Motor cortex

“ How The Brain Produces Muscular Movements
The motor area is itself aroused to action by nerve currents entering it through axons coming from other parts of the cortex; and it is by way of the motor area that any other part of the cortex produces bodily movement. There are a few exceptions, as, for example, the movements of the eyes are produced generally by the "visual area" acting directly on the lower motor centers for the eye in the brain stem; but, in the main, any motor effect of brain action is exerted through the motor area. ”

“ This last tract is the main medium by which impulses initiated by electrical stimulation of the motor cortex reach the moto-neurones of the cord and through them influence the activity of the skeletal muscles. Of the function of the other tracts descending from the brain into the cord little is known except that mediately or immediately they excite or inhibit the spinal moto-neurones by various levels. How they harmonize one with another in their action or what their purpose in normal life may be is at present little more than conjecture. ”

“ The activity of these sensor centres awakes a corresponding activity in the motor centres of the bulb or spinal cord. There is no reason for postulating motor centres in the cortex; the so-called motor centres of the Rolandic tract are of the kinesthetic type and intimately connected with visual, auditory, and other sensor centres. The process of attention is accordingly essentially sensory, and volition, representing certain phases in the association of ideas, is likewise sensory. The motor processes taking place in the bulb and spinal cord are, however, outside of consciousness. ”

“ The highest centres do probably contain nothing but arrangements for representing impressions and movements, and other arrangements for coupling the activity of these arrangements together. Currents pouring in from the sense-organs first excite some arrangements, which in turn excite others, until at last a discharge downwards of some sort occurs. When this is once clearly grasped there remains little ground for asking whether the motor zone is exclusively motor, or sensitive as well. The whole cortex, inasmuch as{118} currents run through it, is both. ”

“ The motor localization seems to be the most perfect. Indeed, experimentation on the brains of monkeys has been successful in mapping out motor areas so accurately that such small centers as those connected with the bending of one particular leg or the flexing of a thumb have been located. Yet each area of the cortex is so connected with every other area by the millions of association fibers that the whole brain is capable of working together as a unit, thus unifying and harmonizing our thoughts, emotions, and acts. ”

“ Much of the cortex is apparently not very directly connected with either the sensory or the motor areas, and probably is concerned somehow in the recognition of facts that are only very indirectly indicated by any single sensory stimulus, or with the planning of actions that only indirectly issue in muscular movement.
On the sensory and intellectual side, the higher reactions follow the lower: sensation arouses perception and perception thought. On the motor side, the lower reactions are aroused by the higher. ”

“ If we suppose now that the cortex ceases to act as a surface of peripheral excitation, and becomes itself a reflex centre, we note at once a complete change. The modification effected by the cortex on afferent impressions is obvious in altered motor reactions, which appear with the stamp of cortical intervention, herein differing from bulbo-spinal phenomena. ”

“ Hence one of the goals to which each afferent fibre from a receptive organ leads is a number of motor nerves. Their conducting paths must, therefore, converge in passing to the starting-points of the motor nerves; because these latter are instruments common to the use of a number of different receptive organs in so far as they excite reflex actions. On the other hand those of their conducting paths which are concerned in the genesis of sensation, instead of converging, diverge, at least as far as the cortex cerebri, or if not divergent, remain separate. ”

“ Electrical stimulation of this motor strand in any accessible part of its course has been shown in dogs to produce movements analogous to those which excitement of the cortical surface calls forth.
One of the most instructive proofs of motor localization in the cortex is that furnished by the disease now called aphemia, or motor Aphasia. Motor aphasia is neither loss of voice nor paralysis of the tongue or lips. The patient's voice is as strong as ever, and all the innervations of his hypoglossal and facial nerves, except those necessary for speaking, may go on perfectly well. ”

“ It is, in the cases noted, at once sensory and motor, or at any rate involves motor elements. And the effect of the activity of such motor elements is both to increase the distinctness of the image and to prolong the duration of the process by which it is apprehended. The sensory process thus stands in intimate dependence on the motor. Nor would failure to move the eyes or any other organ with the movement of attention, if established, be conclusive as against the presence of motor elements. A motor impulse or idea does not always result in apparent peripheral movement. ”

“ Many physiologists claim that these cells are only excitable by the will. For the theory of motor centers they substitute that of psycho-motors. Reserving this discussion for another time and place, we may say that, whether we excite the cells or the fibers that arise from them, the result is the same.
All stimulation, whatever its nature or origin, acts upon a nerve according to its functions. Excite a motor nerve at any point of its course, and you produce movement; excite a sensory nerve, and the subject will feel a sensation which will vary with the nature of the nerve. ”

“ Thus, a sensation is propagated along the sensory nerve to the sensory nerve-centre, whence it is transmitted to the motor-centre, which acts as an accumulator of stored-up energy, a large flow of which is sent through the large conductor of the motor-nerve to the muscle, which it causes to contract and thus produces motion. It is thus that the simpler involuntary actions are produced by a process which is purely mechanical. In the more complex cases, in which consciousness and will are involved, the process is essentially the same, though more complicated. ”

“ From the motor nerve-cells an impulse or command travels by motor nerve-fibres, one from each cell, to the muscles, which contract. If this took as long to happen as it takes to describe, even in outline, it would not be of much use to the earthworm. But the motor answer follows the sensory stimulus almost instantaneously. The great advantage of establishing or enregistering these reflex chains is that the answers are practically ready-made or inborn, not requiring to be learned. It is not necessary that the brain should be stimulated if there is a brain ”

“ The "weaker and purer" sensory currents furnish "ideas of the environment," while the "motor feelings" only furnish "ideas of the organism." What he means by motor feelings as distinguished from motor impulses following normally upon the discharges of the sensori-motor centers it is difficult to see. He seems to think there can be motor without sensory phenomena, and to forget that all sensory currents, whether peripheral or internal may produce motor effects. ”

“ A lower motor center is a group of motor and central neurones, lying anywhere in the cord or brain stem, and capable of directly arousing a certain coördinated muscular movement. One such unit gives flexion of the leg, another gives extension of the leg, a third gives the rapid alternation of flexion and extension that we see in the scratching movement of the dog. Such a motor center can be aroused to activity by a sensory stimulus, and the resulting movement is then called a reflex. ”

“ The discharge from the motor centre into the motor nerve is supposed to give a sensation sui generis, opposed to all our other sensations. These accompany incoming currents, whilst that, it is said, accompanies an outgoing current, and no movement is supposed to be totally defined in our mind, unless an anticipation of this feeling enter into our idea. The movement's degree of strength, and the effort required to perform it, are supposed to be specially revealed by the feeling of innervation. ”

“ It is rarely only one muscle that is thus excited to activity, there are usually several, and here we have the transition to instinctive action, which consists in a longer or shorter series of actions, that is, of motor combinations. These, too, are originally, at least, set a-going by a sense impression, an external stimulus which affects a sensory nerve exactly in the same way as in the reflex mechanism, and this stimulus is carried to a particular group of sensory nerve-cells in the central nervous organ, and from these transmitted by very fine inter-connexions to motor centres. ”

“ When such an impression is thus conducted simply along a nerve fibre, and thence to a muscle, it induces or excites, as we have seen, the contraction of that muscle, and so exercises what is called a motor function.
The nerve cells appear to possess, beyond the simple excitability to general stimuli, conductility, and the peculiar receptivity which is essential to sensation, a special or more exalted kind of excitability which is called into play under mental or psychical stimuli by the changes produced in the gray matter [5] in the formation of ideas, emotions, and the will. ”

“ The physiologist, having only the material aspect of the organism in view, says, “A cerebral process initiates a motor process”; the psychologist says, “A sensation determines an action.” Unless the two processes have been linked together by an organic disposition, native or acquired, there will be no such motor process following the cerebral process. A dog standing outside the gate is unable to ring the bell, though having seen another dog ring it, he may wish to do so; but the cerebral process (his wish) is not linked on to the needful motor process—he has not learned to realize the wish ”

“ The far-reaching effects of motor expressions are too infrequently emphasized, but psychology forces us to give them prime consideration. We are first apprised of their importance when we study the nervous system, and find that every incoming sensory message pushes on and on until it finds a motor pathway over which it may travel and produce movement. This is inevitable. The very structure and arrangement of the neurones is such that we are obliged to make some movement in response to objects affecting our sense organs. ”

“ The complications may be such as actually to double the reaction time in the case of a sensory reaction, and yet a motor reaction, under precisely the same conditions as far as they may be the same for a motor, shows no increase in time. The "set" of the attention in the motor reaction is, no doubt, such a change in the order of succession of the parts of the process that some of those, which come after the stimulus is received in the case of the sensory reaction, are made to come before the stimulus in the motor. ”

“ Modern physiology, from its objective point of view, is engaged over finer and finer issues of microscopic and experimental work; and, from its subjective point of view, is becoming more and more psychological, seeking a higher level of interpretation, and a statement of the departmental life of the brain in terms of ever-increasing complexity. The motor centres, governing the voluntary purposeful movements of the body, are considered to be not simply motor, but “psycho-motor” ”

“ The difference at first appears to be only a difference of size and complexity. But, judging by function, there must be something else besides. In the animal, the motor mechanisms that the brain succeeds in setting up, or, in other words, the habits contracted voluntarily, have no other object nor effect than the accomplishment of the movements marked out in these habits, stored in these mechanisms. But, in man, the motor habit may have a second result, out of proportion to the first: it can hold other motor habits in check, and thereby, in overcoming automatism, set consciousness free. ”

“ Each of these successive degrees, which measures a growing intensity of life, corresponds to a higher tension of duration and is made manifest externally by a greater development of the sensori-motor system. But let us consider this nervous system itself: we note that its increasing complexity appears to allow an ever greater latitude to the activity of the living being, the faculty of waiting before reacting, and of putting the excitation received into relation with an ever richer variety of motor mechanisms. ”

“ How can we help being struck by the fact that, while man is capable of learning any sort of exercise, of constructing any sort of object, in short of acquiring any kind of motor habit whatsoever, the faculty of combining new movements is strictly limited in the best-endowed animal, even in the ape? The cerebral characteristic of man is there. The human brain is made, like every brain, to set up motor mechanisms and to enable us to choose among them, at any instant, the one we shall put in motion by the pull of a trigger. ”