Physiology of the Nervous System : Part 5

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 22 of 27)

Respiration. - Efferent fibers connect the different muscles of respiration with a cluster of cell-bodies in the bulb, called the respiratory center. This center together with the nerves and muscles in question form an automatic, or self-acting, mechanism similar in some respects to that of the heart. Through the impulses passing from the respiratory center to the muscles, a rhythmic action is maintained sufficient to satisfy the usual needs of the body for oxygen. The demand of the body for oxygen, however, varies with its activities, and to such variations the respiratory center alone is unable to respond.

The regulating factor in the respiratory movements has been found to be the condition of the blood with reference to the presence of oxygen and carbon dioxide. If the blood contains much carbon dioxide and little oxygen, it acts as a strong stimulus to the respiratory center, causing it, in turn, to stimulate the respiratory muscles with greater intensity and frequency. On the other hand, if the blood contains much oxygen and little carbon dioxide, it acts only as a mild stimulus. This explains how physical exercise increases the breathing, since the muscles at work consume more oxygen than when resting and give more carbon dioxide and other wastes to the blood.

The respiratory center is also connected by afferent nerves with the mucous membrane of the air passages. Irritation of the nerve endings in this membrane causes impulses to pass to the center, and this leads, by reflex action, to such modifications of the respiratory acts as sneezing and coughing. There is also a connection between the cerebrum and the respiratory center. This is shown by the fact that one can voluntarily change the rate and force of the respiratory movements, and further by the fact that emotions affect the breathing.

Regulation of the Body Temperature. - As explained in the study of the skin, the nervous system regulates the body temperature by controlling the circulation of the blood through the skin and the internal organs. This is accomplished by stimulating in a reflex manner the muscles in the walls of certain arteries. To prevent the body from getting too hot, muscles in the arteries going to the skin relax, thereby allowing more blood to flow to the surface, where the heat can be disposed of through radiation and through the evaporation of the perspiration. On the other hand, if the body is in danger of losing too much heat, the muscles in the walls of arteries going to the skin are made to contract and those to internal organs relax, so that less blood flows to the skin and more to the internal organs. In this way the nervous system adjusts the circulation to suit the conditions of temperature outside of and within the body and, in so doing, maintains the normal body temperature.

Summary. - The nervous system is able to control, coordinate, and adjust the different organs of the body through its intimate connection with all parts and through a stimulus (the nervous impulse) which it supplies and transmits. Nervous impulses, excited by external stimuli, follow definite paths and cause activity in the different parts of the body. All such pathways are through the central nervous system. In reflex action the impulses are mainly through the spinal cord, but to some extent through the bulb, pons, and midbrain. In voluntary action they pass through the cerebrum - a condition that leads to important modifications in the results. The cerebrum, in addition to controlling the voluntary movements, is able to establish the necessary conditions for secondary reflex actions, such as walking, writing, etc. Although certain of the divisions of the nervous system exercise special functions, all parts of it are closely related.

Exercises. - 1. Give the function of each of the parts of a neuron.

2. State the purpose of the nervous impulse.

3. Show that the exciting cause of bodily action is outside of the nervous system and, to a large extent, outside of the body.

4. Describe the arrangement that enables stimuli outside of the body to cause action within the body.

5. Describe a reflex action and show how it is brought about.

6. Distinguish between afferent, efferent, and intermediate neurons.

7. Draw diagrams showing the impulse pathways in voluntary and in reflex action.

8. What purposes are served by the sympathetic neurons?

9. Describe the method of control of the circulatory and digestive processes. How do reflex actions protect the body?

10. Compare voluntary and reflex action. In what sense are all the activities of the body reflex?

11. In what sense is walking voluntary? In what sense is it reflex?

12. How does secondary reflex action lessen the work of the nervous system?

13. State the special functions of the nerves, ganglia, spinal cord, bulb, cerebellum, and cerebrum.

14. State the importance of the formation of correct habits.

Practical Work

To demonstrate Nerve Pathways. - A smooth board, 2x6 ft., is painted black, and upon this is drawn in white a life-size outline of the body. Pieces of cord of different colors and lengths are knotted to represent mon-axonic and di-axonic neurons. These are then pinned or tacked to the board in such a manner as to represent the connections in the different kinds of nerve pathways. Fig. 143 shows such a board with connections for a reflex action and a voluntary action of the same muscle.

Study of the "Knee Jerk" Reflex. - A boy is seated on a chair with the legs crossed. With a small pointer he is given a light, quick blow on the upper margin of the patella at the point of connection of the tendon. The stroke will usually be followed by a reflex movement of the foot. Does this take place independently of the mind? (The one upon whom the experiment is being performed should assume a relaxed condition and make no effort either to cause or prevent the movement.) Can the movement be inhibited (prevented)? Repeat the experiment, effort being made to prevent the movement, but not by contracting opposing muscles.

Other reflex actions adapted to class study are those of the eyes, such as the closing of the lids on moving objects near them and the dilating of the pupils when the eyes are shaded. The involuntary jerking of the head on bringing the prongs of a vibrating tuning fork in contact with the end of the nose is also a reflex action which can be studied to advantage.

To determine the Reaction Time. - Have several pupils join hands, facing outwards, making a complete circle, excepting one gap. Give a signal by touching the hand of one pupil at the end of the line. Let this pupil communicate the signal, by pressure of the other hand, to the next pupil and so on around, having the last pupil raise the free hand at close of the experiment. Note carefully the time, preferably with a stop watch, required to complete the experiment and divide this by the number of pupils, to get the average reaction time. The experiment may be repeated with boys only and then with girls, comparing their average reaction time.

Reflex Action of the Salivary Glands. - Place a small pinch of salt upon the tongue and note the flow of saliva into the mouth. Try other substances, as starch, bits of wood, and sugar. What appears to be the natural stimulus for these glands? Compare with reflex actions of the muscles.

D.C. Heath and Co. - Publishers
Original copyright 1909