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Nerves (Motor Depressant)

Bryan Applications, N

According to the popular use of the expression, a nervous breakdown may mean anything from neurasthenia or some other neurotic state to actual insanity. See: Mental Disease.. Neurosis..

Nervous headache:

When no apparent reason exists for a headache it is sometimes referred to as a nervous headache. In neurasthenia, for example, there may be a headache of this description. A search for some definite cause should always be made, however, before labeling a headache as nervous.

Nervousness:

Different characteristics are popularly described as nervousness: shyness and timidity, excitability, apprehension and the anxiety state. These qualities are more or less related, however, though individuals exhibit one or other of them more particularly.

Abnormal shyness or excitability may be inborn or may result from excessive strain, worry, much excitement, mental or physical shock, the abuse of drugs or some other experience; the remedy would vary accordingly. A nervous child should be overhauled and physical defects put right; it may be eye strain, adenoids, worms, commencing chorea or something else.

When the disposition is nervous the fact that there is no discoverable cause of this kind is no reason for regarding the child as capable of living the life of ordinary children or of attempting to bully it into doing so.

The natural tendency of ordinary children to tease a sensitive, diffident child and make its life a misery should be borne in mind, and any risk of the sort should be avoided in the choice of a school and otherwise. At the same time a hot-house existence is undesirable, and the child should be encouraged to exercise freely in the open air and to mix with children who are congenial to it. A child should not be forced to sleep in the dark or do anything else that terrifies it. A sense of confidence may be fostered, but it cannot be forced. See: Neurosis..

Nervous system:

Under their own headings accounts are given of the brain and the spinal cord, which together with their nerves form the central nervous system, and of the sympathetic nervous system; also of disorders that affect them. Here it is proposed to take a general survey of the nervous system, the principle of its construction and of the working of its various parts, the morbid changes that occur in nervous tissue and the alterations of function resulting from these.

Nerve cells, with their fibers, are the units of the nervous system. These are of various shapes and sizes, but each consists of a mass of protoplasm, the body of the cell, containing a nucleus and a long, slender prolongation outward of its substance, known as the axon, which forms the conducting part of a nerve fiber. Most of the cells have also other processes connected with their body, relatively short and with numerous branches. These are called dendrons, or dendrites, and do not conduct impulses away from the cell, as the axon in general does, but into the cell. They may also have to do with the nutrition of the cell.

The axon is known as the axis cylinder in a nerve fiber, and is covered with a thin sheath, the primitive sheath, or neurilemma, but many of the nerves have, in addition, within this sheath, a coating of a complex fatty substance, which is known as the white or medullary sheath. This sheath is interrupted, however, at intervals, such points being described as the nodes of Ranvier (after the French histologist, Louis-Antoine Ranvier). Nerves with a white sheath are called medullated nerves, the others, non-medullated. When many of the medullated nerves are massed together in the brain or spinal cord they form what is called white matter.

Gray matter, on the other hand, consists mainly of nerve cells and non-medullated fibers. In the brain and spinal cord the cells and fibers are supported by a peculiar tissue known as neuroglia, in which are numerous cells with small bodies and a large number of radiating processes, and known as spider cells.

Metabolic changes in the body of a nerve cell can produce a current of energy along the nerve that springs from it, and this may excite movement in a muscle, activity of a gland and so on. The nature of this energy, which is termed nerve impulse, is not known, but, although it possesses certain analogies to electrical energy, it is not an electric current. For one thing, it travels more slowly than the latter. The rate of travel, it may be noted in passing, is faster in a medullated than in a non-medullated nerve.

An electric shock will, however, provoke a current of nerve impulse in a nerve, as will thermal, chemical or mechanical stimulation, and the impulse may go in either direction, up or down a nerve. Normally, however, the impulse will only flow in one direction. In one nerve it flows towards the nerve centers and is called an afferent nerve, in another it flows away from the cell and is called an efferent nerve. A motor nerve, that is, one activating muscle, is efferent, a sensory nerve is afferent. A peculiar effect of an impulse along an efferent nerve is not activation of any-thing but the reverse, inhibition or restraint of activity; this power may be called into operation as and when required.

A nerve cell with its various processes, axon and dendron, is called a neuron, and a characteristic of the structure of the nervous system is that it is made up of relays of neurons. The large cells in the cortex of the brain which originate voluntary movements, for example, are not directly in contact with the muscles, but the terminations of their axons come into relationship with other nerve cells in what is called the anterior, horn of the gray matter in the spinal cord, and it is the axons of the latter cells that actually come into contact with the muscles, through structures, known as end-plates, on the fibers of a muscle.

The brain cell with its processes is called the upper motor neuron, and the spinal cord with its processes the lower motor neuron. An impulse in the upper is communicated to the lower nervous system neuron, and thus a muscle is called into action. But, when necessary, an impulse in the upper neuron may inhibit the activity of the lower neuron, either altogether or partially.

One neuron is not actually joined to another but effects a communication through its axon, at its termination, breaking up into fine tendrils, forming a sort of brush; these tendrils surround the cell body of the second neuron or, at any rate, intertwine with its dendrons. Such an arrangement is described as a synapse.

The simplest form of nervous activity occurs in what is called reflex action. If the sole of the foot of a sleeping person is pricked the foot is drawn away, although the person may be unconscious either of the pain of the prick or of the drawing away of the foot. What happens is that an impulse is carried along an afferent nerve to motor cells in the spinal cord with which the fibers of the nerve are in communication by synapses.

The cells are provoked to activity and in their turn cause a movement of the muscles to draw away the foot. This movement is coordinated and purposeful. It is meant to withdraw the foot from what is hurting it, but it is not voluntary. If the person were to wake he be aware of what is taking place and would probably also withdraw his foot, the withdrawal taking place unless the person willed strongly that it should not do so.

Reflex actions of many kinds, such, for example, as the blinking of an eye, the movements of the intestines, the secretion of saliva and changes in the size of the pupil, are always going on more or less. In numerous instances there are chains of reflex actions, one setting another in motion. Some of the instances of such action are very complicated, and when one moves into the psychical sphere it will be observed that impulsive acts partake largely of this character. Even inhibition may be reflex, as when, for example, the muscles which bend an arm are at work it is necessary that those which straighten it should be relaxed, this being brought about by reflex action.

Reflex action was at one time spoken of as sympathetic action, and therefore the name sympathetic system was applied to those ganglia, or masses of nerve cells which, with their fibers, appeared to control the activities of the internal organs, the dilatation and contraction of blood vessels, and so on. Whether the cells in the sympathetic ganglia can act as true reflex centers is doubtful, however, though it is possible. A nerve consists of bundles, or funiculi, of nerve fibers. The whole nerve is surrounded by a sheath of connective tissue, the epineurium, and there is also connective tissue, the perineurium, between the funiculi and the delicate endoneurium, between the fibers.

A nerve is supplied with blood vessels, with lymphatics, and with nerves of its own. A nerve may be purely afferent, purely efferent or be mixed. The optic nerve, for example, is purely afferent, the 3rd cranial nerve purely efferent, but the 5th cranial nerve is mixed, having, both afferent and efferent fibers.

Injuries and diseases of the nerves:

If a nerve is divided, the part which is separated from the nerve cells undergoes changes, referred to as Wallerian degeneration, The axis cylinder and the sheath break up and are absorbed, the primitive sheaths of the fibers remain alive. The regeneration of the nerve is made possible by two circumstances. In the first place, new nerve fibers grow out from the divided end which is still connected with the nerve cells, and, in the second place, the survival of the primitive sheaths in the other portion of the nerve provides a scaffolding along which the new fibers may grow.

Regeneration may not take place, however, if the space between the divided ends is too wide, or if they are separated by scar tissue. When it does take place it is many months before it is complete, though the time varies for different nerves. Division of fibers within the brain and spinal cord is not followed by regeneration.

The activity of nerve cells may be diminished by certain drugs, bromides, for example, and can be increased by others, such as strychnine. Similar effects are produced by the toxins of various bacteria. If a poison is sufficiently virulent the nerve cells may actually be killed. Nervous tissue may also be damaged by physical or other kinds of violence.

Inflammation in gray nervous matter is associated with dilatation of blood vessels, exudation of lymph and possibly overgrowth of the connecting substance, so that the nerve cells are damaged by compression, apart from any damage caused by the toxins which may be responsible for the inflammation. If these damaging causes are removed sufficiently early the nerve cell may recover, and its function, which was weakened for the time being, may be regained. If the nerve cell is destroyed, however, it cannot be replaced, a fact exemplified in infantile paralysis (q.v.).

An overgrowth of the supporting tissue in the brain or spinal cord, whether due to chronic inflammation or to degeneration of the nervous tissue proper from other causes, is called sclerosis.

In a nerve also inflammation may similarly cause pressure or the nerve fibers may be affected by poisoning, with impairment or actual loss of function. Sometimes the whole nerve is converted into a mere fibrous cord. Nervous tissue may be damaged also by pressure from tumors within it or from tumors or other swellings of adjoining structures.

A poverty of the general nutrition may weaken nervous tissue, though in prolonged starvation it is the tissue which suffers last of all. Deprivation of the blood supply, however, from blocking of the arteries causes a degenerative softening of nervous tissue. The metabolism of nerve cells may also be affected by disorders of the endocrine glands.

The symptoms of nervous disorder depend upon the situation of the mischief and upon whether the irritability of the nerve cell is increased, lessened or destroyed, or upon whether or not the conductivity of nerve fibers is maintained.

Disease of the upper motor neuron, if irrigative, leads to spasms of the muscles supplied by the affected cells, and possibly general convulsions; if depressive it leads to weakness or loss of voluntary movements, reflex actions being preserved and, in fact, exaggerated; the muscles are in a stiff or spastic condition and their nutrition is maintained, apart from the consequences of disuse. An irritative lesion of the lower motor neuron also causes spasm, but a destruction of the nerve cells leads to loss not only of voluntary but of reflex movements, and the muscles waste because good nutrition of the muscles depends on the integrity of these nerve cells.

The effects of irritation of sensory centers in the brain have not been so definitely marked out as those of the motor centers. Destruction of the centers causes paralysis of sensation, and this follows also if the sensory tracts in the brain and cord are affected. As, however, the various kinds of sensation, tactile, heat, pain and so on, run in different tracts in the cord, there may be a loss of some kinds of sensation but not of others. In some instances of disorder of the sensory nerves or tracts there are alterations of sensation, or paresthesia.

The cortex of the brain exercises a controlling influence over cells in the optic thalamus, as regards sensation, for if certain fibers passing from the cortex to the thalamus are blocked by disease there is a great increase in the intensity of sensations, whether pleasurable or not.

Diseases affecting the bulb of the brain are very dangerous, as here are the affecting centers for the heart, respiration and blood pressure, in addition to others for various important functions.

Disease of a mixed nerve may cause pain, but if the nerve is blocked there is loss of all kinds of sensation and paralysis and wasting of muscles. There is also atrophy of other tissues, such as the skin and its appendages, the hair and nails.

Application and treatment:

See: Ataxia.. Infantile Paralysis.. Neuralgia.. Neuritis.. Sciatica..