The Central Nervous System

The central nervous system is the means by which our various bodily activities are co-ordinated and by which we respond to changes in our environment. In the course of their evolutionary development the cells of the nervous system and their long processes, the nerve-fibres, have become endowed with an exceptional degree of those two properties of all living cells, excitability and conductivity. Thanks to this, the nervous system is able to transmit an excitatory state with great speed and precision to other parts of the body and allows it to adjust itself rapidly to changes in its environment.

Functionally speaking, the nervous system can be divided into three parts, the afferent, the central, and the efferent. The afferent part of the central nervous system includes all those nervous structures which carry incoming impulses going towards headquarters, and the efferent those which conduct outgoing messages towards the periphery. Anatomically speaking, the system is divided into the brain, the spinal cord, and the various afferent and efferent nerve's to which these two main structures give rise.

The brain is usually likened to a great collection of central offices which is kept in constant touch with the working of the body and which co-ordinates its various activities through the medium of an elaborate net of telephone wires. Incoming messages are continually arriving at headquarters by the afferent nerves and instructions are passing in return along the efferent system. Different parts of the brain co-ordinate the work of different organs. The cerebral hemispheres deal with messages arriving at headquarters from the various special senses and are also regarded as being the chief seat of man's mental processes. The spinal cord is mainly concerned with the life of automatic movement. To the bulb, situated at the junction of the spinal cord with the brain, is delegated the all-important work of co-ordinating a great many of the vital activities of the body, such as respiration, blood pressure and body temperature.

Before dealing with the anatomy of the central nervous system, it will be necessary to describe the morphological unit of which it is built up, namely, the nerve cell, or neurone. This is a much-specialised cello from which have grown out one or more branches, or processes. In atypical neurone the cell body gives rise to two processes, one short and branched called the dendron, and the other long and less branched called the axon. What is popularly known as a nerve is really a big collection of axons of nerve cells bound together by a packing of connective tissue. Since a nerve contains many nerve fibres, a section through it resembles a section of a marine cable. On the cut surface we note a great number of insulated wires held together by a certain amount of packing material. Nor is the resemblance to a cable only a superficial one. If the long process (axon) that grows from a nerve cell is suitably stained and examined microscopically it will be seen to be made up of two parts, a central core, or axis cylinder, and a thick covering of fat known as the myelin sheath. Outside this fatty covering is a thin outer layer of connective tissue, called the nerilemma. The central core, or axis cylinder, is the active part of the nerve fibre along which the nerve impulse passes, and the myelin and connective tissue sheaths are insulating material which has the same function as the rubber which surrounds the marine cable. The sheath prevents leakage of current. It is to the fat in the myelin coat that the white colour of a nerve is due. The nerves of the sympathetic system are devoid of fat and consequently are greyer in colour than the nerves in other parts of the body. These grey nerves are described as non-medullated, in contradistinction to the white medullated nerves which make up the rest of the central nervous system.

The nerve fibre, or axon, is an offshoot of the cell body and depends for its nutrition on this structure. When a large composite nerve, such as the sciatic, is divided, those parts of its constituent nerve fibres which still retain connexion with the parent cell survive, whilst those parts which have been separated from them die. This does not mean that once a nerve has been cut it is for ever destroyed. Fortunately, repair of a divided nerve takes place by the nerve fibres which still retain connexion with their cells growing until they reach their original destination. Surgeons take advantage of this power of a nerve to regenerate in dealing with severed nerves. They suture together the two halves of the divided nerve, not with any hope that they will be welded together, but in order that the dead half of the nerve may provide a path along which the living half grows. The dead portion of the nerve merely acts as a guide to the growing portion.

From the end of the nerve cell opposite to the axis cylinder is given off the shorter branch, the dendron. Some cells have several dendrons and others only one. These much-branched processes serve to form connexions with the dendrons in neighbouring cells. They are the links along which nerve impulses pass from one cell to another. Just as in a telephone system the operator can connect up one subscriber with another by 'plugging in' on a switchboard, so in the human body impulses may be switched in one direction or another by the connexions between dendrons. There is only contact between the dendrons of the two nerve-cells. At one time the idea of continuity of structure was favoured, and on it was based a theory of sleep. Sleep, it was supposed, was caused by the retractions of dendrons in the nerve cells of the higher centres of the brain. This separation of dendrons interrupted the flow of nerve impulses from one cell to another and consequently brought to rest all mental processes. Ingenious as this theory may be, it cannot be supported by any reliable observations.

Having studied the minute structure of the central nervous system, it will now be necessary to look at the system as a whole. As we have seen, it consists of the brain and spinal cord and the innumerable nerves to which these give rise. From the spinal cord, housed within the vertebral column, there issue thirty-one pairs of nerves which run to different areas in the body. The brain gives rise to twelve pairs of cranial nerves. These are chiefly concerned with the special senses of sight, hearing, taste, and smell. But there exists yet another system of nerves, the sympathetic system, which is formed as a kind of offshoot from the spinal nerves. It takes its origin from the spinal nerves at a short distance from their emergence from the spinal cord. Whilst the main central nervous system (the brain, spinal cord, and their nerves) is chiefly concerned with sensations and movements, the sympathetic system regulates the purely automatic functions of the body, such as the activity of glands, the constriction and dilation of the blood vessels and the movements of the viscera. It is distributed widely through the body as a fine network which in certain areas becomes more dense, forming what are called ganglia and plexuses. The best known plexus is the solar plexus, which lies in the upper part of the abdomen. This appears as a great tangle of grey threads, and in ancient medicine it was believed to be the seat of the emotions. Nor is this belief without some justification. It has now been found that the sympathetic system establishes connexion with a very important mass of grey matter lying at the base of the brain, the thalamus. Modern research has shown that this great collection of grey matter is intimately associated with the emotional life.

If we could see the whole living web of the central nervous system spread out before us and if nervous impulses were visible, say, as light travelling along nerves, we should see this system in ceaseless activity. There would be an uninterrupted passage of sensation from all parts of the body towards the spinal cord and brain, and an unending procession of return messages in the opposite direction. For the web is a double one containing fibres of two kinds, sensory or afferent fibres, which carry incoming impulses, and motor and efferent fibres, which bear outgoing impulses. These two kinds of fibres are bound up together within the same nerve trunk, and in structure appear similar. They differ only in the direction along which the impulses travel, and in the destination to which they are being carried. Some of the nerves carrying afferent (incoming) messages leave their origin in the special sense organs. These are scattered over the surface of the body and also amongst the muscles, joints, and viscera. They are the intelligence agents which report to the spinal cord and the brain the state of affairs in different parts of the body. The central government is thereby informed of the temperature of the skin, the position of the limbs, the nature of the object with which the skin is in contact and, through the eye and the ear, of the impact on the body of undulations of light and sound. These intelligence agents may be divided into internal sense organs and external sense organs. The former report on the state of the body and the latter take note of conditions external to the body. The external sense organs are of particular importance to us, for they include the special organs of sight, hearing, taste, smell, and touch. (Kenneth Walker: Human Physiology).