Physiology of the Nervous System : Part 3

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 20 of 27)

Not only does the element of choice enter into the selection of the proper stimulus, but it also enters into the time, nature, and intensity of the action. For these reasons it is frequently impossible to trace voluntary actions back to their actual stimuli. The pupil will recognize the element of choice in such simple acts as picking up some object from the street, complying with a request, and purchasing some article from a store.

Reflex and Voluntary Action Compared. - Certain likenesses and differences, already suggested in these two forms of action, may now be more fully pointed out. Reflex and voluntary action are alike in that the primary cause of each is some outside force or condition which has impressed itself upon the nervous system. They are also alike in the general direction taken by the impulses in producing the action. The impulses are, first, from the surface of the body to the central nervous system; second, through the central system; and third, from the central nervous system to the active tissues of the body.

Their chief differences are to be found, first, in the pathways followed by the impulses, which are through the cerebrum (the organ of the mind) in voluntary action, but in reflex action are only through the spinal cord or the lower parts of the brain; and second, in the fact that voluntary action is under the direction of the mind, while reflex action is not. It would seem, therefore, that the statement sometimes made that "voluntary action is reflex action plus the mind" is not far from correct. Mind, however, is the important factor in this kind of action.

Secondary Reflex Action. - Everyday experience teaches that any voluntary action becomes easier by repetition. A given act performed a number of times under conscious direction establishes a condition in the nervous system that enables it to occur without that direction and very much as reflex actions occur. Actions of this kind are known as secondary reflex actions, or as acquired reflexes. Walking, writing, and numerous other movements pertaining to the occupation which one follows are examples of such reflexes. These activities are at first entirely voluntary, but by repetition they gradually become reflex, requiring only the stimulus to start them.

The advantages to the body of its acquired reflexes are quite apparent. The mind does not have to attend to the selection and direction of stimuli and, to that extent, is left free for other work. A good example of this is found in writing, where the mind apparently gives no heed to the movements of the hand and is only concerned in what is being written. The student will easily supply other illustrations of the advantages of secondary reflex action.

The development of secondary reflexes probably consists in the establishment of fixed pathways for impulses through the nervous system. Through the branching of the nerve fibers many pathways are open to the impulses. But in repeating the same kind of action the impulses are guided into particulary paths, or channels. In time these paths become so well established that the impulses flow along them without conscious direction and it is then simply necessary that some stimulus starts the impulses. By following the established pathways, these reach the right destination and produce the desired result. According to this view, secondary reflex action is but a higher phase of ordinary reflex action - a kind of reflex action, the conditions of which have been established by the mind through repetition.

Habits. - People are observed to act differently when exposed to the same conditions, or when acted upon by the same stimuli. This is explained by saying they have different habits. By habits are meant certain general modes of action that have been acquired by repetition. Certain acts repeated again and again have established conditions in the nervous system which enable definite forms of action to be excited, somewhat after the manner of reflex action. On account of habits, therefore, the actions of the individual are more or less predisposed. What he will do under certain conditions may be foretold from his habits. Habits simply represent, a higher order of secondary reflexes - those more closely associated with the mental life and character than are the lower forms.

Habits, in common with other forms of secondary reflex action, serve the important purpose of economizing the nervous energy. However, if pernicious habits are formed instead of those that are useful, they are detrimental from both a moral and physical standpoint. Youth is recognized as the period in which fundamental habits are formed and character is largely determined. Therefore parents and teachers do wisely when they insist upon the formation of right habits by the young.

Functions of Divisions of the Nervous System. - The relationship between the different parts of the nervous system is very close and one part does not work independently of other parts. At the same time the general work of the nervous system requires that its different divisions serve different purposes:

1. The peripheral divisions of the nervous system are concerned in the transmission of impulses between the surface of the body and the central system and between the central system and the active tissues. The nerves are the carriers of the impulses. The ganglia contain the cell-bodies which serve as nutritive centers; and, in the case of the sympathetic ganglia, these cell-bodies are the places where the fiber terminations of one neuron connect with, and stimulate, other neurons.

2. The gray matter in the spinal cord, bulb, pons, and midbrain (through the cell-bodies, fiber terminations, and short neurons which they contain) completes the reflex action pathways between the surface of the body and the voluntary muscles, and also between the surface of the body and the organs of circulation and digestion.

3. The white matter of the spinal cord, bulb, pons, and midbrain (by means of the fibers of which they are largely composed) forms connections with, and passes impulses between, the various parts of the central nervous system.

4. The bulb, because of certain special reflex-action pathways completed through it, is the portion of the central nervous system concerned in the control of respiration, circulation, and the secretion of liquids.

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