Nervous System



The electronic computer used universally in business offices is a relatively modern invention. Each year they are becoming smaller, cheaper, more efficient, and requiring less sophisticated electronics. Indeed, today hardly a school student is without a computer at school (or at home, for that matter), and they have long since become as much a part of the daily human scene as transistor radios and television sets. However, the idea of the computer is nothing new. Indeed, the basic ingredients have been around for as long as the human race.

In fact, every human possesses one of the world’s finest computers. It is quick, reliable, requires minimum servicing, and flashes out the answers in record time. We refer, of course, to the human brain, and the intricate apparatus known as the central nervous system.



This is basically a complex communications network and, fortunately for the human race, because it is so specialized and so efficient, it makes living a relatively easy pastime.

Housed in the protective bony vault of the cranium is the nerve centre of this computer system. From here, branches radiate. Some nerves go directly to certain specialized areas providing specific functions—such as the optic nerve caring for vision that goes to the eye, and the auditory nerve attached to the ear structure that cares for the sense of hearing.



There are 12 so-called “cranial nerves” leading directly from the brain itself. A massive ropelike structure of nervous material then leaves the brain via the stem, and courses through a canal made up of a central hole in the vertebral bones of the spine.
Depending on their location, the vertebrae have been given different names, purely for descriptive purposes. Those in the neck are referred to as the cervical vertebrae. Those in the chest region are the dorsal or thoracic vertebrae. In the lower part of the trunk, they arc large, having to support the body, and these are the big lumbar vertebrae. Then comes a series of fused vertebrae located in the pelvic region, called the sacrum. Finally, at the very tip, there is a series of tiny, apparently unimportant bones collectively called the coccyx (or tail). In animals, the coccygeal bones are for the tail structure. As the spinal column of nervous tissue descends through this strongly protected (but movable) canal, nerves are given off at each side, between successive vertebrae. These supply structures on each side of the body.

In the lower cervical, upper dorsal region, several major roots (as they are called) are given off, and these join, divide and redivide in a maze of nervous tissue called the brachial plexus. In turn this gives rise to the major nerves of the upper limbs. In this manner each part of the arms, hands and fingers is supplied with nerve fibers. They are all part of the total nervous network.



Further down, in the lumbar region, massive nerve roots are given off on either side to form the giant sciatic nerve. This supplies the lower limbs, including feet and toes. The large nerve gradually divides into smaller and smaller branches until every part, skin, blood vessel walls, muscles etc, receives a nervous supply.

There are many functions of the nervous system, but only the chief ones will be pointed out.



Motor Function

This part of the brain controls the action (or motor function) of the various parts of the body. It acts by producing electrical contraction of the muscle fibers that make up muscles. Contraction causes a shortening of the muscles. In turn this produces active movement.

If we desire to move the upper limb, the conscious idea goes to the part of the brain (called the cerebral hemisphere) governing limb movement. Electrical impulses are channeled along the correct nerve that goes to the muscle groups involving the upper limbs – a fraction later, the muscle fibers contract. The limb moves, or bends, or does whatever activity is required.



Similarly, if we desire to bend it in the opposite direction, impulses are sent to Micrograph of human nerve cells. There are two types of branching processes: an axon extends from each cell body to transmit nerve impulses away from the cell body, and dendrites receive incoming impulses and act as sensory receptors for the cell, the opposite muscle groups. The first set of muscles relax as no more impulses are sent, and the opposite ones then carry out a similar function, so enabling an opposite movement.

This all happens in a fraction of a second. In an incredible manner, our computerized system knows which messages to send to which muscle fibers, merely by a conscious thought. It surely represents the finest coordination and smoothest working ever devised.

Sensory Function

Apart from the strictly motor activity, the nerves have a sensory function as well. Most parts of the body have receptors to the various sensations. For example, we are able to appreciate heat (or lack of heat, which equals cold), pressure, pain, the awareness of space. The hands and fingers are plentifully endowed with sense end organs (as they are called), for this all forms a part of nature’s inbuilt protection system of the body.

The cranial nerves also have specialized senses, such as being able to appreciate vision, hearing, the sense of smell and taste.



When stimulated, similar electrical impulses are conveyed through the nervous system back to the brain, and an awareness of these different sensations is possible.

When the two systems are coordinated, it will be seen that they are basically operational for our own welfare. We may see, hear, feel and sense danger. This will immediately give us the message consciously to set our motor system into action and take appropriate steps.

If we are exposed to excessive heat, we quickly move from the spot. We immediately remove any part of the body from a source of pain. If we see danger approaching (whatever the form), our legs immediately work and bodily remove us from this threat.

The gradations may be on a small scale (such as pulling the finger from a hot stove) or a more major one (when we run across the road to escape an oncoming vehicle, or flee for our lives in the event of a more ominous life-endangering threat from hostile foes). There are many degrees. But the essential basic principles are the same. The nervous system is geared to keep the body protected and in first-class working order at all times.



Just to make the total operation more effective, there are certain actions that are governed by so-called “reflex” movements. This circumvents the brain, and acts at the same level as the activity. For example, if the knee is tapped just below the kneecap, a “reflex” movement occurs and the foot jumps forward.

Many such reflex circuits occur in the body. They are automatic protective devices geared to preserve the body.

Sympathetic Nervous System

This is another silently working system that is also operating at all times. It is basically geared to carry out many unconscious functions necessary to the normal everyday wellbeing of the body. For example, the sweat glands of the skin are under the control of this system. Nobody consciously tells the sweat glands to start to work when the weather becomes hot, and the body desires to lose fluid. It just happens. This is because the sympathetic system automatically operates when stimulated by certain situations (e.g. temperatures rising). Sweat is produced, and pours onto the skin surface.

Here it dries, and in so doing, takes a lot of heat from the body. The result is that the body cools down, and so does not reach dangerously high levels that could have an adverse effect on the general function. As the temperatures (internally and externally) reduce, so the need for sweating is reduced, and we perspire less. Once more this is an entirely automatic, computerized function and result. It occurs in the specialized tissues of the brain cells.



This simple example is multiplied thousands of times so that every activity and possible function is covered. Fortunately, this is quite out of conscious control, for if we had to think each time we needed the assistance of one of these functions, we would be hard pressed.

The circulation of the blood, the production of hormones and chemicals by the internal glands, the beating of the heart and in fact nearly every part of the system is, to a greater or lesser extent, under this automatic nervous system control.