Motor nerve

“ A single nerve impulse generated in a motor nerve causes in the muscle a brief contraction which is called a twitch, and lasts a tenth of a second. A single nerve impulse generated in an afferent nerve sometimes fails on arriving at the spinal centre to evoke any observable reflex effect at all, but if it is effective the muscle contraction tends to be longer than a " twitch," often much longer. It is therefore questioned whether the spinal centre when excited even most briefly ever discharges one single centrifugal impulse only; it seems usually to discharge a short series of such impulses. ”

“ The different motor nerves most commonly excited are the facial nerves, the nervous branches of the forearm or the fingers, which are affected in "writer's cramp," and the branches of the spinal nerve, whose irritation occasions tic-douloureux, chronic wryneck, etc. Now, electricity cures, or at least noticeably benefits these different morbid states, and exerts the like influence over neuralgic and neuritic affections, wherever these disorders are not the symptoms of other deeper maladies. Currents restore the normal activity of nutrition in the diseased nerves, and the corresponding muscles ”

“ One of the strangest facts known about the system of inhibitory nerves is their marked tendency to leave the central nervous system at a different level to the corresponding motor nerves, as is well known in the case of the heart, where the inhibitory nerve—the vagus—arises from the medulla oblongata, while the motor nerve—the augmentor or accelerator—leaves the spinal cord in the upper thoracic region. ”

“ Mechanical stimuli, blows, concussions, pressure, traction, etc., excite the nerves, causing sensations when applied to sensitive nerves, or contraction of muscles when applied to motor nerves. When mechanical stimuli are pushed farther, the irritability of the nerves may be destroyed. The gradual application of mechanical stimuli may destroy the irritability of nerve-fibres without any exhibition of excitation, as in paralysis from pressure. In nerve-stretching it is probable that many of the results depend upon the mechanical stimulation of the nerve-fibres by the traction. ”

“ As motor nerves, when irritated, produce movement, because they are connected with muscles, and glandular nerves secretion, because they lead to glands, so do sensitive nerves, when they are irritated, produce sensation, because they are connected with sensitive organs.... Whether by the irritation of a nerve we produce a muscular movement, a secretion or a sensation depends upon whether we are handling a motor, a glandular, or a sensitive nerve, and not at all upon what means of irritation we may use. ”

“ Paralysis occurs whenever the motor nerves are separated from the nervous centres by any injuring cause, or by any modification of texture impairing their sensitiveness. With a destroyed nerve, paralysis is incurable, but, in case of its disease only, its functions can almost always be restored by electric treatment. As there is always some degree of muscular atrophy in the case, electricity is directed upon the nerves and the muscles at once, and the battery and the induction current usually employed together. ”

“ Although the stimulation of muscles usually comes through a nerve-centre, yet since the muscles do not derive their Contractility from nerve-centres any stimulation will suffice. Now since we have abundant proof that sensory nerves are stimulated by certain organic changes, by poisons in the blood, excess of carbonic acid, etc., we are justified in concluding that motor nerves will be stimulated in like manner, and thus muscular movement be produced occasionally without the intervention of a centre. ”

“ 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. ”

“ It will, of course, be objected that the muscular element receives its conditions of activity from the motor nerves, and that the blood-globules only vivified it indirectly, by restoring excitability to the nerves; but has not the mode of action of curare proved that the life of the muscle persisted after the physiological death of the nerve? If the arteries of the lower limbs of a living animal are tightened by a compress, the withdrawal of the blood will in the same way cause the properties of the nerves to disappear before those of the muscles ”

“ The lower motor centers, called also reflex centers, are located in the cord or brain stem, and their nerve cells give rise to the axons that form the motor nerves and connect with the muscles and glands. A muscle is thrown into action by nerve currents from its lower motor center. ”

“ 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 ”

“ When the interrupted current is used to stimulate the skin over a motor nerve, all the muscles supplied by that nerve are thrown into rapid tetanic contraction, the contraction both beginning and ceasing sharply and suddenly with the current. This is the normal reaction of the nerve to faradism. If the muscle be wasted from disuse or some local cause unconnected with its nerve-supply, the contraction is smaller, and both arises and relaxes more slowly. ”

“ The pinching irritates the sensory nerve, something, as you now know, travels along it to the nerve-centre, which, in this case, is in the frog's spinal marrow, and from the spinal marrow, after the lapse of a little time (the latent period in the marrow) a new impulse starts outwards along motor nerves to the muscles, reaches these, and causes them to contract. There are many varieties of these reflex acts. We may be quite unconscious of them, or we may feel the stimulus, and we may feel that we make a movement, and yet we may be unable to restrain the movement. ”

“ On passing through the network of nerves in the brain area, the current passes back again through a complicated maze of connections to a motor nerve which insures a muscular response. The first time a stimulus passes through this network the resistance offered at the nerve junction or synapse is very high; at succeeding repetitions of the stimulus the resistance is reduced, the nerve current passes more rapidly and fluently over the paths it has already traveled, and the action resulting becomes as direct and automatic as if it were an original reflex action. ”

“ 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. ”

“ 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. ”

“ One of the most interesting discoveries of physiology was the discovery, made simultaneously in France and Germany fifty years ago, that nerve currents do not only start muscles into action, but may check action already going on or keep it from occurring as it otherwise might. Nerves of arrest were thus distinguished alongside of motor nerves. The pneumogastric nerve, for example, if stimulated, arrests the movements of the heart: the splanchnic nerve arrests those of the intestines, if already begun. ”

“ If one stimulates the nerve running through the frog’s leg by very slight electric currents which are incapable of producing a contraction of the muscles, the force of the current may be slowly and evenly increased without the leg moving or in any way responding. This experiment shows us that the motor nerves do not respond to the stimulus as such, because the latter may be very strong without producing any visible effect, but that it is the rapid variations and changes which cause the convulsive movements. ”

“ Further, although this extraordinary change takes place in the peripheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements. ”

“ It is also ascertained, that of the two sorts of nerves which supply every muscle, the nerve of sensibility is a positive pole of a Voltaic circuit, while the nerve of motion, or the muscle into which it passes, is a negative pole. So that the sensor nerves act as electric telegraphs to carry the sensations to the brain, and inform it what is needed, while the motor nerves bring back the volition to the muscles—the brain acting as a galvanic battery, very much like the electric organs of certain fishes. ”

“ He also demonstrated that no motor nerve ever passes through a ganglion. Lastly, he showed, both from theoretical considerations and from the result of actual experiment on the living animal, that the anterior roots of the spinal nerves are motor, while the posterior are sensory. ”

“ From it in turn come a great many connecting nerve cells whose axons pass along the spinal cord and form nerve junctions with the motor nerve cells; and so nervous disturbances coming from the cerebellum can arouse the muscles to activity. We all know that walking or any other locomotor act, after it has once been learned, is carried on successfully without our performing any intellectual work in connection with it. What we do have to do is to start the act, and this is done through a discharge from the cerebrum ”

“ How then are the muscles informed that their contraction is called for? They have no independent volition; each muscular fiber obeys the impulse transmitted to it by the nerve, from the nerve center governing its action. These nerve centers are in their turn controlled by the central nervous mechanism. And in complex voluntary movements the central nervous mechanism is under the control of the higher cerebral centers. The wish to raise the hand appears to the mind as an idea of the hand being raised. This idea is translated by the central nervous mechanism into a set of motor nerve impulses. ”

“ 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. ”

“ We know little as yet of the way in which we impart a definite duration to the energy of the muscles, for our researches have so far informed us upon little else than the convulsions following extremely brief excitations, and upon tetanus. Since the nerves only transmit the impulses coming from the motor-ganglion cells, it is evident that the peculiar mechanism of the composite movements resides in the central nerve-system, and that, consequently, exercise in such movements is really nothing else than exercise of the central nerve-system. ”

“ The contractions of the muscle, which are produced by stimulation of the periphery region of the nerve with stimuli of a definite strength, remain unaltered, even when the asphyxiated stretch of nerve in the chamber is already completely degenerated. The central depression of a ganglion cell of a motory neuron is likewise wholly without influence on the degree of excitability of its nerve fiber, as I was able to demonstrate79 in the reflex inhibition of the motor neurons of the spinal cord of the dog. (Figure 14.) That which is conducted by the nerves is solely the process of excitation. ”

“ In the single induction shock he has at command a stimulus of extreme brevity, lasting only a few millionths of a second. With such stimulus a lower limit is soon found to the brevity of the reflex effect as expressed 1;,y muscles, It is found difficult to evoke with brief stimuli reflex contractions so brief as those evoked from the muscle by similar stimuli applied direct to the motor nerve of the muscle. There is reason to think that such stimuli applied to a nerve may evoke one single nerve-impulse. ”