Neuromuscular system

“ That that muscle in modern adult vertebrates is under a certain amount of nervous control is unquestionable, but this control is not of the kind usually seen in other neuromuscular combinations. The nerves that enter the heart are probably not ordinarily directly concerned with its beat, for, as already pointed out, this continues after they are cut. ”

“ So uniformly is a separate instrument provided for every additional function, that there is strong reason to regard the muscular nerves, although running in one sheath, as in reality double, and performing distinct functions. Sir Charles Bell, in his work on the Nervous System, endeavors to show, 352 that one set of nervous fibres conveys the mandate from the brain to the muscle, and excites the contraction ”

“ In the lines of junction between the plates layers of connective-tissue cells appear, which form the commencements of the intermuscular septa.
The growth of the plates is very rapid, and their upper ends [Pg 673] soon extend to the summit of the neural canal, and their lower ones nearly meet in the median ventral line. The original band of muscles, whose growth at first is very slow, now increases with great rapidity, and forms the nucleus of the whole voluntary muscular system (fig. 380, mp ́) . ”

“ When, however, any one of the various muscles whose integrity is necessary for the automatic performance of any act becomes affected, let the lesion be in the muscle itself or anywhere in the nerve-substance between it and its centre, or in that centre itself in such a way as to hinder its free response to the nervous stimulus, then the will has to be especially directed to the act in order to counteract the effect of the disability, and some other muscle or group of muscles must be substituted in the place of the one incapacitated. ”

“ This is manifested by the muscular walls of the hollow viscera and of the heart, where it is the expression of a continuous liberation of energy in process in the muscular tissue, the outcome of the latter’s own intrinsic life, and largely independent of any connexion with the nervous system. The muscular wall of the blood-vessels also exhibits tonic contraction, which, however, seems to be mainly traceable to a continual excitation of the muscle cells by nervous influence conveyed to them along their nerves, and originating in the great vaso motor centre in the bulb. ”

“ Sometimes a muscle disappears entirely; usually the connective-tissue elements, including the interstitial cellular and adipose tissue, undergo proliferation, so as to mask the wasting of the muscles to some extent.
The morbid process in the nervous system is also a simple degenerative process. The nerve-fibres and cells atrophy first, and the connective-tissue proliferation which marks the sclerotic change of the diseased area is a secondary process. There is still considerable dispute among authorities as to which segment of the motor-conduits the degeneration begins in. ”

“ Each myotome is divided into a dorsal or epiaxial portion, and a ventral or hypaxial portion, by a longitudinal, horizontal, fibrous septum extending outwards from the vertebral centra to the skin. The muscles of the pectoral and pelvic fins are derivatives from more or fewer of the adjacent myotomes. The splanchnic muscles include the musculature of the walls of the alimentary canal, as well as those specialised portions of the visceral system which are represented by the muscles of the branchial arches and the jaws, and are composed of striated fibres. ”

“ The significance of the various parts of the neuromuscular mechanism in such an animal as Aurelia can be determined by experiment. If the eight sense-bodies are removed, the animal will no longer pulse spontaneously, though its muscles may be made to contract by direct stimulation. If all but one sense-body are removed, the bell will pulse with regularity and by artificially stimulating the single remaining body a wave of muscular contraction can be sent over it. ”

“ Those few cases that exhibit great difficulty in performing one set of movements that requires a certain combination of muscles, but are able to use those same muscles in combination with others with ease, are difficult of explanation by the peripheral theory of the disease alone; but less difficulty is encountered if a superexcitability of the spinal centres is supposed to exist, for here we can imagine that when all the centres so affected are called into use, and only then, there will be an undue amount of nervous energy transmitted to the periphery, and disordered movement be the result. ”

“ Under this division it only remains further to be said, that section of the nerve-ring in the disc, or the nerve-trunks of the rays, although, as we have seen, so completely destroying physiological continuity in the rows of ambulacral feet and muscular system of the animal, does not destroy physiological continuity in the external nerve-plexus; for however much the nerve-ring and nerve-trunks may be injured, stimulation of the dorsal surface of the animal throws all the ambulacral feet and all the muscular system of the rays into active movement. ”

“ The fibers of the muscles themselves may be the end organs of the motor system, and it may be that nerve fibers everywhere accompany muscular fibers. Thus we know that the motor system is connected usually through ancillary ganglia with the cortex. The end organs of this system, be they the muscles themselves or specialized parts of them, are the organs for conveying to the cortex impressions of muscular force. ”

“ The Involuntary Muscles are influenced by the sympathetic nervous system, and their action pertains to the nutritive functions of the body. They differ from the voluntary muscles in not being striated, having no tendons, and in the net-work arrangements of their fibers. The Intermediate Muscles are composed of striated and unstriated fibers; they are, therefore, both voluntary and involuntary in their functions. The muscles employed in respiration are of this class, for we can breathe rapidly or slowly, and, for a short time, even suspend their action ”

“ They dwindle in size; they do not, however, die. Similarly, when the axons of the motor spinal cells are by severance of the nerve trunk of a muscle broken through, the muscle cells undergo “degeneration”—dwindle, become fatty, and alter almost beyond recognition. This trophic influence which one neuron exerts upon others, or upon the cells of an extrinsic tissue, such as muscle, is exerted in that direction which is the one normally taken by the natural nerve impulses. ”

“ When motor symptoms make their appearance they begin with paresis of the muscles, which may increase rapidly to paralysis. As this is the result of destructive changes, more or less complete, in the motor nerve-fibres, we will have, as would be expected, accompanying the paralysis the symptoms already detailed in the consideration of nerve-injuries with destruction of continuity—namely, absence of muscular tone, loss of skin and tendon reflexes, increased mechanical excitability, atrophy of muscles, and the different forms of degenerative reaction, with loss of faradic contractility. ”

“ In many cases, as has been remarked before, the pathological process descends into the spinal cord. Here, as in the medulla oblongata, the degeneration is principally confined to the motor centres located in the anterior horns, while the posterior horns, together with the lateral and posterior white columns, remain free from disease. The degree of degeneration taking place in the fibres of the respectively paralyzed muscles also corresponds to that of the elements of the nerve-centres from which they are supplied. ”

“ We have encountered many difficulties in the process of defining classes of anatomical entities, often because commonly used categories are “fuzzy.” For example, most of the large muscles of the body are “skeletal” muscles. “Skeletal muscles” are attached to skeletal elements, are “voluntary” muscles innervated by somatic motor neurons, and are derived from somites. ”

“ Thus, from the foot up to the spinal column the morbid imminence is pronounced on the side of the extensor muscles. The liability to invasion on the part of the flexors is greatest at the foot, where dorsal flexion is early impeded, and diminishes upward toward the abdomen.
Most important for the theory of the disease is the fact that the hypertrophic appearance of the muscles is never accompanied by even a transitory period of increased strength.28 Some degree of paresis usually precedes the hypertrophy, and becomes intensified when this sets in. ”

“ A comparison between the muscular and the nervous systems from the point of view of their connection with heat is a bold one for many reasons. Between nerve and muscle there exists this enormous difference—that the former is endowed with a spontaneity denied to the latter. Muscular fibre never contracts of its own accord; it needs a stimulus—its energy is borrowed. The nerve-cell, on the contrary, has in itself an ever-present, never-exhausted power of action, of which the energy is its peculiar property. ”

“ Reflex arcs arising in manifold sense organs can pour their influence into one and the same muscle. A limb muscle is the terminus ad quem of nervous arcs arising not only in the right eye, but in the left; not only in the eyes, but in the organs of smell and hearing; not only in these, but in the geotropic labyrinth, in the skin and in the muscles and joints of the limb itself and of the other limbs as well. Its motor nerve is a path common to all these. ”

“ A prominent and practically important illustration of neural tonus is given by the skeletal muscles. These muscles exhibit a certain constant condition of slight contraction, which disappears on severance of the nerve that innervates the muscle. It is a muscular tonus of central source consequent on the continual glow of excitement in the spinal motor neuron, whose outgoing end plays upon the muscle cells, whose ingoing end is played upon by other neurons—spinal, cerebral and cerebellar. ”

“ 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. ”

“ Muscular action, however, cannot take place without nervous action. These two tissues are dependent one on the other. Yet the muscular tissue may be considered as subordinate to the nervous tissue. While the muscular tissue may become totally inactive or incapacitated, or even removed, the brain tissue may retain its activity and continue to perform its functions. The very reverse takes place when the brain is either injured or removed. We know by experience, experiments, that injuries or other pathological changes will cause impairment to muscular tissue. ”

“ Now the peculiarity of a muscle nerve, or motor nerve, as it is called, is that irritation of the nerve fibre at any part of its length, however distant from the muscle, brings about muscular contraction, just as if the muscle itself were irritated. A change is produced in the molecular condition of the nerve at the point of irritation; and this change is propagated along the nerve, until it reaches the muscle, in which it gives rise to that change in the arrangement of its molecules, the most obvious effect of which is the sudden alteration of form which we call muscular contraction. ”

“ When the nerve discharges its influence into the muscles, a very rapid molecular change takes place in the substance contained in the cartouches or muscular boxes, which contract, pressing their ends close together. Scarcely has the action of the nerves ceased than they relax and resume their previous form. It is well known, that when a muscle contracts, it becomes thicker and shorter; it suffices to take hold of the arm a little above the elbow and then to bend it in order to feel how the biceps muscle swells and hardens. ”

“ The pyramidal tract conveys impulses that inhibit muscle tonus, while the rubro-spinal tract is the path by which excitatory impulses travel. When the inhibitory influences are cut off, as in a lesion of the internal capsule, the paralysed muscles become spastic, and the deep reflexes are exaggerated. When the excitatory impulses are also lost, as in a total transverse lesion of the cord, the paralysed muscles are flaccid and the deep reflexes disappear. In destructive lesions of the lower neurones, the muscles are always flaccid. ”

“ To explain the function of nerve-tissue, it is necessary to begin by explaining what physiologists mean by the term "excitability." Suppose that a 24 muscle has been cut from the body of a freshly killed animal; so long as it is not interfered with in any way, so long will it remain quite passive. But every time a stimulus is supplied to it, either by means of a pinch, a burn, an electrical shock, or a chemical irritant, the muscle will give a single contraction in response to every stimulation. ”

“ The diseased muscle will respond much more readily to the Voltaic current than the corresponding muscle of the healthy side; while at the same time it will be found on examining the nerve that there is equal diminution to both currents.|Diagnosis of Peripheral from Central Disease.| This increased muscle reaction is due to the special irritability of muscular tissue, and is quite independent of any nerve influence. ”