Studies in the Osteopathic Sciences
The Physiology of Consciousness: Volume 3
Louisa Burns, M.S., D.O., D.Sc.O.
    To a very great extent the development of the human brain is a matter not of increased variety of possible reactions to given environmental changes, but to the increased development of inhibitory powers. The difference between man and the higher animals, then, is seen to be, not that he can do more different things than they can, but that he can refrain from doing more things than they can. And the difference between the higher and lower among human kind is, not that the higher are able to do more than the lower, though this is probably true, but the essential difference is, that the more highly civilized a nation is, in the best use of that term, the more able its citizens are to refrain from doing certain things.
Function of the Inhibitions

    Inefficiency along certain lines of mentality is associated with a lessened number of possible associative reactions. But for the most part a loss of mentality, either as positive insanity or as the neurasthenic or psychasthenic states, is more characterized by a loss of the inhibitory powers than by a loss of the powers of active cerebration. Or, if the mental faculties do seem to be lessened, it is largely because of the lack of the inhibitory powers, thus permitting unwise and improper reactions to occur to the exclusion of those more carefully determined.

    It is by means of the inhibitory activities that the adjustment of complex mechanisms becomes possible. With even the most complex possibilities of reaction, were every incoming impulse to be followed immediately by one of these, the powers of choice must be certainly limited. But with the action of the cells which inhibit activity, those nerve impulses derived from past experience, even though they be revived only through the activity of many cells, are enabled to affect the reaction, whether it affects consciousness or whether merely some lower center be affected.

Frontal Inhibitions

    There is reason to believe that the frontal lobes of the brain are concerned with those associative processes which relate the personality of an individual with his environment. Other investigators see in the frontal lobes an immense center for inhibitory impulses. It is possible that both these views are correct. If the frontal lobes are chiefly concerned with the inhibition of the nerve impulses too hastily transformed into efferent impulses by the lower centers, then it is evident that this very function of retardation renders these lobes most efficient as controlling agents for the relating of the person to his environment. On the other hand, if the frontal lobes were, as their phylogenetic development and their structural relations seem to indicate, chiefly concerned with the determination of the individual’s own place in the midst of things, then the increase of their inhibitory powers would follow speedily.

    That the activity of the frontal lobes does exert an inhibitory power upon the activities of the lower centers is shown by the results of the injury of these lobes. Patients whose frontal lobes have been subjected to injury by trauma, pressure, or other pathological conditions show very great lack of the normal self-control. They yield unquestioned obedience to the dictates of anger, amusement, suspicion, or any other passion. The loss of this self-control is not seen in like degree in injuries of other parts of the brain, though it must not be forgotten that pressure abnormalities are transmitted by the almost fluid brain substance without interference.
The inhibitory action of the frontal lobes renders them of tremendous value n adding to the wisdom of all conscious reactions, and of many of those which are not present in consciousness.

Physiology of the Inhibitions

    When the lower centers are stimulated by incoming impulses reaching them, the tendency is, of course, toward immediate discharge of the nerve impulses resulting from that stimulation as efferent or motor impulses; that is, if any center whose duty is the coordination of any certain action receives impulses increasing its activity from other parts of the nervous system, the effect of that stimulation is to cause immediate activity of that center, and thus an immediate reaction of a type depending upon the nature of the stimulation and the physiological condition of the center affected. This immediate reaction is exactly what occurs in the simpler reflexes and in the lower animals, for the most part. But just to the extent that the inhibitory powers of the higher centers, and especially the frontal lobes, are developed, just to that extent there is produced, not the prevention of the reaction, but its slight postponement. Thus, the center affected may receive impulses from other parts of the cerebral cortex, from the nerve cells in which are stored memories of previous related reactions and their results, from other nerve cells in which are stored memories of the nerve activity dependent upon the association of these experiences, from other cells in which are stored memories of the experiences of other people, of historical events, of various ethical or esthetical teachings. As a result of this inhibition, this temporary postponement of the reaction, the nature of the efferent impulses may be considerably modified. Perhaps it may be altogether prevented, perhaps much reinforced, perhaps modified in various manners. In any event the resultant reaction is the effect, not only of the certain immediate stimulation, but also of all the experiences of the past of the individual, and of the past histories of many other people. Thus the reaction is, or ought to be, much wiser than would be possible if sensory stimulation were followed by immediate motor activity.
It must not be forgotten that the physiological value of the inhibitions is to postpone temporarily the motor impulses and not to repress indefinitely any reactions. The temporary inhibition may be followed by indefinite postponement of the reaction, and the nature of the reaction may be greatly modified. If the algebraic sum of the impulses reaching the center whose activity has been postponed should equal zero, then probably no reaction would occur.

Inhibition of the Emotions

    The centers most affected by this inhibition are those concerned in the emotional reactions. The physiology of these centers is discussed elsewhere. Inasmuch as these centers are those in which are coordinated the impulses concerned in securing the safety of the person and his race, and since in these centers the manner of the reaction is that dependent upon racial rather than upon individual history, we have these presenting, in their unmodified activities, the spectacles of the most destructive, the most suicidal, and the most dangerous of all reactions possible to animals or man. It is by the uncontrolled activity of the emotional centers that the most destructive actions are performed. By means of the inhibitory action of the higher centers these emotional reactions are subject to control, and thus serve as a source of much strength and pleasure in the performance of life’s duties. It is, however, only when the relationship between the higher and inhibitory centers is normal and exact that these reactions give all of their proper force to life, and all of their normal enjoyment to those who live. With unimpeded expression, without guidance, these powers are frittered and lost, and in their expression destruction is certain. With the postponing effects of the inhibitions, the excessive and destructive action of the emotions is prevented, while under other circumstances the sane and chosen expression of the same emotions may be increased to a force beyond the unimpeded reactions, and the pleasure derived from that expression becomes increased to a degree unimagined by the person in whom the effects of inhibition are not manifest.

    The activity of many of the reactions which are not ever or not usually a part of consciousness is postponed in the same manner. Even the action of the spinal centers is subject to the inhibition of higher centers. It thus occurs that under normal conditions there is probably never an immediate response to afferent impulses, nor an immediate reaction following even the most exactly coordinated series of complex associative impulses. In other words, there is present always, in the action of the very lowest as well as of the highest centers, some inhibitory, some postponing impulses, by means of which too hasty or too unguarded, or too unreserved reactions are prevented. Thus, there is afforded the time needful for each center to receive from other related centers those impulses which are concerned in the coordinate reactions of many centers; thus, too, the effects of the memory impulses, whether present in consciousness or not, are permitted to affect the ultimate reaction.

    Because of this postponing condition the reaction of any center may be greatly increased. If the experience of the individual and of his race should be of a nature to show that the proposed reaction is for great good, for the prevention of great evil, then the effect of the postponement of the reaction increases its force and renders the action itself more permanent. On the other hand, if the experience of the individual and of the race is such as to indicate that the proposed reaction is better modified, then that modification must inevitably ensue. It is not probable that a complete prevention of any reaction often occurs. But the proposed reaction may be so modified as to become unrecognizable. For instance, the first effect of the presence of a burglar may be to kill him, but the second thought may lead to efforts toward his greater comfort and his reformation. So anger may be turned to pity, or pity to anger, under other circumstances.

Education of the Inhibitions

    The inhibitory powers, like any others, are subject to educational influences. The child whose every impulse is afforded immediate and unimpeded expression becomes erratic, uncertain, easy to anger, and not in the least to be depended upon as a member of society. On the other hand, the child whose every expression is made the subject of discussion and reproof, who is permitted no speedy and unthoughtful reaction, who stops to consider the effects of every action, such a one has excessively developed his inhibitory powers, and becomes inert, helpless, undecided. A rational development of the inhibitory powers is greatly to be desired. The ability to decide quickly and yet wisely, that is the desideratum. This is to be secured only by training the postponement and not the prevention of reactions. Let the child be taught to think about the effects of his performances, then let him be taught to decide quickly and finally the nature of his subsequent performances. The rational uses of the inhibitory powers, it must be recognized, are to postpone the reactions only just long enough to permit the receipt of other impulses from other centers, and not to prevent reactions. Thus, the nature of any reaction is subject to the effects of many experiences, but the fact that some reaction must occur as an expression of the recognition of certain environmental changes is probably inevitable. For example, if anything occurs which under ordinary circumstances would arouse the emotion of fear, there must, probably, be some reaction in answer to this environmental change. The inhibitions postpone immediate reaction. The effects of the impulses from other centers, from nerve cells concerned with the memories of the past experiences of the person himself and others with whom he has been associated and of whom he has heard or read, may cause his first impulse toward flight to be superseded by movements toward investigation, or the effect of his further consideration may cause him to run the faster. But it is very doubtful if ever the need for some answer to the original stimulation is neglected.

The Nature of the Inhibitions

    The nature of the inhibitions has long been discussed. No satisfactory explanation has yet been offered for the phenomenon; the following is only tentative and is given with a full appreciation of the difficulties of explaining the matter in any feasible manner:

    In the first place, it is known that the subminimal stimulation of a neuron may produce a latent period similar to that produced by efficient stimulation. This latent period may be increased by recurring slight stimuli, if these occur at intervals too great for the summation of stimuli to occur. It is known, also, that impulses which are oridinarily inhibitory are carried over the same nerves and tracts as those employed in transmitting impulses which increase the activity of the same structures. The descending impulses from the cortex to the spinal centers, for example, seem to be carried by the same tracts which carry the ordinary stimulating impulses. The effects of the removal or the disease of the cortical areas seem to show that the cell and fiber systems which transmit inhibitory impulses may also, under conditions leading to more powerful stimulation, carry impulses which are efficient in promoting neuron activity. Experimentally, it is shown that inefficient stimuli at longer intervals than those necessary for summation may prevent the stimulation of the neuron group for a considerable time, even in the presence of other sources of stimulation ordinarily effective.

    The psychical side of the phenomenon is shown in a common experience; if inefficient reasons for the performance of any act are brought before any one for a number of times, and the act is not performed, even more powerful reasons—reasons which at first would have easily persuaded the individual to forceful reaction—are unable to arouse his activity. The indifference with which most persons view social and governmental conditions, which would be seen to be unendurable if they had been suddenly produced, is due to this physiological factor. “Too often seen, familiar with its face,” is due, not to the frequent seeing, but to the fact that no efficient stimulus to opposition was produced by the sight of vice. To put inefficient reasons for performing any given act before an unthoughtful person is equivalent to increasing the difficulty of persuading him to that act at a later time. In teaching people, as well as children, or in persuading others to engage in any given line of effort, it is better to present no arguments at all until one has some hope of success in that persuasion. Also, when any given line of argument fails, it is better to cease referring to the matter again until other and more cogent arguments are at hand. This does not apply in those cases in which the repetition of certain teachings may lead to their acceptance. In this case there is no question of reasoning; there can be only the use of the demand or the statement in securing summation of stimuli in affecting the reactions of others. Arguments fail in producing the phenomena of summation probably because all reasoning is associated more or less vividly with coordinating impulses, both stimulatory and inhibitory. The appeal to the feelings and the repetition of facts may be summated, since the nerve reactions concerned in these conditions are not thoroughly coordinated, and the inhibitions are correspondingly less powerful.

Structural Basis

    The structure of the cerebral cortex has been studied with much care, yet very little is known concerning the place of the neurons in affecting consciousness or in causing inhibition. The following schema has been built upon the facts of histology. It is offered with a vivid appreciation of its very slender foundation, yet it is perhaps as well verified as any such schema can be, with our present very limited knowledge and the handicaps under which investigations into the phenomena of consciousness and the inhibitions must be made.

    The external layer of cells of the cortex, the layer of stellate and polymorphic cells, seems to be concerned in consciousness probably altogether. The primary sensory areas, except for smell, contain comparatively few of the cells of the external layer. The sensory overflows, the intermediate and motor areas, all are very well supplied with this layer. The cells are found best developed among the higher races of mankind, and are very feebly developed in the brains of animals. They are first affected by disease in certain degenerative processes associated with deficiencies of consciousness and of self-control. For all these reasons it is believed that these cells are the ones chiefly, if not altogether, concerned in producing the phenomenon called consciousness.

    Inhibition seems to be a function of almost all nerve cells, under certain conditions. The inverted pyramids of Martinotti are especially adapted to the performance of this function. (Fig. 7.) These cells are sometimes found in the brains of animals, but are much more frequently found in the human brain, and are then larger, with more complex structure. They are found most plentifully in the overflow and intermediate areas.

    Fig. 7. Diagram of the elements of the cortex. The arrows show the direction of the nerve impulses. A. Incoming axon from other parts of the nervous system: B. Large pyramidal cell, which may receive the impulse directly from A, or indirectly by the interpolated neurons. The axon of B may transmit the impulse thus received to other parts of the nervous system, without affecting the neurons of the external layers of the cortex. It is probable that consciousness is not affected by these impulses.
    A may transmit the impulses to C, as inverted pyramid of Martinotti, which carries the impulses over its axon to the cells of the stratum zonale, D. The dendrites of either the small, the medium, the large pyramidal cells, as E, may receive the impulses from D, and the large pyramidal cells, as E, may transmit the impulse thus received to other parts of the nervous system.
    F, as an axon from another part of the nervous system. The impulses carried by F may affect the inverted pyramid, G, which in turn may stimulate the cells of the more peripheral layers of the cortex, including H, a cell of the stratum zonale. The large, small and medium pyramids are affected by the action of H and of G, and these affect the cells of the lower layers again. The impulse descending from the stratum zonale may be transmitted to other inverted pyramids, as G and J, and the impulses thus again be carried to the cells of the stratum zonale. There is no way of determining the number of times this reaction may occur. It is probable that this series of impulses passing around this circular path is the physiological basis of inhibition, and thus of the mental process of “thinking things over.” Ultimately the stimulation of I, a large pyramid, or any of the cells of the seventh layer, may carry the impulses to the other parts of the nervous system, and the final destination of these must be the motor neurons.

Physiology of the Inhibitory Neurons

    The impulses concerned in the phenomena of consciousness and the cortical coordinations are probably as follows:

    The sensory impulses from any environmental or physiological change are carried upward by the sensory tracts to the cortex, in which is located the primary area for the class of sensations under consideration. The fibers of the centrum ovale enter the gray matter, passing as far toward the external surface as the line of Bailarger. (In the auditory area the incoming fibers reach the external layer.) The impulses carried by these fibers then affect the cells, with which they form synapses. These cells include the pyramids, the Golgi Type II cells, and the polymorphic cells. The large pyramidal cells thus stimulated may initiate efferent impulses directly, without the intermediation of neurons of other layers. It is probable that when the incoming sensory impulse is of sufficient force, or if the large pyramidal cells have a low enough liminal value, the efferent impulse may be initiated immediately, and the arc thus concerned in the reaction need not include any effect in consciousness. In this way are produced those complex reactions, often the result of habit or of certain slightly abnormal conditions, in which activities which are usually considered necessarily conscious are thus effected without the intermediation of consciousness in any degree.

    The impulses carried by the incoming fibers may affect the inverted pyramids of Martinotti. The axons of these cells pass to the outer layer of the cortex and there form synapses with the spindle-shaped cells and the Golgi Type II cells of that layer. These cells send impulses to the pyramidal cells, which all send dendrites into the stratum zonale. The impulses are carried downward by way of these dendrites to the deeper layers. The giant pyramids may be affected directly by this relationship, and the efferent impulses initiated by these may cause immediate reaction to the incoming stimulus. Or the impulses may be carried by way of the small pyramids; the axons of these send collaterals to the cells of the larger pyramids lying in the deeper layers; the axons of these, in turn, may send impulses to others, until a large number of neurons have been subjected to the stimulation. The efferent impulses are thus very notably increased over those which have resulted if the first incoming stimulus should affect the large pyramids directly. At the same time and in the same manner the impulses carried from cell to cell in the cortex affect those cells, the giant pyramids and the polymorphic cells of the seventh layer, and the axons of these carry the stimulation to the overflow of that sensory area. In the overflow areas the cells are adapted to the retention of the effects of previous stimulations; that is, these cells, probably of the stratum zonale, are capable of being so changed by repeated stimulation that their liminal value is lowered more or less permanently. When the liminal value thus is lowered constantly, less amounts of stimulation are needful for the initiation of nerve impulses by these cells. They send efferent impulses, as all nerve cells do when stimulated, and these impulses are carried to other parts of the cortex. Again, the cells in the stratum zonale are affected, and these bring about the phenomenon of consciousness again. When this occurs, the individual is said to “remember” the circumstances whose occurrence originally stimulated the neuron systems concerned.

    It may happen that the immediate discharge of the efferent impulses does not occur. This postponement of reaction is called inhibition. The impulses sent, either to the primary sense areas or to the overflow areas or to the intermediate areas of the cortex, may be carried to the external layer, as already described, and the descending impulses may be carried to the small pyramids, then by the axons of these to the inverted pyramids of Martinotti. These send axons toward the cortex, the cells of the stratum zonale are again stimulated, and the impulses may be again transmitted by the same pathway of small pyramids, inverted pyramids, and again to the cortex. The efferent impulses may thus be postponed. Each time the stimulation is carried over any given pathway the neuron systems of that pathway have lower liminal value than before, and the tendency is for the succeeding streams of impulses to follow that pathway. So, if any given reaction is once prevented, it becomes easier to prevent it again.

    The reactions just described are probably never so simple as has been given. It seems probable that even the simplest reactions of which we are conscious require the associated activity of many parts of the cortex. But the series of occurrences is probably as described, except that many areas are active at the same time, and consciousness at any given time is the sum of the effects of the cortical activities.

    If the reaction occurs as described for inhibition, the effect produced in consciousness by the series of impulses passing through cells of differing degrees of liminal value, causing repeated stimulations due to the effects of past reactions, is that of “thinking the matter over.” The feeling in consciousness is that of deliberately withholding reaction, whereas really it is the condition of the cortical neurons which prevents the reaction, and the conscious phenomena are merely the effects of this withholding.

    This inhibition permits the impulses sent to other parts of the cortex to affect the cells there, and the impulses initiated by the activities of these cells, affected as they are by the sum of all past activities, result in sending increased stimuli to the overflow area, or the intermediate area, whose cells are concerned in the reaction. These impulses may reinforce the effect of the original stimulation, or they may stimulate more forcibly the inverted pyramids, and thus perpetuate the inhibition. Or the impulses from the area finally affected may initiate the stimulation of other cell groups, and some very different reaction, only distantly related to the original stimulus, may occur. This reaction may be directly opposite to that which would have been expected to follow the original stimulus, if no complicating factors had arisen.

    Any given impulses or series of impulses, or the whole complex of groups of impulses arising from any circumstances, may be held within one of the circular phases of neuron action for some time. It is a matter of common experience that certain reactions may be a part of consciousness for a long time, and that the reaction may be subjected to very much consideration before a judgment is finally decided upon. Now, the physiological action is that of a series of very complex neuron actions. The impulses passing through the circle of neurons, the cells of the stratum zonale, the pyramidal and Golgi Type II and other cells of associational function, reach the inverted pyramids, by means of which the impulses are again carried to the cortex, and so on. At the same time impulses are being carried to other parts of the cortex by these same associational cells; these are in turn stimulated, and the impulses thus initiated add to the delicacy and propriety of the final reaction.

    It may occur that the balance between the liminal value of neuron groups and the effects of the activities of the association areas is so exact that no reaction at all is permitted. Under such circumstances a sort of fatigue results. The neuron groups being subjected to the constant stream of impulses, which are not varied nor permitted to cease, become affected, as the neurons are under similar conditions throughout the nervous system. First, the liminal value is lowered, so that amounts of stimulation originally inefficient may initiate excessive reactions. Under such conditions, whatever environmental changes occur in the life of the individual are apt to affect the cells associated with the active circle. This condition has its place in consciousness as the feeling people have when they are intently pondering some problem; if anything else is perceived at all, it is apt to be perceived as being in some way related to the matter in hand at the time. This is especially true of all matters associated with emotional states, or with feelings of an intensely personal character.

    The tendency seems to be for arcs of nervous pathways to become smaller. This is due to the fact that in complex pathways those cell groups whose liminal value is lowest are most efficiently stimulated. Now, when these circular streams of impulses are produced, the cells most liable to stimulation from other parts of the nervous system react to those impulses, are less irritable to the circular streams, and are finally left out of the circle. The path of the impulses is more easily understood by reference to the diagrams. The series of neuron activities thus produced is the condition known as “split-off” complexes, etc., by various authors.
There may be harm in these circular reactions if the nervous system is slightly abnormal. It is extremely improbable that a normal person should suffer from such conditions unless the original stimulation had been of a very intense and injurious nature.

    If the circular stream of impulses should be permitted to continue indefinitely, it is probable that the neuron groups would be fatigued to the point of inactivity. The further history of the neurons would depend upon their nutritive conditions, the amount and nature of the stimulation reaching them later, and the integrity of the associational structures.