shaped proteins called tangles and plaques - that are recognized as the hallmark of the disease. Not all brain regions show these characteristic formations. The areas most prominently affected are those related to memory.
Tangles are long, slender tendrils found inside nerve cells, or neurons. Scientists have learned that when a protein-called tau becomes altered, it may cause the characteristic tangles in the brain of the Alzheimer’s patient. In healthy brains provides structural support for neurons, but in Alzheimer's patients this structural support collapses.
Plaques, or clumps of fibers, form outside the neurons in the adjacent brain tissue. Scientists found that a type of protein, called amyloid precursor protein, forms toxic plaques when it is cut in two places. Researchers have isolated the enzyme beta-secretes, which is believed to make one of the cuts in the amyloid precursor protein. Researchers also identified another enzyme, called gamma secretes, that makes the second cut in the amyloid precursor protein. These two enzymes snip the amyloid precursor protein into fragments that then accumulate to form plaques that are toxic to neurons.
Scientists have found that tangles and plaques cause neurons in the brains of Alzheimer's patients to shrink and eventually die, first in the memory and language centers and finally throughout the brain. This widespread neuron degeneration leaves gaps in the brain's messaging network that may interfere with communication between cells, causing some of the symptoms of Alzheimer’s disease.
Alzheimer's patients have lower levels of neurotransmitters, chemicals that carry complex messages back and forth between the nerve cells. For instance, Alzheimer's disease seems to decrease the level of the neurotransmitter acetylcholine, which is known to influence memory. A deficiency in other neurotransmitters, including somatostatin and corticotropin-releasing factor, and, particularly in younger patients, serotonin and norepinephrine, also interferes with normal communication between brain cells.
The causes of Alzheimer's disease remain a mystery, but researchers have found that particular groups of people have risk factors that make them more likely to develop the disease than the general population. For example, people with a family history of Alzheimer's are more likely to develop Alzheimer's disease.
Some of the most promising Alzheimer's research is being conducted in the field of genetics to learn the role a family history of the disease has in its development. Scientists have learned that people who are carriers of a specific version of the apolipoprotein E gene (apoE genes), found on chromosome 19, are several times more likely to develop Alzheimer's than carriers of other versions of the apoE gene. The most common version of this gene in the general population is apoE3. Nearly half of all late-onset Alzheimer’s patients have the fewer in common apoE4 versions, however, and research has shown that this gene plays a role in Alzheimer's disease. Scientists have also found evidence that variations in one or more genes located on chromosomes 1, 10, and 14 may increase a person’s risk for Alzheimer's disease. Scientists have identified the gene variations on chromosomes 1 and 14 and learned that these genes produce mutations in proteins called presenilins. These mutated proteins apparently trigger the activity of the enzyme gamma secretase, which splices the amyloid precursor protein.
Researchers have made similar strides in the investigation of early-onset Alzheimer's disease. A series of genetic mutations in patients with early-onset Alzheimer's has been linked to the production of amyloid precursor protein, the protein in plaques that may be implicated in the destruction of neurons. One mutation is particularly interesting to geneticists because it occurs on a gene involved in the genetic disorder Down syndrome. People with Down syndrome usually develop plaques and tangles in their brains as they get older, and researchers believe that learning more about the similarities between Down syndrome and Alzheimer's may further our understanding of the genetic elements of the disease.
Some studies suggest that one or more factors other than heredity may determine whether people develop the disease. One study published in February 2001 compared residents of Ibadan, Nigeria, who eat a mostly low-fat vegetarian diet, with African Americans living in Indianapolis, Indiana, whose diet included a variety of high-fat foods. The Nigerians were less likely to develop Alzheimer’s disease compared to their U.S. counterparts. Some researchers suspect that health imposes on high blood pressure, atherosclerosis (arteries clogged by fatty deposits), high cholesterol levels, or other cardiovascular problems may play a role in the development of the disease.
Other studies have suggested that environmental agents may be a possible cause of Alzheimer's disease; for example, one study suggested that high levels of aluminum in the brain may be a risk factor. Several scientists initiated research projects to further investigate this connection, but no conclusive evidence has been found linking aluminum with Alzheimer's disease. Similarly, investigations into other potential environmental causes, such as zinc exposure, viral agents, and food-borne poisons, while initially promising, have generally turned up inconclusive results.
Some studies indicate that brain trauma can trigger a degenerative process that results in Alzheimer's disease. In one study, an analysis of the medical records scribed upon veterans of World War II (1939-1945) linked serious head injury in early adulthood with Alzheimer's disease in later life. The study also looked at other factors that could possibly influence the development of the disease among the veterans, such as the presence of the apoE gene, but no other factors were identified.
Alzheimer’s disease is only positively diagnosed by examining brain tissue under a microscope to see the hallmark plaques and tangles, and this is only possible after a patient dies. As a result, physicians rely on a series of other techniques to diagnose probable Alzheimer's disease in living patients. Diagnosis begins by ruling out other problems that cause memory loss, such as stroke, depression, alcoholism, and the use of certain prescription drugs. The patient undergoes a thorough examination, including specialized brain scans, to eliminate other disorders. The patient may be given a detailed evaluation called a neuropsychological examination, which is designed to evaluate a patient’s ability to perform specific mental tasks. This helps the physician determine whether the patient is showing the characteristic symptoms of Alzheimer's disease - progressively worsening memory problems, language difficulties, and trouble with spatial direction and time. The physician also asks about the patient's family medical history to learn about any past serious illnesses, which may give a hint about the patient's current symptoms.
Evidence shows that there is inflammation in the brains of Alzheimer's patients, which may be associated with the production of amyloid precursor protein. Studies are underway to find drugs that prevent this inflammation, to possibly slow or even halt the progress of the disease. Other promising approaches center on mechanisms that manipulate amyloid precursor protein production or accumulation. Drugs are in development that may block the activity of the enzymes that cut the amyloid precursor protein, halting amyloid production. Other studies in mice suggest those vaccinating animals with amyloid precursor protein can produce a reaction that clears amyloid precursor protein from the brain. Physicians have started vaccination studies in humans to determine if the same potentially beneficial effects can be obtained. There is still much to be learned, but as scientists better understand the genetic components of Alzheimer’s, the roles of the amyloid precursor protein and the tau protein in the disease, and the mechanisms of nerve cell degeneration, the possibility that a treatment will be developed is more likely.
The responsibility for caring for Alzheimer's patients generally falls on their spouses and children. Care givers must constantly be on guard for the possibility of Alzheimer's patients wandering away or becoming agitated or confused in a manner that jeopardizes the patient or others. Coping with a loved one's decline and inability to recognize familiar face causes enormous pain.
The increased burden faced by families is intense, and the life of the Alzheimer's care giver is often called a 36-hour day. Not surprisingly, care givers often develop health and psychological problems of their own as a result of this stress. The Alzheimer's Association, a national organization with local chapters throughout the United States, was formed in 1980 in large measure to provide support for Alzheimer's care givers. Today, national and local chapters are a valuable source for information, referral, and advice.
Not to long ago, most approaches to the philosophy of science were ‘cognitive’. This includes ‘logical positivism’, as nearly all of those who wrote about the nature of science would have been in agreement that science ought to be ‘value-free’. This had been a particular emphasis on the part of the first positivist, as it would be upon twentieth-century successors. Science, so it was said, deals with ‘facts’, and facts and values and irreducibly distinct. Facts are objective, they are what we seek in our knowledge of the world. Values are subjective: They bear the mark of human interest, they are the radically individual products of feeling and desire. Fact and value cannot, therefore, be inferred from fact, fact ought not be influenced by value. There were philosophers, notably some in the Kantian tradition, who viewed the relation of the human individual to the universalist aspiration of difference rather differently. But the legacy of three centuries of largely empiricist reflection of the ‘new’ sciences ushered in by Galilee Galileo (1564-1642), the Italian scientist whose distinction belongs to the history of physics and astronomy, rather than natural philosophy.
The philosophical importance of Galileo’s science rests largely upon the following closely related achievements: (1) His stunning successful arguments against Aristotelean science, (2) his proofs that mathematics is applicable to the real world. (3) his conceptually powerful use of experiments, both actual and employed regulatively, (4) his treatment of causality, replacing appeal to hypothesized natural ends with a quest for efficient causes, and (5) his unwavering confidence in the new style of theorizing that would come to be known as mechanical explanation.
A century later, the maxim that scientific knowledge is ‘value-laded’ seems almost as entrenched as its opposite was earlier. It is supposed that between fact and value has been breached, and philosophers of science seem quite at home with the thought that science and value may be closely intertwined after all. What has happened to bring about such an apparently radial change? What are its implications for the objectivity of science, the prized characteristic that, from Plato’s time onwards, has been assumed to set off real knowledge (epistēmē) from mere opinion (doxa)? To answer these questions adequately, one would first have to know something of the reasons behind the decline of logical positivism, as, well as of the diversity of the philosophies of science that have succeeded it.
More general, the interdisciplinary field of cognitive science is burgeoning on several fronts. Contemporary philosophical reelection about the mind - which has been quite intensive - has been influenced by this empirical inquiry, to the extent that the boundary lines between them are blurred in places.
Nonetheless, the philosophy of mind at its core remains a branch of metaphysics, traditionally conceived. Philosophers continue to debate foundational issues in terms not radically different from those in vogue in previous eras. Many issues in the metaphysics of science hinge on the notion of ‘causation’. This notion is as important in science as it is in everyday thinking, and much scientific theorizing is concerned specifically to identify the ‘causes’ of various phenomena. However, there is little philosophical agreement on what it is to say that one event is the cause of some other.
Modern discussion of causation starts with the Scottish philosopher, historian, and essayist David Hume (1711-76),who argued that causation is simply a matter for which he denies that we have innate ideas, that the causal relation is observably anything other than ‘constant conjunction’ wherefore, that there are observable necessary connections anywhere, and that there is either an empirical or demonstrative proof for the assumptions that the future will resemble the past, and that every event has a cause. That is to say, that there is an irresolvable dispute between advocates of free-will and determinism, that extreme scepticism is coherent and that we can find the experiential source of our ideas of self-substance or God.
According to Hume (1978), on event causes another if only if events of the type to which the first event belongs regularly occur in conjunctive events of the type to which the second event belongs. The formulation, however, leaves a number of questions open. Firstly, there is a problem of distinguishing genuine ‘causal law’ from ‘accidental regularities’. Not all regularities are sufficient lawlike to underpin causal relationships. Being a screw in my desk could well be constantly conjoined with being made of copper, without its being true that these screws are made of copper because they are in my desk. Secondly, the idea of constant conjunction does not give a ‘direction’ to causation. Causes need to be distinguished from effects. But knowing that A-type events are constantly conjoined with B-type events does not tell us which of ‘A’ and ‘B’ is the cause and which the effect, since constant conjunction is itself a symmetric relation. Thirdly, there is a problem about ‘probabilistic causation’. When we say that causes and effects are constantly conjoined, do we mean that the effects are always found with the causes, or is it enough that the causes make the effect probable?
Many philosophers of science during the past century have preferred to talk about ‘explanation’ than causation. According to the covering-law model of explanation, something is explained if it can be deduced from premises which include one or more laws. As applied to the explanation of particular events this implies that one particular event can be explained it if is linked by a law to some other particular event. However, while they are often treated as separate theories, the covering-law account of explanation is at bottom little more than a variant of Hume’s constant conjunction account of causation. This affinity shows up in the fact at the covering-law account faces essentially the same difficulties as Hume: (1) In appealing to deduction from ‘laws’, it needs to explain the difference between genuine laws and accidentally true regularities: (2) It omits by effects, as swell as effects by causes, after all, it is as easy to deduce the height of flag-pole from the length of its shadow and the law of optics: (3) Are the laws invoked in explanation required to be exceptionalness and deterministic, or is it acceptable, say, to appeal to the merely probabilistic fact that smoking makes cancer more likely, in explaining why some particular person develops cancer?
Nevertheless, one of the centrally obtainable achievements for which the philosophy of science is to provide explicit and systematic accounts of the theories and explanatory strategies exploited in the science. Another common goal is to construct philosophically illuminating analyses or explanations of central theoretical concepts invoked in one or another science. In the philosophy of biology, for example, there is a rich literature aimed at understanding teleological explanations, and there has been a great deal of work on the structure of evolutionary theory and on such crucial concepts as fitness and biological function. By introducing ‘teleological considerations’, this account views beliefs as states with biological purpose and analyses their truth conditions specifically as those conditions that they are biologically supposed to covary with.
A teleological theory of representation needs to be supplemental with a philosophical account of biological representation generally a selectionism account of biological purpose, according to which item ‘F’ has purpose ‘G’ if and only if it is now present as a result of past selection by some process which favoured item with ‘G’. So, a given belief type will have the purpose of covarying with ‘P’, say. If and only if some mechanism has selected it because it has covaried with ‘P’ the past.
Along the same lines, teleological theory holds that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’, teleological theories differ depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions and a-historical theories. Historical theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicate ‘x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. thus, a state physically indistinguishable from ‘r’ (physical states being a-historical) but lacking r’s historical origins would not represent ‘x’ according to historical theories.
The American philosopher of mind (1935-) Fodor, is known for a resolute ‘realism’ about the nature of mental functioning, taking the analogy between thought and computation seriously. Fodor believes that mental representations should be conceived as individual states with their own identities and structures, like formulae transformed by processes of computation or thought. His views are frequently contrasted with those of ‘holist s’ such as the American philosopher Herbert Donald Davidson (1917-2003), or ‘instrumentalists about mental ascription, such as the British philosopher of logic and language, Eardley Anthony Michael Dummett (1925-). In recent years he has become a vocal critic of some of the aspirations of cognitive science.
Nonetheless, a suggestion extrapolating the solution of teleology is continually queried by points as owing to ‘causation’ and ‘content’, and ultimately a fundamental appreciation is to be considered, is that: We suppose that there’s a causal path from A’s to ‘A’s’ and a causal path from B’s to ‘A’s’, and our problem is to find some difference between B-caused ‘A’s’ and A-caused ‘A’s’ in virtue of which the former but not the latter misrepresented. Perhaps, the two paths differ in their counter-factual properties. In particular, though A’s and B’s both cause ‘A’s’ as a matter of fact, perhaps can assume that only A’s would cause ‘A’s’ in - as one can say - ,‘optimal circumstances’. We could then hold that a symbol expresses its ‘optimal property’, viz., the property that would causally control its tokening in optimal circumstances. Correspondingly, when the tokening of a symbol is causally controlled by properties other than its optimal property, the tokens that eventuate are ipso facto wild.
Suppose at the present time, that this story about ‘optimal circumstances’ is proposed as part of a naturalized semantics for mental representations. In which case it is, of course, essential that it be possible to say that the optimal circumstances for tokening a mental representation are in terms that are not themselves either semantical nor intentional. (It would not do, for example, to identify the optimal circumstances for tokening a symbol as those in which the tokens are true, that would be to assume precisely the sort of semantical notions that the theory is supposed to naturalize.) Befittingly, the suggestion - to put it in a nutshell - is that appeals to ‘optimality’ should be buttressed by appeals to ‘teleology’: Optimal circumstances are the ones in which the mechanisms that mediate symbol tokening are functioning ‘as they are supposed to’. In the case of mental representations, these would be paradigmatically circumstances where the mechanisms of belief fixation are functioning as they are supposed to.
So, then: The teleology o the cognitive mechanisms determines the optimal condition for belief fixation, and the optimal condition for belief fixation determines the content of beliefs. So the story goes.
To put this objection in slightly other words: The teleology story perhaps strikes one as plausible in that it understands one normative notion - truth - in terms of another normative notion - optimality. But this appearance e of fit is spurious there is no guarantee that the kind of optimality that teleology reconstructs has much to do with the kind of optimality that the explication of ‘truth’ requires. When mechanisms of repression are working ‘optimally’ - when they’re working ‘as they’re supposed to’ - what they deliver are likely to be ‘falsehoods’.
Or again: There’s no obvious reason why coitions that are optimal for the tokening of one sort of mental symbol need be optimal for the tokening of other sorts. Perhaps the optimal conditions for fixing beliefs about very large objects, are different from the optimal conditions for fixing beliefs about very small ones, are different from the optimal conditions for fixing beliefs sights. But this raises the possibility that if we’re to say which conditions are optimal for the fixation of a belief, we’ll have to know what the content of the belief is - what it’s a belief about. Our explication of content would then require a notion of optimality, whose explication in turn requires a notion of content, and the resulting pile would clearly be unstable.
Teleological theories hold that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’. Teleological theories differ, depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions: Historically, theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicates ’x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. Thus, a state physically indistinguishable from ‘r’ (physical states being a-historical), but lacking r’s historical origins would not represent ‘x’ according to historical theories.
Just as functional role theories hold that r’s representing ‘x’ is grounded in the functional role ‘r’ has in the representing system, i.e., on the relations imposed by specified cognitive processes between ‘r’ and other representations in the system’s repertoire. Functional role theories take their cue from such common-sense ideas as that people cannot believe that cats are furry if they do not know that cats are animals or that fur is like hair.
That being said, that nowhere is the new period of collaboration between philosophy and other disciplines more evident than in the new subject of cognitive science. Cognitive science from its very beginning has been ‘interdisciplinary’ in character, and is in effect the joint property of psychology, linguistics, philosophy, computer science and anthropology. There is, therefore, a great variety of different research projects within cognitive science, but the central area of cognitive science, its hard-coded ideology rests on the assumption that the mind is best viewed as analogous to a digital computer. The basic idea behind cognitive science is that recent developments in computer science and artificial intelligence have enormous importance for our conception of human beings. The basic inspiration for cognitive science went something like this: Human beings do information processing. Computers are designed precisely do information processing. Therefore, one way to study human cognition - perhaps the best way to study it - is to study. It as a matter of computational information processing. Some cognitive scientists think that the computer is just a metaphor for the human mind: Others think that the mind is literally a computer program. But it is fair to say, that without the computational model there would not have been a cognitive science as we now understand it.
In, Essay Concerning Human Understanding is the first modern systematic presentation of an empiricist epistemology, and as such had important implications for the natural sciences and for philosophy of science generally. Like his predecessor, Descartes, the English philosopher (1632-104) John Locke began his account of knowledge from the conscious mind aware of ideas. Unlike Descartes, however, he was concerned not to build a system based on certainty, but to identify the mind’s scope and limits. The premise upon which Locke built his account, including his account of the natural sciences, is that the ideas which furnish the mind are all derived from experience. He thus, totally rejected any kind of innate knowledge. In this he consciously opposing Descartes, who had argued that it is possible to come to knowledge of fundamental truths about the natural world through reason alone. Descartes (1596-1650) had argued, that we can come to know the essential nature of both ‘mind’ and ‘matter’ by pure reason. John Locke accepted Descartes’s criterion of clear and distinct ideas as the basis for knowledge, but denied any source for them other than experience. It was information that came in via the five senses (ideas of sensation) and ideas engendered from pure inner experiences (ideas of reflection) came the building blocks of the understanding.
Locke combined his commitment to ‘the new way of ideas’ with a te native espousal of the ‘corpuscular philosophy’ of the Irish scientist (1627-92) Robert Boyle. This, in essence, was an acceptance of a revised, more sophisticated account of matter and its properties that had been advocated by the ancient atomists and recently supported by Galileo (1564-1642) and Pierre Gassendi (1592-1655). Boyle argued from theory and experiment that there were powerful reasons to justify some kind of corpuscular account of matter and its properties. He called the latter qualities, which he distinguished as primary and secondary - the distinction between primary and secondary qualities may be reached by two rather different routes: Either from the nature or essence of matter or from the nature and essence of experience, though practising these have tended to run together. The former considerations make the distinction seem like an a priori, or necessary, truth about the nature of matter, while the latter make it appears to be an empirical hypothesis -. Locke, too, accepted this account, arguing that the ideas we have of the primary qualities of bodies resemble those qualities as they are in the subject, whereas the ideas of the secondary qualities, such as colour, taste, and smell, do not resemble their causes in the object.
There is no strong connection between acceptance of the primary-secondary quality distinction and Locke’s empiricism and Descartes had also argued strongly for universal acceptance by natural philosophers, and Locke embraced it within his more comprehensive empirical philosophy. But Locke’ empiricism did have major implications for the natural sciences, as he well realized. His account begins with an analysis of experience. all ideas, he argues, are either simple or complex. Simple ideas are those like the red of a particular rose or the roundness of a snowball. Complex ideas, our ideas of the rose or the snowball, are combinations of simple ideas. We may create new complex ideas in our imagination - a dragon, for example. But simple ideas can never be created by us: We just have them or not, and characteristically they are caused, for example, the impact on our senses of rays of light or vibrations of sound in the air coming from a particular physical object. Since we cannot create simple ideas, and they are determined by our experience. Our knowledge is in a very strict uncompromising way limited. Besides, our experiences are always of the particular, never of the general. It is this particular simple idea or that particular complex idea that we apprehend. We never in that sense apprehend a universal truth about the natural world, but only particular instances. It follows from this that all claims to generality about that world - for example, all claims to identity what were then beginning to be called the laws of nature - must to that extent go beyond our experience and thus be less than certain.
The Scottish philosopher, historian, and essayist, (1711-76) David Hume, whose famous discussion appears in both his major philosophical works, the ‘Treatise’ (1739) and the ‘Enquiry’(1777). The distinction is couched in terms of the concept of causality, so that where we are accustomed to talk of laws, Hume contends, involves three ideas:
1. That there should be a regular concomitance between events of the type of the cause and those of the type of the effect.
2. That the cause event should be contiguous with the effect event.
3. That the cause event should necessitate the effect event.
The tenets (1) and (2) occasion no differently for Hume, since he believes that there are patterns of sensory impressions un-problematically related to the idea of regularity concomitance and of contiguity. But the third requirement is deeply problematic, in that the idea of necessarily that figures in it seems to have no sensory impression correlated with it. However, carefully and attentively we scrutinize a causal process, we do not seem to observe anything that might be the observed correlates of the idea of necessity. We do not observe any kind of activity, power, or necessitation. All we ever observe is one event following another, which is logically independent of it. Nor is this necessity logical, since, as, Hume observes, one can jointly assert the existence of the cause and a denial of the existence of the effect, as specified in the causal statement or the law of nature, without contradiction. What, then, are we to make of the seemingly central notion of necessity that is deeply embedded in the very idea of causation, or lawfulness? To this query, Hume gives an ingenious and telling story. There is an impression corresponding to the idea of causal necessity, but it is a psychological phenomenon: Our exception that an even similar to those we have already observed to be correlated with the cause-type of events will come to be in this cas e too. Where does that impression come from? It is created as a kind of mental habit by the repeated experience of regular concomitance between events of the type of the effect and the occurring of event s of the type of the cause. And then, the impression that corresponds to the idea of regular concomitance - the law of nature then asserts nothing but the existence of the regular concomitance.
At this point in our narrative, the question at once arises as to whether this factor of life in nature, thus interpreted, corresponds to anything that we observe in nature. All philosophy is an endeavour to obtain a self-consistent understanding of things observed. Thus, its development is guided in two ways, one is demand for coherent self-consistency, and the other is the elucidation of things observed. With our direct observations how are we to conduct such comparison? Should we turn to science? No. There is no way in which the scientific endeavour can detect the aliveness of things: Its methodology rules out the possibility of such a finding. On this point, the English mathematician and philosopher (1861-1947) Alfred Whitehead, comments: That science can find no individual enjoyment in nature, as science can find no creativity in nature, it finds mere rules of succession. These negations are true of natural science. They are inherent in its methodology. The reason for this blindness of physical science lies in the fact that such science only deals with half the evidence provided by human experience. It divides the seamless coat - or, to change the metaphor into a happier form, it examines the coat, which is superficial, and neglects the body which is fundamental.
Whitehead claims that the methodology of science makes it blind to a fundamental aspect of reality, namely, the primacy of experience, it neglected half of the evidence. Working within Descartes’ dualistic framework of matter and mind as separate and incommensurate, science limits itself to the study of objectivised phenomena, neglecting the subject and the mental events that are his or her experience.
Both the adoption of the Cartesian paradigm and the neglect of mental events are reason enough to suspect ‘blindness’, but there is no need to rely on suspicions. This blindness is clearly evident. Scientific discoveries, impressive as they are, are fundamentally superficial. Science can express regularities observed in nature, but it cannot explain the reasons for their occurrence. Consider, for example, Newton’s law of gravity. It shows that such apparently disparate phenomena as the falling of an apple and the revolution of the earth around the sun are aspects of the same regularity - gravity. According to this law the gravitational attraction between two objects deceases in proportion to the square of the distance between them. Why is that so? Newton could not provide an answer. Simpler still, why does space have three dimensions? Why is time one-dimensional? Whitehead notes, ‘None of these laws of nature gives the slightest evidence of necessity. They are [merely] the modes of procedure which within the scale of observation do in fact prevail’.
This analysis reveals that the capacity of science to fathom the depths of reality is limited. For example, if reality is, in fact, made up of discrete units, and these units have the fundamental character in being ’throbs of experience’, then science may be in a position to discover the discreteness: But it has no access to the subjective side of nature, since, as the Austrian physicist(1887-1961) Erin Schrödinger points out, we ‘exclude the subject of cognizance from the domain of nature that we endeavour to understand’. It follows that in order to find ‘the elucidation of things observed’ in relation to the experiential or aliveness aspect, we cannot rely on science, we need to look elsewhere.
If, instead of relying on science, we rely on our immediate observation of nature and of ourselves, we find, first, that this [i.e., Descartes’] stark division between mentality and nature has no ground in our fundamental observation. We find ourselves living within nature. Secondly, in that we should conceive mental operations as among the factors which make up the constitution of nature, and thirdly, that we should reject the notion of idle wheels in the process of nature. Every factor which makes a difference, and that difference can only be expressed in terms of the individual character of that factor.
Whitehead proceeds to analyse our experiences in general, and our observations of nature in particular, and ends up with ‘mutual immanence’ as a central theme. This mutual immanence is obvious in the case of an experience, that, I am a part of the universe, and, since I experience the universe, the experienced universe is part of me. Whitehead gives an example’ ‘I am in the room, and the room is an item in my present experience. But my present experience is what I am now’. A generalization of this relationship to the case of any actual occasions yields the conclusion that ‘the world is included within the occasion in one sense, and the occasion is included in the world in another sense’. The idea that each actual occasion appropriates its universe follows naturally from such considerations.
The description of an actual entity as being a distinct unit is, therefore, only one part of the story. The other, complementary part is this: The very nature of each and every actual entity is one of interdependence with all the other actual entities in the universe. Each and every actual entity is a process of prehending or appropriating all the other actual entities and creating one new entity out of them all, namely, itself.
There are two general strategies for distinguishing laws from accidentally true generalizations. The first stands by Hume’s idea that causal connections are mere constant conjunctions, and then seeks to explain why some constant conjunctions are better than others. That is, this first strategy accepts the principle that causation involves nothing more than certain events always happening together with certain others, and then seeks to explain why some such patterns - the ‘laws’ - matter more than others - the ‘accidents’ -. The second strategy, by contrast, rejects the Humean presupposition that causation involves nothing more than happen-stantial co-occurrence, and instead postulates a relationship ‘necessitation’, a kind of ‘cement, which links events that are connected by law, but not those events (like being a screw in my desk ad being made of copper) that are only accidentally conjoined.
There are a number of versions of the first Human strategy. The most successful, originally proposed by the Cambridge mathematician and philosopher F.P. Ramsey (1903-30), and later revived by the American philosopher David Lewis (1941-2002), who holds that laws are those true generalizations that can be fitted into an ideal system of knowledge. The thought is, that, the laws are those patterns that are somewhat explicated in terms of basic science, either as fundamental principles themselves, or as consequences of those principles, while accidents, although true, have no such explanation. Thus, ‘All water at standard pressure boils at 1000 C’ is a consequence of the laws governing molecular bonding: But the fact that ‘All the screws in my desk are copper’ is not part of the deductive structure of any satisfactory science. Ramsey neatly encapsulated this idea by saying that laws are ‘consequences of those proposition which we should take as axioms if we knew everything and organized it as simply as possible in a deductive system’.
Advocates of the alternative non-Humean strategy object that the difference between laws and accidents is not a ‘linguistic’ matter of deductive systematization, but rather a ‘metaphysical’ contrast between the kind of links they report. They argue that there is a link in nature between being at 1000 C and boiling, but not between being ‘in my desk’ and being ‘made of copper’, and that this is nothing to do with how the description of this link may fit into theories. According to D.M. Armstrong (1983), the most prominent defender of this view, the real difference between laws and accidentals, is simply that laws report relationships of natural ‘necessitation’, while accidents only report that two types of events happen to occur together.
Armstrong’s view may seem intuitively plausible, but it is arguable that the notion of necessitation simply restates the problem, than solving it. Armstrong says that necessitation involves something more than constant conjunction: If two events e related by necessitation, then it follows that they are constantly conjoined, but two events can be constantly conjoined without being related by necessitation, as when the constant conjunction is just a matter of accident. So necessitation is a stronger relationships than constant conjunction. However, Armstrong and other defenders of this view say ver y little about what this extra strength amounts to, except that it distinguishes laws from accidents. Armstrong’s critics argue that a satisfactory account of laws ought to cast more light than this on the nature of laws.
Hume said that the earlier of two causally related events is always the cause, and the later effect. However, there are a number of objections to using the earlier-later ‘arow of time’ to analyse the directional ‘arrow of causation’. For a start, it seems in principle, possible that some causes and effects could be simultaneous. That more, in the idea that time is directed from ‘earlier’ to ‘later’ itself stands in need of philosophical explanation - and one of the most popular explanations is that the idea of ‘movement’ from earlier to later depends on the fact that cause-effect pairs always have a time, and explain ‘earlier’ as the direction in which causes lie, and ‘later’ as the direction of effects, that we will clearly need to find some account of the direction of causation which does not itself assume the direction of time.
A number of such accounts have been proposed. David Lewis (1979) has argued that the asymmetry of causation derives from an ‘asymmetry of over-determination’. The over-determination of present events by past events - consider a person who dies after simultaneously being shot and struck by lightning - is a very rare occurrence, by contrast, the multiple ‘over-determination’ of present events by future events is absolutely normal. This is because the future, unlike the past, will always contain multiple traces of any present event. To use Lewis’s example, when the president presses the red button in the White House, the future effects do not only include the dispatch of nuclear missiles, but also the fingerprint on the button, his trembling, the further depletion of his gin bottle, the recording of the button’s click on tape, he emission of light waves bearing the image of his action through the window, the warnings of the wave from the passage often signal current, and so on, and so on, and on.
Lewis relates this asymmetry of over-determination to the asymmetry of causation as follows. If we suppose the cause of a given effect to have been absent, then this implies the effect would have been absent too, since (apart from freaks like the lightning-shooting case) there will not be any other causes left to ‘fix’ the effect. By contrast, if we suppose a given effect of some cause to have been absent, this does not imply the cause would have been absent, for there are still all the other traces left to ’fix’ the causes. Lewis argues that these counterfactual considerations suffice to show why causes are different from effects.
Other philosophers appeal to a probabilistic variant of Lewis’s asymmetry. Following, the philosopher of science and probability theorists, Hans Reichenbach (1891-1953), they note that the different causes of any given type of effect are normally probabilistically independent of each other, by contrast, the different effects of any given type of cause are normally probabilistically correlated. For example, both obesity and high excitement can cause heart attacks, but this does not imply that fat people are more likely to get excited than thin ones: Its facts, that both lung cancer and nicotine-stained fingers can result from smoking does imply that lung cancer is more likely among people with nicotine-stained fingers. So this account distinguishes effects from causes by the fact that the former, but not the latter are probabilistically dependent on each other.
However, there is another course of thought in philosophy of science, the tradition of negative or eliminative induction. From the English statesman and philosopher Francis Bacon (1561-1626) and in modern time the philosopher of science Karl Raimund Popper (1902-1994), we have the idea of using logic to bring falsifying evidence to bear on hypotheses about what must universally be the case that many thinkers accept in essence his solution to the problem of demarcating proper science from its imitators, namely that the former results in genuinely falsifiable theories whereas the latter do not. Although falsely allowed many people’s objections to such ideologies as psychoanalysis and Marxism.
Hume was interested in the processes by which we acquire knowledge: The processes of perceiving and thinking, of feeling and reasoning. He recognized that much of what we claim to know derives from other people secondhand, thirdhand or worse: Moreover, our perceptions and judgements can be distorted by many factors - by w hat we are studying, as well as by the very act of study itself., the main reason, however, behind his emphasis on ‘probabilities and those other measures of evidence on which life and action entirely depend’ is this:
It is evident that all reasoning concerning ‘matter of fact’ are founded on the relation of cause and effect, and that we can never infer the existence of one object from another unless the are connected together, either mediately or immediately.
When we apparently observe a whole sequence, say of one ball hitting another, what exactly do we observe? And in the much commoner cases, when we wonder about the unobserved causes or effects of the events we observe, what precisely are we doing?
Hume recognized that a notion of ‘must’ or necessity is a peculiar feature of causal relation, inference and principles, and challenges us to explain and justify the notion. He argued that there is no observable feature of events, nothing like a physical bond, which can be properly labelled the ‘necessary connection’ between a given cause and its effect: Events simply are, they merely occur, and there is in ‘must’ or ‘ought’ about therm. However, repeated experience of pairs of events sets up the habit of expectation in us, such that when one of the pair occurs we inescapably expect the other. This expectation makes us infer the unobserved cause or unobserved effect of the observed event, and we mistakenly project this mental inference on to the events themselves. There is no necessity observable in causal relations; all that can be observed is regular sequence, here is necessity in causal inferences, but only in the mind. Once we realize that causation is a relation between pairs of events. We also realize that often we are not present for the whole sequence e which we want to divide into ‘cause’ and ‘effect’. Our understanding of the casual relation is thus intimately linked with the role of the causal inference cause only causal inferences entitle us to ‘go beyond what is immediately present to the senses’. But now two very important assumptions emerge behind the causal inference: The assumptions that ‘like causes, in like circumstances, will always produce like effects’, and the assumption that ‘the course of nature will continue uniformly the same’ - or, briefly that the future will resemble the past. Unfortunately, this last assumption lacks either empirical or a priori proof, that is, it can be conclusively established neither by experience nor by thought alone.
Hume frequently endorsed a standard seventeenth-century view that all our ideas are ultimately traceable, by analysis, to sensory impressions of an internal or external kind. Accordingly, he claimed that all his theses are based on ‘experience’, understood as sensory awareness together with memory, since only experience establishes matters of fact. But is our belief that the future will resemble the past properly construed as a belief concerning only a mater of fact? As the English philosopher Bertrand Russell (1872-1970) remarked, earlier this century, the real problem that Hume raises is whether future futures will resemble future pasts, in the way that past futures really did resemble past pasts. Hume declares that ‘if . . . the past may be no rule for the future, all experience become useless and can give rise to inference or conclusion. And yet, he held, the supposition cannot stem from innate ideas, since there are no innate ideas in his view nor can it stem from any abstract formal reasoning. For one thing, the future can surprise us, and no formal reasoning seems able to embrace such contingencies: For another, even animals and unthinkable people conduct their lives as if they assume the future resembles the past: Dogs return for buried bones, children avoid a painful fire, and so forth. Hume is not deploring the fact that we have to conduct our lives on the basis of probabilities, and he is not saying that inductive reasoning could or should be avoided or rejected. Rather, he accepted inductive reasoning but tried to show that whereas formal reasoning of the kind associated with mathematics cannot establish or prove matters of fact, factual or inductive reasoning lacks the ‘necessity’ and ‘certainty’ associated with mathematics. His position, therefore clear; because ‘every effect is a distinct event from its cause’, only investigation can settle whether any two particular events are causally related: Causal inferences cannot be drawn with the force of logical necessity familiar to us from a priori reasoning, but, although they lack such force, they should not be discarded. In the context of causation, inductive inferences are inescapable and invaluable. What, then, makes ‘past experience’ the standard of our future judgement? The answer is ‘custom’, it is a brute psychological fact, without which even animal life of a simple kind would be more or less impossible. ‘We are determined by custom to suppose the future conformable to the past’ (Hume, 1978), nevertheless, whenever we need to calculate likely events we must supplement and correct such custom by self-conscious reasoning.
Nonetheless, the problem that the causal theory of reference will fail once it is recognized that all representations must occur under some aspect or that the extentionality of causal relations is inadequate to capture the aspectual character of reference. The only kind of causation that could be adequate to the task of reference is intentional causal or mental causation, but the causal theory of reference cannot concede that ultimately reference is achieved by some met device, since the whole approach behind the causal theory was to try to eliminate the traditional mentalism of theories of reference and meaning in favour of objective causal relations in the world, though it is at present by far the most influential theory of reference, will prove to be a failure for these reasons.
If mental states are identical with physical states, presumably the relevant physical states are various sorts of neural states. Our concepts of mental states such as thinking, sensing, and feeling are of course, different from our concepts of neural states, of whatever sort. But that is no problem for the identity theory. As J.J.C. Smart (1962), who first argue for the identity theory, emphasized, the requisite identities do not depend on understanding concepts of mental states or the meanings of mental terms. For ‘a’ to be the identical with ‘b’, ‘a’, and ‘b’ must have exactly the same properties, but the terms ‘a’ and ‘b’ need not mean the same. Its principal means by measure can be accorded within the indiscernibility of identicals, in that, if ‘A’ is identical with ‘B’, then every property that ‘A’ has ’B’, and vice versa. This is, sometimes known as Leibniz’ s Law.
But a problem does seem to arise about the properties of mental states. Suppose pain is identical with a certain firing of c-fibres. Although a particular pain is the very same as a neural-firing, we identify that state in two different ways: As a pain and as neural-firing. that the state will therefore have certain properties in virtue of which wee identify it as pain and others in virtue of which we identify it as an excitability of neural firings. The properties in virtue of which we identify it as a pain will be mental properties, whereas those in virtue of which ewe identify it as neural excitability firing, will be physical properties. This has seemed to many to lead to a kind of dualism at the level of the properties of mental states, even if we reject dualism of substances and take people simply to be physical organisms, those organisms still have both mental and physical states. Similarly, even if we identify those mental states with certain physical states, those states will, nonetheless have both mental and physical properties. So disallowing dualism with respect to substances and their states simply es to its reappearance at the level of the properties of those states.
There are two broad categories of mental property. Mental states such as thoughts and desires, often called ‘propositional altitudes’, have ‘content’ that can be de scribed by ‘that’ clauses. For example, one can have a thought, or desire, that it will rain. These states are said to have intentional properties, or ‘intentionality sensations’, such as pains and sense impressions, lack intentional content, and have instead qualitative properties of various sorts.
The problem about mental properties is widely thought to be most pressing for sensations, since the painful qualities of pains and the red quality of visual sensations seem to be irretrievably non-physical. And if mental states do actually have non-physical properties, the identity of mental states generate to physical states as they would not sustain a thoroughgoing mind-body materialism.
The Cartesian doctrine that the mental is in some way non-physical is so pervasive that even advocates of the identity theory sometimes accepted it, for the ideas that the mental is non-physical underlies, for example, the insistence by some identity theorists that mental properties are really neural as between being mental or physical. To be neural is in this way, a property would have to be neutral as to whether its mental at all. Only if one thought that being meant being non-physical would one hold that defending materialism required showing the ostensible mental properties are neutral as regards whether or not they’re mental.
But holding that mental properties are non-physical has a cost that is usually not noticed. A phenomenon is mental only if it has some distinctively mental property. So, strictly speaking, a materialist who claims that mental properties are non-physical phenomena exist. This is the ‘Eliminative-Materialist position advanced by the American philosopher and critic Richard Rorty (1979).
According to Rorty (1931-) ‘mental’ and ‘physical’ are incompatible terms. Nothing can be both mental and physical, so mental states cannot be identical with bodily states. Rorty traces this incompatibly to our views about incorrigibility: ‘Mental’ and ‘physical’ are incorrigible reports of one’s own mental states, but not reports of physical occurrences, but he also argues that we can imagine a people who describe themselves and each other using terms just like our mental vocabulary, except that those people do not take the reports made with that vocabulary to be incorrigible. Since Rorty takes a state to be a mental state only if one’s reports about it are taken to be incorrigible, his imaginary people do not ascribe mental states to themselves or each other. Nonetheless, the only difference between their language and ours is that we take as incorrigible certain reports which they do not. So their language as no less descriptive or explanatory power than ours. Rorty concludes that our mental vocabulary is idle, and that there are no distinctively mental phenomena.
This argument hinges on building incorrigibly into the meaning of the term ‘mental’. If we do not, the way is open to interpret Rorty’s imaginary people as simply having a different theory of mind from ours, on which reports of one’s own mental stares are not incorrigible. Their reports would this be about mental states, as construed by their theory. Rorty’s thought experiment would then provide to conclude not that our terminology is idle, but only that this alternative theory of mental phenomena is correct. His thought experiment would thus sustain the non-eliminativist view that mental states are bodily states. Whether Rorty’s argument supports his eliminativist conclusion or the standard identity theory, therefore, depends solely on whether or not one holds that the mental is in some way non-physical.
Paul M. Churchlands (1981) advances a different argument for eliminative materialism. According to Churchlands, the common-sense concepts of mental states contained in our present folk psychology are, from a scientific point of view, radically defective. But we can expect that eventually a more sophisticated theoretical account will relace those folk-psychological concepts, showing that mental phenomena, as described by current folk psychology, do not exist. Since, that account would be integrated into the rest of science, we would have a thoroughgoing materialist treatment of all phenomena, unlike Rorty’s, does not rely of assuming that the mental is non-physical.
But even if current folk psychology is mistaken, that does not show that mental phenomena does not exist, but only that they are of the way folk psychology described them as being. We could conclude they do not exist only if the folk-psychological claims that turn out to be mistaken actually define what it is for a phenomena to be mental. Otherwise, the new theory would be about mental phenomena, and would help show that they’re identical with physical phenomena. Churchlands argument, like Rorty’s, depends on a special way of defining the mental, which we need not adopt, its likely that any argument for Eliminative materialism will require some such definition, without which the argument would instead support the identity theory.
Despite initial appearances, the distinctive properties of sensations are neutral as between being mental or physical, in that borrowed from the English philosopher and classicist Gilbert Ryle (1900-76), they are topic neutral: My having a sensation of red consists in my being in a state that is similar, in respect that we need not specify, even so, to something that occurs in me when I am in the presence of certain stimuli. Because the respect of similarity is not specified, the property is neither distinctively mental nor distinctively physical. But everything is similar to everything else in some respect or other. So leaving the respect of similarity unspecified makes this account too weak to capture the distinguishing properties of sensation.
A more sophisticated reply to the difficultly about mental properties is due independently to forthright Australian David Malet Armstrong (1926-) and American philosopher David Lewis (1941-2002), who argued that for a state to be a particular sort of intentional state or sensation is for that state to bear characteristic causal relations to other particular occurrences. The properties in virtue of which e identify states as thoughts or sensations will still be neural as between being mental or physical, since anything can bear a causal relation to anything else. But causal connections have a better chance than similarity in some unspecified respect to capturing the distinguishing properties of sensations and thought.
This casual theory is appealing, but is misguided to attempt to construe the distinctive properties of mental states as being neutral as between being mental; or physical. To be neutral as regards being mental or physical is to be neither distinctively mental nor distinctively physical. But since thoughts and sensations are distinctively mental states, for a state to be a thought or a sensation is perforce for it to have some characteristically mental property. We inevitably lose the distinctively mental if we construe these properties as being neither mental nor physical.
Not only is the topic-neutral construal misguided: The problem it was designed to solve is equally so, only to say, that problem stemmed from the idea that mental must have some non-physical aspects. If not at the level of people or their mental states, then at the level of the distinctively mental properties of those states. However, it should ne mentioned, that properties can be more complicated, for example, in the sentence, ‘John is married to Mary’, we are attributing to John the property of being married, and unlike the property of John is bald. Consider the sentence: John is bearded. The word ‘John’ in this sentence is a bit of language - a name of some individual human being - and more some would be tempted to confuse the word with what it names. Consider the expression ‘is bald’, this too is a bit of language - philosopher call it a ‘predicate’ - and it brings to our attention some property or feature which, if the sentence is true,. Is possessed by John. Understood in this ay, a property is not its self linguist though it is expressed, or conveyed by something that is, namely a predicate. What might be said that a property is a real feature of the word, and that it should be contrasted just as sharply with any predicates we use to express it as the name ‘John’ is contrasted with the person himself. Controversially, just what sort of ontological status should be accorded to properties by describing ‘anomalous monism’, - while its conceivably given to a better understanding the similarity with the American philosopher Herbert Donald Davidson (1917-2003wherefore he adopts a position that explicitly repudiates reductive physicalism, yet purports to be a version of materialism, nonetheless, Davidson holds that although token mental event nd states are identical to those of physical events and states - mental ‘types’ - i.e., kinds, and/or properties - are neither to, nor nomically co-existensive with, physical types. In other words, his argument for this position relies largely on the contention that the correct assignment of mental a actionable properties to a person is always a holistic matter, involving a global, temporally diachronic, ‘intentional interpretation’ of the person. But as many philosophers have in effect pointed out, accommodating claims of materialism evidently requires more than just repercussions of mental/physical identities. Mentalistic explanation presupposes not merely that metal events are causes but also that they have causal/explanatory relevance as mental - i.e., relevance insofar as they fall under metal kinds or types. Nonetheless, Davidson’s position, which denies there are strict psychological or psychological laws, can accommodate the causal/explanation relevance of the mental quo mental: If to ‘epiphenomenalism’ with respect to mental properties.
But the idea that the mental is in some respect non-physical cannot be assumed without argument. Plainly, the distinctively mental properties of the mental states are unlikely any other properties we know about. Only mental states have properties that are at all like the qualitative properties that anything like the intentional properties of thoughts and desires. Bu t this does not show that the mental properties are not physical properties, not all physical properties like the standard states: So, mental properties might still be special kinds of physical properties. Its question beginning to assume otherwise. The doctrine that the mental properties is simply an expression of the Cartesian doctrine that the mental is automatically non-physical.
Its sometimes held that properties should count as physical properties inly if they can be defined using the terms of physics. This to far to restrictively. Nobody would hold that to reduce biology to physics, for example, we must define all biological properties using only terms that occur in physics. And even putting ‘reduction’ aside, I certain biological properties could have been defined, that would not mean that those properties were in n way non-physical. The sense of ‘physical’ that is relevant, that is of its situation it must be broad enough to include not only biological properties, but also most common-sense, macroscopic properties. Bodily states are uncontroversially physical in the relevant way. So, we can recast the identity theory as asserting that mental states are identical with bodily state.
In the course of reaching conclusions about the origin and limits of knowledge, Locke had occasioned concern himself with topics which are of philosophical interest in themselves. On of these is the question of identity, which includes, more specifically, the question of personal identity: What are the criteria by which a person at one time is numerically the same person as a person encountering of time? Locke points out whether ‘this is what was here before, it matters what kind of thing ‘this’ is meant to be. If ‘this’ is meant as a mass of matter then it is what was before so long as it consists of the same material panicles, but if it is meant as a living body then its considering of the same particles does mot matter and the case is different. ‘A colt grown up to a horse, sometimes fat, sometimes lean, is all the while the same horse though . . . there may be a manifest change of the parts. So, when we think about personal identity, we need to be clear about a distinction between two things which ‘the ordinary way of speaking runs together’ - the idea of ‘man’ and the idea of ‘person’. As with any other animal, the identity of a man consists ‘in nothing but a participation of the same continued life, by constantly fleeting particles of matter, in succession initially united to the same organized body, however, the idea of a person is not that of a living body of a certain kind. A person is a ‘thinking’. ‘intelligent being, that has son and reflection and such a being ‘will be the same self as far as the same consciousness can extend to action past or to come’ . Locke is at pains to argue that this continuity of delf-consciousness does not necessarily involve the continuity of some immaterial substance, ion the way that Descartes had held, fo we all know, says Locke, consciousness and thought may be powers which can be possessed by ‘systems of matter fitly disposed’, and even if this is not so the question of the identity of person is not the same as the question of the identity of an ‘immaterial; substance’. For just as the identity of as horse can be preserved through changes of matter and depends not on the identity of a continued material substance of its unity of one continued life. So the identity of a person does not depend on the continuity of a immaterial; substance. The unity of one continued consciousness does not depend on its being ‘annexed’ only to one individual substance, [and not] . . . continued in a succession of several substances. For Lock e, then, personal identity consists in an identity of consciousness, and not in the identity of some substance whose essence it is to be conscious
In as far as he does discover healthy maturational changes at work in his body and personality, changes that he realizes to be wonderful and priceless, he experiences the poignant accompanying realization that there is no one there to welcome these changes and to share his joy. The parents, if sufficiently free from anxiety to recognize such changes at all, have a tendency to accept them as evidence that their child is rejecting then by growing functionally. Also to be noted, in this connexion, is their lack of trust in him, their lack of assurance that he is elementally good and can be trusted to maturational bases of a good healthy adult. Instead they are alert to find, and warn him against, manifestations in him that can be construed as evidence that he is on a predestined, downward path into an adulthood of criminality, insanity, more at best ineptitude for living.
Moreover, he emergences change not as something within his own power to wield, for the benefit of himself and others but as something imposed from without. This is due not only to structures that the parents place upon his autonomy, but also to the process of increasing repression of his emotions and life as, such that when this latter manifest themselves, they do so in a projected expressive style, for being uncontrollable changed, inflicted upon him from the surrounding world? We see extreme examples of this mechanism later on. In the full-blown schizophrenic person who experiences sexual feelings not as such but as electric shocks sent into him from the outside world, and who experiences anger not as an emerging emotion directorially fittingly as in a way up from within, but a massive and sudden blow coming somehow from the outer world. In fewer extreme instances, in the life of the yet-to-become-schizophrenic youth, he finds repeatedly that when he reaches out to another person, the other suddenly undergoes a change in demeanour, from friendliness to antagonism, in reaction to an unwitting manifestation of the youths’ unconscious hostility. The youth himself, if unable to recognize his own hostility, can only be left feeling increased helplessness in face of an unpredictably changeable world of people.
The final incident that occurs before his admission to the hospital, giving him still further reason for anxiety as for change, is his experience of the psychotic symptoms as an overwhelming anxiety-laden and mysterious change. His own anxiety about this frightened away by the seismic disturbance and horror of the members of his family who finds hi ‘changed’ by what they see as an unmitigated catastrophe, a nervous or mental ‘breakdown’. Although the therapist can come to see, in retrospect, a potential positive element via this occurrence - namely, the emergence of onetime-repressed insights concerning the true state of affairs involving the patient and his family, none of those participants can integrate so radically changed a picture at that time. Over the preceding years the family members could not tolerate their child’s seeing himself and them with the eyes of a normally maturing offspring, and when repressed percepts emerge from repression in him, neither they nor he possesses the requisite ego-strength to accept them as badly needed changes in his picture of himself and of them. Instead, the tumult of depressed percepts foes into the formation of such psychotic phenomena as misidentifications, hallucinations, and delusions in which neither he nor the member of his family can discern the links to reality that we, upon investigation in individual psychotherapy with him, can find in these psychotic phenomena - links, that is, to the state of affairs that has really held sway in the family. Paretically, it should be marked and noted that the psychotic episode often occurs in such ac way as to leave the patient especially fearful of sudden change, for in many instances the de-repressed material emerges suddenly and leads him to damage, in the short space of a few hours or even moments, his life situation so grievously that repair can be affected only very slowly and painfully, over many subsequent months of treatment in the confines of a hospital.
It should be conveyed, in that the regression of the thought-processes, which occurs as one of the features of the developing schizophrenia, results in an experience of the world so kaleidoscopic as to make up still another reason for the individual’s anxiety concerning change. That is, as much as he has lost thee capacity to grasp the essentials of a given whole - to the extent that he has regressed to what Goldstein (1946) terms the ‘concrete attitude’ - he experiences any change, even if it is only in an insignificant (by mature standards) detail of that which he perceives, as a metamorphosis that leaves him with no sense of continuity between the present perception and that immediately preceding. This thought disorder, various aspects of which have been described also by Angyal (1946), Kasanin (1946), Zucker (1958), and others, is compared by Werner with the modes of thought that are found in members of so-called primitive cultures (and in healthy children of our own culture): . . . in the primitive mentality, particulars often as self-subsisting things that do not necessarily become synthized into larger entities. . . . The natives of the Kilimanjaro region do not have a word for the whole mountain range that they inhabit, only words for its peaks. . . . The same is reported of the aborigines of East Australia. From each twist and turn of a river has a name, but the language does not permit of a single all-embracing differentiation for the whole river. . . . [He] quotes Radin (1927) as saying that for the primitive man: 'A mountain is not thought of as a unified whole. It is a continually changing entity’ . . . [and, Radin continues, such a man lives in a world that is] ‘dynamic and ever-changing . . . Since he sees the same objects changing in their appearance from day to day, the primitive man regards this phenomenon as definitely depriving them of immutability and self-subsistence’ (Werner 1957).
Langer (1942) has called the symbolic-making function ‘one of man’s primary activities, like eating, looking, or moving about. It is the fundamental process of his mind’, she says, as she terms the need of symbolization ‘a primary need in man, which other creatures probably do not have’. Kubie (1953) terms the symbolizing capacity ‘the unique hallmark of man . . . capacities’, and he states that it is in impairment of this capacity to symbolize that all adult psychopathology essentially consists.
As for schizophrenia, we find that since 1911 this disease was described by Bleuler (1911) as involving an impairment of the thinking capacities, and in the thirty years many psychologists and psychiatrists, including Vigotsky (1934) Hanfmann and Kasanin (1942) Goldstein (1946) Norman Cameron (1946) Benjamin (1946) Beck (1946) von Domarus (1946) and Angtal (1946) - to mention but a few - has described various aspects of this thinking disorder. These writers, agreeing that one aspect of the disorder consists in over -concreteness or literalness of thought, have variously described the schizophrenic as unable to think in figurative (including metaphorical) terms, or in abstractions, or in consensually validated concepts and symbols, mor in categorical generalizations. Bateson (1956) described the schizophrenic as using metaphor, but unlabelled metaphor.
Werner (1940) has understood this most accurately matter of regression to a primitive level of thinking, comparable with the found in children and in members of so-called primitive cultures, a level of thinking in which there is a lack of differentiation between the concrete and the metaphorical. Thus we might say that just as the schizophrenic is unable to think in effective, consensually validated metaphor, as too as he is unable to think in terms that are genuinely concrete, free from an animistic forbear of a so-called metaphorical overlay.
The defensive function of the dedifferentiation that in so characterized of schizophrenic experience, and one find that this fragmentation o experience, justly lends itself to the repression of various motions that are too intense, and in particular too complex, for the weak ego to endure, which must be faced as one becomes aware of change as involving continuity rather than total discontinuity.
That is, the deeply schizophrenic patient who, when her beloved therapist makes a unkind or stupid remark, experiences him now for being a different person from the one who was there a moment ago - who experiences that a Bad Therapist has replaced the Good Therapist - is by that spared the complex feeling of disillusionment and hurt, the complex mixture of love and anger and contempt that a healthier patient would feel then. Similarly, if she experiences it in tomorrow’s session - or even later in the same session - that another good therapist has now come on the scene. The bad therapist is now totally gone, she will feel none of the guilt and self-reproach that a healthier patient would feel at finding that this therapist, whom she has just now been hated or despising, is after all a person capable of genuine kindness. Likewise, when she experiences a therapist’s departure on vacation for being a total deletion of him from her awareness, this bit of discontinuity, or fragmentation, in her subjective experience spars her from feeling the complex mixture of longing, grief, separation-anxiety, rejection, rage and so on, which a less ill patient feels toward a therapist who is absent but of whose existence he continues to be only too keenly aware.
Finally, such repressed emotions as hostility and lust may readily be seen, as these feelings not easy to hear expressed, as, for instance, the woman, who, at the beginning of her therapy, had been encased for years I flint lock paranoid defenses, become able to express her despair by saying that 'If I had something to get well for, it would make a difference,' her grief, by saying, 'The reason I am afraid to be close to people is because I feel so much like crying': Her loneliness, by expressing a wish that she would turn an insect into a person, so then she would have a friend. Her helplessness in face of her ambivalence by saying, to her efforts to communicate with other persons, 'I feel just like a little child, at the edge of the Atlantic or Pacific Ocean, trying to build a castle - right next to the water. Something just starts to be gasped [by the other person], and then bang! It has gone - another wave. As joining the mainstream of fellow human beings.
In the compliant charge of bringing forward three hypotheses are to be shown, they're errelated or portray in words as their interconnectivity, are as (1) in the course of a successful psychoanalysis, the analyst goes through a phase of reacting to, and eventually relinquishing, the patient as his oedipal love-object, (2) in normal personality development, the parent reciprocates the child's oedipal love with greater intensity than we have recognized before, and (3) in such normal developments, the passing of the Oedipus complex is at least important a phase in ego-development as in superego-development.
While doing psycho-analysis, time and again patients who have progressed to, or very far toward, a thorough going analysis to cure, become aware of experiential romantic and erotic desires and fantasies. Such fantasizing and emotions have appeared in a usual but of late in the course of treatment, have been preset not briefly but usually for several months, and have subsided only after having experienced a variety of feelings - frustration, separation anxiety, grief and so forth - entirely akin to those that attended as the resolution of an Oedipus complex late in the personal analysis.
Psycho-analysis literature is, in the main. Such as to make one feel more, rather than less, troubled at finding in oneself such feelings toward one's patient. As Lucia Tower (1956) has recently noted, . . . Virtually every writer on the subject of countertransference . . . states unequivocally that no form of erotic reaction to a patient is to be tolerated . . .
Still, in recent years, many writers, such as P. Heimann (1950), M. B. Cohen (1952) and E. Weigert (1952, 1954), have emphasized how much the analyst can learn about the patient from noticing his own feelings, of whatever sort, in the analytic relationship. Weigert (1952), defining countertransference as emphatic identification with the analysand, has stated that . . . 'In terminal phases of analyses the resolution of countertransference goes hand in hand with the resolution of transference.'
Respectfully, these additional passages are shown in view of countertransference, in the special sense in which defines the analyst for being innate, inevitable ingredients in the psycho-analytic relationship, in particular, the feelings of loss that the analyst experiences with the termination of the analysis. However, case in point, that the particular variety of countertransference with which are under approach is concerned that of the analyst's reacting as a loving and protective parent to the analysand, reacted too as an infant: There are plausible reasons why in the last phase it is especially difficult to achieve and maintain analytic frankness. The end of analysis is an experience of loss that mobilizes all the resistances in the transference (and in the counter-transference too), for a final struggle. . . . Recently, Adelaide Johnson (1951) described the terminal conflict of analysis as fully reliving the Oedipus conflict in which the quest for the genitally gratifying parent is poignantly expressed and the intense grief, anxiety and wrath of its definitive loss are fully reactivated. . . . Unless the patient dares to be exposed to such an ultimate frustration he may cling to the tacit permission that his relation to the analyst will remain his refuge from the hardships of his libidinal cravings to an aim-inhibited, tender attachment to the analyst as an idealized parent, he can get past the conflicts of genital temptation and frustration.
. . . . The resolution of the counter-transference permits the analyst to be emotionally freer and spontaneous with the patient, and this is an additional indication of the approaching end of an analysis.
. . . . When the analyst observes that he can be unrestrained with the patient, when he no longer weighs his words to maintain as cautious objectivity, this empathic countertransference and the transference of the patient are in a process of resolution. The analyst can treat the analysand on terms of equality; he is no longer needed as an auxiliary superego, an unrealistic deity in the clouds of detached neutrality. These are signs that the patient's labour of mourning for infantile attachments nears completion.
In stressing the point, which before an analysis can properly bring to an end, the analyst must have experienced a resolution of his countertransference to the patient for being a deep beloved, and desired, figure not only on this infantile level that Weigert has emphasized valuably, but also on an oedipal-genital level. Weigeret's paper, which helped to formulate the views that are set down, that is, as expressing the total point that a successful psycho-analysis involves the analyst's deeply felt relinquishment of the patient both as a cherished infant, and for being a fellow adult who is responded to at the level of genital love?
The paper by L. E. Tower (1956) comes similarly close to the view that, unlike Weigert, limits the term counter-transference to those phenomena that are transferences of the analyst to the patient. It is much more striking, therefore, that she finds even this classification defined countertransference to be innate to the analytic process: . . . . That there is inevitably, naturally, and often desirable, many countertransference developments in every analysis (some evanescent - some sustained), which is a counterpart of the transference phenomena. Interactions (or transactions) between the transference of the patient and the countertransference of the analyst, going on at unconscious levels, may be - or perhaps are always - of vital significance for the outcome of the treatment. . . .
. . . . Virtually every writer on the subject of countertransference. States unequivocally that no form of erotic reaction to a patient is to be tolerated. This would suggest that temptations in this area are great, and perhaps ubiquitous. This is the one subject about which almost every author is very certain to state his position. Other 'counter-transference' manifestations are not routinely condemned. Therefore, it must be to assume that erotic responses to some extent trouble nearly every analyst. This is an interesting phenomenon and one that call for investigation; nearly all physicians, when they gain enough confidence in their analysts, report erotic feelings and imply toward their patients, but usually do so with a good deal of fear and conflict. . . .
Of our tending purposes, we are to pay close attention to the libidinal resources that are of our applicative theory, in that large amounts of resulting available libido are necessary to tolerate the heavy task of many intensive analyses. While, we deride almost every detectable libidinal investment made by an analyst in a patient . . . various forms of erotic fantasy and erotic countertransference phenomena of a fantasy and of an affective character are in some experiential ubiquitous and presumably normal. Which lead to suspect that in many - perhaps every - intensive analytic treatment there develops something like countertransference structures (perhaps even a 'neurosis') which are essential and inevitable counterparts of the transference neurosis. These countertransference structures may be large or small in their quantitative aspects, but in the total picture they may be of considerable significance for the outcome of the treatment. They function in the manner of a catalytic agent in the treatment process. Their understanding by the analyst may be as important to the final working through of the transference neurosis as is the analyst's intellectual understanding of the transference neurosis itself, perhaps because they are, so to speak, the vehicle for the analyst's emotional understanding of the transference neurosis. Both transference neurosis and countertransference structure seem intimately bound together in a living process and both must be considered continually in the work that is the psychoanalysis. . . .
. . . . Seemingly questionable, is any thorough working through a deep transference neurosis, in the strictest sense, which does not involve some form of emotional upheaval in which both patient and analysts are involved. In other words, there are both a transference neurosis and a corresponding Countertransference 'neurosis' (no matter how small and temporary) which are both analyzed in the treatment situation, with eventual feelings of a new orientation by both one another toward any other but themselves.
Freud, in his description of the Oedipus complex (1900, 1921, 1923), tended largely to give us a picture of the child as having an innate, self-determined tendency to experience, under the conditions of a normal home, feelings of passionate love toward the parent of the opposite sex; we get little hints, from his writings, that in this regard the child enters a mutual relatedness of passionate love with that parent, a relatedness in which the parent's feelings may be of much the same quality and intensity as those in the child (although this relatedness must be very important in the life of the developing child than it is in the life of the mature adult, with his much stronger, more highly differentiated ego and with his having behind him the experience of a successfully resolved oedipal experience during his own maturation).
Nevertheless, in the earliest of his publications concerning the Oedipus complex, namely The Interpretation of Dreams (1900), Freud makes a fuller acknowledgements of the parent's participation in the oedipal phase of the child's life than does in any of his later writings on the subject'. . . a child's sexual wishes - if in their embryonic stage they deserve to be so described - awaken very early. . . . A girl's first affection is for her father and boy's first childish desires are for his mother. Accordingly, the father becomes a disturbing rival to the boy and the mother to the girl. The parents too give evidence as a rule of sexual partiality: A natural predilection usually sees to it that a man tends to spoil his little daughters, while his wife takes her sons' part; though both of them, where their judgement is not disturbed by the magic of sex, keep a strict eye upon their children's education. The child is very well aware of this patriality and turns against that one of his parents who is opposed to showing it. Being loved by an adult does not merely bring a child the satisfaction of a special need; it also means that he will get what he wants in every other respect as well. Thus, he will be following his own sexual instinct and while giving fresh strength to the inclination shown by his parents if his choice between them falls in with theirs (1900).
Theodor Reik, in his accounts of his coming to sense something of the depths of possessiveness, jealousy, fury at rivals, and anxiety in the face of impending loss, in himself regarding his two daughters, conveys a much more adequate picture of the emotions that genuinely grip the parent in the oedipal relationship than is conveyed by Freud's sketchy account, as Reik's deeply moving descriptions occupy a chapter in his Listening with the Third Ear (1949), written at the time when his daughters were twelve and six years of age; and a chapter in his The Secret Self (1952), when the oldest daughter was now seventeen.
Returning to a further consideration of the therapist's oedipal-love responses to the patient, it seems that these response flows from four different sources. In actual practice the responses from these four tributaries are probably so commingled in the therapists that it is difficult of impossible fully to distinguish one kind from another; the important thing is that he is maximally open to the recognition of these feelings in himself, no matter what their origin, for he can probably discern, in as far as is possible, from where they flow they signify, therefore, concerning the patient's analysis.
First among these four sources may be mentioned the analyst's feeling-responses to the patient's transference. This, when, as the analysis progresses and the patient enter an experiencing of oedipal love, ongoing, jealousy y, frustration and loss as for the analyst as a parent in the transference, the analyst will experience to at least some degree, response's reciprocally th those of the patient-responses, that is, such for being present within the parent in questions, during the patient's childhood and adolescence, which the parent presumably was not ably to recognize freely and accept within himself. Some writers apply the term 'counter-transference' to such analyst-responese to the patient's transference, unlike others some do not do so.
The second source consists in the countertransference in the classical sense in which this term is most often used: The analyst's responding to the patient about transference-feelings carried over from a figure out of the analyst 's own earlier years, without awareness that his response springs predominantly from this early-life, rather than being based mainly upon the reality of the patient analyst-patient relationship. It is this source, of course, which we wish to reduce to a minimum, by means of thoroughgoing personal analysis and ever-continuing subsequent alertness for indications that our work with a patient has come up against, in us, unanalyzed emotional residues from our past. This source is so very important, in fact, as to make the writing of such a paper as a somewhat precarious venture. Must expect that some readers will charge him with trying to portray, as natural and necessary to the annalistic process generally, certain analyst-responese that in actuality is purely the result of an unworked-through? Oedipus' complex in himself, which are dangerously out of place in his own work with patients that have no place in the well-analysed analyst's experience with his patient.
It can only be surmised that although this source may play an insignificant role in the responses of a well-analysed analyst who has conducted many analyses through to completion - to an intensified inclusion as a thoroughgoing resolution of the patient's Oedipus complex - it is probably to be found, in some measure, in every analyst. This is, it seems that the nature and conflictual feeling-experience in this regard - a fostering of his deepest love toward the fellow human being with whom she participates in such prolonged and deeply personal work, and a simultaneous, unceasing, and rigorous taboo against his behavioural expression of any of the romantic or erotic components of his love - as to require almost any analyst's tending to relegate the deepest intensities of these conflictual feelings to his own unconscious mind, much as were the deepest intensities of his oedipal strivings toward a similar beloved, and similarly unobtainable and rigorously tabooed, parent in particular, and in the hope of the remaining in the analyst's unconscious. That is hoping that this will help analysts - in particular, to a lesser extent-experienced analyst - whereas to some readers awareness, and by that diminution, of this countertransference feeling, as justly dealing with other kinds of countertransference feelings, by such as those wrote by P. Heumann (1950, M. B., Cohen (19520 and E. Weigert (1952?)
A third source is to be found in the appeal that the gratifyingly improving patient makes to the narcissistic residue in the analyst's personality, the Pygmalion in him. He tends to fall in love with this beautifully developing patient, regarded at this narcissistic level as his own creation, just as Pygmalion fell in love with the beautiful statu e of Galatea that he had sculptured. This source, like the second one that we can expect to holds little sways in the well-analysed practitioner of long experience, but it, too, is probably never absent of great experience and professional standing, than we may like to think. Particularly in articles and books that describe the author's new technique or theoretical concept as an outgrowth of the work with a particular patient, or a very few patients, do we see this source very prominently present in many instances.
The fourth source, based on the genuine reality of the analyst-patient situation, consists in the circumstance that nearly becomes, per se, a likeable, admirable and insightfully speaking lovable, human being from whom the analyst will soon become separated. If he is not himself a psychiatrist, the analyst may very likely never see him again. Even if he is a professional colleague, the relationship with him will become in many respects far more superficial, far less intimate, than it has been. This real and unavoidable circumstance of the closing analytic work tends powerfully to arouse within the analyst feelings of painfully frustrated love that deserve to be compared with the feelings of ungratifiable love that both child and parent experience in the oedipal phase of the child's development. Feelings from this source cannot properly be called countertransference. They may flow from the reality of the present circumstances but they may be difficult or impossible e to distinguish fully from countertransference.
There are, then four essentially powerful sources having to promote of the tendency toward the feelings of deep love with romantic and erotic overtones, and with accompanying feelings of jealousy, anxiety, frustration-rage, separation-anxiety, and grief, in the analyst about the patient. These feelings come to him, like all feelings, without tags showing from where they have come, and only if he is open and accepting to their emergence into his awareness does he have a chance to set about finding out their origin and thus their significance in his work with the patient.
Finally, with which the considerations have been presented so far, a few remarks concerning the passing of the Oedipus complex in normal development and in a successful psycho-analysis.
In the Ego and the Id (1923) we find italicized a passage in which Freud stresses that the oedipus phase results in the formation of the superego; we find that he stresses the patient's opposition to ther child's oedipal swosh, and lastly, we see this resultant suprerego to be predominantly a severe and forbidding one: The broad general outcome of the sexual phase dominated by the Oedipus complex may, therefore, be taken to be the forming of a precipitating in the ego . . . This modification of the ego
. . . comforts the other contents of the ego as an ego ideal or super-ego.
. . . . The child's parents, and especially his father, were perceived as the obstacle to verbalizations of his Oedipus wishes, so his infantile ego fortified itself for the carrying out of the repression by building this obstacle within itself. It borrowed the strength to do this, so to seek, from the father, and this loan was an extraordinarily nonentous act. The super-ego retains the character of the father, while the more powerful the Oedipus complex was and the more rapid succumbed to repression (under the influence of authority, religious teachings, schooling and reading), this strictly will be the domination of the super-ego over the ego later on - as conscience or perhaps of an unconscious sense of guilt. . . .
The subject dealt within the subjective matter through which generative pre-oedipal origins are to be found of the superego, on which has been dealt by M. Klein (1955). E. Jacobson (1954) and others, also apart from that subject, a regard for Freud's above-quoted description as more applicable to the child who later becomes neurotic or psychotic, than to the 'normal'; child. Since we can assume that there is virtually a wholly complimentary neurotic difficulty, we may then have in assuming that Freud's formation holds true to some degree in every instance. Still, to the extent that a child's relationships with his parents are healthy, he finds the strength to accept the unrealizibilityy of his oedipal strivings, not mainly through the identification with the forbidding rival-parent, but mainly, as an alternative, the ego-strengthening experiences of finding the beloved parent reciprocate his love - responds to him, that is, for being a worthwhile and loveable individual, for being, a conceivably desirable love-partner - and renounces him only with an accompanying sense of loss on the parent's own part. The renunciation, again, something that is mutual experience for the chid and parent, and is made in deference to a recognizedly greater limiting realty, a reality that includes not only the taboo maintained by the rival-parent, but also the love of the oedipal desired parent toward his or her spouse - a love that undeterred the child's birth and a love to which, in a sense, he owes his very existence?
Out of such an oedipal situation the child emerges, with no matter how deep and painful sense of loss at the recognition that he can never displace the rival-parent and posses the beloved on e in a romantic-and-erotic relationship, in a state differently from the ego-diminished, superego-domination state that Freud described. This child that his love, however unrealized, is reciprocated. Strengthened, too, out of the realization, which his relationship with the beloved parent has helped him to achieve, that he lives in a wold in which any individual's strivings are encompassed by a reality much larger than he: Freud, when he stressed that the oedipal phase normally results mainly in the formations of a forbidding superego, and if it is resulting mainly in enchantments of the ego's ability to test both inner and outer reality.
All experiences with both neurotic and psychotic patients had shown that, in every individual instance, in as far as the oedipal phase was entered the course of their past elements, it led to ego impairment rather than ego functioning as primarily because the beloved parent had to repress his or her reciprocal desire for the child, chiefly through the mechanism of unconscious denial of the child's importance to the parent. More often than not, in these instancies, that suggested that the parent would unwittingly act out his or her repressed desires in the unduly seductive behaviour toward the child; yet whenever the parents come close to the recognition of such desires within him, he would unpredictably start reacting to the child as unlovable - undesirable.
With many of these parents, appears that, primarily because of the parent's own unresolved Oedipus complex, his marriage proved too unsatisfying, and his emotional relationship to his own culture too tenuous, for him to dare to recognize the strength of his reciprocal feelings toward his child during the latter's oedipal phase of development. The child is reacting too as a little mother or father transference-figure to the parent, a transference-figure toward whom the parent's repressed oedipal love feelings are directed. If the parent had achieved the inner reassurance of a deep and enduring love toward his wife, and a deeply felt relatedness with his culture including the incest taboos to which his culture adheres, he would have been able to participate in as deeply felt, but minimally acted out, relationship with the chid in a way that fostered the healthy resolutions of the child's Oedipus complex. Instead, what usually happens in such instances, in that the child's Oedipus complex remains unresolved because the child stubbornly - and naturally - refuses to accept defeat within these particular family circumstances, whereas the acceptance of oedipal defeat is tantamount to the acceptance of irrevocable personal worthlessness and unlovability.
It seems much clearer, then this former child, now neurotic or psychotic adult, requires from us for the successful resolution to his unresolved Oedipus complex: Not such a repression of desire, acted-out seductiveness, and denial of his own worth as he met in the relationship with his parent, but a maximal awareness on our part of the reciprocal feelings while we develop in response to his oedipal strivings. Our main job remains always, of course, to further the analysis of his transference, but what might be described seems to be the optimal feeling background in the analyst for such analytic work.
Formidably, when applied not to a moderate degree found in the background of the neurotic person but invested with all the weight of actual biological attributes, have much ado with the person's unconscious refusal to relinquish, in adolescence and young adulthood, his or her fantasied infantile omnipotence in exchange for a sexual identity of - in these-described terms - a 'man' or a 'woman'. It would be like having to accept only certain dispensations as well as salvageable sights, if ony to see the whole fabric ruined into the bargin. A person cannot deeply accept an adult sexual identity until he has been able to find that this identity can express all the feeling-potentialities of his comparatively boundless infancy. This implies that he has become able to blend, for example, his infantile - dependent needs into his more adult erotic strivings, than regard these as mutually exclusive in the way that the mother of the future patient or the persons infant frighteningly feels that her lust has been placed in her mothering. Another difficult facet of this situation resides in a patient's youngful conviction, based on his intrafamiliar experiences, which he can win parental love only if he can become or, perhaps, at an unconscious level remain - a girl; accepting her sexuality as a woman is equated with the abandonment of the hope of being loved.
Concerning the warped experiences their persons have and with the oedipal phase of development, calls to our attention of two features. First, the child whose parents are more narcissistic than truly object-related in faced with the basically hopeless challenge of trying to compete with the mother's own narcissistic love for herself, and with the father's similar love for himself, than being presented with a competitive challenge involving separate, flesh-and-blood human beings. Secondly, concerning warped oedipal experiences, in, as far as the parents succeeded in achieving object-relatedness, this has often become only weakly established as a genital level, so that it remains much more prominently at the mother-infant level of ego-development. Thus, the mother, for example, is much more able to love her infant son than her adult husband, and the oedipal competition between husband and son are in terms of who can better become, or remain, the infant whom the mother is capable of loving. When the infant becomes chronologically a young man, having learned that one wins a woman not through genial assertiveness but through regression, he is apt to shy away from entering into true adult genitality, and is tempted to settle for what amounts to 'regressive victory' in the oedipal struggle
We write much about the analyst’s or therapist’s being able to identify or empathize with the patient for helping in the resolution of the neurotic or psychotic difficulties. Such writings always portray a merely transitory identification, an empathic sensing of the patient’s conflicts, an identification that is of essentially communicative value only. However, it should be seen that we inevitably identify with the patient another fashion also, we identify with the healthy elements in him, in a way that entails enduing, constructive additions to our own personality. Patients - above all schizophrenic patients - need and welcome our acknowledgement, simply and undemonstratively, that they have contributed, and are contributing, in some such significant way, to our existence.
Increasing maturity involves increasing ability not merely to embrace change in the world around one, but to realize that one is oneself in a constant state of change. By contrast, the recovering, maturing patiently becomes less and less dependent upon any such sharply delineated, static self-image or even a constellation of such images, the answer to the question, 'Who are you?' is almost as small, solid, and well defined as a stone, but is a larger, fluid, richly-laden, and sniffingly outlined as an ocean? As the individual becomes well, he comes to realize that, as Henri Bergson (1944) puts it, 'reality is a perpetual growth, a creation pursued without end. . . . A perpetual becoming,' and to the extent that he can actively welcome change and let it become part of him, he comes to know that - again in Bergson’s phrase - 'to exist is to change, to change is too mature, to mature is to go on creating oneself endlessly.'
Philosophical issues about ‘perception’ tend to be issues specifically about ‘sense-perception’. In England (and the same is true of comparable terms in many other languages) the term ‘perception’ has a wider connotation than anything that has to do with the senses and sense-organs, though it generally involves the idea of what may imply, if only in a metaphorical sense, a point of view. Thus it is now increasingly common for news-commentators, for example, to speak of events, even though those people have not been witnesses of them. In one sense, however, there is nothing new about this, in seventeenth-and-eighteenth-century philosophical usage, words for perception were used with a much wider coverage than sense-perception alone. It is, however, sense-perception that has typically raised the largest and most obvious philosophical problems.
Such problems may be said to fall into two categories. These are, for the epistemological problems about the role of sense-perception in connection with the acquisition and possession of knowledge of the world around us. These problems - does perception give us knowledge of the so-called ‘external world’, and to what extent? - have become dominant in epistemology since Descartes because of his invocation of the method of doubt, although they undoubtedly existed in philosophers’ minds in one way or another before that. In early and middle twentieth-century Anglo-Saxon philosophy such problems centred on the question whether there are firm data provided by the senses - so-called sense-data - and if so what is the relation of such sense-data to so-called material objects. Such problems are not essentially problems for the philosophy of mind, although certain answers to questions about perception which undoubtedly belong to the philosophy of mind can certainly add to epistemological differences. If perception is assimilated, for example, to sensation, there is an obvious temptation to think that in perception we are restricted, at any rate initially, to the contents of our own minds.
The second category of problems about perception - those that fall directly under the heading of the philosophy of mind - are thus in a sense prior to the problems that exercised many empiricist in the first half of this century. They are problems about how perception is to be construed and how it relates to a number of other aspects of the mind’s functioning - sensations, concepts and other things involved in our understanding of things, beliefs and judgements, and the imagination, our action in relation to the world around us, and the causal processes involved in the physics, biology and psychology of perception. Some of the last were central to the considerations that Aristotle raised about perception in his ‘De Anima’.
It is obvious enough that sense-perception involves some kind of stimulation of sense organs by stimuli that are themselves the product of physical processes which are biological in character are then initiated. Moreover, only if the organism in which this takes place is adapted to such excitation, for which the stimulation can perception ensue. Aristotle had something to say about such matters, but it was evident to him that such an account was insufficient to explain what perception itself is. It might be thought that the most obvious thing is missing in such an account is some reference to consciousness. But while it may be the case that perception can take place only in creatures that have consciousness in some sense, it is not clear that every case of perception directly involves consciousness. There is such a thing as unconscious perception and psychologists have recently drawn attention to the phenomenon which is described as ‘blind-sight’ - an ability, generally manifested in patients with certain kinds of brain-damage, to discriminate sources of light, even when the people concerned have no consciousness of the lights and think that they are guessing about them. It is important, then, not to confuse the plausible claim that perception can take place only in conscious beings with the less plausible claim that perception always involves consciousness of objects. A similar point may apply to the relation of perception to some of the other items exposed to concept-possession.
Consciousness may possibly be the most challenging and persuasive source of problems in the whole of philosophy. Our own consciousness seems to be the most basic fact confronting us, yet it is almost impossible to say what consciousness is. Is mine like yours? Is ours like that of animals? Might machines come to have consciousness? Is it possible for there to be disembodied consciousness? Whatever complex biological and neural processes go on back-stage, it is my consciousness that provides the theatre where my experiences and thoughts have their existence, where my desires are felt and where my intentions are formed. But then how am I to conceive that ‘I’, or ‘self’ that is the spectator, or at any rate the owner of this theatre? There problems together make up what is sometimes called ‘the hard problem’ of consciousness. One of the difficulties in thinking about consciousness is that the problems seem not to be scientific ones. Gottfried Wilhelm Leibniz (1646-1716) remarked that if we could construct a field or machine, per se, and find to its expansive area, we still would not be able to find consciousness, so that consciousness resides in simple subjects, not complex ones. Even if we are convinced that consciousness somehow emerges from the complexity of brain functioning, we may still feel baffled about the way the emergence takes place, or why it takes place in just the way it does.
The nature of conscious experience has been the largest single obstacle to physicalism, behaviourism and functionalism in the philosophy of mind: These are all views that according to their opponents, can only be believed by feigning permanent anaesthesia. But many philosophers are convinced that we can divide and conquer: We may make progress not by thinking of one ‘hard’ problem, but by breaking the subject up into different skills and recognizing that rather than a single self or observer we would do better to think of a relatively undirected whirl of cerebral activity, with no inner theatre, no inner lights, and above all no inner spectator.
Til most recently it has been thought that in the study of how nerve cells, or neurons, receives and transmits information. Two types of phenomena are involved in processing nerve signals: Electrical and chemical. Electrical events propagate a signal within a neuron, and chemical processes transmit the signal from one neuron to another neuron or to a muscle cell.
A neuron is a long cell that has a thick central area containing the nucleus, it also has one long process called an axon and one or more short, bushy processes called dendrites. Dendrites receive impulses from other neurons. (The exceptions are sensory neurons, such as those that transmit information about temperature or touch, in which the signal is generated by specialized receptors in the skin.) These impulses are propagated electrically along the cell membrane to the end of the axon. At the tip of the axon the signal is chemically transmitted to an adjacent neuron or muscle cell.
Like all other cells, neurons contain charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they can produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes called membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory information or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negative too positively. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process. Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; These points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
When the electrical signal reaches the tip of an axon, it stimulates small presynaptic vesicles in the cell. These vesicles contain chemicals called neurotransmitters, which are released into the microscopic space between neurons (the synaptic cleft). The neurotransmitter attaches on the surface of the adjacent neuron. This stimulus causes the adjacent cell to depolarize and propagate an action potential of its own. The duration of a stimulus from a neurotransmitter is limited by the breakdown of the chemicals in the synaptic cleft and the reuptake by the neuron that produced them. Formerly, each neuron was thought to make only one transmitter, but recent studies have shown that some cells make two or more.
During the early 1900s, in examining the workings of the nervous system, physiologists were beginning to explore the idea that the transmission of nerve impulses takes place, in part, via chemical means. Loewi decided to explore this idea. During a stay in London in 1903, he met Sir Dale, who was also interested in the chemical transmission of nerve impulses. However, for Loewi, Dale, and all the other researchers pursuing a chemical transmitter of nerve impulses, years of effort produced no solid evidence. In 1921 Loewi suspended two frogs' hearts in solution, one with a major nerve removed. Removing fluid from the heart that still contained the nerve, and injecting the fluid into the nerveless heart, Loewi observed that the second heart behaved as if the missing nerve were present. The nerves, he concluded, do not act directly on the heart - it is the action of chemicals, freed by the stimulation of nerves, that causes increases in heart rate and other functional changes. In 1926 Loewi and his colleagues identified one of the chemicals in his experiment as acetylcholine. This was indisputably a neurotransmitter - a chemical that serves to transmit nerve impulses in the involuntary nervous system.
We acknowledge the neurotransmitters are inherently made by chemically induced neurons, or nerve cells. Neurons send out neurotransmitters as chemical signals to activate or inhibit the function of neighboring cells.
Within the central nervous system, which consists of the brain and the spinal cord, neurotransmitters pass from neuron to neuron. In the peripheral nervous system, which is made up of the nerves that run from the central nervous system to the rest of the body, the chemical signals pass between a neuron and an adjacent muscle or gland cells.
Chemical compounds - belonging to three chemical families - are widely recognized as neurotransmitters. In addition, certain other body chemicals, including adenosine, histamine, enkephalins, endorphins, and epinephrine, have neurotransmitterlike properties. Experts believe that there are many more neurotransmitters yet undiscovered.
The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Among the amine neurotransmitters are acetylcholine, norepinephrine, dopamine, and serotonin. Acetylcholine is the most widely used neurotransmitter in the body, and neurons that leave the central nervous system (for example, those running to skeletal muscle) use acetylcholine as their neurotransmitter; neurons that run to the heart, blood vessels, and other organs may use acetylcholine or norepinephrine. Dopamine is involved in the movement of muscles, and it controls the secretion of the pituitary hormone prolactin, which triggers milk production in nursing mothers.
The second neurotransmitter family is composed of amino acids, organic compounds containing both an amino group (NH2) and a carboxylic acid group (COOH). Amino acids that serve as neurotransmitters include glycine, glutamic and aspartic acids, and gamma-amino butyric acid (GABA). Glutamic acid and GABA are the most abundant neurotransmitters within the central nervous system, and especially in the cerebral cortex, which is largely responsible for such higher brain functions as thought and interpreting sensations.
The third neurotransmitter family is composed of peptides, which are compounds that contain at least two, and sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly understood, but scientists know that the peptide neurotransmitter called substance P influences the sensation of pain.
Overall, each neuron uses only a single compound as its neurotransmitter. However, some neurons outside the central nervous system can release both an amine and a peptide neurotransmitter.
Neurotransmitters are manufactured from precursor compounds like amino acids, glucose, and the dietary amine-called choline. Neurons modify the structure of these precursor compounds in a series of reactions with enzymes. Neurotransmitters that comes from amino acids include serotonin, for which it is derived from tryptophan. Dopamine and norepinephrine, under which are derived from tyrosine, and glycine, which is derived from threonine. Among the neurotransmitters made from glucose are glutamate, aspartate, and GABA. The choline serves as the precursor for acetylcholine
Neurotransmitters are released into a microscopic gap, called a synapse, that separates the transmitting neuron from the cell receiving the chemical signal. The cell that generates the signal is called the presynaptic cell, while the receiving cell is termed the postsynaptic cell.
After their release into the synapse, neurotransmitters combine chemically with highly specific protein molecules, termed receptors, embedded in the surface membranes of the postsynaptic cell. When this combination occurs, the voltage, or electrical force, of the postsynaptic cell is either increased (excited) or decreased (inhibited).
When a neuron is in its resting state, its voltage is about -70 millivolts. An excitatory neurotransmitter alters the membrane of the postsynaptic neuron, making it possible for ions (electrically charged molecules) to move back and forth across the neuron’s membranes. This flow of ions makes the neuron’s voltage rise toward zero. If enough excitatory receptors have been activated, the postsynaptic neuron responds by firing, generating a nerve impulse that causes its own neurotransmitter to be released into the next synapse. An inhibitory neurotransmitter causes different ions to pass back and forth across the postsynaptic neuron’s membrane, lowering the nerve cell’s voltage to -80 or -90 millivolts. The drop in voltage makes it less likely that the postsynaptic cell will fire.
If the postsynaptic cell is a muscle cell rather than a neuron, an excitatory neurotransmitter will cause the muscle to contract. If the postsynaptic cell is a gland cell, an excitatory neurotransmitter will cause the cell to secrete its contents.
While most neurotransmitters interact with their receptors to create new electrical nerve impulses that energize or inhibit the adjoining cell, some neurotransmitter interactions do not generate or suppress nerve impulses. Instead, they interact with a second type of receptor that changes the internal chemistry of the postsynaptic cell by either causing or blocking the formation of chemicals called second messenger molecules. These second messengers regulate the postsynaptic cell’s biochemical processes and enable it to conduct the maintenance necessary to continue synthesizing neurotransmitters and conducting nerve impulses. Examples of second messengers, which are formed and entirely contained within the postsynaptic cell, include cyclic adenosine monophosphate, diacylglycerol, and inositol phosphates.
Once neurotransmitters have been secreted into synapses and have passed on their chemical signals, the presynaptic neuron clears the synapse of neurotransmitter molecules. For example, acetylcholine is broken down by the enzyme acetylcholinesterase into choline and acetate. Neurotransmitters like dopamine, serotonin, and GABA is removed by a physical process called reuptake. In reuptake, a protein in the presynaptic membrane acts as a sort of sponge, causing the neurotransmitters to reenter the presynaptic neuron, where they can be broken down by enzymes or repackaged for reuse.
Neurotransmitters are known to be involved in many disorders, including Alzheimer’s disease. Victims of Alzheimer’s disease suffer from loss of intellectual capacity, disintegration of personality, mental confusion, hallucinations, and aggressive - even violent - behavior. These symptoms are the result of progressive degeneration in many types of neurons in the brain. Forgetfulness, one of the earliest symptoms of Alzheimer’s disease, is partly caused by the destruction of neurons that normally release the neurotransmitter acetylcholine. Medications that increase brain levels of acetylcholine have helped restore short-term memory and reduce mood swings in some Alzheimer’s patients.
Neurotransmitters also play a role in Parkinson disease, which slowly attacks the nervous system, causing symptoms that worsen over time. Fatigue, mental confusion, a masklike facial expression, stooping posture, shuffling gait, and problems with eating and speaking are among the difficulties suffered by Parkinson victims. These symptoms have been partly linked to the deterioration and eventual death of neurons that run from the base of the brain to the basal ganglia, a collection of nerve cells that manufacture the neurotransmitter dopamine. The reasons why such neurons die are yet to be understood, but the related symptoms can be alleviated. L-dopa, or levodopa, widely used to treat Parkinson disease, acts as a supplementary precursor for dopamine. It causes the surviving neurons in the basal ganglia to increase their production of dopamine, by that compensating to some extent for the disabled neurons.
Many other effective drugs have been shown to act by influencing neurotransmitter behavior. Some drugs work by interfering with the interactions between neurotransmitters and intestinal receptors. For example, belladonna decreases intestinal cramps in such disorders as irritable bowel syndrome by blocking acetylcholine from combining with receptors. This process reduces nerve signals to the bowel wall, which prevents painful spasms.
Other drugs block the reuptake process. One well-known example is the drug fluoxetine (Prozac), which blocks the reuptake of serotonin. Serotonin then remains in the synapse for a longer time, and its ability to act as a signal is prolonged, which contributes to the relief of depression and the control of obsessive-compulsive behaviors.
Dopamine, chemical known as a neurotransmitter essential to the functioning of the central nervous system. During neurotransmission, dopamine is transferred from one nerve cell, or neuron, to another, playing a key role in brain function and human behavior.
Dopamine forms from a precursor molecule called dopa, which is manufactured in the liver from the amino acid tyrosine. Dopa is then transported by the circulatory system to neurons in the brain, where the conversion to dopamine takes place.
Dopamine is a versatile neurotransmitter. Among its many functions, it plays a major role in two activities of the central nervous system: one that helps control movement, and a second that are strongly associated with emotion-based behaviors.
The pathway involved in movement control is called the nigrostriatal pathway. Dopamine is released by neurons that originate from an area of the brain called the substantia nigra and connect to the part of the brain known as the corpora striata, an area known to be important in controlling the musculoskeletal system.
The second brain pathway in which dopamine plays a major role is called the mesocorticolimbic pathway. Neurons in an area of the brain called the ventral tegmentalarea transmits dopamine to other neurons connected to various parts of the limbic system, which is responsible for regulating emotion, motivation, behavior, the sense of smell, and variously autonomic, or involuntary, functions like heartbeat and breathing. A growing body of evidence suggests that dopamine be involved in several major brain disorders. Narcolepsy, a disorder characterized by brief, recurring episodes of sudden, deep sleep, is associated with abnormally high levels of both dopamine and a second neurotransmitter, acetylcholine. Huntington’s chorea, an inherited, fatal illness in which neurons in the base of the brain are progressively destroyed, is also linked to an excess of dopamine.
Commonly known as shaking palsy, Parkinson disease is another brain disorder in which dopamine is involved. Besides tremors of the limbs, Parkinson patients suffer from muscular rigidity, which leads to difficulties in walking, writing, and speaking. This disorder results from the degeneration and death of neurons in the nigrostriatal pathway, resulting in low levels of dopamine. The symptoms of Parkinson disease can be reduced by treatment with a drug called levodopa, or L-dopa, which converts to dopamine in the brain.
Schizophrenia is a psychiatric disorder characterized by loss of contact with reality and major changes in personality. Schizophrenics have normal levels of dopamine in the brain, but because they are highly sensitive to this neurotransmitter, these normal levels of dopamine triggers unusual behaviors. Drugs such as thorazine that blocks the action of dopamine have been found to decrease the symptoms of schizophrenia.
Studies suggest that people who are addicted to alcohol and other drugs like, cocaine and nicotine have less dopamine in the mesocorticolimbic pathway. These drugs appear to increase dopamine levels, resulting in the pleasurable feelings associated with the drugs.
Serotonin, neurotransmitter, or chemical that transmits messages across the synapses, or gaps, between adjacent cells. Among its many functions, serotonin is released from blood cells called platelets to activate blood vessel constriction and blood clotting. In the gastrointestinal tract, serotonin inhibits gastric acid production and stimulates muscle contraction in the intestinal wall. Its functions in the central nervous system and effects on human behavior - including mood, memory, and appetite control - have been the subject of a great deal of research. This intensive study of serotonin has revealed important knowledge about the serotonin-related cause and treatment of many illnesses.
Serotonin is produced in the brain from the amino acid tryptophan, which is derived from foods high in protein, such as meat and dairy products. Tryptophan is transported to the brain, where it is broken down by enzymes to produce serotonin. During neurotransmission, serotonin is transferred from one nerve cell, or neuron, to another, triggering an electrical impulse that stimulates or inhibits cell activity as needed. Serotonin is then reabsorbed by the first neuron, in a process known as reuptake, where it is recycled and used again or converted into an inactive chemical form and excreted.
While the complete picture of serotonin’s function in the body is still being investigated, many disorders are known to be associated with an imbalance of serotonin in the brain. Drugs that manipulate serotonin levels have been used to alleviate the symptoms of serotonin imbalances. Some of these drugs, known as selective serotonin reuptake inhibitors (SSRIs), block or inhibit the reuptake of serotonin into neurons, enabling serotonin to remain active in the synapses for a longer period. These medications are used to treat such psychiatric disorders as depression; Obsessive-compulsive disorder, in which repetitive and disturbing thoughts trigger bizarre, ritualistic behaviors, and impulsive aggressive behaviors. Fluoxetine (more commonly known by the brand name Prozac), is a widely prescribed SSRI used to treat depression, and more recently, obsessive-compulsive disorder.
Drugs that affect serotonin levels may prove beneficial in the treatment of nonpsychiatric disorders as well, including diabetic neuropathy (degeneration of nerves outside the central nervous system in diabetics) and premenstrual syndrome. Recently the serotonin-releasing agent dexfenfluramine has been approved for patients who are 30 percent or more over their ideal body weight. By preventing serotonin reuptake, dexfenfluramine promotes satiety, or fullness, after eating less food.
Other drugs serve as agonists that react with neurons to produce effects similar to those of serotonin. Serotonin agonists have been used to treat migraine headaches, in which low levels of serotonin cause arteries in the brain to swell, resulting in a headache. Sumatriptan is an agonist drug that mimics the effects of serotonin in the brain, constricting blood vessels and alleviating pain.
Drugs known as antagonists bind with neurons to prevent serotonin neurotransmission. Some antagonists have been found effective in treating the nausea that typically accompanies radiation and chemotherapy in cancer treatment. Antagonists are also being tested to treat high blood pressure and other cardiovascular disorders by blocking serotonin’s ability to constrict blood vessels. Other antagonists may produce an effect on learning and memory in age-associated memory impairment.
The Synapse is the signal conveying everything that human beings sense and think, and every motion they make, follows nerve pathways in the human body as waves of ions (atoms or groups of atoms that carries electric charges). Australian physiologist Sir John Eccles discovered many intricacies of this electrochemical signaling process, particularly the pivotal step in which a signal is conveyed from one nerve cell to another. He shared the 1963 Nobel Prize in physiology or medicine for this work, which he described in a 1965 Scientific American article.
How does one nerve cell transmit the nerve impulse to another cell? Electron microscopy and other methods show that it does so by means of special extensions that deliver a squirt of transmitter substance
The human brain is the most highly organized form of matter known, and in complexity the brains of the other higher animals are not greatly inferior. For certain purposes regarding the brain for being analogous to a machine is expedient. Even if it is so regarded, however, it is a machine of a totally different kind from those made by man. In trying to understand the workings of his own brain man meets his highest challenge. Nothing is given; There are no operating diagrams, no maker's instructions.
The first step in trying to understand the brain is to examine its structure to discover the components from which it is built and how they are related to each another. After that one can attempt to understand the mode of operation of the simplest components. These two modes of investigation - the morphological and the physiological - have now become complementary. In studying the nervous system with today's sensitive electrical device, however, finding physiological events that cannot be correlated with any known anatomical structure is all too easy. Conversely, the electron microscope reveals many structural details whose physiological significance is obscure or unknown.
At the close of the past century the Spanish anatomist Santiago Ramón Cajal showed how all parts of the nervous system are built up of individual nerve cells of many different shapes and sizes. Like other cells, each nerve cell has a nucleus and the surrounding cytoplasm. Its outer surface consists of many fine branches - the dendrites - that receive nerve impulses from other nerve cells, and one relatively long branch - the axon - that transmits nerve impulses. Near its end the axon divides into branches that end at the dendrites or bodies of other nerve cells. The axon can be as short as a fraction of a millimeter or if a meter, depending on its place and function. It has many properties of an electric cable and is uniquely specialized to conduct the brief electrical waves called nerve impulses. In very thin axons these impulses travel at less than one meter per second; In others, for example in the large axons of the nerve cells that activate muscles, they travel as fast as 100 meters per second.
The electrical impulse that travels along the axon ceases abruptly when it comes to the point where the axon's terminal fibers contact another nerve cell. These junction points were given the name ‘synapses’ by Sir Charles Sherrington, who laid the foundations of what is sometimes called synaptology. If the nerve impulse is to continue beyond the synapse, it must be regenerated afresh on the other side. As recently as 15 years ago some physiologists held that transmission at the synapse was predominantly, if not exclusively, an electrical phenomenon. Now, however, there is abundant evidence that transmission is made by the release of specific chemical substances that trigger a regeneration of the impulse. In fact, the first strong evidence showing that some transmitter substance act across the synapse was provided more than 40 years ago by Sir Henry Dale and Otto Loewi.
It has been estimated that the human central nervous system, which of course includes the spinal cord and the brain itself, consists of about 10 billion (1010) nerve cells. With rare exceptions each nerve cell receives information directly as impulses from many other nerve cells - often hundreds - and transmits information to a like number. Depending on its threshold of response, a given nerve cell may fire an impulse when stimulated by only a few incoming fibers or it may not fire until stimulated by many incoming fibers. It has long been known that this threshold can be raised or lowered by various factors. Moreover, it was supposed some 60 years ago that some incoming fibers must inhibit the firing of the receiving cell rather than excite it. The conjecture was subsequently confirmed, and the mechanism of the inhibitory effect has now been clarified. This mechanism and its equally fundamental counterpart - nerve-cell excitation - are of its topic.
In the levels of anatomy there are some clues to show how the fine axon terminals impinging on a nerve cell can make the cell regenerate a nerve impulse of its own nerve cell and its dendrites are covered by fine branches of nerve fibers that end in knob-like structures. These structures are the synapses.
The electron microscope has revealed structural details of synapses that fit in nicely with the view that a chemical transmitter is involved in nerve transmission. Enclosed in the synaptic knob are many vesicles, or tiny sacs, which appear to contain the transmitter substances that induce synaptic transmission. Between the synaptic knob and the synaptic membrane of the adjoining nerve cell is a remarkably uniform space of about 20 millimicrons that is termed the synaptic cleft. Many of the synaptic vesicles are concentrated adjacent to this cleft; It seems plausible that the transmitter substance is discharged from the nearest vesicles into the cleft, where it can act on the adjacent cell membrane. This hypothesis is supported by the discovery that the transmitter is released in packets of a few thousand molecules.
The study of synaptic transmission was revolutionized in 1951 by the introduction of delicate techniques for recording electrically from the interior of single nerve cells. This is done by inserting into the nerve cell an extremely fine glass pipette with a diameter of .5 microns - about a fifty-thousandth of an inch. The pipette is filled with an electrically conducting salt solution such as concentrated potassium chloride. If the pipette is carefully inserted and held rigidly in place, the cell membrane appears to seal quickly around the glass, thus preventing the flow of a short-circuiting current through the puncture in the cell membrane. Impaled in this fashion, nerve cells can function normally for hours. Although there is no way of observing the cells during the insertion of the pipette, the insertion can be guided by using as clues the electric signals that the pipette picks up when close to active nerve cells.
At the John Curtin School of Medical Research in Canberra first employed this technique, choosing to study the large nerve cells called motoneurons, which lie in the spinal cord whose function is to activate muscles. This was a fortunate choice: Intracellular investigations with motoneurons are easier and more rewarding than those with any other kind of mammalian nerve cell.
Finding that when the nerve cell responds to the chemical synaptic transmitter, the response depends in part on characteristic features of ionic composition that are also concerned with the transmission of impulses in the cell and along its axon. When the nerve cell is at rest, its physiological makeup resembles that of most other cells in that the water solution inside the cell is quite different in composition from the solution in which the cell is bathed. The nerve cell can exploit this difference between external and internal composition and use it in quite different ways for generating an electrical impulse and for synaptic transmission.
The composition of the external solution is well established because the solution is essentially the same as blood from which cells and proteins have been removed. The composition of the internal solution is known only approximately. Indirect evidence suggests that the concentrations of sodium and chloride ions outside the cell are respectively some 10 and 14 times higher than the concentrations inside the cell. In contrast, the concentration of potassium ions inside the cell is about 30 times higher than the concentration outside.
How can one account for this remarkable state of affairs? Part of the explanation is that inside the cell is negatively charged with the respect of the cell about 70 millivolts. Since like charges repel each other, this internal negative charge tends to drive chloride ions (Cl-) outward through the cell membrane and, at the same time, to impede their inward movement. In fact, a potential difference of 70 millivolts is just sufficient to maintain the observed disparity in the concentration of chloride ions inside the cell and outside it; Chloride ions diffuse inward and outward at equal rates. A drop of 70 millivolts across the membrane therefore defines the ‘equilibrium potential’ for chloride ions.
To obtain a concentration of potassium ions (K) that is 30 times higher inside the cell than outside would require that the interior of the cell membrane be about 90 millivolts negative with respect to the exterior. Since the actual interior is only 70 millivolts negative, it falls short of the equilibrium potential for potassium ions by 20 millivolts. Evidently the thirtyfold concentration can be achieved and maintained only if there is some auxiliary mechanism for ‘pumping’ potassium ions into the cell at a rate equal to their spontaneous net outward diffusion.
The pumping mechanisms have fewer, but more difficult tasks of pumping sodium ions (Na) out of the cell against a potential gradient of 130 millivolts. This figure is obtained by adding the 70 millivolts of internal negative charge to the equilibrium potential for sodium ions, which is 60 millivolts of internal positive charge. If it were not for this postulated pump, the concentration of sodium ions inside and outside the cell would be almost the reverse of what is observed.
In their classic studies of nerve-impulse transmission in the giant axon of the squid, A. L. Hodgkin, A. F. Huxley and Bernhard Katz of Britain proved that the propagation of the impulse coincides with abrupt changes in the permeability of the axon membrane. When a nerve impulse has been triggered in some way, what can be described as a gate opens and lets sodium ions pour into the axon during the advance of the impulse, making the interior of the axon locally positive. The process is self-reinforcing in that the flow of some sodium ions through the membrane opens the gate further and makes it easier for others to follow. The sharp reversal of the internal polarity of the membrane makes up the nerve impulse, which moves like a wave until it has traveled the length of the axon. In the wake of the impulse the sodium gate closes and a potassium gate opens, by that restoring the normal polarity of the membrane within a millisecond or less.
With this understanding of the nerve impulse in hand, one is ready to follow the electrical events at the excitatory synapse. One might guess that if the nerve impulse results from an abrupt inflow of sodium ions and a rapid change in the electrical polarity of the axon's interior, something similar must happen at the body and dendrites of the nerve cell in order to generate the impulse in the first place. Indeed, the function of the excitatory synaptic terminals on the cell body and its dendrites is to depolarize the interior of the cell membrane essentially by permitting an inflow of sodium ions. When the depolarization reaches a threshold value, a nerve impulse is triggered.
As a simple instance of this phenomenon we have recorded the depolarization that occurs in a single motoneuron activated directly by the large nerve fibers that enter the spinal cord from special stretch-receptors known as annulospiral endings. These receptors in turn are found in the same muscle that is activated by the motoneuron under study. Thus the whole system forms a typical reflex arc, such as the arc responsible for the patellar reflex, or ‘knee jerk.’
To conduct the experiment we anesthetize an animal (most often a cat) and free by dissection a muscle nerves that contains these large nerve fibers. By applying a mild electric shock to the exposed nerve one can produce a single impulse in each of the fibers; Since the impulses travel to the spinal cord almost synchronously, they are referred to collectively as a volley. The number of impulses contained in the volley can be reduced by reducing the stimulation applied to the nerve. The volley strength is measured at a point just outside the spinal cord and is displayed on an oscilloscope. About half a millisecond after detection of a volley there is a wavelike change in the voltage inside the motoneuron that has received the volley. The change is detected by a microelectrode inserted in the motoneuron and is displayed on another oscilloscope.
What we find is that the negative voltage inside the cell becomes progressively fewer negative as more of the fibers impinging on the cell are stimulated to fire. This observed depolarization is in fact a simple summation of the depolarizations produced by each individual synapse. When the depolarization of the interior of the motoneuron reaches a critical point, a ‘spike’ suddenly appears on the second oscilloscope, showing that a nerve impulse has been generated. During the spike the voltage inside the cell changes from about 70 millivolts negative to as much as 30 millivolts positive. The spike regularly appears when the depolarization, or reduction of membrane potential, reaches a critical level, which is usually between 10 and 18 millivolts. The only effect of a further strengthening of the synaptic stimulus is to shorten the time needed for the motoneuron to reach the firing threshold. The depolarizing potentials produced in the cell membrane by excitatory synapses are called excitatory postsynaptic potentials, or EPSP's.
Through one barrel of a double-barreled microelectrode one can apply a background current to change the resting potential of the interior of the cell membrane, either increasing it or decreasing it. When the potential is made more negative, the EPSP rises more steeply to an earlier peak. When the potential is made less negative, the EPSP rises more slowly to a lower peak. Finally, when the charge inside the cell is reversed so as to be positive with respect to the exterior, the excitatory synapses give rise to an EPSP that is actually the reverse of the normal one.
These observations support the hypothesis that excitatory synapses produce what amounts virtually to a short circuit in the synaptic membrane potential. When this occurs, the membrane no longer acts as a barrier to the passage of ions but lets them flow through in response to the differing electric potential on the two sides of the membrane. In other words, the ions are momentarily allowed to travel freely down their electrochemical gradients, which means that the sodium ions flow into the cell and, to a lesser degree, potassium ions flow out. It is this net flow of positive ions that creates the excitatory postsynaptic potential. The flow of negative ions, such as the chloride ion, is apparently not involved. By artificially altering the potential inside the cell one can establish that there is no flow of ions, and therefore no EPSP, when the voltage drop across the membrane is zero.
How is the synaptic membrane converted from a strong ionic barrier into an ion-permeable state? It is currently accepted that the agency of conversion is the chemical transmitter substance contained in the vesicles inside the synaptic knob. When a nerve impulse reaches the synaptic knob, some of the vesicles are caused to eject the transmitter substance into the synaptic cleft. The molecules of the substance would take only a few microseconds to diffuse across the cleft and become attached to specific receptor sites on the surface membrane of the adjacent nerve cell.
Presumably the receptor sites are associated with fine channels in the membrane that are opened in some way by the attachment of the transmitter-substance molecules to the receptor sites. With the channels thus opened, sodium and potassium ions flow through the membrane thousands of times more readily than they normally do, by that producing the intense ionic flux that depolarizes the cell membrane and produces the EPSP. In many synapses the current flows strongly for only about a millisecond before the transmitter substance is eliminated from the synaptic cleft, either by diffusion into the surrounding regions or as a result of being destroyed by enzymes. The latter process is known to occur when the transmitter substance is acetylcholine, which is destroyed by the enzyme acetylcholinesterase.
The substantiation of this general picture of synaptic transmission requires the solution of many fundamental problems. Since we do not know the specific transmitter substance for the vast majority of synapses in the nervous system, we do not know whether there are many different substances or only a few. The only one identified with reasonable certainty in the mammalian central nervous system is acetylcholine. We know practically nothing about the mechanism by which a presynaptic nerve impulse causes the transmitter substance to be injected into the synaptic cleft. Nor do we know how the synaptic vesicles not immediately next to the synaptic cleft follow to moved up to the firing line to replace the emptied vesicles. It is supposed that the vesicles contain the enzyme systems needed to recharge themselves. The entire process must be swift and efficient: The total amount of transmitter substance in synaptic terminals is enough for only a few minutes of synaptic activity at normal operating rates. There are also knotty problems to be solved on the other side of the synaptic cleft. What, for example, is the nature of the receptor sites? How are the ionic channels in the membrane opened?
The second type of synapse that has been identified in the nervous system. These are the synapses that can inhibit the firing of a nerve cell even though it may be receiving a volley of excitatory impulses. When inhibitory synapses are examined in the electron microscope, they look very much like excitatory synapses. (There are probably some subtle differences, but they need not concern us here.) Microelectrode recordings of the activity of single motoneurons and other nerve cells have now shown that the inhibitory postsynaptic potential (IPSP) is virtually a mirror image of the EPSP. Moreover, individual inhibitory synapses, like excitatory synapses, have a cumulative effect. The chief difference is simply that the IPSP makes the cell's internal voltage more negative than it is normally, which is in a direction opposite to that needed for generating a spike discharge.
By driving the internal voltage of a nerve cell in the negative direction inhibitory synapses oppose the action of excitatory synapses, which of course drive it in the positive direction. So if the potential inside a resting cell is 70 millivolts negative, a strong volley of inhibitory impulses can drive the potential to 75 or 80 millivolts depreciating count. One can easily see that if the potential is made more negative in this way the excitatory synapses find it more difficult to raise the internal voltage to the threshold point for the generation of a spike. Thus, the nerve cell responds to the algebraic sum of the internal voltage changes produced by excitatory and inhibitory synapses.
If, as in the experiment described earlier, the internal membrane potential is altered by the flow of an electric current through one barrel of a double-barreled microelectrode, one can observe the effect of such changes on the inhibitory postsynaptic potential. When the internal potential is made less negative, the inhibitory postsynaptic potential is deepened. Conversely, when the potential is made more negative, the IPSP diminishes; it finally reverses when the internal potential is driven below minus 80 millivolts.
One can therefore assume that inhibitory synapse’s share with excitatory synapses the ability to change the ionic permeability of the synaptic membrane. The difference is that inhibitory synapses enable ions to flow freely down an electrochemical gradient that has an equilibrium point at minus 80 millivolts rather than at zero, as is the case for excitatory synapses. This effect could be achieved by the outward flow of positively charged ions such as potassium or the inward flow of negatively charged ions such as chloride, or by a combination of negative and positive ionic flows such that the interior reaches equilibrium at minus 80 millivolts.
If the concentration of chloride ions within the cell is increased as much as three times, the inhibitory postsynaptic potential reverses and acts as a depolarizing current; that is, it resembles an excitatory potential. On the other hand, if the cell is heavily injected with sulfate ions, which are also negatively charged, there is no such reversal. This simple test shows that under the influence of the inhibitory transmitter substance, which is still unidentified, the subsynaptic membrane becomes permeable momentarily to chloride ions but not to sulfate ions. During the generation of the IPSP the outflow of chloride ions is so rapid that it more than outweighs the flow of other ions that generate the normal inhibitory potential.
The effect of injecting motoneurons with more than 30 kinds of negatively lunged ions. With one exception the hydrated ions (ions bound to water) to which the cell membrane is permeable under the influence of the inhibitory transmitter substance are smaller than the hydrated ions to which the membrane is impermeable. The exception is the formate ion (HCO2-), which may have an ellipsoidal shape and so be able to pass through membrane pores that block smaller spherical ions.
Apart from the formate ion all the ions to which the membrane is permeable have a diameter not greater than 1.14 times the diameter of the potassium ion; That is, they are less than 2.9 angstrom units in diameter. Comparable investigations in other laboratories have found the same permeability effects, including the exceptional behavior of the formate ion, in fishes, toads and snails. It might be that the ionic mechanism responsible for synaptic inhibition is the same throughout the animal kingdom.
The significance of these and other studies is that they strongly suggest that the inhibitory transmitter substance open the membrane to the flow of potassium ions but not to sodium ions. It is known that the sodium ion is somewhat larger than any of the negatively charged ions, including the formate ion, that are able to pass through the membrane during synaptic inhibition. Testing the effectiveness of potassium ions by injecting excess amounts into the cell is not possible, however, because the excess is immediately diluted by an osmotic flow of water into the cell.
The concentration of potassium ions inside the nerve cell is about 30 times greater than the concentration outside, and to maintain this large difference in concentration without the help of some metabolic pumps inside of the membrane would have to be charged 90 millivolts negative with respect to the exterior. This implies that if the membrane were suddenly made porous to potassium ions, the resulting outflow of ions would make the inside potential of the membrane even more negative than it is in the resting state, and that is just what happens during synaptic inhibition. The membrane must not simultaneously become porous to sodium ions, because they exist in much higher concentration outside the cell than inside and their rapid inflow would more than compensate for the potassium outflow. In fact, the fundamental difference between synaptic excitation and synaptic inhibition is that the membrane freely passes sodium ions in response to the former and largely excludes the passage of sodium ions in response to the latter.
This fine discrimination between ions that are not very different in size must be explained by any hypothesis of synaptic action. It is most unlikely that the channels through the membrane are created afresh and accurately maintained for a thousandth of a second every time a burst of transmitter substance is released into the synaptic cleft. It is more likely that channels of at least two different sizes are built directly into the membrane structure. In some way the excitatory transmitter substance would selectively unplug the larger channels and permit the free inflow of sodium ions. Potassium ions would simultaneously flow out and thus would tend to counteract the large potential change that would be produced by the massive sodium inflow. The inhibitory transmitter substance would selectively unplug the smaller channels that are large enough to pass potassium and chloride ions but not sodium ions.
To explain certain types of inhibition other features must be added to this hypothesis of synaptic transmission. In the simple hypothesis chloride and potassium ions can flow freely through pores of all inhibitory synapses. It has been shown, however, that the inhibition of the contraction of heart muscle by the vagus nerve is due almost exclusively to potassium-ion flow. On the other hand, in the muscles of crustaceans and in nerve cells in the snail's brain synaptic inhibition is due largely to the flow of chloride ions. This selective permeability could be explained if there were fixed charges along the walls of the channels. If such charges were negative, they would repel negatively charged ions and prevent their passage; if they were positive, they would similarly prevent the passage of positively charged ions. One can now suggest that the channels opened by the excitatory transmitter are negatively charged and so do not permit the passage of the negatively charged chloride ion, even though it is small enough to move through the channel freely.
One might wonder if a given nerve cell can have excitatory synaptic action at some of its axon terminals and inhibitory action at others. The answer is no. Two different kinds of nerve cells are needed, one for each type of transmission and synaptic transmitter substance. This can readily be shown by the effect of strychnine and tetanus toxins in the spinal cord; They specifically prevent inhibitory synaptic action and leave excitatory action unaltered. As a result the synaptic excitation of nerve cells is uncontrolled and convulsions result. The special types of cells responsible for inhibitory synaptic action are now being recognized in many parts of the central nervous system.
This account of communication between nerve cells is necessarily oversimplified, yet it shows that some significant advances are being made at the level of individual components of the nervous system. By selecting the most favorable situations we have been able to throw light on some details of nerve-cell behavior. We can be encouraged by these limited successes. Nevertheless, the task of understanding in a comprehensive way how the human brain operates staggers its own imagination.
Our brain begins with its portion of the central nervous system contained within the skull. The brain is the control center for movement, sleep, hunger, thirst, and virtually every other vital activity necessary to survival. All human emotions - including love, hate, fear, anger, elation, and sadness - are controlled by the brain. It also receives and interprets the countless signals that are sent to it from other parts of the body and from the external environment. The brain makes us conscious, emotional, and intelligent.
The human brain has three major structural components: the large dome-shaped cerebrum, the smaller somewhat spherical cerebellum, and the brainstem. Prominent in the brainstem are the medulla oblongata and the thalamus - between the medulla and the cerebrum. The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus act as a relay center for electrical impulses traveling to and from the cerebral cortex.
The adult human brain is a 1.3-kg. (3-lb.) Mass of pinkish-gray jellylike tissue made up of approximately 100 billion nerve cells or neurons: The Neuroglia (supporting-tissue) cells, and vascular (blood-carrying) and other tissues.
Between the brain and the cranium - the part of the skull that directly covers the brain - are three protective membranes, or meninges. The outermost membrane, the dura mater, is the toughest and thickest. Below the dura mater is a middle membrane, called the arachnoid layer. The innermost membrane, the pia mater, consists mainly of small blood vessels and follows the contours of the surface of the brain.
A clear liquid, the cerebrospinal fluid, bathes the entire brain and fills a series of four cavities, called ventricles, near the center of the brain. The cerebrospinal fluid protects the internal portion of the brain from varying pressures and transports chemical substances within the nervous system.
From the outside, the brain appears as three associatively distinct but connected parts, the cerebrum (the Latin word for brain) - two large, almost symmetrical hemispheres; the cerebellum ('little brain') - two smaller hemispheres located at the back of the cerebrum; and the brain stem - a central core that gradually becomes the spinal cord, exiting the skull through an opening at its base called the foramen magnum. Two other major parts of the brain, the thalamus and the hypothalamus, lie in the midline above the brain stem underneath the cerebellum.
The brain and the spinal cord together make up the central nervous system, which communicates with the rest of the body through the peripheral nervous system. The peripheral nervous system consists of 12 pairs of cranial nerves extending from the cerebrum and brain stem; a system of other nerves branching throughout the body from the spinal cord, and the autonomic nervous system, which regulates vital functions is not very consciously of its own control, such as the activity of the heart muscle, smooth muscle (involuntary muscle found in the skin, blood vessels, and internal organs), and glands.
Many motor and sensory functions have been ‘mapped’ to specific areas of the cerebral cortex, some of which are indicated here. In general, these areas exist in both hemispheres of the cerebrum, each serving the opposite side of the body. Fewer defined are the areas of association, located mainly in the frontal cortex, operatives in functions of thought and emotion and responsible for linking input from different senses. The areas of language are an exception: Both Wernicke’s area, concerned with the comprehension of spoken language, and Broca’s area, governing the production of speech, have been pinpointed on the cortex.
Most high-level brain functions take place in the cerebrum. Its two large hemispheres make up approximately 85 percent of the brain's weight. The exterior surface of the cerebrum, the cerebral cortex, is a convoluted, or folded, grayish layer of cell bodies known as the gray matter. The gray matter covers an underlying mass of fibers called the white matter. The convolutions are made up of ridgelike bulges, known as gyri, separated by small grooves called sulci and larger grooves called fissures. Approximately two-thirds of the cortical surface is hidden in the folds of the sulci. The extensive convolutions enable a very large surface area of brain cortices - roughly, 1.5 m2 (16 ft2) in an adult - to fit within the cranium. The pattern of these convolutions is similar, although not identical, in all humans.
The two cerebral hemispheres are partially separated from each other by a deep fold known as the longitudinal fissure. Communication between the two hemispheres is through several concentrated bundles of axons, called commissures, the largest of which is the corpus callosum.
Several major sulci divides the cortex into distinguishable regions. The central sulcus, or Rolandic fissure, runs from the middle of the top of each hemisphere downward, forwards, and toward another major sulcus, the lateral (side), or Sylvian, sulcus. These and other sulci and gyri divide the cerebrum into five lobes: The frontal, parietal, temporal, and occipital lobes and the insula.
Although the cerebrum is symmetrical in structure, with two lobes emerging from the brain stem and matching motor and sensory areas in each, certain intellectual functions are restricted to one hemisphere. A person’s dominant hemisphere is usually occupied with language and logical operations, while the other hemisphere controls emotion and artistic and spatial skills. In nearly all right-handed and many left-handed people, the left hemisphere is dominant.
The frontal lobe is the largest of the five and consists of all the cortices in front of the central sulcus. Broca's area, a part of the cortex related to speech, is located in the frontal lobe. The parietal lobe consists of the cortex behind the central sulcus to some sulcus near the back of the cerebrum known as the parieto-occipital sulcus. The parieto-occipital sulcus, in turn, forms the front border of the occipital lobe, which are the rearmost part of the cerebrum. The temporal lobe is to the side of and below the lateral sulcus. Wernicke's area, a part of the cortex related to the understanding of language, is located in the temporal lobe. The insula lies deep within the folds of the lateral sulcus.
The cerebrum receives information from all the sense organs and sends motor commands (signals that results in activity in the muscles or glands) to other parts of the brain and the rest of the body. Motor commands are transmitted by the motor cortex, a strip of cerebral cortex extending from side to side across the top of the cerebrum just in front of the central sulcus. The sensory cortex, parallel strips of cerebral cortex just in back of the central sulcus, receives input from the sense organs.
Many other areas of the cerebral cortex have also been mapped according to their specific functions, such as vision, hearing, speech, emotions, language, and other aspects of perceiving, thinking, and remembering. Cortical regions known as associative cortices are responsible for integrating multiple inputs, processing the information, and carrying out complex responses.
The cerebellum coordinates body movements. Located at the lower back of the brain beneath the occipital lobes, the cerebellum is divided into two lateral (side-by-side) lobes connected by a fingerlike bundle of white fibers called the vermis. The outer layer, or cortex, of the cerebellum consists of fine folds called folia. As in the cerebrum, the outer layer of cortical gray matter surrounds a deeper layer of white matter and nuclei (groups of nerve cells). Three fiber bundles called cerebellar peduncles connect the cerebellum to the three parts of the brain stem - the midbrain, the pons, and the medulla oblongata.
The cerebellum coordinates voluntary movements by fine-tuning commands from the motor cortex in the cerebrum. The cerebellum also maintains posture and balance by controlling muscle tone and sensing the position of the limbs. All motor activity, from hitting a baseball to fingering a violin, depends on the cerebellum.
The limbic system is a group of brain structures that play a role in emotion, memory, and motivation. For example, electrical stimulation of the amygdala in laboratory animals can provoke fear, anger, and aggression. The hypothalamus regulates hunger, thirst, sleep, body temperature, sexual drive, and other functions.
The thalamus and the hypothalamus lie underneath the cerebrum and connect it to the brain stem. The thalamus consist of two rounded masses of gray tissue lying within the middle of the brain, between the two cerebral hemispheres. The thalamus are the main relay station for incoming sensory signals to the cerebral cortex and for outgoing motor signals from it. All sensory input to the brain, except that of the sense of smell, connects to individual nuclei of the thalamus.
The hypothalamus lies beneath the thalamus on the midline at the base of the brain. It regulates or is involved directly in the control of many of the body's vital drives and activities, such as eating, drinking, temperature regulation, sleep, emotional behavior, and sexual activity. It also controls the function of internal body organs by means of the autonomic nervous system, interacts closely with the pituitary gland, and helps coordinate activities of the brain stem.
The brain stem, shown here in colored cross section, is the lowest part of the brain. It serves as the path for messages traveling between the upper brain and spinal cord but is also the seat of basic and vital functions such as breathing, blood pressure, and heart rates, as well as reflexes like eye movement and vomiting. The brain stem has three main parts: the medulla, pons, and midbrain. A canal runs longitudinally through these structures carrying cerebrospinal fluid. Also distributed along its length is a network of cells, referred to as the reticular formation, that governs the state of alertness.
The brain stem is revolutionarily the most primitive part of the brain and is responsible for sustaining the basic functions of life, such as breathing and blood pressure. It includes three main structures lying between and below the two cerebral hemispheres - the midbrain, pons, and medulla oblongata.
The topmost structure of the brain stem is the midbrain. It contains major relay stations for neurons transmitting signals to the cerebral cortex, as well as many reflex centers - pathways carrying sensory (input) information and motor (output) command. Relays and reflex centers for visual and auditory (hearing) functions are located in the top portion of the midbrain. A pair of nuclei called the superior colliculus control reflex actions of the eye, such as blinking, opening and closing the pupil, and focusing the lens. A second pair of nuclei, called the inferior colliculus, controls auditory reflexes, such as adjusting the ear to the volume of sound. At the bottom of the midbrain are reflex and relay centers relating to pain, temperature, and touch, as well as several regions associated with the control of movement, such as the red nucleus and the substantia nigra.
Continuous with and below the midbrain and directly in front of the cerebellum is a prominent bulge in the brain stem called the pons. The pons consists of large bundles of nerve fibers that connect the two halves of the cerebellum and also connect each side of the cerebellum with the opposite-side cerebral hemisphere. The pons serves mainly as a relay station linking the cerebral cortex and the medulla oblongata.
The long, stalklike lowermost portion of the brain stem is called the medulla oblongata. At the top, it is continuous with the pons and the midbrain; at the bottom, it makes a gradual transition into the spinal cord at the foramen magnum. Sensory and motor nerve fibers connecting the brain and the rest of the body cross over to the opposite side as they pass through the medulla. Thus, the left half of the brain communicates with the right half of the body, and the right half of the brain with the left half of the body.
Running up the brain stem from the medulla oblongata through the pons and the midbrain is a netlike formation of nuclei known as the reticular formation. The reticular formation controls respiration, cardiovascular function, digestion, levels of alertness, and patterns of sleep. It also determines which parts of the constant flow of sensory information into the body are received by the cerebrum.
There are two main types of brain cells, neurons and neuroglia. Neurons are responsible for the transmission and analysis of all electrochemical communication within the brain and other parts of the nervous system. Each neuron is composed of a cell body called a soma, and a major fiber called an axon, and a system of branches called dendrites. Axons, also called nerve fibers, convey electrical signals away from the soma and can be up to 1 m. (3.3 ft.) in length. Most axons are covered with a protective sheath of myelin, a substance made of fats and protein, which insulates the axon. Myelinated axons conduct neuronal signals faster than do unmyelinated axons. Dendrites convey electrical signals toward the soma, are shorter than axons, and are usually multiple and branching.
Neuroglial cells are twice as numerous as neurons and account for half of the brain's weight. Neuroglia (from glia, Greek for 'glue') provides structural support to the neurons. Neuroglial cells also form myelin, guide developing neurons, take up chemicals involved in cell-to-cell communication, and contribute to the maintenance of the environment around neurons.
Twelve pairs of cranial nerves arise symmetrically from the base of the brain and are numbered, from front to back, in the order in which they arise. They connect mainly with structures of the head and neck, such as the eyes, ears, nose, mouth, tongue, and throat. Some are motor nerves, controlling muscle movement; some are sensory nerves, conveying information from the sense organs; and others contain fibers for both sensory and motor impulses. The first and second pairs of cranial nerves - the olfactory (smell) nerves and the optic (vision) nerve - carry sensory information from the nose and eyes, respectively, to the undersurface of the cerebral hemispheres. The other ten pairs of cranial nerves originate in or end in the brain stem.
The brain functions by complex neuronal, or nerve cell, circuits. Communication between neurons is both electrical and chemical and always travels from the dendrites of a neuron, through its soma, and out its axon to the dendrites of another neuron.
Dendrites of one neuron receive signals from the axons of other neurons through chemicals known as neurotransmitters. The neurotransmitters set off electrical charges in the dendrites, which then carry the signals electrochemically to the soma. The soma integrates the information, which is then transmitted electrochemically down the axon to its tip.
At the tip of the axon, small, bubble-like structures called vesicles’ release neurotransmitters that carries the signal across the synapse, or gap, between two neurons. There are many types of neurotransmitters, including norepinephrine, dopamine, and serotonin. Neurotransmitters can be excitatory (that is, they excite an electrochemical response in the dendrite receptors) or inhibitory (they block the response of the dendrite receptors).
One neuron may communicate with thousands of other neurons, and many thousands of neurons are involved with even the simplest behavior. It is believed that these connections and their efficiency can be modified, or altered, by experience.
Scientists have used two primary approaches to studying how the brain works. One approach is to study brain function after parts of the brain have been damaged. Functions that disappear or that is no longer normal after injury to specific regions of the brain can often be associated with the damaged areas. The second approach is to study the response of the brain to direct stimulation or to stimulation of various sense organs.
Neurons are grouped by function into collections of cells called nuclei. These nuclei are connected to form sensory, motor, and other systems. Scientists can study the function of somatosensory (pain and touch), motor, olfactory, visual, auditory, language, and other systems by measuring the physiological (physical and chemical) change that occur in the brain when these senses are activated. For example, electroencephalography (EEG) measures the electrical activity of specific groups of neurons through electrodes attached to the surface of the skull. Electrodes incorporate directly into the brain can give readings of individual neurons. Changes in blood flow, glucose (sugar), or oxygen consumption in groups of active cells can also be mapped.
Although the brain appears symmetrical, how it functions is not. Each hemisphere is specializing and dominates the other in certain functions. Research has shown that hemispheric dominance is related to whether a person is predominantly right-handed or left-handed. In most right-handed people, the left hemisphere processes arithmetic, language, and speech. The right hemisphere interprets music, complex imagery, and spatial relationships and recognizes and expresses emotion. In left-handed people, the pattern of brain organization is more variable.
Hemispheric specialization has traditionally been studied in people who have sustained damage to the connections between the two hemispheres, as may occur with a stroke, an interruption of blood flow to an area of the brain that causes the death of nerve cells in that area. The division of functions between the two hemispheres has also been studied in people who have had to have the connection between the two hemispheres surgically cut in order to control severe epilepsy, a neurological disease characterized by convulsions and loss of consciousness.
The visual system of humans is one of the most advanced sensory systems in the body. More information is conveyed visually than by any other means. In addition to the structures of the eye itself, several cortical regions - collectively called a primary visual and visual associative cortex - as well as the midbrain are involved in the visual system. Conscious processing of visual input occurs in the primary visual cortex, but reflexive - that is, immediate and unconscious - responses occur at the superior colliculus in the midbrain. Associative cortical regions - specialized regions that can associate, or integrate, multiple inputs - in the parietal and frontal lobes along with parts of the temporal lobe are also involved in the processing of visual information and the establishment of visual memories.
Language involves specialized cortical regions in a complex interaction that allows the brain to comprehend and communicate abstract ideas. The motor cortex initiates impulses that travel through the brain stem to produce audible sounds. Neighboring regions of motor cortices, called the supplemental motor cortex, are involved in sequencing and coordinating sounds. Broca's area of the frontal lobe is responsible for the sequencing of language elements for output. The comprehension of language is dependent upon Wernicke's area of the temporal lobe. Other cortical circuits connect these areas.
Memory is usually considered a diffusely stored associative process - that is, it puts together information from many different sources. Although research has failed to identify specific sites in the brain as locations of individual memories, certain brain areas are critical for memory to function. Immediate recall - the ability to repeat short series of words or numbers immediately after hearing them - is thought to be located in the auditory associative cortex. Short-term memory - the ability to retain a limited amount of information for up to an hour - is located in the deep temporal lobe. Long-term memory probably involves exchanges between the medial temporal lobe, various cortical regions, and the midbrain.
The autonomic nervous system regulates the life support systems of the body reflexively - that is, without conscious direction. It automatically controls the muscles of the heart, digestive system, and lungs; Certain glands, and homeostasis - that is, the equilibrium of the internal environment of the body. The autonomic nervous system itself is controlled by nerve centers in the spinal cord and brain stem and is fine-tuned by regions higher in the brain, such as the midbrain and cortex. Reactions such as blushing indicate that cognitive, or thinking, centers of the brain are also involved in autonomic responses.
The brain is guarded by several highly developed protective mechanisms. The bony cranium, the surrounding meninges, and the cerebrospinal fluid all contribute to the mechanical protection of the brain. In addition, a filtration system called the blood-brain barrier protects the brain from exposure to potentially harmful substances carried in the bloodstream.
Brain disorders have a wide range of causes, including head injury, stroke, bacterial diseases, complex chemical imbalances, and changes associated with aging.
Head injury can initiate a cascade of damaging events. After a blow to the head, a person may be stunned or may become unconscious for a moment. This injury, called - concussion, - usually leaves no permanent damage. If the blow is more severe and hemorrhage (excessive bleeding) and swelling occurs, however, severe headache, dizziness, paralysis, a convulsion, or temporary blindness may result, depending on the area of the brain affected. Damage to the cerebrum can also result in profound personality changes.
Damage to Broca's area in the frontal lobe causes difficulty in speaking and writing, a problem known as Broca's aphasia. Injury to Wernicke's area in the left temporal lobe results in an inability to comprehend spoken language, called Wernicke's aphasia.
An injury or disturbance to a part of the hypothalamus may cause a variety of different symptoms, such as loss of appetite with an extreme drop in body weight, increase in appetite leading to obesity; Extraordinary thirst with excessive urination (diabetes insipidus), failure in body-temperature control, resulting in either low temperature (hypothermia) or high temperature (fever), excessive emotionality, and uncontrolled anger or aggression. If the relationship between the hypothalamus and the pituitary gland is damaged, other vital bodily functions may be disturbed, such as sexual function, metabolism, and cardiovascular activity.
Injury to the brain stem is even more serious because it houses the nerve centers that control breathing and heart action. Damage to the medulla oblongata usually results in immediate death.
A stroke is damage to the brain due to an interruption in blood flow. The interruption may be caused by a blood clot, constriction of a blood vessel, or rupture of a vessel accompanied by bleeding. A pouchlike expansion of the wall of a blood vessel, called an aneurysm, may weaken and burst, for example, because of high blood pressure.
Sufficient quantities of glucose and oxygen, transported through the bloodstream, are needed to keep nerve cells alive. When the blood supply to a small part of the brain is interrupted, the cells in that area die and the function of the area is lost. A massive stroke can cause a one-sided paralysis (hemiplegia) and sensory loss on the side of the body opposite the hemisphere damaged by the stroke.
Some brain diseases, such as multiple sclerosis and Parkinson disease, are progressive, becoming worse over time. Multiple sclerosis damages the myelin sheath around axons in the brain and spinal cord. As a result, the affected axons cannot transmit nerve impulses properly. Parkinson disease destroys the cells of the substantia nigra in the midbrain, resulting in a deficiency in the neurotransmitter dopamine that affects motor functions.
Cerebral palsy is a broad term for brain damage sustained close to birth that permanently affects motor function. The damage may take place either in the developing fetus, during birth, or just after birth and is the result of the faulty development or breaking down of motor pathways. Cerebral palsy is nonprogressive - that is, it does not worsen with time.
A bacterial infection in the cerebrum or in the coverings of the brain, swelling of the brain, or an abnormal growth of healthy brain tissue can all cause an increase in intracranial pressure and result in serious damage to the brain.
Scientists are finding that certain brain chemical imbalances are associated with mental disorders such as schizophrenia and depression. Such findings have changed scientific understanding of mental health and have resulted in new treatments that chemically correct these imbalances.
During childhood development, the brain is particularly susceptible to damage because of the rapid growth and reorganization of nerve connections. Problems that originate in the immature brain can appear as epilepsy or other brain-function problems in adulthood.
Several neurological problems are common in aging. Alzheimer's disease damages many areas of the brain, including the frontal, temporal, and parietal lobes. The brain tissue of people with Alzheimer's disease shows characteristic patterns of damaged neurons, known as plaques and tangles. Alzheimer's disease produces progressive dementia, characterized by symptoms such as failing attention and memory, loss of mathematical ability, irritability, and poor orientation in space and time.
A magnetic resonance imaging (MRI) scan of the human brain reveals the contours of one of the brain’s hemispheres. The gyri, or ridges, appear in red, while the sulci, or valleys, are shown in blue. Each person has slightly different patterns of gyri and sulci, which reflect individual differences in brain development.
Several commonly used diagnostic methods give images of the brain without invading the skull. Some portray anatomy - that is, the structure of the brain - whereas others measure brain function. Two or more methods may be used to complement each other, together providing a more complete picture than would be possible by one method alone.
Magnetic resonance imaging (MRI), introduced in the early 1980s, beams high-frequency radio waves into the brain in a highly magnetized field that causes the protons that form the nuclei of hydrogen atoms in the brain to reemit the radio waves. The reemitted radio waves are analyzed by computer to create thin cross-sectional images of the brain. MRI provides the most detailed images of the brain and is safer than imaging methods that use X-rays. However, MRI is a lengthy process and also cannot be used with people who have pacemakers or metal implants, both of which are adversely affected by the magnetic field.
Computed tomography (CT), also known as CT scans, developed in the early 1970s. This imaging method X-rays the brain from many different angles, feeding the information into a computer that produces a series of cross-sectional images. CT is particularly useful for diagnosing blood clots and brain tumors. It is a much quicker process than magnetic resonance imaging and is therefore advantageous in certain situations - for example, with people who are extremely ill.
This positron emission tomography (PET) scans of the brain shows the activity of brain cells in the resting state and during three types of auditory stimulation. PET uses radioactive substances introduced within the brain to measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. This imaging method collects data from many different angles, feeding the information into a computer that produces a series of cross-sectional images.
Changes in brain function due to brain disorders can be visualized in several ways. Magnetic resonance spectroscopy measures the concentration of specific chemical compounds in the brain that may change during specific behaviors. Functional magnetic resonance imaging (fMRI) maps changes in oxygen concentration that correspond to nerve cell activity.
Positron emission tomography (PET), developed in the mid-1970s, uses computed tomography to visualize radioactive tracers, radioactive substances are introduced into the brain intravenously or by inhalation. PET can measure such brain functions as cerebral metabolism, blood flow and volume, oxygen use, and the formation of neurotransmitters. Single photon emission computed tomography (SPECT), developed in the 1950s and 1960s, used radioactive tracers to visualize the circulation and volume of blood in the brain.
Brain-imaging studies have provided new insights into sensory, motor, language, and memory processes, as well as brain disorders such as epilepsy, cerebrovascular disease; Alzheimer's, Parkinson, and Huntington's diseases, and various mental disorders, such as schizophrenia.
Although all vertebrate brains share the same basic three-part structure, the development of their constituent parts varies across the evolutionary scale. In fish, the cerebrum is dwarfed by the rest of the brain and serves mostly to process input from the senses. In reptiles and amphibians, the cerebrum is proportionally larger and begins to connect and form conclusions about this input. Birds have well-developed optic lobes, making the cerebrum even larger. Among mammals, the cerebrum dominates the brain. It is most developed among primates, in whom cognitive ability is the highest.
In lower vertebrates, such as fish and reptiles, the brain is often tubular and bears a striking resemblance to the early embryonic stages of the brains of more highly evolved animals. In all vertebrates, the brain is divided into three regions: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). These three regions further sub-divide into different structures, systems, nuclei, and layers.
The more highly evolved the animal, the more complex is the brain structure. Human beings have the most complex brains of all animals. Evolutionary forces have also resulted in a progressive increase in the size of the brain. In vertebrates lower than mammals, the brain is small. In meat-eating animals, particularly primates, the brain increases dramatically in size.
The cerebrum and cerebellum of higher mammals are highly convoluted in order to fit the most gray matter surface within the confines of the cranium. Such highly convoluted brains are called gyrencephalic. Many lower mammals have a smooth, or lissencephalic (smooth head), cortical surfaces.
There is also evidence of evolutionary adaption of the brain. For example, many birds depend on an advanced visual system to identify food at great distances while in flight. Consequently, their optic lobes and cerebellum are well developed, giving them keen sight and outstanding motor coordination in flight. Rodents, on the other hand, as nocturnal animals, do not have a well-developed visual system. Instead, they rely more heavily on other sensory systems, such as a highly-developed sense of smell and facial whiskers.
Recent research in brain function suggests that there may be sexual differences in both brain anatomy and brain function. One study indicated that men and women may use their brains differently while thinking. Researchers used functional magnetic resonance imaging to observe which parts of the brain were activated as groups of men and women tried to determine whether sets of nonsense words rhymed. Men used only Broca's area in this task, whereas women used Broca's area plus an area on the right side of the brain.
The Cell, in [biology] is the most basic unit of life. Cells are the smallest structures capable of basic life processes, such as taking in nutrients, expelling waste, and reproducing. All living things are composed of cells. Some microscopic organisms, such as bacteria and protozoa, are unicellular, meaning they consist of a single cell. Plants, animals, and fungi are multicellular; that is, they are composed of a great many cells working in concert. But whether it makes up an entire bacterium or is just one of the trillions in a human being, the cell is a marvel of design and efficiency. Cells carry out thousands of biochemical reactions each minute and reproduce new cells that perpetuate life.
The word cell refers to several types of organisms. Cells such as paramecia, dinoflagellates, diatoms, and spirochetes are self-maintaining organisms; cells such as lymphocytes, erythrocytes, muscle cells, nerve cells, cardiac muscle, and chromoplasts are more specializing cells that are a part of higher multicellular organisms. Nonetheless, of its size or whether the cell is a complete organism or just part of an organism, all cells have certain structural components in common. All cells have some type of outer cell boundary that permits some materials to leave and enter the cell and a cell interior composed of a water-rich, fluid material called cytoplasm that contains hereditary material in the form of deoxyribonucleic acid (DNA).
Cells vary considerably in size. The smallest cell, a type of bacterium known as a mycoplasma, measures 0.0001 mm. (0.000004 in.) in diameter; 10,000 mycoplasmas in a row are only as wide as the diameter of a human hair. Among the largest cells are the nerve cells that run down a giraffe’s neck; these cells can exceed 3 m. (9.7 ft.) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm. (0.00003 in.) to liver cells that may be ten times larger. About 10,000 average-sized human cells can fit on the head of a pin.
Along with their differences in size, cells present an array of shapes. Some, such as the bacterium Escherichia coli, resemble rods. The paramecium, a type of protozoan, is a slipper shaped. The amoeba, another protozoan, has an irregular form that changes shape as it moves around. Plant cells typically resemble boxes or cubes. In humans, the outermost layers of skin cells are flat, while muscle cells are long and thin. Some nerve cells, with their elongated, tentacle-like extensions, suggest an octopus.
In multicellular organisms, shape is typically tailored to the cell’s job. For example, flat skin cells pack tightly into a layer that protects the underlying tissues from invasions by bacteria. Long, thin muscle cells’ contract readily to move bones. The numerous extensions from a nerve cell enable it to connect to several other nerve cells in order to send and receive messages rapidly and efficiently.
By itself, each cell is a model of independence and self-containment. Like some miniature, walled city in perpetual rush hour, the cell constantly bustles with traffic, shuttling essential molecules from place to place to carry out the business of living. Despite their individuality, however, cells also display a remarkable ability to join, communicate, and coordinate with other cells. The human body, for example, consists of an estimated 20 to 30 trillion cells. Dozens of different kinds of cells are organized into specialized groups called tissues. Tendons and bones, for example, are composed of connective tissue, whereas skin and mucous membranes are built from epithelial tissue. Different tissue types are assembled into organs, which are structures specialized to perform particular functions. Examples of organs include the heart, stomach, and brain. Organs, in turn, are organized into systems such as the circulatory, digestive, or nervous systems. All together, these assembled organ systems form the human body.
The components of cells are molecules, nonliving structures formed by the union of atoms. Small molecules serve as building blocks for larger molecules. Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure and also participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and protein-sugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membrane-bound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes, specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid (RNA), works with DNA to build the thousands of proteins the cell needs.
Cells fall into one of two categories: Prokaryotic or eukaryotic, in a prokaryotic cell, found only in bacteria and archaebacteria, all the components, including the DNA, mingle freely in the cell’s interior, a single compartment. Eukaryotic cells, which make up plants, animals, fungi, and all other life forms, contain numerous compartments, or organelles, within each cell. The DNA in eukaryotic cells is enclosed in a special organelle called the nucleus, which serves as the cell’s command center and information library. The term prokaryote comes from Greek words that mean ‘before the nucleus’ or ‘prenucleus,’ while eukaryote means ‘a true nucleus.’
Bacteria’s cells typically are surrounded by a rigid, protective cell wall. The cell membrane, also called the plasma membrane, regulates passage of materials into and out of the cytoplasm, the semi-fluid that fill the cell. The DNA, located in the nucleoid region, contains the genetic information for the cell. Ribosomes carry out protein synthesis. Many bacteria contain some pilus (plural pili), a structure that extends out of the cell to transfer DNA to another bacterium. The flagellum, found in numerous species, is used for the locomotion. Some bacteria contain a plasmid, a small chromosomes with extra genes. Others have a capsule, a sticky substance external to the cell wall that protects bacteria from attack by white blood cells. Mesosomes were formerly thought to be structures with unknown functions, but now are known to be artifacts created when cells are prepared for viewing with electron microscopes.
Prokaryotic cells are among the tiniest of all cells, ranging in size from 0.0001 to 0.003 mm. (0.000004 to 0.0001 in.) in diameter. About a hundred typical prokaryotic cells lined up in a row would match the thickness of a book page. These cells, which can be rod-like, spherical, or spiral in shape, are surrounded by a protective cell wall. Like most cells, prokaryotic cells live in a watery environment, whether it is soil moisture, a pond, or the fluid surrounding cells in the human body. Tiny pores in the cell wall enable water and the substances dissolved in it, such as oxygen, to flow into the cell; these pores also allow wastes to flow out.
Pushed up against the inner surface of the prokaryotic cell wall is a thin membrane called the plasma membrane. The plasma membrane, composed of two layers of flexible lipid molecules and interspersed with durable proteins, is both supple and strong. Unlike the cell wall, whose open pores allow the unregulated traffic of materials in and out of the cell, the plasma membrane is selectively permeable, meaning it allows only certain substances to pass through. Thus, the plasma membrane actively separates the cell’s contents from its surrounding fluids.
While small molecules such as water, oxygen, and carbon dioxide diffuse freely across the plasma membrane, the passage of many larger molecules, including amino acids (the building blocks’ of proteins) and sugars, is carefully regulated. Specialized transport proteins accomplish this task. The transport proteins span the plasma membrane, forming an intricate system of pumps and channels through which traffic is conducted. Some substances swirling in the fluid around the cell can enter it only if they bind to and are escorted in by specific transport proteins. In this way, the cell fine-tunes its internal environment.
The plasma membrane encloses the cytoplasm, the semifluid that fill the cell. Composed of about 65 percent water, the cytoplasm is packed with up to a billion molecules per cell, a rich storehouse that includes enzymes and dissolved nutrients, such as sugars and amino acids. The water provides a favorable environment for the thousands of biochemical reactions that take place in the cell.
Within the cytoplasm of all prokaryote is deoxyribonucleic acid (DNA), a complex molecule in the form of a double helix, a shape similar to a spiral staircase. The DNA is about 1,000 times the length of the cell, and to fit inside, it repeatedly twists and folds to form a compact structure called a chromosome. The chromosome in prokaryote is circular, and is located in a region of the cell called the nucleoid. Often, smaller chromosomes called plasmids are located in the cytoplasm. The DNA is divided into units called genes, just like a long train is divided into separate cars. Depending on the species, the DNA contains several hundred or even thousands of genes. Typically, one gene contains coded instructions for building all or part of a single protein. Enzymes, which are specialized proteins, determine virtually all the biochemical reactions that support and sustain the cell.
Also, immersed in the cytoplasm are the only organelles in prokaryotic cells. Tiny bead-like structures called ribosomes. These are the cell’s protein factories. Following the instructions encoded in the DNA, ribosomes churn out proteins by the hundreds every minute, providing needed enzymes, the replacements for worn-out transport proteins, or other proteins required by the cell.
While relatively simple in construction, prokaryotic cells display extremely complex activity. They have a greater range of biochemical reactions than those found in their larger relatives, the eukaryotic cells. The extraordinary biochemical diversity of prokaryotic cells is manifested in the wide-ranging lifestyles of the archaebacteria and the bacteria, whose habitats include polar ice, deserts, and hydrothermal vents - deep regions of the ocean under great pressure where hot water geysers erupt from cracks in the ocean floor.
An animal cell typically contains several types of membrane-bound organs, or organelles. The nucleus directs activities of the cell and carries genetic information from generation to generation. The mitochondria generates energy for the cell. Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free in the cytoplasm. The Golgi apparatus modifies, packages, and distributes proteins while lysosomes store enzymes for digesting food. The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the cytoplasm.
Eukaryotic cells are typically about ten times larger than prokaryotic cells. In animal cells, the plasma membrane, rather than a cell wall, forms the cell’s outer boundary. With a design similar to the plasma membrane of prokaryotic cells, it separates the cell from its surroundings and regulates the traffic across the membrane.
The eukaryotic cell cytoplasm is similar to that of the prokaryote cell except for one major difference: Eukaryotic cells house a nucleus and numerous other membrane-enclosed organelles. Like separate rooms of a house, these organelles enable specialized functions to be carried out efficiently. The building of proteins and lipids, for example, takes place in separate organelles where specialized enzymes geared for each job are located.
The plasma membrane that surrounds eukaryotic cells is a dynamic structure composed of two layers of phospholipid molecules interspersed with cholesterol and proteins. Phospholipids are composed of a hydrophilic, or water-loving, head and two tails, which are hydrophobic, or water-hating. The two phospholipid layers face each other in the membrane, with the heads directed outward and the tails pointing inward. The water-attracting heads anchor the membrane to the cytoplasm, the watery fluid inside the cell, and also to the water surrounding the cell. The water-hating tails block large water-soluble molecules from passing through the membrane while permitting fat-soluble molecules, including medications such as tranquilizers and sleeping pills, to freely cross the membrane. Proteins embedded in the plasma membrane carry out a variety of functions, including transport of large water soluble molecules such as sugars and certain amino acids. Glycoproteins, proteins bonded to carbohydrates, serve in part to identify the cell as belonging to a unique organism, enabling the immune system to detect foreign cells, such as invading bacteria, which carry different glycoproteins. Cholesterol molecules in the plasma membrane act as stabilizers that limit the movement of the two slippery phospholipids layer, which slide back and forth in the membrane. Tiny gaps in the membrane enable small molecules such as oxygen to diffuse readily into and out of the cell. Since cells constantly use up oxygen, decreasing its concentration within the cell, the higher concentration of oxygen outside the cell causes a net flow of oxygen into the cell. The steady stream of oxygen into the cell enables it to carry out aerobic respiration continually, a process that provides the cell with the energy needed to carry out its functions.
The nucleus is the largest organelle in an animal cell. It contains numerous strands of DNA, the length of each strand being many times the diameter of the cell. Unlike the circular prokaryotic DNA, long sectors of eukaryotic DNA pack into the nucleus by wrapping around proteins. As a cell begins to divide, each DNA strand folds over onto itself several times, forming a rod-shaped chromosome.
The nucleus is surrounded by a double-layered membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. Messages pass between the cytoplasm and the nucleus through nuclear pores, which are holes in the membrane of the nucleus. In each nuclear pore, molecular signals flash back and forth as often as ten times per second. For example, a signal to activate a specific gene comes into the nucleus and instructions for production of the necessary protein go out to the cytoplasm.
The nucleus, present in eukaryotic cells, is a discrete structure containing chromosomes, which hold the genetic information for the cell. Separated from the cytoplasm of the cell by a double-layered membrane called the nuclear envelope, and the nucleus contains a cellular material called nucleoplasm. Nuclear pores, present around the circumference of the nuclear membrane, allow the exchange of cellular materials between the nucleoplasm and the cytoplasm.
Attached to the nuclear membrane is an elongated membranous sac called the endoplasmic reticulum. This organelle tunnels through the cytoplasm, folding back and forth on itself to form a series of membranous stacks. Endoplasmic reticulums take two forms: Rough and smooth. A rough endoplasmic reticulum (RER) is so called because it appears bumpy under a microscope. The bumps are actually thousands of ribosomes attached to the membrane’s surface. The ribosomes in eukaryotic cells have the same function as those in prokaryotic cells - protein synthesis - but they differ slightly in structure. Eukaryote ribosomes bound to the endoplasmic reticulum help assemble proteins that typically are exported from the cell. The ribosomes work with other molecules to link amino acids to partially completed proteins. These incomplete proteins then travel to the inner chamber of the endoplasmic reticulum, where chemical modifications, such as the addition of a sugar, are carried out. Chemical modifications of lipids are also carried out in the endoplasmic reticulum.
The endoplasmic reticulum and its bound ribosomes are particularly dense in cells that produce many proteins for export, such as the white blood cells of the immune system, which produce and secrete antibodies. Some ribosomes that manufacture proteins are not attached to the endoplasmic reticulum. These so-called free ribosomes are dispersed in the cytoplasm and typically make proteins - many of them enzymes - that remain in the cell.
The second form of an endoplasmic reticulum, the smooth endoplasmic reticulum (SER), lacks ribosomes and has an even surface. Within the winding channels of the smooth endoplasmic reticulum are the enzymes needed for the construction of molecules such as carbohydrates and lipids. The smooth endoplasmic reticulum is prominent in liver cells, where it also serves to detoxify substances such as alcohol, drugs, and other poisons.
Proteins are transported from free and bound ribosomes to the Golgi apparatus, an organelle that resembles a stack of deflated balloons. It is packed with enzymes that complete the processing of proteins. These enzymes add sulfur or phosphorus atoms to certain regions of the protein, for example, or chop off tiny pieces from the ends of the proteins. The completed protein then leaves the Golgi apparatus for its final destination inside or outside the cell. During its assembly on the ribosome, each protein has acquired a group of from 4 to 100 amino acids called a signal. The signal works as a molecular shipping label to direct the protein to its proper location.
Lysosomes are small, often spherical organelles that function as the cell’s recycling center and garbage disposal. Powerful digestive enzymes concentrated in the lysosome break down worn-out organelles and ship their building blocks to the cytoplasm where they are used to construct new organelles. Lysosomes also dismantle and recycle proteins, lipids, and other molecules.
The mitochondria is the powerhouse of the cell. Within these long, slender organelles, which can appear oval or bean shaped under the electron microscope, enzymes convert the sugar glucose and other nutrients into adenosine triphosphate (ATP). This molecule, in turn, serves as an energy battery for countless cellular processes, including the shuttling of substances across the plasma membrane, the building and transport of proteins and lipids, the recycling of molecules and organelles, and the dividing of cells. Muscle and liver cells are particularly active and require dozens and sometimes up to hundreds mitochondria per cell to meet their energy needs. Mitochondria is unusual in that they contain their own DNA in the form of a prokaryote-like circular chromosome; Have their own ribosomes, which resemble prokaryotic ribosomes, and divide independently of the cell.
Unlike the tiny prokaryotic cell, the relatively large eukaryotic cell requires structural support. The cytoskeleton, a dynamic network of protein tubes, filaments, and fibers, crisscrosses the cytoplasm, anchoring the organelles in place and providing shape and structure to the cell. Many components of the cytoskeleton are assembled and disassembled by the cell as needed. During cell division, for example, a special structure called a spindle is built to move chromosomes around. After cell division, the spindle, no longer needed, is dismantled. Some components of the cytoskeleton serve as microscopic tracks along which proteins and other molecules travel like miniature trains. Recent research suggests that the cytoskeleton also may be a mechanical communication structure that converses with the nucleus to help organize events in the cell.
Plant cells have all the components of animal cells and boast several added features, including chromoplasts, a central vacuole, and a cell wall. Chromoplasts convert light energy - typically from the Sun - into the sugar glucose, a form of chemical energy, in a process known as photosynthesis. Chromoplasts, like mitochondria, possess a circular chromosome and prokaryote-like ribosomes, which manufacture the proteins that the chloroplasts typically need.
The central vacuole of a mature plant cell typically takes up most of the room in the cell. The vacuole, a membranous bag, crowds the cytoplasm and organelles to the edges of the cell. The central vacuole stores water, salts, sugars, proteins, and other nutrients. In addition, it stores the blue, red, and purple pigments that give certain flowers their colors. The central vacuole also contains plant wastes that taste bitter to certain insects, thus discouraging the insects from feasting on the plant.
In plant cells, a sturdy cell wall surrounds and protects the plasma membrane. Its pores enable materials to pass freely into and out of the cell. The strength of the wall also enables a cell to absorb water into the central vacuole and swell without bursting. The resulting pressure in the cells provides plants with rigidity and support for stems, leaves, and flowers. Without sufficient water pressure, the cells collapse and the plant wilts.
To stay alive, cells must be able to carry out a variety of functions. Some cells must be able to move, and most cells must be able to divide. All cells must maintain the right concentration of chemicals in their cytoplasm, ingest food and use it for energy, recycle molecules, expel wastes, and construct proteins. Cells must also be able to respond to changes in their environment.
Although many forms of bacteria are not capable of independent movement, species such as the Salmonella bacterium pictured here can move by means of fine threadlike projections called flagella. The arrangement of flagella across the surface of the bacterium differs from species to species; they can be present at the ends of the bacterium or all across the body surface. Forward movement is accomplished either by a tumbling motion or in a forward manner without tumbling.
Many unicellular organisms swim, glide, thrash, or crawl to search for food and escape enemies. Swimming organisms often move by means of a flagellum, a long tail-like structure made of protein. Many bacteria, for example, have one, two, or many flagella that rotate like propellers to drive the organism along. Some single-celled eukaryotic organisms, such as the euglena, also have a flagellum, but it is longer and thicker than the prokaryotic flagellum. The eukaryotic flagellums work by waving up and down like a whip. In higher animals, the sperm cell uses a flagellum to swim toward the female egg for fertilization.
Movement in eukaryotes is also accomplished with cilia, short, hairlike proteins built by centrioles, which are barrel-shaped structures located in the cytoplasm that assemble and break down protein filaments. Typically, thousands of cilia extend through the plasma membrane and cover the surface of the cell, giving it a dense, hairy appearance. By beating its cilia as if they were oars, an organism such as the paramecium propels itself through its watery environment. In cells that do not move, cilia are used for other purposes. In the respiratory tract of humans, for example, millions of ciliated cells prevent inhaled dust, smog, and microorganisms from entering the lungs by sweeping them up on a current of mucus into the throat, where they are swallowed. Eukaryotic flagella and cilia are formed from basal bodies, small protein structures located just inside the plasma membrane. Basal bodies also help to anchor flagella and cilia.
Still other eukaryotic cells, such as amoebas and white blood cells, move by amoeboid motion, or crawling. They extrude their cytoplasm to form temporary pseudopodia, or false feet, which actually are placed in front of the cell, rather like extended arms. They then drag the trailing end of their cytoplasm up to the pseudopodia. A cell using amoeboid motion would lose a race to a euglena or paramecium. But while it is slow, amoeboid motion is strong enough to move cells against a current, enabling water-dwelling organisms to pursue and devour prey, for example, or white blood cells roaming the blood stream to stalk and engulf a bacterium or virus.
An amoeba, a single-celled organism lacking internal organs, is shown approaching a much smaller paramecium, which it begins to engulf with large outflowings of its cytoplasm, called pseudopodia. Once the paramecium is completely engulfed, a primitive digestive cavity, called a vacuole, forms around it. In the vacuole, acids break the paramecium down into chemicals that the amoeba can diffuse back into its cytoplasm for nourishment.
All cells require nutrients for energy, and they display a variety of methods for ingesting them. Simple nutrients dissolved in pond water, for example, can be carried through the plasma membrane of pond-dwelling organisms via a series of molecular pumps. In humans, the cavity of the small intestine contains the nutrients from digested food, and cells that form the walls of the intestine use similar pumps to pull amino acids and other nutrients from the cavity into the bloodstream. Certain unicellular organisms, such as amoebas, are also capable of reaching out and grabbing food. They used a process known as endocytosis, in which the plasma membrane surrounds and engulfed the food particle, enclosing it in a sac, called a vesicle, that is within the amoeba’s interior.
Cells require energy for a variety of functions, including moving, building up and breaking down molecules, and transporting substances across the plasma membrane. Nutrients contain energy, but cells must convert the energy locked in nutrients to another form - specifically, the ATP molecule, the cell’s energy battery - before it is useful. In single-celled eukaryotic organisms, such as the paramecium, and in multicellular eukaryotic organisms, such as plants, animals, and fungi, mitochondria is responsible for this task. The interior of each mitochondrion consists of an inner membrane that is folded into a mazelike arrangement of separate compartments called cristae. Within the cristae, enzymes form an assembly line where the energy in glucose and other energy-rich nutrients is harnessed to build ATP; thousands of ATP molecules are constructed each second in a typical cell. In most eukaryotic cells, this process requires oxygen and is known as aerobic respiration.
Some prokaryotic organisms also carry out aerobic respiration. They lack mitochondria, however, and carry out aerobic respiration in the cytoplasm with the help of enzymes sequestered there. Many prokaryote species live in environments where there is little or no oxygen, environments such as mud, stagnant ponds, or within the intestines of animals. Some of these organisms produce ATP without oxygen in a process known as anaerobic respiration, where sulfur or other substances take the place of oxygen. Still other prokaryotes, and yeast, a single-celled eukaryote, build ATP without oxygen in a process known as fermentation.
Almost all organisms rely on the sugar glucose to produce ATP. Glucose is made by the process of photosynthesis, in which light energy is transformed to the chemical energy of glucose. Animals and fungi cannot carry out photosynthesis and depend on plants and other photosynthetic organisms for this task. In plants, as we have seen, photosynthesis takes place in organelles called chloroplasts. Chloroplasts contain numerous internal compartments called thylakoids where enzymes aid in the energy conversion process. A single leaf cell contains 40 to 50 chloroplasts. With sufficient sunlight, one large tree is capable of producing upwards of two tons of sugar in a single day. Photosynthesis in prokaryotic organisms - typically aquatic bacteria - is carried out with enzymes clustered in plasma membrane folds called chromatophores. Aquatic bacteria produce the food consumed by tiny organisms living in ponds, rivers, lakes, and seas.
A typical cell must have on hand, about. 30,000 proteins at any-one time. Many of these proteins are enzymes needed to construct the major molecules used by cells - carbohydrates, lipids, proteins, and nucleic acids - nor to aid in the breakdown of such molecules after they have worn out. Other proteins are part of the cell’s structure - the plasma membrane and ribosomes, for example. In animals, proteins also function as hormones and antibodies, and they function like delivery trucks to transport other molecules around the body. Hemoglobin, for example, is a protein that transports oxygen in red blood cells. The cell’s demand for proteins never ceases.
Before a protein can be made, however, the molecular directions to build, it must be extracted from one or more genes. In humans, for example, one gene holds the information for the protein insulin, the hormone that cells need to import glucose from the bloodstream, while at least two genes hold the information for collagen, the protein that imparts strength to skin, tendons, and ligaments. The process of building proteins begins when enzymes, in response to a signal from the cell, bind to the gene that carries the code for the required protein, or part of the protein. The enzymes transfer the code to a new molecule called messenger RNA, which carries the code from the nucleus to the cytoplasm. This enables the original genetic code to remain safe in the nucleus, with messenger RNA delivering small bits and pieces of information from the DNA to the cytoplasm as needed. Depending on the cell type, hundreds or even thousands of molecules of messenger RNA are produced each minute.
Once in the cytoplasm, the messenger RNA molecule links up with a ribosome. The ribosome moves along the messenger RNA like a monorail car along a track, stimulating another form of RNA - transfer RNA - to gather and link the necessary amino acids, pooled in the cytoplasm, to form the specific protein, or section of protein. The protein is modified as necessary by the endoplasmic reticulum and Golgi apparatus before embarking on its mission. Cells teem with activity as they forge the numerous, diverse proteins that are indispensable for life. For a more detailed discussion about protein synthesis, When there are a hundred or more cells, they formed a hollow ball of cells, called a blastula, surrounding a fluid-filled cavity. Later divisions produce three layers of cells - endoderm (inner), mesoderm (middle), and ectoderm (outer) - from which the principal features of the animal will differentiate.
Most cells divide at some time during their life cycle, and some divide dozens of times before they die. Organisms rely on cell division for reproduction, growth, and repair and replacement of damaged or worn out cells. Three types of cell division occur: Binary fission, mitosis, and meiosis. Binary fission, the method used by prokaryotes, produces two identical cells from one cell. The more complex process of mitosis, which also produces two genetically identical cells from a single cell, is used by many unicellular eukaryotic organisms for reproduction. Multicellular organisms use mitosis for growth, cell repair, and cell replacement. In the human body, for example, an estimated 25 million mitotic cell divisions occur every second in order to replace cells that have completed their normal life cycles. Cells of the liver, intestine, and skin may be replaced every few days. Recent research indicates that even brain cell, once thought to be incapable of mitosis, undergo cell division in the part of the brain associated with memory.
In a landmark intersection of science and fiction, cloning leapt from the world’s imagination to its front page in February 1997. It arrived in the innocent form of a sheep named Dolly: The first exact genetic duplicate of an adult mammal due to genetic engineering. Scottish scientists had created Dolly from deoxyribonucleic acid (DNA) - the basic unit of heredity - taken from a single adult sheep cell. The accomplishment threw open the door to profoundly ethical as well as scientific controversy over the potential uses and abuses of cloning. ‘However the debate is resolved,’ wrote Los Angeles Times science reporter Thomas H. Maugh II, ‘the genie is irretrievably out of the bottle.’
The type of cell division required for sexual reproduction is meiosis. Sexually reproducing organisms include seaweeds, fungi, plants, and animals - including, of course, human beings. Meiosis differs from mitosis in that cell division begins with a cell that has a full complement of chromosomes and ends with gamete cells, such as sperm and eggs, that have only half the complement of chromosomes. When a sperm and egg unite during fertilization, the cell resulting from the union, called a zygote, contains the full number of chromosomes.
The story of how cells evolved remains an open and actively investigated question in science. The combined expertise of physicists, geologists, chemists, and evolutionary biologists has been required to shed light on the evolution of cells from the nonliving matter of early Earth. The planet formed about 4.5 billion years ago, and for millions of years, violent volcanic eruptions blasted substances such as carbon dioxide, nitrogen, water, and other small molecules into the air. These small molecules, bombarded by ultraviolet radiation and lightning from intense storms, collided to form the stable chemical bonds of larger molecules, such as amino acids and nucleotides - the building blocks of proteins and nucleic acids. Experiments indicate that these larger molecules form spontaneously under laboratory conditions that simulate the probable early environment of Earth.
Scientists speculate that rain may have carried these molecules into lakes to create a primordial soup - the breeding ground for the assembly of proteins, the nucleic acid RNA, and lipids. Some scientists postulate that these more complex molecules formed in hydrothermal vents rather than in lakes. Other scientists propose that these key substances may have reached Earth on meteorites from outer space. Regardless of the origin or environment, however, scientists do agree that proteins, nucleic acids, and lipids provided the raw materials for the first cells. In the laboratory, scientists have observed lipid molecules joining to form spheres that resemble a cell’s plasma membrane. As a result of these observations, scientists postulate that millions of years of molecular collisions resulted in lipid spheres enclosing RNA, the simplest molecule capable of self-replication. These primitive aggregations would have been the ancestors of the first prokaryotic cells.
Fossil studies indicate that Cyanobacteria, bacteria capable of photosynthesis, were among the earliest bacteria to evolve, an estimated 3.4 billion to 3.5 billion years ago. In the environment of the early Earth, there were no oxygen, and cyanobacteria probably used fermentation to produce ATP. Over the eons, cyanobacteria performed photosynthesis, which produces oxygen as a byproduct; The result was the gradual accumulation of oxygen in the atmosphere. The presence of oxygen set the stage for the evolution of bacteria that used oxygen in aerobic respiration, a more efficient ATP-producing process than fermentation. Some molecular studies of the evolution of genes in archaebacteria suggest that these organisms may have evolved in the hot waters of hydrothermal vents or hot springs slightly earlier than cyanobacteria, around 3.5 billion years ago. Like cyanobacteria, archaebacteria probably relied on fermentation to synthesize ATP.
Eukaryotic cells may have evolved from primitive prokaryotes about 2 billion years ago. One hypothesis suggests that some prokaryotic cells lost their cell walls, permitting the cell’s plasma membrane to expand and fold. These folds, ultimately, may have given rise to separate compartments within the cell - the forerunners of the nucleus and other organelles now found in eukaryotic cells. Another key hypothesis is known as endosymbiosis. Molecular studies of the bacteria-like DNA and ribosomes in mitochondria and chloroplasts indicate that mitochondrion and chloroplast ancestors were once free-living bacteria. Scientists propose that these free-living bacteria were engulfed and maintained by other prokaryotic cells for their ability to produce ATP efficiently and to provide a steady supply of glucose. Over generations, eukaryotic cells situated with mitochondria - the ancestors of animals - or with both mitochondria and chloroplasts - the ancestors of plants - evolved.
The first observations of cells were made in 1665 by English scientist Robert Hooke, who used a crude microscope of his own invention to examine a variety of objects, including a thin piece of cork. Noting the rows of tiny boxes that made up the dead wood’s tissue, Hooke coined the term cell because the boxes reminded him of the small cells occupied by monks in a monastery. While Hooke was the first to observe and describe cells, he did not comprehend their significance. At about the same time, the Dutch maker of microscopes Antoni van Leeuwenhoek pioneered the invention of one of the best microscopes of the time. Using his invention, Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva, one-celled organisms cavorting in pond water, and sperm swimming in semen. Two centuries passed, however, before scientists grasped the true importance of cells.
Many advances have been made in microscope technology. This article from the 1994 Collier’s Year Book begins with the microscope most young students are familiar with and tracks the breakthroughs in the development of new types of microscopes - including those that use ultrasonic imaging and those that ‘feel’ an object’s surface.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. Working together, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between plant and animal cells. In 1839 they proposed the revolutionary idea that all living things are made up of cells. Their theory gave rise to modern biology: a whole new way of seeing and investigating the natural world.
By the late 1800s, as light microscopes improved still further, scientists were able to observe chromosomes within the cell. Their research was aided by new techniques for staining parts of the cell, which made possible the first detailed observations of cell division, including observations of the differences between mitosis and meiosis in the 1880s. In the first few decades of the 20th century, many scientists focused on the behavior of chromosomes during cell division. At that time, it was generally held that mitochondria transmitted the hereditary information. By 1920, however, scientists determined that chromosomes carry genes and that genes transmit hereditary information from generation to generation.
During this period, scientists began to understand some of the chemical processes in cells. In the 1920s, the ultracentrifuge was developed. The ultracentrifuge is an instrument that spins cells or other substances in test tubes at high speeds, which causes the heavier parts of the substance to fall to the bottom of the test tube. This instrument enabled scientists to separate the relatively abundant and heavy mitochondria from the rest of the cell and study their chemical reactions. By the late 1940s, scientists were able to explain the role of mitochondria in the cell. Using refined techniques with the ultracentrifuge, scientists subsequently isolated the smaller organelles and gained an understanding of their functions.
The deoxyribonucleic acid (DNA) molecule is the genetic blueprint for each cell and ultimately the blueprint that determines every characteristic of a living organism. In 1953 American biochemist James Watson, left, and British biophysicist Francis Crick, right, described the structure of the DNA molecule as a double helix, somewhat like a spiral staircase with many individual steps. Their work was aided by X-ray diffraction pictures of the DNA molecule taken by British biophysicist Maurice Wilkins and British physical chemist Rosalind Franklin. In 1962 Crick, Watson, and Wilkins received the Nobel Prize for their pioneering work on the structure of the DNA molecule.
While some scientists were studying the functions of cells, others were examining details of their structure. They were aided by a crucial technological development in the 1940s, the invention of the electron microscope, which uses high-energy electrons instead of light waves to view specimens. New generations of electron microscopes have provided resolution, or the differentiation of separate objects, thousands of times more powerful than that available in light microscopes. This powerful resolution revealed organelles such as the endoplasmic reticulum, lysosomes, the Golgi apparatus, and the cytoskeleton. The scientific fields of cell structure and function continue to complement each other as scientists explore the enormous complexity of cells.
The discovery of the structure of DNA in 1953 by American biochemist James D. Watson and British biophysicist Francis Crick ushered in the era of molecular biology. Today, investigation inside the world of cells - of genes and proteins at the molecular level - constitutes one of the largest and fastest moving areas in all of science. One particularly active field in recent years has been the investigation of cell signaling, the process by which molecular messages find their way into the cell via a series of complex protein pathways in the cell.
Another busy area in cell biology concerns programmed cell death, or apoptosis. Millions of times per second in the human body, cells commit suicide as an essential part of the normal cycle of cellular replacement. This also seems to be a check against disease: When mutations build up within a cell, the cell will usually self-destruct. If this fails to occur, the cell may divide and give rise to mutated daughter cells, which continue to divide and spread, gradually forming a growth called a tumor. This unregulated growth by rogue cells can be benign, or harmless, or cancerous, which may threaten healthy tissue. The study of apoptosis is one avenue that scientists explore in an effort to understand how cells become cancerous.
Scientists are also discovering exciting aspects of the physical forces within cells. Cells employ a form of architecture called tensegrity, which enables them to withstand battering by a variety of mechanical stresses, such as the pressure of blood flowing around cells or the movement of organelles within the cell. Tensegrity stabilizes cells by evenly distributing mechanical stresses to the cytoskeleton and other cell components. Tensegrity also may explain how a change in the cytoskeleton, where certain enzymes are anchored, initiates biochemical reactions within the cell, and can even influence the action of genes. The mechanical rules of tensegrity may also account for the assembly of molecules into the first cells. Such new insights - made some 300 years after the tiny universe of cells was first glimpsed - show that cells continue to yield fascinating new worlds of discovery.
The Nervous System signifies of those elements within the animal organism that are concerned with the reception of stimuli, the transmission of nerve impulses, or the activation of muscle mechanisms.
The reception of stimuli is the function of special sensory cells. The conducting elements of the nervous system are cells called neurons; these may be capable of only slow and generalized activity, or they may be highly efficient and rapidly conducting units. The specific response of the neuron—the nerve impulse - and the capacities of the cell to be stimulated make this cell a receiving and transmitting unit capable of transferring information from one part of the body to another.
Each nerve cell consists of a central portion containing the nucleus, known as the cell body, and one or more structures referred to as axons and dendrites. The dendrites are rather short extensions of the cell body and are involved in the reception of stimuli. The axon, by contrast, is usually a single elongated extension, it is especially important in the transmission of nerve impulses from the region of the cell body to other cells.
Although all many-celled animals have some kind of nervous system, the complexity of its organization varies considerably among different animal types. In simple animals such as jellyfish, the nerve cells form a network capable of mediating only a relatively stereotyped response. In more complex animals, such as shellfish, insects, and spiders, the nervous system is more complicated. The cell bodies of neurons are organized in clusters called ganglia. These clusters are interconnected by the neuronal processes to form a ganglionated chain. Such chains are found in all vertebrates, in which they represent a special part of the nervous system, related especially to the regulation of the activities of the heart, the glands, and the involuntary Vertebrate animals have a bony spine and skull in which the central part of the nervous system is housed; The peripheral part extends throughout the remainder of the body. That part of the nervous system located in the skull is referred to as the brain that found in the spine is called the spinal cord. The brain and the spinal cord are continuous through an opening in the base of the skull; Both are also in contact with other parts of the body through the nerves. The distinction made between the central nervous system and the peripheral nervous system is based on the different locations of the two intimately related parts of a single system. Some of the processes of the cell bodies conduct sense impressions and others conduct muscle responses, called reflexes, such as those caused by pain.
In the skin are cells of several types called receptors; each is especially sensitive to particular stimuli. Free nerve endings are sensitive to pain and are directly activated. The neurons so activated send impulses into the central nervous system and have junctions with other cells that have axons extending back into the periphery. Impulses are carried from processes of these cells to motor endings within the muscles. These neuromuscular endings excite the muscles, resulting in muscular contraction and appropriate movement. The pathway taken by the nerve impulse in mediating this simple response is in the form of a two-neuron arc that begins and ends in the periphery. Many of the actions of the nervous system can be explained on the basis of such reflex arcs, which are chains of interconnected nerve cells, stimulated at one end and capable of bringing about movement or glandular secretion at the other.
The cranial nerves connect to the brain by passing through openings in the skull, or cranium. Nerves associated with the spinal cord pass through openings in the vertebral column and are called spinal nerves. Both cranial and spinal nerves consist of large numbers of processes that convey impulses to the central nervous system and also carry messages outward; the former processes are called afferent, and the latter are called efferent. Afferent impulses are referred to as sensory; efferent impulses are referred to as either somatic or visceral motor, according to what part of the body they reach. Most nerves are mixed nerves made up of both sensory and motor elements.
The cranial and spinal nerves are paired; The number in humans are 12 and 31, respectively. Cranial nerves are distributed to the head and neck regions of the body, with one conspicuous exception: the tenth cranial nerve, called the vagus. In addition to supplying structures in the neck, the vagus is distributed to structures located in the chest and abdomen. Vision, auditory and vestibular sensation, and taste is mediated by the second, eighth, and seventh cranial nerves, respectively. Cranial nerves also mediate motor functions of the head, the eyes, the face, the tongue, and the larynx, as well as the muscles that function in chewing and swallowing. Spinal nerves, after they exit from the vertebrae, are distributed in a band-like fashion to regions of the trunk and to the limbs. They interconnect extensively, thereby forming the brachial plexus, which runs to the upper extremities, and the lumbar plexus, which passes to the lower limbs.
Among the motor’s fibers may be found groups that carry impulses to viscera. These fibers are designated by the special name of autonomic nervous system. That system consists of two divisions, more or less antagonistic in function, that emerge from the central nervous system at different points of origin. One division, the sympathetic, arises from the middle portion of the spinal cord, joins the sympathetic ganglionated chain, courses through the spinal nerves, and is widely distributed throughout the body. The other division, the parasympathetic, arises both above and below the sympathetic, that is, from the brain and from the lower part of the spinal cord. These two divisions control the functions of the respiratory, circulatory, digestive, and urogenital systems.
Consideration of disorders of the nervous system is the province of neurology; Psychiatry deals with behavioral disturbances of a functional nature. The division between these two medical specialties cannot be sharply defined, because neurological disorders often manifest both organic and mental symptoms.
Diseases of the nervous system include genetic malformations, poisonings, metabolic defects, vascular disorders, inflammations, degeneration, and tumors, and they involve either nerve cells or their supporting elements. Vascular disorders, such as cerebral hemorrhage or other forms of a stroke, are among the most common causes of paralysis and other neurologic complications. Some diseases exhibit peculiar geographic and age distribution. In temperate zones, multiple sclerosis is a common degenerative disease of the nervous system, but it is rare in the Tropics.
The nervous system is subject to infection by a great variety of bacteria, parasites, and viruses. For example, meningitis, or infection of the meninges investing the brain and spinal cord, can be caused by many different agents. On the other hand, one specific virus causes rabies. Some viruses causing neurological ills effect only certain parts of the nervous system. For example, the virus causing poliomyelitis commonly affects the spinal cord, as Viruses manufacturing encephalitis attack the brain.
Inflammations of the nervous system are named according to the part affected. Myelitis is an inflammation of the spinal cord; Neuritis is an inflammation of a nerve. It may be caused not only by infection but also by poisoning, alcoholism, or injury. Tumors originating in the nervous system usually are composed of meningeal tissue or neuroglia (supporting tissue) cells, depending on the specific part of the nervous system affected, but other types of a tumor may metastasize to or invade the nervous system. In certain disorders of the nervous system, such as neuralgia, migraine, and epilepsy, no evidence may exist of organic damage. Another disorder, cerebral palsy, is associated with birth defects.
Pain, is an unpleasant sensory or emotional experience caused by real or potential injury or damage to the body or described in terms of such damage. Scientists believe that pain evolved in the animal kingdom as a valuable three-part warning system. First, it warns of injury. Second, pain protects against further injury by causing a reflexive withdrawal from the source of injury. Finally, pain leads to a period of reduced activity, enabling injuries to heal more efficiently.
Pain is difficult to measure in humans because it has an emotional, or psychological component as well as a physical component. Some people express extreme discomfort from relatively small injuries, while others show little or no pain even after suffering severe injury. Sometimes pain is present even though no injury is apparent at all, or pain lingers long after an injury appears to have healed.
The signals that warn the body of tissue damage are transmitted through the nervous system. In this system, the basic unit is the nerve cell or neuron. A nerve cell is composed of three parts: a central cell body, a single major branching fiber called an axon, and a series of smaller branching fibers known as dendrites. Each nerve cell meets other nerve cells at certain points on the axons and dendrites, forming a dense network of interconnected nerve fibers that transmit sensory information about touch, pressure, or warmth, as well as pain.
Sensory information is transmitted from the different parts of the body to the brain via the spinal cord, which is a complex set of nerves that extend from the brain down along the back, protected by the bones of the spine. About as wide as a finger, the spinal cord is like a cable packed with many bundles of wires. The bundles are nerve pathways for transmitting information. But the spinal cord is more than just a message transmitter, it is also an extension of the brain. It contains neurons that process incoming sensory information, and generates messages to be sent back down to cells in other parts of the body.
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to ‘fire,’ or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Information being transmitted between and within the brain and spinal cord travels through the nervous system using both chemical and electrical mechanisms. A message-carrying impulse travels from one end of a nerve cell to another by means of an electric signal. When the electric signal reaches the terminal end of a nerve cell, a gap called a synapse prevents the electric signal from crossing to the next cell. The electric signal triggers the cell to release chemicals called neurotransmitters, which float across the synapse to the neighboring nerve cell. These neurotransmitters fit into specialized receptors found on the adjacent nerve cell, much as a key fits into a lock, generating an electric impulse in the neighboring cell. This new impulse travels to the end of the long cell, in turn triggering the release of neurotransmitters to carry the message across the next synapse. Not all neurotransmitters initiate a message in a neighboring nerve cell. Some specialize in preventing neighboring cells from generating an electrical signal, while others function as helpers, facilitating the message's journey to the brain.
While most of the sensory nerves in the skin and other body tissues have special structures covering their nerve endings, those nerves that signal injury have free nerve endings. These simple nerve endings specialize in detecting noxious stimuli - a catchall term for injury-causing stimuli such as intense heat, extreme pressure, or sharp pricks or cuts. The nerve endings that detect pain are called nociceptors, and the process of transmitting pain signals when harmful stimulation occurs is called nociception. Several million nociceptors are interlaced through the tissues and organs of the body.
When a person experiences an injury, such as a stubbed toe, specialized cells called nociceptors sense potential tissue damage (1) and send an electric signal, called an impulse, to the spinal cord via a sensory nerve (2). A specialized region of the spinal cord known as the dorsal horn (3) processes the pain signal, immediately sending another impulse back down the leg via a motor nerve (4). This causes the muscles in the leg to contract and pull the toe away from the source of injury (6). At the same time, the dorsal horn sends another impulse up the spinal cord to the brain. During this trip, the impulse travels between nerve cells. When the impulse reaches a nerve ending (7), the nerve released chemical messengers, called neurotransmitters, which carry the message to the adjacent nerve. When the impulse reaches the brain (8), it is analyzed and processed as an unpleasant physical and emotional sensation.
An injury triggers pain signals in two types of nociceptors, one with large, insulated axons known as A-delta fibers and one with small, uninsulated axons known as C fibers. The large A-delta fibers conduct signals quickly, and the smaller C fibers transmit information slowly. The difference in the functions of these two fibers becomes obvious to a person who stubs a toe. At first the injured person is aware of a sharp, flashing pain at the point of injury. Generated by the A-delta fibers, this short-lived pain intrudes upon the thoughts and perceptions occurring in the brain. Just as this first pain subsides, a second pain begins that is vague, throbbing, and persistent. This sensation is derived from the C fibers.
Pain information from the A-delta and C fibers travels through the spinal cord to the brain. When it receives the pain message, the spinal cord generates impulses that travel back down to muscles, which lead to a reflexive contraction that pulls the body away from the source of injury. Other reflexes may affect skin temperature, blood flow, sweating, and other changes.
While this reflex action is underway, the pain message continues up the spinal cord to relay centers in the brain. The sensory information is routed to many other parts of the brain, including the cortex, where thinking processes occur
The Adrenal Gland is the vital endocrine gland that secretes hormones into the bloodstream, situated, in humans, on top of the upper end of each kidney. The two parts of the gland - the inner portion, or medulla, and the outer portion, or the cortex - are like separate organs: They are composed of different types of tissue and perform different functions. The adrenal medulla, composed of chromaffin cells secretes the hormone epinephrine, also called adrenaline, in response to stimulation of the sympathetic nervous system at times of stress. The medulla also secretes the hormone norepinephrine, which plays a role in maintaining normal blood circulation. The hormones of the medulla are called catecholamines. Unlike the adrenal cortex, the medulla can be removed without endangering the life of an individual.
The adrenal outer layer, or cortex, secretes about 30 steroid hormones, but only a few are secreted in significant amounts. Aldosterone, one of the most important hormones, regulates the balance of salt and water in the body. Cortisone and hydrocortisone are necessary to regulate fat, carbohydrate, and protein metabolism. Adrenal sex steroids have a minor influence on the reproductive system. Modified steroids, now produced synthetically, are superior to naturally secreted steroids for treatment of Addison's disease and other disorders.
Adrenocorticotropic Hormone is also known as corticotropin, hormones secreted by the anterior part of the pituitary gland. The specific function of ACTH is to stimulate the growth and secretions of the cortex (outer layers) of the adrenal gland. One of these secretions is cortisone, a hormone involved in carbohydrate and protein metabolisms. ACTH is used medically for its anti-inflammatory action to alleviate symptoms of allergies and arthritis. ACTH is a complex protein molecule containing 39 amino acids. Its molecular weight is approximately 5000. The biological activity of the ACTH of various animal species is similar to that of humans, but the sequence of amino acids has been found to vary somewhat among species. ACTH production is controlled in part by the hypothalamus and in part by the existing levels of adrenal gland hormones. ACTH levels increased in response to stress, disease, and decreased blood pressure.
The Pituitary Gland is the master endocrine gland in vertebrate animals. The hormones secreted by the pituitary stimulate and control the functioning of almost all the other endocrine glands in the body. Pituitary hormones also promote growth and control the water balance of the body.
The pituitary is a small bean-shaped, reddish-gray organ located in the saddle-shaped depression (sella turcica) in the floor of the skull (the sphenoid bone) and attached to the base of the brain by a stalk; it is located near the hypothalamus. The pituitary has two lobes - the anterior lobe, or adenohypophysis, and the posterior lobe, or neurohypophysis - which differ in structure and function. The anterior lobe is derived embryologically from the roof of the pharynx and is composed of groups of epithelial cells separated by blood channels; the posterior lobe is derived from the base of the brain and is composed of nervous connective tissue and nerve-like secreting cells. The area between the anterior and posterior lobes of the pituitary is called the intermediate lobe; it has the same embryological origin as the anterior lobe.
Concentrated chemical substances, or hormones, which control 10 to 12 functions in the body, have been obtained as extracts from the anterior pituitary glands of cattle, sheep, and swine. Eight hormones have been isolated, purified, and identified; All of them are peptides, that is, they are composed of amino acids. A growth hormone (GH), or the somatotropic hormone (STH), is essential for normal skeletal growth and is neutralized during adolescence by the gonadal sex hormones. Thyroid-stimulating hormones (TSH) control the normal functioning of the thyroid gland, and the adrenocorticotropic hormone (ACTH) controls the activity of the cortex of the adrenal glands and takes part in the stress reaction. Prolactin, also called lactogenic, luteotropic, or mammotropic hormone, initiates milk secretion in the mammary gland after the mammary tissues have been prepared during pregnancy by the secretion of other pituitary and sex hormones. The two gonadotropic hormones are follicle-stimulating hormones (FSH) and a luteinizing hormone (LH). Follicle-stimulating hormones stimulates the formation of the Graafian follicle in the female ovary and the development of spermatozoa in the male. The luteinizing hormone stimulates the formation of ovarian hormones after ovulation and initiates lactation in the female, in the male, it stimulates the tissues of the testes to elaborate testosterone. In 1975 scientists identified the pituitary peptide endorphin, which acts in experimental animals as a natural pain reliever in times of stress. Endorphin and ACTH are made as parts of a single large protein, which subsequently splits. This may be the body's mechanism for coordinating the physiological activities of two stress-induced hormones. The same large prohormone that contains ACTH and endorphin also contains short peptides called melanocyte-stimulating hormones. These substances are analogous to the hormone that regulates pigmentation in fish and amphibians, but in humans they have no known function.
Research has shown that the hormonal activity of the anterior lobe is controlled by chemical messengers sent from the hypothalamus through tiny blood vessels to the anterior lobe. In the 1950s, the British neurologist Geoffrey Harris discovered that cutting the blood supply from the hypothalamus to the pituitary impaired the function of the pituitary. In 1964, chemical agents called releasing factors were found in the hypothalamus; These substances, it was learned, affect the secretion of growth hormones, a thyroid-stimulating hormone called thyrotropin, and the gonadotropic hormones involving the testes and ovaries. In 1969 the American endocrinologist Roger Guillemin and colleagues isolated and characterized thyrotropin-releasing factors, which stimulates the secretion of thyroid-stimulating hormones from the pituitary. In the next few years his group and that of the American physiologist Andrew Victor Schally isolated the luteinizing hormone-releasing factor, which stimulates secretion of both LH and FSH, and somatostatin, which inhibits release of growth hormones. For this work, which proved that the brain and the endocrine system are linked, they shared the Nobel Prize in physiology or medicine in 1977. Human somatostatin was one of the first substances to be grown in bacteria by recombinant DNA.
The presence of the releasing factors in the hypothalamus helped to explain the action of the female sex hormones, estrogen and progesterone, and their synthetic versions contained in oral contraceptives, or birth-control pills. During a woman's normal monthly cycle, several hormonal changes are needed for the ovary to produce an egg cell for possible fertilization. When the estrogen level in the body declines, the follicle-releasing factor (FRF) flows to the pituitary and stimulates the secretion of the follicle-stimulating hormone. Through a similar feedback principle, the declining level of progesterone causes a release of luteal-releasing factors (LRF), which stimulates secretion of the luteinizing hormone. The ripening follicle in the ovary then produces estrogen, and the high level of that hormone influences the hypothalamus to shut down temporarily the production of FSH. Increased progesterone feedback to the hypothalamus shuts down LH production by the pituitary. The daily doses of synthetic estrogen and progesterone in oral contraceptives, or injections of the actual hormones, inhibit the normal reproductive activity of the ovaries by mimicking the effect of these hormones on the hypothalamus.
In lower vertebrates this part of the pituitary secretes melanocyte-stimulating hormones, which brings about skin-color changes. In humans, it is present only for a short time early in life and during pregnancy, and is not known to have any function.
Two hormones are secreted by the posterior lobe. One of these is the antidiuretic hormone (ADH), vasopressin. Vasopressin stimulates the kidney tubules to absorb water from the filtered plasma that passes through the kidneys and thus controls the amount of urine secreted by the kidneys. The other posterior pituitary hormone is oxytocin, which causes the contraction of the smooth muscles in the uterus, intestines, and blood arterioles. Oxytocin stimulates the contractions of the uterine muscles during the final stage of pregnancy to stimulate the expulsion of the fetus, and it also stimulates the ejection, or let-down, of milk from the mammary gland following pregnancy. Synthesized in 1953, oxytocin was the first pituitary hormone to be produced artificially. Vasopressin was synthesized in 1956.
Pituitary functioning may be disturbed by such conditions as tumors, blood poisoning, blood clots, and certain infectious diseases. Conditions resulting from a decrease in anterior-lobe secretion include dwarfism, acromicria, Simmonds's disease, and Fröhlich's syndrome. The dwarfism occurs when anterior pituitary deficiencies occur during childhood; acromicria, in which the bones of the extremities are small and delicate, results when the deficiency occurs after puberty. Simmonds's disease, which is caused by extensive damage to the anterior pituitary, is characterized by premature aging, loss of hair and teeth, anemia, and emaciation; it can be fatal. Fröhlich's syndrome, also called adiposogenital dystrophy, is caused by both anterior pituitary deficiency and a lesion of the posterior lobe or hypothalamus. The result is obesity, dwarfism, and retarded sexual development. Glands under the influence of anterior pituitary hormones are also affected by anterior pituitary deficiency.
Over secretion of one of the anterior pituitary hormones, somatotropin, results in a progressive chronic disease called acromegaly, which is characterized by enlargement of some parts of the body. Posterior-lobe deficiency results in diabetes insipidus.
Tissue
Tissue, - group of associated, similarly structured cells that perform specialized functions for the survival of the organism. Animal tissues, to which this article is limited, take their first form when the blastula cells, arising from the fertilized ovum, differentiate into three germ layers: the ectoderm, mesoderm, and endoderm. Through further cell differentiation, or histogenesis, groups of cells grow into more specialized units to form organs made up, usually, of several tissues of similarly performing cells. Animal tissues are classified into four main groups.
These tissues include the skin and the inner surfaces of the body, such as those of the lungs, stomach, intestines, and blood vessels. Because its primary function is to protect the body from injury and infection, epitheliums are made up of tightly packed cells with little intercellular substance between them.
About 12 kinds of epithelial tissue occur. One kind is stratified squamous tissue found in the skin and the linings of the esophagus and vagina. It is made up of thin layers of flat, scalelike cells that form rapidly above the blood capillaries and is pushed toward the tissue surface, where they die and are shed. Another is a simple columnar epithelium, which lines the digestive system from the stomach to the anus; Simple columnar epithelium cells stand upright and not only control the absorption of nutrients but also secrete mucus through individual goblet cells. Glands are formed by the inward growth of epithelium-for examples, the sweat glands of the skin and the gastric glands of the stomach. Outward growth results in hair, nails, and other structures.
These tissues, which support and hold parts of the body together, comprises the fibrous and elastic connective tissues, the adipose (fatty) tissues, and cartilage and bone. In contrast to an epithelium, the cells of these tissues are widely separated from one another, with a large amount of intercellular substance between them. The cells of fibrous tissue, found throughout the body, connect to one another by an irregular network of strands, forming a soft, cushiony layer that also supports blood vessels, nerves, and other organs. Adipose tissue has a similar function, except that its fibroblasts also contain store fat. Elastic tissue, found in ligaments, the trachea, and the arterial walls, stretches and contracts again with each pulse beat. In the human embryo, the fibroblast cells that originally secreted collagen for the formation of fibrous tissue later change to secrete a different form of protein called chondrion, for the formation of cartilage, some cartilage later becomes calcified by the action of osteoblast to form bones. Blood and lymph are also often considered connective tissues.
Tissues, which contract and relax, comprise the striated, smooth, and cardiac muscles. Striated muscles, also called skeletal or voluntary muscles, include those that are activated by the somatic, or voluntary, nervous system. They are joined together without cell walls and have several nuclei. The smooth, or involuntary muscles, which are activated by the autonomic nervous system, are found in the internal organs and consist of simple sheets of cells. Cardiac muscles, which have characteristics of both striated and smooth muscles, are joined together in a vast network. These highly complex groups of cells, called ganglia, transfer information from one part of the body to another. Each neuron, or nerve cell, consists of a cell body with branching dendrites and one long fiber, or axons. The dendrites connect one neuron to another; The axon transmits impulses to an organ or collects impulses from a sensory organ.
Crossing a Synapse
In the nervous system, a message-carrying impulse travels from one end of a nerve cell to the other by means of an electrical impulse. When it reaches the terminal end of a nerve cell, the impulse trigger’s tiny sacs called presynaptic vessicles to release their contents, chemical messengers called neurotransmitters. The neurotransmitters float across the synapse, or gap between adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to fire or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic versicles in a process called neurotransmitter reuptake.
Reflex
Reflex, in physiology, is the involuntary response to a stimulus by the animal organism. In its simplest form, it consisted of the stimulation of an afferent nerve through a sense organ, or receptor, followed by transmission of the stimulus, usually through a nerve center, to an efferent motor nerve, resulting in action of a muscle or gland, called the effector. In most reflex action, however, the stimulus passes through one or more intermediate nerve cells, which modify and direct its action, sometimes to the extent of involving the muscular activity of the entire organism. For example, a painful stimulus applied to the hand causes a reflex withdrawal of the hand, which involves contraction of the flexor group of muscles and reflexation of the opposing extensor group; if the stimulus is strong, the coordinating nerve cells pass it to the arm muscles and also to the muscles of the trunk and legs, the result being a jump that removes not only the arm, but the entire person from the vicinity of the painful stimulus.
The system of coordinating nerve cells is such that several different kinds of stimuli may produce the same result. For example, the stimulus produced by the sight of food and that caused by the smell of food travel different afferent pathways, but both have a common final path that stimulates the salivary glands to secretion. The final common path may also be activated through associated nerve tracts by a stimulus that ordinarily is not directly connected with the response. This type of reflex was named conditioned reflex by its discoverer, the Russian physiologist Ivan Pavlov, about 1904. Pavlov found that sounding a bell every time a dog was about to be given food eventually caused a reflex flow of saliva, which later persisted even when no food was produced. Elaborations of this habituative type of reflex are regarded by some physiologists and psychologists as an important basis for many behaviors, both voluntary and involuntary.
The normal pathways of many reflexes are generally known, and the presence, absence, or exaggerations of the normal physical responses to certain stimuli are symptoms used by neurologists to determine the condition of the neural pathways involved. A familiar reflex commonly tested by physicians is the patellar reflex, in which an involuntary jerk of the knee is evoked by lightly striking the tendon of the patella, or kneecap, indicating the efficiency of certain nerve tracts in the spinal cord.
Like all other cells, neurons contain charged ions: Potassium and sodium (positively charged) and chlorine (negatively charged). Neurons differ from other cells in that they are able to produce a nerve impulse. A neuron is polarized - that is, it has an overall negative charge inside the cell membrane because of the high concentration of chlorine ions and low concentration of potassium and sodium ions. The concentration of these same ions is exactly reversed outside the cell. This charge differential represents stored electrical energy, sometimes referred to as membrane potential or resting potential. The negative charge inside the cell is maintained by two features. The first is the selective permeability of the cell membrane, which is more permeable to potassium than sodium. The second feature is sodium pumps within the cell membrane that actively pump sodium out of the cell. When depolarization occurs, this charge differential across the membrane is reversed, and a nerve impulse is produced.
Depolarization is a rapid change in the permeability of the cell membrane. When sensory input or any other kind of stimulating current is received by the neuron, the membrane permeability is changed, allowing a sudden influx of sodium ions into the cell. The high concentration of sodium, or action potential, changes the overall charges within the cell from negative to positive. The local changes in ion concentration triggers similar reactions along the membrane, propagating the nerve impulse. After a brief period called the refractory period, during which the ionic concentration returned to resting potential, the neuron can repeat this process.
Nerve impulses travel at different speeds, depending on the cellular composition of a neuron. Where speed of impulse is important, as in the nervous system, axons are insulated with a membranous substance called myelin. The insulation provided by myelin maintains the ionic charge over long distances. Nerve impulses are propagated at specific points along the myelin sheath; These points are called the nodes of Ranvier. Examples of myelinated axons are those in sensory nerve fibers and nerves connected to skeletal muscles. In non-myelinated cells, the nerve impulse is propagated more diffusely.
The nervous system has two divisions: The somatic, which allow voluntary control over skeletal muscle, and the autonomic, which is involuntary and controls cardiac and smooth muscle and glands. The autonomic nervous system has two divisions: The sympathetic and the parasympathetic. Many, but not all, of the muscles and glands that distribute nerve impulses to the larger interior organs possess a double nerve supply; in such cases the two divisions may exert opposing effects. Thus, the sympathetic system increases heartbeat, and the parasympathetic system decreases heartbeat. The two nervous systems are not always antagonistic, however. For example, both nerve supplies to the salivary glands excite the cells of secretion. Furthermore, a single division of the autonomic nervous system may both excite and inhibit a single effector, as in the sympathetic supply to the blood vessels of skeletal muscle. Finally, the sweat glands, the muscles that cause involuntary erection or bristling of the hair, the smooth muscle of the spleen, and the blood vessels of the skin and skeletal muscle are actuated only by the sympathetic division.
Voluntary movement of head, limbs, and body is caused by nerve impulses arising in the motor area of the cortex of the brain and carried by cranial nerves or by nerves that emerge from the spinal cord to connect with skeletal muscles. The reaction involves both excitation of nerve cells stimulating the muscles involved and inhibition of the cells that stimulate opposing muscles. A nerve impulse is an electrical change within a nerve cell or fiber; Measured in millivolts, it lasts a few milliseconds and can be recorded by electrodes.
The human brain has three major structural components: The large dome-shaped cerebrum, the smaller somewhat spherical cerebellum, and the brainstem. Prominent in the brainstem is the medulla oblongata (the egg-shaped enlargement at the center) and the thalamus (between the medulla and the cerebrum). The cerebrum is responsible for intelligence and reasoning. The cerebellum helps to maintain balance and posture. The medulla is involved in maintaining involuntary functions such as respiration, and the thalamus act as a relay center for electrical impulses traveling to and from the cerebral cortex. Lack of blood flow to any part of the brain results in a stroke, permanent damage that interferes with the functions of the affected part of the brain.
Movement may occur also in direct response to an outside stimulus, thus, a tap on the knee causes a jerk, and a light shone into the eye makes the pupil contract. These involuntary responses are called reflexes. Various nerve terminals called receptors constantly send impulses into the central nervous system. These are of three classes: exteroceptors, which are sensitive to pain, temperature, touch, and pressure; interoceptors, which react to changes in the internal environment; and proprioceptors, which respond to variations in movement, position, and tension. These impulses terminate in special areas of the brain, as do of those special receptors concerned with sight, hearing, smell, and taste.
Whereas most major nerves emerge from the spinal cord, the 12 pairs of cranial nerves project directly from the brain. All but 1 pair relay motor or sensory information (or both); the tenth, or vagus nerve, affects visceral functions such as heart rate, vasoconstriction, and contraction of the smooth muscle found in the walls of the trachea, stomach, and intestine.
Muscular contractions do not always cause actual movement. A small fraction of the total number of fibers in most muscles is usually contracting. This serves to maintain the posture of a limb and enables the limb to resist passive elongation or stretch. This slight continuous contraction is called muscle tone.
In 1946 Axelrod joined the laboratory of American pharmacologist Bernard Brodie at Goldwater Memorial Hospital in New York. The pair conducted research on pain-relieving drugs called analgesics. They identified a pain-relieving chemical known as acetaminophen. This drug was later developed and marketed by the drug company Johnson & Johnson under the brand-name Tylenol.
In 1949 Axelrod took a position at the National Heart Institute, a branch of the National Institutes of Health (NIH). Their Axelrod studied how the body processes certain drugs that cause behavioral changes, including amphetamines, ephedrine, and mescaline. He identified a group of enzymes that help these drugs break down in the body. These enzymes, called cytochrome-P450 monoxygenases, have been studied extensively by other scientists, particularly in cancer research.
Realizing that career advancement in the sciences requires a doctoral degree, in 1954 Axelrod took a leave of absence from his job at the National Heart Institute to attend The George Washington University. He earned his doctorate in pharmacology in 1955. That same year he was named chief of pharmacology at the National Institute of Mental Health (NIMH) another branch of NIH.
At NIMH, Joseph Axelrod began research on neurotransmitters. A nerve cell releases a neurotransmitter to spur a neighboring cell into action. In the 1950s most scientists believed that a neurotransmitter became inactive once it stimulated a neighboring cell. But Axelrod’s research found that the neurotransmitter returns to the first nerve cell, in a process known as reuptake, where it is broken down by enzymes or repackaged for reuse. This research led to the creation of a number of drugs that prevent the reuptake process, enabling a neurotransmitter to remain active for a longer period of time.
Axelrod’s research revolutionized the understanding of many mental-health disorders, including depression, anxiety, and schizophrenia. Prior to his research, psychiatry focused on the relationship of life experiences to mental health problems. But Axelrod's research proved that mental-health disorders were often the result of complicated brain chemistry. His research spurred the development of new drugs that advanced the treatment of mental-health conditions. Among these are selective serotonin reuptake inhibitors, including the antidepressants fluoxetine, sold under the brand name Prozac, sertraline(Zoloft) and paroxetine (Paxil).
The study of the biochemistry of memory is another exciting scientific enterprise, but one that can only be touched upon here. Scientists estimate that an adult human brain contains about 100 billion neurons. Each of these is connected to hundreds or thousands of other neurons, forming trillions of neural connections. Neurons communicate by chemical messengers called neurotransmitters. An electrical signal travels along the neuron, triggering the release of neurotransmitters at the synapse, the small gap between neurons. The neurotransmitters travel across the synapse and act on the next neuron by binding with protein molecules called receptors. Most scientists believe that memories are somehow stored among the brain's trillions of synapses, rather than in the neurons themselves.
Scientists who study the biochemistry of learning and memory often focus on the marine snail Aplysia because its simple nervous system allows them to study the effects of various stimuli on specific synapses. A change in the snail's behavior due to learning can be correlated with a change at the level of the synapse. One exciting scientific frontier is discovering the changes in neurotransmitters that occur at the level of the synapse.
Other researchers have implicated glucose, a sugar and insulin(a hormone secreted by the pancreas) as important to learning and memory. Humans and other animals given these substances show an improved capacity to learn and remember. Typically, when animals or humans ingest glucose, the pancreas responds by increasing insulin production, so it is difficult to determine which substance contributes to improved performance. Some studies in humans that have systematically varied the amount of glucose and insulin in the blood have shown that insulin may be the more important of the two substances for learning.
Scientists also have examined the influence of genes on learning and memory. In one study, scientists bred strains of mice with extra copies of a gene that helps build a protein called N-methyl-D-aspartate, or NMDA. This protein acts as a receptor for certain neurotransmitters. The genetically altered mice outperformed normal mice on a variety of tests of learning and memory. In addition, other studies have found that chemically blocking NMDA receptor impairs learning in laboratory rats. Future discoveries from genetic and biochemical studies may lead to treatments for memory deficits from Alzheimer's disease and other conditions that affect memory.
Alzheimer's Disease, progressive brain disorders that causes a gradual and irreversible decline in memory, language skills, perception of time and space, and, eventually, the ability to care for oneself. First described by German psychiatrist Alois Alzheimer in 1906, Alzheimer's disease was initially thought to be a rare condition affecting only young people, and was referred to as prehensile dementia. Today late-onset Alzheimer's disease is recognized as the most common cause of the loss of mental function in those aged 65 and over. Alzheimer's in people in their 30s, 40s, and 50s, called early-onset Alzheimer's disease, occurs less frequently, accountings for less than 10 percent of the estimated 4 million Alzheimer's cases in the United States.
Although Alzheimer's disease is not a normal part of the aging process, the risk of developing the disease increases as people grow older. About 10 percent of the United States population over the age of 65 is affected by Alzheimer's disease, and nearly 50 percent of those over age 85 may have the disease.
Alzheimer's disease takes a devastating toll, not only on the patients, but also on those who love and care for them. Some patients experience immense fear and frustration as they struggle with once commonplace tasks and slowly lose their independence. Family, friends, and especially those who provide daily care suffer immeasurable pain and stress as they witness Alzheimer's disease slowly take their loved one from them.
The onset of Alzheimer's disease is usually very gradual. In the early stages, Alzheimer's patients have relatively mild problems learning new information and remembering where they have left common objects, such as keys or a wallet. In time, they begin to have trouble recollecting recent events and finding the right words to express themselves. As the disease progresses, patients may have difficulty remembering what day or month it is, or finding their way around familiar surroundings. They may develop a tendency to wander off and then be unable to find their way back. Patients often become irritable or withdrawn as they struggle with fear and frustration when once commonplace tasks become unfamiliar and intimidating. Behavioral changes may become more pronounced as patients become paranoid or delusional and unable to engage in normal conversation.
Eventually Alzheimer's patients become completely incapacitated and unable to take care of their most basic life functions, such as eating and using the bathroom. Alzheimer's patients may live many years with the disease, usually dying from other disorders that may develop, such as pneumonia. Typically the time from initial diagnosis until death is seven to ten years, but this is quite variable and can range from three to twenty years, depending on the age of the onset, other medical conditions present, and the care patients receive.
The brains of patients with Alzheimer's have distinctive formations - abnormally shaped proteins called tangles and plaques - that are recognized as the hallmark of the disease. Not all brain regions show these characteristic formations. The areas most prominently affected are those related to memory.
Tangles are long, slender tendrils found inside nerve cells, or neurons. Scientists have learned that when a protein-called tau becomes altered, it may cause the characteristic tangles in the brain of the Alzheimer’s patient. In healthy brains provides structural support for neurons, but in Alzheimer's patients this structural support collapses.
Plaques, or clumps of fibers, form outside the neurons in the adjacent brain tissue. Scientists found that a type of protein, called amyloid precursor protein, forms toxic plaques when it is cut in two places. Researchers have isolated the enzyme beta-secretes, which is believed to make one of the cuts in the amyloid precursor protein. Researchers also identified another enzyme, called gamma secretes, that makes the second cut in the amyloid precursor protein. These two enzymes snip the amyloid precursor protein into fragments that then accumulate to form plaques that are toxic to neurons.
Scientists have found that tangles and plaques cause neurons in the brains of Alzheimer's patients to shrink and eventually die, first in the memory and language centers and finally throughout the brain. This widespread neuron degeneration leaves gaps in the brain's messaging network that may interfere with communication between cells, causing some of the symptoms of Alzheimer’s disease.
Alzheimer's patients have lower levels of neurotransmitters, chemicals that carry complex messages back and forth between the nerve cells. For instance, Alzheimer's disease seems to decrease the level of the neurotransmitter acetylcholine, which is known to influence memory. A deficiency in other neurotransmitters, including somatostatin and corticotropin-releasing factor, and, particularly in younger patients, serotonin and norepinephrine, also interferes with normal communication between brain cells.
The causes of Alzheimer's disease remain a mystery, but researchers have found that particular groups of people have risk factors that make them more likely to develop the disease than the general population. For example, people with a family history of Alzheimer's are more likely to develop Alzheimer's disease.
Some of the most promising Alzheimer's research is being conducted in the field of genetics to learn the role a family history of the disease has in its development. Scientists have learned that people who are carriers of a specific version of the apolipoprotein E gene (apoE genes), found on chromosome 19, are several times more likely to develop Alzheimer's than carriers of other versions of the apoE gene. The most common version of this gene in the general population is apoE3. Nearly half of all late-onset Alzheimer’s patients have the fewer in common apoE4 versions, however, and research has shown that this gene plays a role in Alzheimer's disease. Scientists have also found evidence that variations in one or more genes located on chromosomes 1, 10, and 14 may increase a person’s risk for Alzheimer's disease. Scientists have identified the gene variations on chromosomes 1 and 14 and learned that these genes produce mutations in proteins called presenilins. These mutated proteins apparently trigger the activity of the enzyme gamma secretase, which splices the amyloid precursor protein.
Researchers have made similar strides in the investigation of early-onset Alzheimer's disease. A series of genetic mutations in patients with early-onset Alzheimer's has been linked to the production of amyloid precursor protein, the protein in plaques that may be implicated in the destruction of neurons. One mutation is particularly interesting to geneticists because it occurs on a gene involved in the genetic disorder Down syndrome. People with Down syndrome usually develop plaques and tangles in their brains as they get older, and researchers believe that learning more about the similarities between Down syndrome and Alzheimer's may further our understanding of the genetic elements of the disease.
Some studies suggest that one or more factors other than heredity may determine whether people develop the disease. One study published in February 2001 compared residents of Ibadan, Nigeria, who eat a mostly low-fat vegetarian diet, with African Americans living in Indianapolis, Indiana, whose diet included a variety of high-fat foods. The Nigerians were less likely to develop Alzheimer’s disease compared to their U.S. counterparts. Some researchers suspect that health imposes on high blood pressure, atherosclerosis (arteries clogged by fatty deposits), high cholesterol levels, or other cardiovascular problems may play a role in the development of the disease.
Other studies have suggested that environmental agents may be a possible cause of Alzheimer's disease; for example, one study suggested that high levels of aluminum in the brain may be a risk factor. Several scientists initiated research projects to further investigate this connection, but no conclusive evidence has been found linking aluminum with Alzheimer's disease. Similarly, investigations into other potential environmental causes, such as zinc exposure, viral agents, and food-borne poisons, while initially promising, have generally turned up inconclusive results.
Some studies indicate that brain trauma can trigger a degenerative process that results in Alzheimer's disease. In one study, an analysis of the medical records scribed upon veterans of World War II (1939-1945) linked serious head injury in early adulthood with Alzheimer's disease in later life. The study also looked at other factors that could possibly influence the development of the disease among the veterans, such as the presence of the apoE gene, but no other factors were identified.
Alzheimer’s disease is only positively diagnosed by examining brain tissue under a microscope to see the hallmark plaques and tangles, and this is only possible after a patient dies. As a result, physicians rely on a series of other techniques to diagnose probable Alzheimer's disease in living patients. Diagnosis begins by ruling out other problems that cause memory loss, such as stroke, depression, alcoholism, and the use of certain prescription drugs. The patient undergoes a thorough examination, including specialized brain scans, to eliminate other disorders. The patient may be given a detailed evaluation called a neuropsychological examination, which is designed to evaluate a patient’s ability to perform specific mental tasks. This helps the physician determine whether the patient is showing the characteristic symptoms of Alzheimer's disease - progressively worsening memory problems, language difficulties, and trouble with spatial direction and time. The physician also asks about the patient's family medical history to learn about any past serious illnesses, which may give a hint about the patient's current symptoms.
Evidence shows that there is inflammation in the brains of Alzheimer's patients, which may be associated with the production of amyloid precursor protein. Studies are underway to find drugs that prevent this inflammation, to possibly slow or even halt the progress of the disease. Other promising approaches center on mechanisms that manipulate amyloid precursor protein production or accumulation. Drugs are in development that may block the activity of the enzymes that cut the amyloid precursor protein, halting amyloid production. Other studies in mice suggest those vaccinating animals with amyloid precursor protein can produce a reaction that clears amyloid precursor protein from the brain. Physicians have started vaccination studies in humans to determine if the same potentially beneficial effects can be obtained. There is still much to be learned, but as scientists better understand the genetic components of Alzheimer’s, the roles of the amyloid precursor protein and the tau protein in the disease, and the mechanisms of nerve cell degeneration, the possibility that a treatment will be developed is more likely.
The responsibility for caring for Alzheimer's patients generally falls on their spouses and children. Care givers must constantly be on guard for the possibility of Alzheimer's patients wandering away or becoming agitated or confused in a manner that jeopardizes the patient or others. Coping with a loved one's decline and inability to recognize familiar face causes enormous pain.
The increased burden faced by families is intense, and the life of the Alzheimer's care giver is often called a 36-hour day. Not surprisingly, care givers often develop health and psychological problems of their own as a result of this stress. The Alzheimer's Association, a national organization with local chapters throughout the United States, was formed in 1980 in large measure to provide support for Alzheimer's care givers. Today, national and local chapters are a valuable source for information, referral, and advice.
Not to long ago, most approaches to the philosophy of science were ‘cognitive’. This includes ‘logical positivism’, as nearly all of those who wrote about the nature of science would have been in agreement that science ought to be ‘value-free’. This had been a particular emphasis on the part of the first positivist, as it would be upon twentieth-century successors. Science, so it was said, deals with ‘facts’, and facts and values and irreducibly distinct. Facts are objective, they are what we seek in our knowledge of the world. Values are subjective: They bear the mark of human interest, they are the radically individual products of feeling and desire. Fact and value cannot, therefore, be inferred from fact, fact ought not be influenced by value. There were philosophers, notably some in the Kantian tradition, who viewed the relation of the human individual to the universalist aspiration of difference rather differently. But the legacy of three centuries of largely empiricist reflection of the ‘new’ sciences ushered in by Galilee Galileo (1564-1642), the Italian scientist whose distinction belongs to the history of physics and astronomy, rather than natural philosophy.
The philosophical importance of Galileo’s science rests largely upon the following closely related achievements: (1) His stunning successful arguments against Aristotelean science, (2) his proofs that mathematics is applicable to the real world. (3) his conceptually powerful use of experiments, both actual and employed regulatively, (4) his treatment of causality, replacing appeal to hypothesized natural ends with a quest for efficient causes, and (5) his unwavering confidence in the new style of theorizing that would come to be known as mechanical explanation.
A century later, the maxim that scientific knowledge is ‘value-laded’ seems almost as entrenched as its opposite was earlier. It is supposed that between fact and value has been breached, and philosophers of science seem quite at home with the thought that science and value may be closely intertwined after all. What has happened to bring about such an apparently radial change? What are its implications for the objectivity of science, the prized characteristic that, from Plato’s time onwards, has been assumed to set off real knowledge (epistēmē) from mere opinion (doxa)? To answer these questions adequately, one would first have to know something of the reasons behind the decline of logical positivism, as, well as of the diversity of the philosophies of science that have succeeded it.
More general, the interdisciplinary field of cognitive science is burgeoning on several fronts. Contemporary philosophical reelection about the mind - which has been quite intensive - has been influenced by this empirical inquiry, to the extent that the boundary lines between them are blurred in places.
Nonetheless, the philosophy of mind at its core remains a branch of metaphysics, traditionally conceived. Philosophers continue to debate foundational issues in terms not radically different from those in vogue in previous eras. Many issues in the metaphysics of science hinge on the notion of ‘causation’. This notion is as important in science as it is in everyday thinking, and much scientific theorizing is concerned specifically to identify the ‘causes’ of various phenomena. However, there is little philosophical agreement on what it is to say that one event is the cause of some other.
Modern discussion of causation starts with the Scottish philosopher, historian, and essayist David Hume (1711-76),who argued that causation is simply a matter for which he denies that we have innate ideas, that the causal relation is observably anything other than ‘constant conjunction’ wherefore, that there are observable necessary connections anywhere, and that there is either an empirical or demonstrative proof for the assumptions that the future will resemble the past, and that every event has a cause. That is to say, that there is an irresolvable dispute between advocates of free-will and determinism, that extreme scepticism is coherent and that we can find the experiential source of our ideas of self-substance or God.
According to Hume (1978), on event causes another if only if events of the type to which the first event belongs regularly occur in conjunctive events of the type to which the second event belongs. The formulation, however, leaves a number of questions open. Firstly, there is a problem of distinguishing genuine ‘causal law’ from ‘accidental regularities’. Not all regularities are sufficient lawlike to underpin causal relationships. Being a screw in my desk could well be constantly conjoined with being made of copper, without its being true that these screws are made of copper because they are in my desk. Secondly, the idea of constant conjunction does not give a ‘direction’ to causation. Causes need to be distinguished from effects. But knowing that A-type events are constantly conjoined with B-type events does not tell us which of ‘A’ and ‘B’ is the cause and which the effect, since constant conjunction is itself a symmetric relation. Thirdly, there is a problem about ‘probabilistic causation’. When we say that causes and effects are constantly conjoined, do we mean that the effects are always found with the causes, or is it enough that the causes make the effect probable?
Many philosophers of science during the past century have preferred to talk about ‘explanation’ than causation. According to the covering-law model of explanation, something is explained if it can be deduced from premises which include one or more laws. As applied to the explanation of particular events this implies that one particular event can be explained it if is linked by a law to some other particular event. However, while they are often treated as separate theories, the covering-law account of explanation is at bottom little more than a variant of Hume’s constant conjunction account of causation. This affinity shows up in the fact at the covering-law account faces essentially the same difficulties as Hume: (1) In appealing to deduction from ‘laws’, it needs to explain the difference between genuine laws and accidentally true regularities: (2) It omits by effects, as swell as effects by causes, after all, it is as easy to deduce the height of flag-pole from the length of its shadow and the law of optics: (3) Are the laws invoked in explanation required to be exceptionalness and deterministic, or is it acceptable, say, to appeal to the merely probabilistic fact that smoking makes cancer more likely, in explaining why some particular person develops cancer?
Nevertheless, one of the centrally obtainable achievements for which the philosophy of science is to provide explicit and systematic accounts of the theories and explanatory strategies exploited in the science. Another common goal is to construct philosophically illuminating analyses or explanations of central theoretical concepts invoked in one or another science. In the philosophy of biology, for example, there is a rich literature aimed at understanding teleological explanations, and there has been a great deal of work on the structure of evolutionary theory and on such crucial concepts as fitness and biological function. By introducing ‘teleological considerations’, this account views beliefs as states with biological purpose and analyses their truth conditions specifically as those conditions that they are biologically supposed to covary with.
A teleological theory of representation needs to be supplemental with a philosophical account of biological representation generally a selectionism account of biological purpose, according to which item ‘F’ has purpose ‘G’ if and only if it is now present as a result of past selection by some process which favoured item with ‘G’. So, a given belief type will have the purpose of covarying with ‘P’, say. If and only if some mechanism has selected it because it has covaried with ‘P’ the past.
Along the same lines, teleological theory holds that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’, teleological theories differ depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions and a-historical theories. Historical theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicate ‘x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. thus, a state physically indistinguishable from ‘r’ (physical states being a-historical) but lacking r’s historical origins would not represent ‘x’ according to historical theories.
The American philosopher of mind (1935-) Fodor, is known for a resolute ‘realism’ about the nature of mental functioning, taking the analogy between thought and computation seriously. Fodor believes that mental representations should be conceived as individual states with their own identities and structures, like formulae transformed by processes of computation or thought. His views are frequently contrasted with those of ‘holist s’ such as the American philosopher Herbert Donald Davidson (1917-2003), or ‘instrumentalists about mental ascription, such as the British philosopher of logic and language, Eardley Anthony Michael Dummett (1925-). In recent years he has become a vocal critic of some of the aspirations of cognitive science.
Nonetheless, a suggestion extrapolating the solution of teleology is continually queried by points as owing to ‘causation’ and ‘content’, and ultimately a fundamental appreciation is to be considered, is that: We suppose that there’s a causal path from A’s to ‘A’s’ and a causal path from B’s to ‘A’s’, and our problem is to find some difference between B-caused ‘A’s’ and A-caused ‘A’s’ in virtue of which the former but not the latter misrepresented. Perhaps, the two paths differ in their counter-factual properties. In particular, though A’s and B’s both cause ‘A’s’ as a matter of fact, perhaps can assume that only A’s would cause ‘A’s’ in - as one can say - ,‘optimal circumstances’. We could then hold that a symbol expresses its ‘optimal property’, viz., the property that would causally control its tokening in optimal circumstances. Correspondingly, when the tokening of a symbol is causally controlled by properties other than its optimal property, the tokens that eventuate are ipso facto wild.
Suppose at the present time, that this story about ‘optimal circumstances’ is proposed as part of a naturalized semantics for mental representations. In which case it is, of course, essential that it be possible to say that the optimal circumstances for tokening a mental representation are in terms that are not themselves either semantical nor intentional. (It would not do, for example, to identify the optimal circumstances for tokening a symbol as those in which the tokens are true, that would be to assume precisely the sort of semantical notions that the theory is supposed to naturalize.) Befittingly, the suggestion - to put it in a nutshell - is that appeals to ‘optimality’ should be buttressed by appeals to ‘teleology’: Optimal circumstances are the ones in which the mechanisms that mediate symbol tokening are functioning ‘as they are supposed to’. In the case of mental representations, these would be paradigmatically circumstances where the mechanisms of belief fixation are functioning as they are supposed to.
So, then: The teleology o the cognitive mechanisms determines the optimal condition for belief fixation, and the optimal condition for belief fixation determines the content of beliefs. So the story goes.
To put this objection in slightly other words: The teleology story perhaps strikes one as plausible in that it understands one normative notion - truth - in terms of another normative notion - optimality. But this appearance e of fit is spurious there is no guarantee that the kind of optimality that teleology reconstructs has much to do with the kind of optimality that the explication of ‘truth’ requires. When mechanisms of repression are working ‘optimally’ - when they’re working ‘as they’re supposed to’ - what they deliver are likely to be ‘falsehoods’.
Or again: There’s no obvious reason why coitions that are optimal for the tokening of one sort of mental symbol need be optimal for the tokening of other sorts. Perhaps the optimal conditions for fixing beliefs about very large objects, are different from the optimal conditions for fixing beliefs about very small ones, are different from the optimal conditions for fixing beliefs sights. But this raises the possibility that if we’re to say which conditions are optimal for the fixation of a belief, we’ll have to know what the content of the belief is - what it’s a belief about. Our explication of content would then require a notion of optimality, whose explication in turn requires a notion of content, and the resulting pile would clearly be unstable.
Teleological theories hold that ‘r’ represents ‘x’ if it is r’s function to indicate (i.e., covary with) ‘x’. Teleological theories differ, depending on the theory of functions they import. Perhaps the most important distinction is that between historical theories of functions: Historically, theories individuate functional states (hence, contents) in a way that is sensitive to the historical development of the state, i.e., to factors such as the way the state was ‘learned’, or the way it evolved. An historical theory might hold that the function of ‘r’ is to indicates ’x’ only if the capacity to token ‘r’ was developed (selected, learned) because it indicates ‘x’. Thus, a state physically indistinguishable from ‘r’ (physical states being a-historical), but lacking r’s historical origins would not represent ‘x’ according to historical theories.
Just as functional role theories hold that r’s representing ‘x’ is grounded in the functional role ‘r’ has in the representing system, i.e., on the relations imposed by specified cognitive processes between ‘r’ and other representations in the system’s repertoire. Functional role theories take their cue from such common-sense ideas as that people cannot believe that cats are furry if they do not know that cats are animals or that fur is like hair.
That being said, that nowhere is the new period of collaboration between philosophy and other disciplines more evident than in the new subject of cognitive science. Cognitive science from its very beginning has been ‘interdisciplinary’ in character, and is in effect the joint property of psychology, linguistics, philosophy, computer science and anthropology. There is, therefore, a great variety of different research projects within cognitive science, but the central area of cognitive science, its hard-coded ideology rests on the assumption that the mind is best viewed as analogous to a digital computer. The basic idea behind cognitive science is that recent developments in computer science and artificial intelligence have enormous importance for our conception of human beings. The basic inspiration for cognitive science went something like this: Human beings do information processing. Computers are designed precisely do information processing. Therefore, one way to study human cognition - perhaps the best way to study it - is to study. It as a matter of computational information processing. Some cognitive scientists think that the computer is just a metaphor for the human mind: Others think that the mind is literally a computer program. But it is fair to say, that without the computational model there would not have been a cognitive science as we now understand it.
In, Essay Concerning Human Understanding is the first modern systematic presentation of an empiricist epistemology, and as such had important implications for the natural sciences and for philosophy of science generally. Like his predecessor, Descartes, the English philosopher (1632-104) John Locke began his account of knowledge from the conscious mind aware of ideas. Unlike Descartes, however, he was concerned not to build a system based on certainty, but to identify the mind’s scope and limits. The premise upon which Locke built his account, including his account of the natural sciences, is that the ideas which furnish the mind are all derived from experience. He thus, totally rejected any kind of innate knowledge. In this he consciously opposing Descartes, who had argued that it is possible to come to knowledge of fundamental truths about the natural world through reason alone. Descartes (1596-1650) had argued, that we can come to know the essential nature of both ‘mind’ and ‘matter’ by pure reason. John Locke accepted Descartes’s criterion of clear and distinct ideas as the basis for knowledge, but denied any source for them other than experience. It was information that came in via the five senses (ideas of sensation) and ideas engendered from pure inner experiences (ideas of reflection) came the building blocks of the understanding.
Locke combined his commitment to ‘the new way of ideas’ with a te native espousal of the ‘corpuscular philosophy’ of the Irish scientist (1627-92) Robert Boyle. This, in essence, was an acceptance of a revised, more sophisticated account of matter and its properties that had been advocated by the ancient atomists and recently supported by Galileo (1564-1642) and Pierre Gassendi (1592-1655). Boyle argued from theory and experiment that there were powerful reasons to justify some kind of corpuscular account of matter and its properties. He called the latter qualities, which he distinguished as primary and secondary - the distinction between primary and secondary qualities may be reached by two rather different routes: Either from the nature or essence of matter or from the nature and essence of experience, though practising these have tended to run together. The former considerations make the distinction seem like an a priori, or necessary, truth about the nature of matter, while the latter make it appears to be an empirical hypothesis -. Locke, too, accepted this account, arguing that the ideas we have of the primary qualities of bodies resemble those qualities as they are in the subject, whereas the ideas of the secondary qualities, such as colour, taste, and smell, do not resemble their causes in the object.
There is no strong connection between acceptance of the primary-secondary quality distinction and Locke’s empiricism and Descartes had also argued strongly for universal acceptance by natural philosophers, and Locke embraced it within his more comprehensive empirical philosophy. But Locke’ empiricism did have major implications for the natural sciences, as he well realized. His account begins with an analysis of experience. all ideas, he argues, are either simple or complex. Simple ideas are those like the red of a particular rose or the roundness of a snowball. Complex ideas, our ideas of the rose or the snowball, are combinations of simple ideas. We may create new complex ideas in our imagination - a dragon, for example. But simple ideas can never be created by us: We just have them or not, and characteristically they are caused, for example, the impact on our senses of rays of light or vibrations of sound in the air coming from a particular physical object. Since we cannot create simple ideas, and they are determined by our experience. Our knowledge is in a very strict uncompromising way limited. Besides, our experiences are always of the particular, never of the general. It is this particular simple idea or that particular complex idea that we apprehend. We never in that sense apprehend a universal truth about the natural world, but only particular instances. It follows from this that all claims to generality about that world - for example, all claims to identity what were then beginning to be called the laws of nature - must to that extent go beyond our experience and thus be less than certain.
The Scottish philosopher, historian, and essayist, (1711-76) David Hume, whose famous discussion appears in both his major philosophical works, the ‘Treatise’ (1739) and the ‘Enquiry’(1777). The distinction is couched in terms of the concept of causality, so that where we are accustomed to talk of laws, Hume contends, involves three ideas:
1. That there should be a regular concomitance between events of the type of the cause and those of the type of the effect.
2. That the cause event should be contiguous with the effect event.
3. That the cause event should necessitate the effect event.
The tenets (1) and (2) occasion no differently for Hume, since he believes that there are patterns of sensory impressions un-problematically related to the idea of regularity concomitance and of contiguity. But the third requirement is deeply problematic, in that the idea of necessarily that figures in it seems to have no sensory impression correlated with it. However, carefully and attentively we scrutinize a causal process, we do not seem to observe anything that might be the observed correlates of the idea of necessity. We do not observe any kind of activity, power, or necessitation. All we ever observe is one event following another, which is logically independent of it. Nor is this necessity logical, since, as, Hume observes, one can jointly assert the existence of the cause and a denial of the existence of the effect, as specified in the causal statement or the law of nature, without contradiction. What, then, are we to make of the seemingly central notion of necessity that is deeply embedded in the very idea of causation, or lawfulness? To this query, Hume gives an ingenious and telling story. There is an impression corresponding to the idea of causal necessity, but it is a psychological phenomenon: Our exception that an even similar to those we have already observed to be correlated with the cause-type of events will come to be in this cas e too. Where does that impression come from? It is created as a kind of mental habit by the repeated experience of regular concomitance between events of the type of the effect and the occurring of event s of the type of the cause. And then, the impression that corresponds to the idea of regular concomitance - the law of nature then asserts nothing but the existence of the regular concomitance.
At this point in our narrative, the question at once arises as to whether this factor of life in nature, thus interpreted, corresponds to anything that we observe in nature. All philosophy is an endeavour to obtain a self-consistent understanding of things observed. Thus, its development is guided in two ways, one is demand for coherent self-consistency, and the other is the elucidation of things observed. With our direct observations how are we to conduct such comparison? Should we turn to science? No. There is no way in which the scientific endeavour can detect the aliveness of things: Its methodology rules out the possibility of such a finding. On this point, the English mathematician and philosopher (1861-1947) Alfred Whitehead, comments: That science can find no individual enjoyment in nature, as science can find no creativity in nature, it finds mere rules of succession. These negations are true of natural science. They are inherent in its methodology. The reason for this blindness of physical science lies in the fact that such science only deals with half the evidence provided by human experience. It divides the seamless coat - or, to change the metaphor into a happier form, it examines the coat, which is superficial, and neglects the body which is fundamental.
Whitehead claims that the methodology of science makes it blind to a fundamental aspect of reality, namely, the primacy of experience, it neglected half of the evidence. Working within Descartes’ dualistic framework of matter and mind as separate and incommensurate, science limits itself to the study of objectivised phenomena, neglecting the subject and the mental events that are his or her experience.
Both the adoption of the Cartesian paradigm and the neglect of mental events are reason enough to suspect ‘blindness’, but there is no need to rely on suspicions. This blindness is clearly evident. Scientific discoveries, impressive as they are, are fundamentally superficial. Science can express regularities observed in nature, but it cannot explain the reasons for their occurrence. Consider, for example, Newton’s law of gravity. It shows that such apparently disparate phenomena as the falling of an apple and the revolution of the earth around the sun are aspects of the same regularity - gravity. According to this law the gravitational attraction between two objects deceases in proportion to the square of the distance between them. Why is that so? Newton could not provide an answer. Simpler still, why does space have three dimensions? Why is time one-dimensional? Whitehead notes, ‘None of these laws of nature gives the slightest evidence of necessity. They are [merely] the modes of procedure which within the scale of observation do in fact prevail’.
This analysis reveals that the capacity of science to fathom the depths of reality is limited. For example, if reality is, in fact, made up of discrete units, and these units have the fundamental character in being ’throbs of experience’, then science may be in a position to discover the discreteness: But it has no access to the subjective side of nature, since, as the Austrian physicist(1887-1961) Erin Schrödinger points out, we ‘exclude the subject of cognizance from the domain of nature that we endeavour to understand’. It follows that in order to find ‘the elucidation of things observed’ in relation to the experiential or aliveness aspect, we cannot rely on science, we need to look elsewhere.
If, instead of relying on science, we rely on our immediate observation of nature and of ourselves, we find, first, that this [i.e., Descartes’] stark division between mentality and nature has no ground in our fundamental observation. We find ourselves living within nature. Secondly, in that we should conceive mental operations as among the factors which make up the constitution of nature, and thirdly, that we should reject the notion of idle wheels in the process of nature. Every factor which makes a difference, and that difference can only be expressed in terms of the individual character of that factor.
Whitehead proceeds to analyse our experiences in general, and our observations of nature in particular, and ends up with ‘mutual immanence’ as a central theme. This mutual immanence is obvious in the case of an experience, that, I am a part of the universe, and, since I experience the universe, the experienced universe is part of me. Whitehead gives an example’ ‘I am in the room, and the room is an item in my present experience. But my present experience is what I am now’. A generalization of this relationship to the case of any actual occasions yields the conclusion that ‘the world is included within the occasion in one sense, and the occasion is included in the world in another sense’. The idea that each actual occasion appropriates its universe follows naturally from such considerations.
The description of an actual entity as being a distinct unit is, therefore, only one part of the story. The other, complementary part is this: The very nature of each and every actual entity is one of interdependence with all the other actual entities in the universe. Each and every actual entity is a process of prehending or appropriating all the other actual entities and creating one new entity out of them all, namely, itself.
There are two general strategies for distinguishing laws from accidentally true generalizations. The first stands by Hume’s idea that causal connections are mere constant conjunctions, and then seeks to explain why some constant conjunctions are better than others. That is, this first strategy accepts the principle that causation involves nothing more than certain events always happening together with certain others, and then seeks to explain why some such patterns - the ‘laws’ - matter more than others - the ‘accidents’ -. The second strategy, by contrast, rejects the Humean presupposition that causation involves nothing more than happen-stantial co-occurrence, and instead postulates a relationship ‘necessitation’, a kind of ‘cement, which links events that are connected by law, but not those events (like being a screw in my desk ad being made of copper) that are only accidentally conjoined.
There are a number of versions of the first Human strategy. The most successful, originally proposed by the Cambridge mathematician and philosopher F.P. Ramsey (1903-30), and later revived by the American philosopher David Lewis (1941-2002), who holds that laws are those true generalizations that can be fitted into an ideal system of knowledge. The thought is, that, the laws are those patterns that are somewhat explicated in terms of basic science, either as fundamental principles themselves, or as consequences of those principles, while accidents, although true, have no such explanation. Thus, ‘All water at standard pressure boils at 1000 C’ is a consequence of the laws governing molecular bonding: But the fact that ‘All the screws in my desk are copper’ is not part of the deductive structure of any satisfactory science. Ramsey neatly encapsulated this idea by saying that laws are ‘consequences of those proposition which we should take as axioms if we knew everything and organized it as simply as possible in a deductive system’.
Advocates of the alternative non-Humean strategy object that the difference between laws and accidents is not a ‘linguistic’ matter of deductive systematization, but rather a ‘metaphysical’ contrast between the kind of links they report. They argue that there is a link in nature between being at 1000 C and boiling, but not between being ‘in my desk’ and being ‘made of copper’, and that this is nothing to do with how the description of this link may fit into theories. According to D.M. Armstrong (1983), the most prominent defender of this view, the real difference between laws and accidentals, is simply that laws report relationships of natural ‘necessitation’, while accidents only report that two types of events happen to occur together.
Armstrong’s view may seem intuitively plausible, but it is arguable that the notion of necessitation simply restates the problem, than solving it. Armstrong says that necessitation involves something more than constant conjunction: If two events e related by necessitation, then it follows that they are constantly conjoined, but two events can be constantly conjoined without being related by necessitation, as when the constant conjunction is just a matter of accident. So necessitation is a stronger relationships than constant conjunction. However, Armstrong and other defenders of this view say ver y little about what this extra strength amounts to, except that it distinguishes laws from accidents. Armstrong’s critics argue that a satisfactory account of laws ought to cast more light than this on the nature of laws.
Hume said that the earlier of two causally related events is always the cause, and the later effect. However, there are a number of objections to using the earlier-later ‘arow of time’ to analyse the directional ‘arrow of causation’. For a start, it seems in principle, possible that some causes and effects could be simultaneous. That more, in the idea that time is directed from ‘earlier’ to ‘later’ itself stands in need of philosophical explanation - and one of the most popular explanations is that the idea of ‘movement’ from earlier to later depends on the fact that cause-effect pairs always have a time, and explain ‘earlier’ as the direction in which causes lie, and ‘later’ as the direction of effects, that we will clearly need to find some account of the direction of causation which does not itself assume the direction of time.
A number of such accounts have been proposed. David Lewis (1979) has argued that the asymmetry of causation derives from an ‘asymmetry of over-determination’. The over-determination of present events by past events - consider a person who dies after simultaneously being shot and struck by lightning - is a very rare occurrence, by contrast, the multiple ‘over-determination’ of present events by future events is absolutely normal. This is because the future, unlike the past, will always contain multiple traces of any present event. To use Lewis’s example, when the president presses the red button in the White House, the future effects do not only include the dispatch of nuclear missiles, but also the fingerprint on the button, his trembling, the further depletion of his gin bottle, the recording of the button’s click on tape, he emission of light waves bearing the image of his action through the window, the warnings of the wave from the passage often signal current, and so on, and so on, and on.
Lewis relates this asymmetry of over-determination to the asymmetry of causation as follows. If we suppose the cause of a given effect to have been absent, then this implies the effect would have been absent too, since (apart from freaks like the lightning-shooting case) there will not be any other causes left to ‘fix’ the effect. By contrast, if we suppose a given effect of some cause to have been absent, this does not imply the cause would have been absent, for there are still all the other traces left to ’fix’ the causes. Lewis argues that these counterfactual considerations suffice to show why causes are different from effects.
Other philosophers appeal to a probabilistic variant of Lewis’s asymmetry. Following, the philosopher of science and probability theorists, Hans Reichenbach (1891-1953), they note that the different causes of any given type of effect are normally probabilistically independent of each other, by contrast, the different effects of any given type of cause are normally probabilistically correlated. For example, both obesity and high excitement can cause heart attacks, but this does not imply that fat people are more likely to get excited than thin ones: Its facts, that both lung cancer and nicotine-stained fingers can result from smoking does imply that lung cancer is more likely among people with nicotine-stained fingers. So this account distinguishes effects from causes by the fact that the former, but not the latter are probabilistically dependent on each other.
However, there is another course of thought in philosophy of science, the tradition of negative or eliminative induction. From the English statesman and philosopher Francis Bacon (1561-1626) and in modern time the philosopher of science Karl Raimund Popper (1902-1994), we have the idea of using logic to bring falsifying evidence to bear on hypotheses about what must universally be the case that many thinkers accept in essence his solution to the problem of demarcating proper science from its imitators, namely that the former results in genuinely falsifiable theories whereas the latter do not. Although falsely allowed many people’s objections to such ideologies as psychoanalysis and Marxism.
Hume was interested in the processes by which we acquire knowledge: The processes of perceiving and thinking, of feeling and reasoning. He recognized that much of what we claim to know derives from other people secondhand, thirdhand or worse: Moreover, our perceptions and judgements can be distorted by many factors - by w hat we are studying, as well as by the very act of study itself., the main reason, however, behind his emphasis on ‘probabilities and those other measures of evidence on which life and action entirely depend’ is this:
It is evident that all reasoning concerning ‘matter of fact’ are founded on the relation of cause and effect, and that we can never infer the existence of one object from another unless the are connected together, either mediately or immediately.
When we apparently observe a whole sequence, say of one ball hitting another, what exactly do we observe? And in the much commoner cases, when we wonder about the unobserved causes or effects of the events we observe, what precisely are we doing?
Hume recognized that a notion of ‘must’ or necessity is a peculiar feature of causal relation, inference and principles, and challenges us to explain and justify the notion. He argued that there is no observable feature of events, nothing like a physical bond, which can be properly labelled the ‘necessary connection’ between a given cause and its effect: Events simply are, they merely occur, and there is in ‘must’ or ‘ought’ about therm. However, repeated experience of pairs of events sets up the habit of expectation in us, such that when one of the pair occurs we inescapably expect the other. This expectation makes us infer the unobserved cause or unobserved effect of the observed event, and we mistakenly project this mental inference on to the events themselves. There is no necessity observable in causal relations; all that can be observed is regular sequence, here is necessity in causal inferences, but only in the mind. Once we realize that causation is a relation between pairs of events. We also realize that often we are not present for the whole sequence e which we want to divide into ‘cause’ and ‘effect’. Our understanding of the casual relation is thus intimately linked with the role of the causal inference cause only causal inferences entitle us to ‘go beyond what is immediately present to the senses’. But now two very important assumptions emerge behind the causal inference: The assumptions that ‘like causes, in like circumstances, will always produce like effects’, and the assumption that ‘the course of nature will continue uniformly the same’ - or, briefly that the future will resemble the past. Unfortunately, this last assumption lacks either empirical or a priori proof, that is, it can be conclusively established neither by experience nor by thought alone.
Hume frequently endorsed a standard seventeenth-century view that all our ideas are ultimately traceable, by analysis, to sensory impressions of an internal or external kind. Accordingly, he claimed that all his theses are based on ‘experience’, understood as sensory awareness together with memory, since only experience establishes matters of fact. But is our belief that the future will resemble the past properly construed as a belief concerning only a mater of fact? As the English philosopher Bertrand Russell (1872-1970) remarked, earlier this century, the real problem that Hume raises is whether future futures will resemble future pasts, in the way that past futures really did resemble past pasts. Hume declares that ‘if . . . the past may be no rule for the future, all experience become useless and can give rise to inference or conclusion. And yet, he held, the supposition cannot stem from innate ideas, since there are no innate ideas in his view nor can it stem from any abstract formal reasoning. For one thing, the future can surprise us, and no formal reasoning seems able to embrace such contingencies: For another, even animals and unthinkable people conduct their lives as if they assume the future resembles the past: Dogs return for buried bones, children avoid a painful fire, and so forth. Hume is not deploring the fact that we have to conduct our lives on the basis of probabilities, and he is not saying that inductive reasoning could or should be avoided or rejected. Rather, he accepted inductive reasoning but tried to show that whereas formal reasoning of the kind associated with mathematics cannot establish or prove matters of fact, factual or inductive reasoning lacks the ‘necessity’ and ‘certainty’ associated with mathematics. His position, therefore clear; because ‘every effect is a distinct event from its cause’, only investigation can settle whether any two particular events are causally related: Causal inferences cannot be drawn with the force of logical necessity familiar to us from a priori reasoning, but, although they lack such force, they should not be discarded. In the context of causation, inductive inferences are inescapable and invaluable. What, then, makes ‘past experience’ the standard of our future judgement? The answer is ‘custom’, it is a brute psychological fact, without which even animal life of a simple kind would be more or less impossible. ‘We are determined by custom to suppose the future conformable to the past’ (Hume, 1978), nevertheless, whenever we need to calculate likely events we must supplement and correct such custom by self-conscious reasoning.
Nonetheless, the problem that the causal theory of reference will fail once it is recognized that all representations must occur under some aspect or that the extentionality of causal relations is inadequate to capture the aspectual character of reference. The only kind of causation that could be adequate to the task of reference is intentional causal or mental causation, but the causal theory of reference cannot concede that ultimately reference is achieved by some met device, since the whole approach behind the causal theory was to try to eliminate the traditional mentalism of theories of reference and meaning in favour of objective causal relations in the world, though it is at present by far the most influential theory of reference, will prove to be a failure for these reasons.
If mental states are identical with physical states, presumably the relevant physical states are various sorts of neural states. Our concepts of mental states such as thinking, sensing, and feeling are of course, different from our concepts of neural states, of whatever sort. But that is no problem for the identity theory. As J.J.C. Smart (1962), who first argue for the identity theory, emphasized, the requisite identities do not depend on understanding concepts of mental states or the meanings of mental terms. For ‘a’ to be the identical with ‘b’, ‘a’, and ‘b’ must have exactly the same properties, but the terms ‘a’ and ‘b’ need not mean the same. Its principal means by measure can be accorded within the indiscernibility of identicals, in that, if ‘A’ is identical with ‘B’, then every property that ‘A’ has ’B’, and vice versa. This is, sometimes known as Leibniz’ s Law.
But a problem does seem to arise about the properties of mental states. Suppose pain is identical with a certain firing of c-fibres. Although a particular pain is the very same as a neural-firing, we identify that state in two different ways: As a pain and as neural-firing. that the state will therefore have certain properties in virtue of which wee identify it as pain and others in virtue of which we identify it as an excitability of neural firings. The properties in virtue of which we identify it as a pain will be mental properties, whereas those in virtue of which ewe identify it as neural excitability firing, will be physical properties. This has seemed to many to lead to a kind of dualism at the level of the properties of mental states, even if we reject dualism of substances and take people simply to be physical organisms, those organisms still have both mental and physical states. Similarly, even if we identify those mental states with certain physical states, those states will, nonetheless have both mental and physical properties. So disallowing dualism with respect to substances and their states simply es to its reappearance at the level of the properties of those states.
There are two broad categories of mental property. Mental states such as thoughts and desires, often called ‘propositional altitudes’, have ‘content’ that can be de scribed by ‘that’ clauses. For example, one can have a thought, or desire, that it will rain. These states are said to have intentional properties, or ‘intentionality sensations’, such as pains and sense impressions, lack intentional content, and have instead qualitative properties of various sorts.
The problem about mental properties is widely thought to be most pressing for sensations, since the painful qualities of pains and the red quality of visual sensations seem to be irretrievably non-physical. And if mental states do actually have non-physical properties, the identity of mental states generate to physical states as they would not sustain a thoroughgoing mind-body materialism.
The Cartesian doctrine that the mental is in some way non-physical is so pervasive that even advocates of the identity theory sometimes accepted it, for the ideas that the mental is non-physical underlies, for example, the insistence by some identity theorists that mental properties are really neural as between being mental or physical. To be neural is in this way, a property would have to be neutral as to whether its mental at all. Only if one thought that being meant being non-physical would one hold that defending materialism required showing the ostensible mental properties are neutral as regards whether or not they’re mental.
But holding that mental properties are non-physical has a cost that is usually not noticed. A phenomenon is mental only if it has some distinctively mental property. So, strictly speaking, a materialist who claims that mental properties are non-physical phenomena exist. This is the ‘Eliminative-Materialist position advanced by the American philosopher and critic Richard Rorty (1979).
According to Rorty (1931-) ‘mental’ and ‘physical’ are incompatible terms. Nothing can be both mental and physical, so mental states cannot be identical with bodily states. Rorty traces this incompatibly to our views about incorrigibility: ‘Mental’ and ‘physical’ are incorrigible reports of one’s own mental states, but not reports of physical occurrences, but he also argues that we can imagine a people who describe themselves and each other using terms just like our mental vocabulary, except that those people do not take the reports made with that vocabulary to be incorrigible. Since Rorty takes a state to be a mental state only if one’s reports about it are taken to be incorrigible, his imaginary people do not ascribe mental states to themselves or each other. Nonetheless, the only difference between their language and ours is that we take as incorrigible certain reports which they do not. So their language as no less descriptive or explanatory power than ours. Rorty concludes that our mental vocabulary is idle, and that there are no distinctively mental phenomena.
This argument hinges on building incorrigibly into the meaning of the term ‘mental’. If we do not, the way is open to interpret Rorty’s imaginary people as simply having a different theory of mind from ours, on which reports of one’s own mental stares are not incorrigible. Their reports would this be about mental states, as construed by their theory. Rorty’s thought experiment would then provide to conclude not that our terminology is idle, but only that this alternative theory of mental phenomena is correct. His thought experiment would thus sustain the non-eliminativist view that mental states are bodily states. Whether Rorty’s argument supports his eliminativist conclusion or the standard identity theory, therefore, depends solely on whether or not one holds that the mental is in some way non-physical.
Paul M. Churchlands (1981) advances a different argument for eliminative materialism. According to Churchlands, the common-sense concepts of mental states contained in our present folk psychology are, from a scientific point of view, radically defective. But we can expect that eventually a more sophisticated theoretical account will relace those folk-psychological concepts, showing that mental phenomena, as described by current folk psychology, do not exist. Since, that account would be integrated into the rest of science, we would have a thoroughgoing materialist treatment of all phenomena, unlike Rorty’s, does not rely of assuming that the mental is non-physical.
But even if current folk psychology is mistaken, that does not show that mental phenomena does not exist, but only that they are of the way folk psychology described them as being. We could conclude they do not exist only if the folk-psychological claims that turn out to be mistaken actually define what it is for a phenomena to be mental. Otherwise, the new theory would be about mental phenomena, and would help show that they’re identical with physical phenomena. Churchlands argument, like Rorty’s, depends on a special way of defining the mental, which we need not adopt, its likely that any argument for Eliminative materialism will require some such definition, without which the argument would instead support the identity theory.
Despite initial appearances, the distinctive properties of sensations are neutral as between being mental or physical, in that borrowed from the English philosopher and classicist Gilbert Ryle (1900-76), they are topic neutral: My having a sensation of red consists in my being in a state that is similar, in respect that we need not specify, even so, to something that occurs in me when I am in the presence of certain stimuli. Because the respect of similarity is not specified, the property is neither distinctively mental nor distinctively physical. But everything is similar to everything else in some respect or other. So leaving the respect of similarity unspecified makes this account too weak to capture the distinguishing properties of sensation.
A more sophisticated reply to the difficultly about mental properties is due independently to forthright Australian David Malet Armstrong (1926-) and American philosopher David Lewis (1941-2002), who argued that for a state to be a particular sort of intentional state or sensation is for that state to bear characteristic causal relations to other particular occurrences. The properties in virtue of which e identify states as thoughts or sensations will still be neural as between being mental or physical, since anything can bear a causal relation to anything else. But causal connections have a better chance than similarity in some unspecified respect to capturing the distinguishing properties of sensations and thought.
This casual theory is appealing, but is misguided to attempt to construe the distinctive properties of mental states as being neutral as between being mental; or physical. To be neutral as regards being mental or physical is to be neither distinctively mental nor distinctively physical. But since thoughts and sensations are distinctively mental states, for a state to be a thought or a sensation is perforce for it to have some characteristically mental property. We inevitably lose the distinctively mental if we construe these properties as being neither mental nor physical.
Not only is the topic-neutral construal misguided: The problem it was designed to solve is equally so, only to say, that problem stemmed from the idea that mental must have some non-physical aspects. If not at the level of people or their mental states, then at the level of the distinctively mental properties of those states. However, it should ne mentioned, that properties can be more complicated, for example, in the sentence, ‘John is married to Mary’, we are attributing to John the property of being married, and unlike the property of John is bald. Consider the sentence: John is bearded. The word ‘John’ in this sentence is a bit of language - a name of some individual human being - and more some would be tempted to confuse the word with what it names. Consider the expression ‘is bald’, this too is a bit of language - philosopher call it a ‘predicate’ - and it brings to our attention some property or feature which, if the sentence is true,. Is possessed by John. Understood in this ay, a property is not its self linguist though it is expressed, or conveyed by something that is, namely a predicate. What might be said that a property is a real feature of the word, and that it should be contrasted just as sharply with any predicates we use to express it as the name ‘John’ is contrasted with the person himself. Controversially, just what sort of ontological status should be accorded to properties by describing ‘anomalous monism’, - while its conceivably given to a better understanding the similarity with the American philosopher Herbert Donald Davidson (1917-2003wherefore he adopts a position that explicitly repudiates reductive physicalism, yet purports to be a version of materialism, nonetheless, Davidson holds that although token mental event nd states are identical to those of physical events and states - mental ‘types’ - i.e., kinds, and/or properties - are neither to, nor nomically co-existensive with, physical types. In other words, his argument for this position relies largely on the contention that the correct assignment of mental a actionable properties to a person is always a holistic matter, involving a global, temporally diachronic, ‘intentional interpretation’ of the person. But as many philosophers have in effect pointed out, accommodating claims of materialism evidently requires more than just repercussions of mental/physical identities. Mentalistic explanation presupposes not merely that metal events are causes but also that they have causal/explanatory relevance as mental - i.e., relevance insofar as they fall under metal kinds or types. Nonetheless, Davidson’s position, which denies there are strict psychological or psychological laws, can accommodate the causal/explanation relevance of the mental quo mental: If to ‘epiphenomenalism’ with respect to mental properties.
But the idea that the mental is in some respect non-physical cannot be assumed without argument. Plainly, the distinctively mental properties of the mental states are unlikely any other properties we know about. Only mental states have properties that are at all like the qualitative properties that anything like the intentional properties of thoughts and desires. Bu t this does not show that the mental properties are not physical properties, not all physical properties like the standard states: So, mental properties might still be special kinds of physical properties. Its question beginning to assume otherwise. The doctrine that the mental properties is simply an expression of the Cartesian doctrine that the mental is automatically non-physical.
Its sometimes held that properties should count as physical properties inly if they can be defined using the terms of physics. This to far to restrictively. Nobody would hold that to reduce biology to physics, for example, we must define all biological properties using only terms that occur in physics. And even putting ‘reduction’ aside, I certain biological properties could have been defined, that would not mean that those properties were in n way non-physical. The sense of ‘physical’ that is relevant, that is of its situation it must be broad enough to include not only biological properties, but also most common-sense, macroscopic properties. Bodily states are uncontroversially physical in the relevant way. So, we can recast the identity theory as asserting that mental states are identical with bodily state.
In the course of reaching conclusions about the origin and limits of knowledge, Locke had occasioned concern himself with topics which are of philosophical interest in themselves. On of these is the question of identity, which includes, more specifically, the question of personal identity: What are the criteria by which a person at one time is numerically the same person as a person encountering of time? Locke points out whether ‘this is what was here before, it matters what kind of thing ‘this’ is meant to be. If ‘this’ is meant as a mass of matter then it is what was before so long as it consists of the same material panicles, but if it is meant as a living body then its considering of the same particles does mot matter and the case is different. ‘A colt grown up to a horse, sometimes fat, sometimes lean, is all the while the same horse though . . . there may be a manifest change of the parts. So, when we think about personal identity, we need to be clear about a distinction between two things which ‘the ordinary way of speaking runs together’ - the idea of ‘man’ and the idea of ‘person’. As with any other animal, the identity of a man consists ‘in nothing but a participation of the same continued life, by constantly fleeting particles of matter, in succession initially united to the same organized body, however, the idea of a person is not that of a living body of a certain kind. A person is a ‘thinking’. ‘intelligent being, that has son and reflection and such a being ‘will be the same self as far as the same consciousness can extend to action past or to come’ . Locke is at pains to argue that this continuity of delf-consciousness does not necessarily involve the continuity of some immaterial substance, ion the way that Descartes had held, fo we all know, says Locke, consciousness and thought may be powers which can be possessed by ‘systems of matter fitly disposed’, and even if this is not so the question of the identity of person is not the same as the question of the identity of an ‘immaterial; substance’. For just as the identity of as horse can be preserved through changes of matter and depends not on the identity of a continued material substance of its unity of one continued life. So the identity of a person does not depend on the continuity of a immaterial; substance. The unity of one continued consciousness does not depend on its being ‘annexed’ only to one individual substance, [and not] . . . continued in a succession of several substances. For Lock e, then, personal identity consists in an identity of consciousness, and not in the identity of some substance whose essence it is to be conscious
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