Production of Sensations : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 20 of 25)

Our study of the nervous system has shown that impulses arising at the surface of the body are able, through connecting neurons, to bring about various activities. Moving along definite pathways, they induce motion in the muscles, and in the glands the secretion of liquids. It is now our purpose to consider the effect produced by afferent impulses upon the brain and, through the brain, upon the mind. This effect is manifested in a variety of similar forms, known as

The Sensations. - Sensations constitute the lowest forms of mental activity. Roughly speaking, they are the states of mind experienced as the direct result of impulses reaching the brain. In a sense, just as impulses passing to the muscles cause motion, impulses passing to the brain cause sensations. The feeling which results from the hand's touching a table is a sensation and so also is the pain which is caused by an injury to the body.

The mental action in each case is due to impulses passing to the brain. Care must be exercised by the beginner, however, not to confuse sensations with the nervous impulses, on the one hand, or with secondary mental effects, such as emotion or imagination, on the other. Sensations are properly regarded as the first conscious effects of the afferent impulses and as the beginning stage in the series of mental processes that may take place on account of them.

In some way, not understood, the mind associates the sensation with the part of the body from which the impulses come. Pain, for example, is not felt at the brain where the sensation is produced, but at the place where the injury occurs. This association, by the mind, of the sensations with different parts of the body, is known as "localizing the sensation."

Sensation Stimuli. - While the sensations are dependent upon the afferent impulses, the afferent impulses are in turn dependent upon causes outside of the nervous system. If these are removed, the sensations cease and they do not start up again unless the exciting influences are again applied. Any agency, such as heat or pressure, which, by acting on the neurons of the body, is able to produce a sensation, may be called a sensation stimulus. It has perhaps already been observed that the stimuli that lead to voluntary action, as well as those that produce reflex action of the muscles, cause sensations at the same time. From this we may conclude that sensation stimuli are the same in character as those that excite motion. On the other hand, it should be noted that sensations are constantly resulting from stimuli that are of too mild a nature to cause motion.

Classes of Sensations. - Perhaps as many as twenty distinct sensations, such as pain, hunger, touch, etc., are recognized. If these are studied with reference to their origin, it will be seen that some of them result from the action of definite forms of stimuli upon the neurons terminating in sense organs; while the others, as a rule, arise from the action of indefinite stimuli upon neurons in parts of the body that do not possess sense organs. The members of the first class - and these include the sensations of touch, temperature, taste, smell, hearing, and sight - are known as the special sensations. The others, including the sensations of pain, hunger, thirst, nausea, fatigue, comfort, discomfort, and those of disease, are known as organic, or general, sensations. These two classes of sensations differ in their purpose in the body as well as in the manner of their origin.

Purposes of Sensations. - Any given sensation is related to the stimulus which excites it as an effect to a cause. It starts up or stops, increases in intensity or diminishes, according to the action of the exciting stimulus. As the stimuli are outside of the nervous system, and in the majority of cases outside of the body, the sensations indicate to the mind what is taking place either in the body itself or in its surroundings. They supply, in other words, the means through which the mind acquires information. By means of the special sensations, a knowledge of the physical surroundings of the body is gained, and through the organic sensations the needs of the body and the state of the various organs are indicated. In general, sensations are made to serve two great purposes in the body, as follows:

1. They provide the necessary conditions for intelligent and purposeful action on the part of the body.

2. They supply the basis for the higher mental activities, as perception, memory, thought, imagination, and emotion.

Intelligent action is impossible without a knowledge both of the bodily organs and of the body's surroundings. Protection and the regulation of the work of an organ necessitate a knowledge of its condition, while the adapting and adjusting of the body to its surroundings require a knowledge of what those surroundings are. The dependence of all the higher forms of mental activity upon sensations is recognized by psychologists and is easily demonstrated by a study of the manner in which we acquire knowledge. "Without sensation there can be no thought."

Steps in the Production of Sensations. - The steps in the production of sensations are not essentially different from those in the production of reflex action. First of all, external stimuli act upon the fiber terminations in the sense organs, or elsewhere, starting impulses in the neurons. These pass into the central nervous system and there excite neurons which in turn discharge impulses into the cerebrum. The result is to arouse an activity of the mind - a sensation. The steps in the production of any special sensation naturally involve the following parts:

1. A sense organ where the terminations of the neurons are acted upon by the stimulus.

2. A chain of neurons which connect the sense organ with the brain.

3. The part of the cerebrum which produces the sensation.

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