A Practical Physiology: A Text-Book for Higher Schools

by Albert F. Blaisdell

279. Reflex Centers. For the purpose of illustration, we might consider the body as made up of so many segments piled one on another, each segment presided over by a similar segment of spinal cord. Each bodily segment would have sensory and motor nerves corresponding to its connection with the spinal cord. The group of cells in each spinal segment is intimately connected with the cells of the segments above and below. Thus an impression reaching the cells of one spinal segment might be so strong as to overflow into the cells of other segments, and thus cause other parts of the body to be affected.

Take as an example the case of a child who has eaten improper food, which irritates its bowels. Sensory nerves of the bowels are disturbed, and powerful impressions are carried up to a center in the spinal cord. These impressions may now overflow into other centers, from which spasmodic discharges of nerve energy may be liberated, which passing to the muscles, throw them into violent and spasmodic contraction. In other words, the child has a fit, or convulsion. All this disturbance being the result of reflex action (the spasmodic motions being quite involuntary, as the brain takes no part in them), the child meanwhile is, of course, entirely unconscious and, however it may seem to be distressed, really suffers no pain.

Scattered along the entire length of the spinal cord, especially in the upper part, are groups of nerve cells which preside over certain specific functions of animal life; that is, definite collections of cells which control definite functions. Thus there are certain centers for maintaining the action of the heart, and the movements of breathing; and low down in the cord, in the lumbar regions, are centers for the control of the various abdominal organs.

Numerous other reflex centers are described by physiologists, but enough has been said to emphasize the great importance of the spinal cord as an independent nerve center, besides its function as a conductor of nervous impulses to and from the brain.

280. The Brain as a Reflex Center. The brain, as we have just stated, is the seat of consciousness and intelligence. It is also the seat of many reflex, automatic, and coordinating centers. These give rise to certain reflex actions which are as entirely independent of consciousness as are those of the spinal cord. These acts take place independently of the will, and often without the consciousness of the individual. Thus, a sudden flash of light causes the eyes to blink, as the result of reflex action. The optic nerves serve as the sensory, and the facial nerves as the motor, conductors. The sudden start of the whole body at some loud noise, the instinctive dodging a threatened blow, and the springing back from sudden danger, are the results of reflex action. The result ensues in these and in many other instances, without the consciousness of the individual, and indeed beyond his power of control.

281. The Importance of Reflex Action. Reflex action is thus a marvelous provision of nature for our comfort, health, and safety. Its vast influence is not realized, as its numberless acts are so continually going on without our knowledge. In fact, the greater part of nerve power is expended to produce reflex action. The brain is thus relieved of a vast amount of work. It would be impossible for the brain to serve as a “thinking center” to control every act of our daily life. If we had to plan and to will every heart-beat or every respiration, the struggle for life would soon be given up.

The fact that the gray cells of the spinal cord can originate a countless number of reflex and automatic activities is not only of great importance in protecting the body from injury, but increases vastly the range of the activities of our daily life.

Even walking, riding the bicycle, playing on a piano, and numberless other such acts may be reflex movements. To learn how, requires, of course, the action of the brain, but with frequent repetition the muscles become so accustomed to certain successive movements, that they are continued by the cord without the control of the brain. Thus we may acquire a sort of artificial reflex action, which in time becomes in a way a part of our organization, and is carried on without will power or even consciousness.

So, while the hands are busily doing one thing, the brain can be intently thinking of another. In fact, any attempt to control reflex action is more apt to hinder than to help. In coming rapidly down stairs, the descent will be made with ease and safety if the spinal cord is allowed entire charge of the act, but the chances of stumbling or of tripping are very much increased if each step be taken as the result of the will power. The reflex action of the cord may be diminished, or inhibited as it is called, but this power is limited. Thus, we can by an effort of the will stop breathing for a certain time, but beyond that the reflex mechanism overcomes our will and we could not, if we would, commit suicide by holding our breath. When we are asleep, if the palm of the hand be tickled, it closes; when we are awake we can prevent it.