Motor response

“ When a stimulus falls upon the appropriate sensory surface, a wave of molecular movement is sent up the attached sensory nerve to a nerve-centre, which thereupon issues another wave of molecular movement down a motor nerve to the group of muscles over whose action it presides; and when the muscles receive this wave of nervous influence they contract. This kind of response to stimuli is purely mechanical, or non-mental, and is ordinarily termed reflex action. The whole of the spinal cord and lower part of the brain are made up of nerve-centres of reflex action ”

“ More than this, it is a fact that many activities originally voluntary become what is known as "acquired reflexes," or "motor habits," by means of certain nervous centers acquiring the habit of sending forth certain motor impulses in response to certain sensory reports. The familiar movements of our lives are largely performed in this way, as, for instance, walk [Pg 20] ing, using knife and fork, operating typewriters, machines of all kinds, writing, etc. ”

“ To start a mechanism that will produce well-ordered movement, as when I lift this book from the table, impulses or messages, whatever you like to call them, must first be transmitted from the body itself to the brain. Such messages may come to the brain by nerve-fibres from some organ of sense, it may be from the skin, or from the eyes, or from the muscles themselves, but they must be sent to the brain before the brain will send out messages along motor nerves to groups of muscles which are called into action, so as to perform a definite 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. ”

“ On the other hand, we can conceive a center consisting of four sensory and four motor elements, in which, however, the junctions between the sensory and the motor elements are so imperfect that the nerve-currents meet with much resistance. ”

“ 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. ”

“ In lifting an object in the hand I can discover no sensation of volitional energy. I perceive in the first place a slight tension about the head, but that this results from a contraction in the head muscles, and not from a feeling of the brain-discharge, is shown by the simple fact that I get the tension on the right side of the head when I move the right arm, whereas the motor discharge takes place in the opposite side of the brain.... ”

“ Moving objects exert, as is well known, a peculiar motor stimulation upon the eye, they draw our attention and our look after them. If the look really follows them ... we assume that they move. But if the eye, instead of following the moving objects, remains steadfastly at rest, it must be that the constant stimulus to motion which it receives is neutralized by an equally constant current of innervation flowing into its motor apparatus. ”

“ Thus, an impulse which originally spread its effects over the whole body, or at least over many of its movable parts, is gradually determined to a single definite organ, in which it effects the contraction of a few limited muscles. In this change the thoughts and perceptions which start the impulse acquire more and more intimate causal relations with a particular group of motor nerves. ”

“ On the other hand, as a great multitude of motor tracks can open simultaneously in this substance to one and the same excitation from the periphery, this disturbance may subdivide to any extent, and consequently dissipate itself in innumerable motor reactions which are merely nascent. Hence the office of the brain is sometimes to conduct the movement received to a chosen organ of reaction, and sometimes to open to this movement the totality of the motor tracks, so that it may manifest there all the potential reactions with which it is charged, and may divide and so disperse. ”

“ 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. ”

“ In either case the nervous disturbance passes over from the cell where the association resides to the motor area, and from there down to start the act of walking; but as soon as it is started, the reflex takes hold and carries it on without any further thought on our part except as some additional associations enter to cause us to change or perhaps to stop the action. It is possible that the complex muscular acts which we group under the head of grasping are operated in similar fashion. For example, the act of writing is essentially a modified grasping movement. ”

“ Although there is still a good deal of uncertainty respecting the exact composition of the feelings of movement, it may be taken as fairly proved that they include an active element or "feeling of innervation," which is correlated with the central excitation of motor fibers, and a passive element or tactual sensation which is connected with a reflex excitation of sensory fibers, consequent on certain differences in the tensions and mutual pressure of various parts of the skin which result from the movement. ”

“ 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. ”

“ Such objects are of value as educational materials to the degree to which they call for necessary muscular adjustments similar to those which the race from the earliest times has experienced. Every sensation or thought, the psychologists affirm, naturally stimulates a motor adjustment which reinforces the original sensory or central impulse which originated the motion. ”

“ This latter, in turn, is propagated through the motor nerve-element, and on reaching its peripheric extremity causes the fibre of the muscle to vibrate, and this, reacting in virtue of its essential property, produces contraction, and consequently motion.
Now, each of the three elements, sensory, motor, and muscular, lives and dies after a fashion of its own, and there are poisons proper to each. ”

“ To accomplish a true reflex movement, a whole mechanism is necessary, set up in the spinal cord or the medulla. To choose voluntarily between several definite courses of action, cerebral centres are necessary, that is, crossways from which paths start, leading to motor mechanisms of diverse form but equal precision. ”

“ Closely connected with these facts, perhaps only a somewhat different aspect of them, is the phenomenon of motor mechanization. Any movement repeated tends to become a circular reaction, as it is called; that is, the end of one repetition serves as a cue for the beginning of the next. ”

“ It is the universal motor adapted in the most exquisite manner to every form and state of the lungs, which communicate their general motion to every part of the body, and set every wheel in its infinitely complex organism revolving upon its axis, and give to every muscle and nerve the power to expand and contract and vibrate in the performance of its use, and in this way to fill every organ in the whole universe of the body with the power of sensation. ”

“ Because the muscles admit of all these possible stimuli, it does not follow that the brain will ever impart them all. The limits of the motor centers would be the limits of the spontaneous impulses. The workings of the system are brought within a narrow routine, from the deficiency of the nervous matter. ”

“ 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. ”

“ We note in the first place the motor manifestations: movements, gestures, and attitude of the body, a modification in the voice, blushing or pallor, tremors, changes in the secretions or excretions, and other bodily phenomena, varying in different cases. We may observe them in ourselves, in our fellows, and in animals. Although they may not always be motor in the strict sense, we may so call them, since they are all the result of a centrifugal action. ”

“ 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 growing complexity of the nervous system shunts the excitation received on to an ever larger variety of motor mechanisms, and so sketches out simultaneously an ever larger number of possible actions. ”

“ It cannot be used to affect the deep-lying structures, such as the brain, spinal cord, etc. The current-strength is not measured by a milliampere meter. It will produce muscular contraction only through the motor nerve supplying the muscle. ”

“ I see an example of it in the way in which many of them account for our thoughts and resolutions: they call them reflex action. The discovery of the nature of automatic motion was one of the most important discoveries of the last age. An action goes along a nerve to the centre of a ganglion, and comes out in motion by another nerve: thus, if a frog's foot is pricked, it is immediately drawn in. Of much the same kind is the reflex action of the sensori-motor system. ”

“ It is by this reflex character that the function to which I have alluded is to be distinguished from every other. There are, in the animal economy, four modes of muscular action, of muscular contraction. The first is that designated VOLUNTARY: volition, originated in the cerebrum and spontaneous in its acts, extends its influence along the spinal marrow and the motor nerves in a DIRECT LINE to the voluntary muscles. The SECOND is that of RESPIRATION: like volition, the motive influence in respiration passes in a DIRECT LINE from one point of the nervous system to certain muscles ”

“ 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. ”

“ The truth is that the muscles have nothing to do with the passage of the mental currents or waves from the Transmitter to the Receiver any more than they have to do with the transmission of nervous sensations from body to brain, or the motor impulses from brain to body. When you wish to close your hand you send a nervous current from your brain to the muscles controlling your hand. The current travels along the nervous system, and is by it distributed over the muscles causing them to contract. A current from a galvanic battery will cause the muscles to act in the same way. ”

“ He studied the nature of muscular contraction, causing a muscle to record its movements on a smoked glass plate, and he worked out the problem of the velocity of the nervous impulse both in the motor nerves of the frog and in the sensory nerves of man. ”

“ The muscular fibre, which in the first instance may have answered but feebly to the stimulus conducted to it by the motor nerve, does so with the greater energy the more often it is stimulated, provided, of course, that reasonable times are allowed for repose. ”