Reflex Action.—When some sudden or strong stimulus acts upon the nerve terminations at the surface of the body, an immediate response is frequently observed in some quick movement. The jerking away of the hand on accidentally touching a hot stove, the winking of the eyes on sudden exposure to danger, and the quick movements from slight electrical shocks are familiar examples. The explanation of reflex action is that external stimuli start impulses in neurons terminating at the surface of the body and these, in turn, excite impulses in neurons which pass from the spinal cord or brain to the muscles (Fig. 138). Since there is an apparent turning back of the impulses by the cord or brain, the resulting movements are termed reflex.[105]
Fig. 138—Diagram illustrating reflex action of an external organ.
Reflex Action and the Mind.—If one carefully studies the reflex actions of his own body, he will find that they[pg 309] occur at the time, or even a little before the time, that he realizes what has happened. If a feather is brought in contact with the more sensitive parts of the face of a sleeping person, there is a twitching of the skin and sometimes a movement of the hand to remove the offending substance. Surgeons operating upon patients completely under the influence of chloroform, and therefore completely unconscious, have observed strong reflex actions. These and other similar cases indicate clearly that reflex action occurs independently of the mind—that the mind neither causes nor controls it. If a further proof of this fact were needed, it is supplied by experiments upon certain of the lower animals,[106] which live for a while after the removal of the brain. These experiments show that the nervous impulses that produce reflex action need only pass through the spinal cord and do not reach the cerebrum, the organ of the mind.
The Reflex Action Pathway.—By study of the impulses that produce any reflex action, a rather definite pathway may be made out, having the following divisions:
1. From the surface of the body to the central nervous system (usually the spinal cord). This, the afferent division, is made up of di-axonic neurons, and these have (in the case of the spinal nerves) their cell-bodies in the dorsal root ganglia (page 295). They are acted upon by external stimuli, while their impulses in turn act on the neurons in the spinal cord.
[pg 310]2. Through the central system (spinal cord or base of brain). This, the intermediate division, may be composed of mon-axonic neurons, or it may consist of branches from the afferent neurons. In the case of separate neurons, these are acted upon by impulses from the afferent neurons, while their impulses serve in turn as stimuli to other neurons within the cord (Fig. 129).
3. From the central nervous system to the muscles. This, the efferent division, is made up of mon-axonic neurons. Most of these have their cell-bodies in the gray matter of the cord, while their fibers pass into the spinal nerves by the ventral roots.[107] They may be stimulated by impulses either from the intermediate neurons, or from branches of the afferent neurons. Their impulses reach and stimulate the muscles.
Reflex Action in Digestion.—The flowing of the saliva, when food is present in the mouth, is an example of reflex action. In this case, however, the organ excited to activity is a gland instead of a muscle. The food starts the impulses, and these, acting through the bulb, reach and stimulate the salivary glands. In a similar manner food excites the glands that empty their fluids into the stomach and intestines, and stimulates the muscular coats of these organs to do their part in the digestive process. To a considerable extent, neurons having their cell-bodies in the sympathetic ganglia are concerned in these actions (Fig. 139).
