Physiology and Hygiene for Secondary Schools

by Francis M. Walters

Properties of Neurons.—The work of the neurons seems to depend mainly upon two properties—the property of irritability and the property of conductivity. Irritability was explained, in the study of the muscles (page 243), as the ability to respond to a stimulus. It has the same meaning here. The neurons, however, respond more readily to stimuli than do the muscles and are therefore more irritable. Moreover, they are stimulated by all the forces that induce muscular contraction and by many others besides. They are by far the most irritable portions of the body.

Conductivity is the property which enables the effect of a stimulus to be transferred from one part of a neuron to another. On account of this property, an excitation, or disturbance, in any part of a neuron is conducted or carried to all the other parts. Thus a disturbance at the distant ends of the dendrites causes a movement toward the cell-body and, reaching the cell-body, the disturbance is[pg 305] passed through it into the axon. This movement through the neuron is called the nervous impulse.

Purpose of the Impulse. —Though the nature of the nervous impulse is not understood, [103] its purpose is quite apparent. It is the means employed by the nervous system for controlling and coördinating the different parts of the body. The arrangement of the neurons enables impulses to be started in certain parts of the nervous system, and the property of conductivity causes them to be passed as stimuli to other parts. This enables excitation at one place to bring about action at another place.

Acting as stimuli, the impulses seem able to produce two distinct effects: first, to throw resting organs into action and to increase the activity of organs already at work; and second, to diminish the rate, or check entirely, the activity of organs. Impulses producing the first effect are called excitant impulses; those producing the second effect, inhibitory impulses.

Functions of the Parts of Neurons.—The cell-body serves as a nutritive center from which the other parts derive nourishment. Proof of this is found in the fact that when any part of the neuron is separated from the cell-body, it dies, while the cell-body and the parts attached to the cell-body[pg 306] may continue to live. In addition to this the cell-body probably reënforces the nervous impulse.

The dendrites serve two purposes: first, they extend the surface of the cell-body, thereby enabling it to absorb a greater amount of nourishment from the surrounding lymph; second, they act as receivers of stimuli from other neurons. The same impulse does not pass from one neuron to another. An impulse in one neuron, however, is able to excite the neuron with which it makes an end-to-end connection, so that a series of impulses is produced along a given nerve path (Fig. 129).

The special function of the axon is to transmit the impulse. By its length, structure, and property of conductivity it is especially adapted to this purpose. The axis cylinder, however, is the only part of the axon concerned in the transmission. The primitive sheath and the medullary layer protect the axis cylinder, and, according to some authorities, serve to insulate it. The medullary sheath may also aid in the nourishment of the axis cylinder.

Nerve Stimuli.—While the properties of irritability and conductivity supply a necessary cause for the production and transmission of nervous impulses, these alone are not sufficient to account for their origin. An additional cause is necessary—a force not found in the nerve protoplasm, but one which, by its action on the protoplasm, makes it produce the impulse. In this respect, the neuron does not differ essentially from the cell of a muscle. Just as the muscle cell requires a stimulus to make it contract, so does the neuron require a stimulus to start the impulse. Hence, in accounting for the activities of the body, it is not sufficient to say they are caused by nervous impulses. We must also investigate the nerve stimuli—the means through which the nervous impulses are started. Most of these[pg 307] are found outside of the body and are known as external stimuli.

Action of External Stimuli.—In the arrangement of the nervous system the most favorable conditions are provided for the reception of external stimuli. Not only do vast numbers of neurons terminate at the surface of the body,[104] but they connect there with delicate structures, called sense organs. The purpose of the sense organs is to sensitize (make sensitive) the terminations of the neurons. This they do by supplying special structures through which the stimuli can act to the best advantage upon the nerve endings. Moreover, there are different kinds of sense organs, and these cause the neurons to be sensitive to different kinds of stimuli. Acting through the sense organs adapted for receiving them, light, sound, heat, cold, and odors all act as stimuli for starting impulses. Indeed, the arrangement is so complete that the nervous system is subjected to the action of external stimuli in some form practically all the time. The work of the sense organs is further considered in Chapters XX, XXI, and XXII.

How External Stimuli act on Internal Organs.—For stimulating the neurons not connected with the body surface we are dependent, so far as known, upon the nervous impulses. An impulse started by the external stimulus goes only so far as its neuron extends. But it serves as a stimulus for the neuron with which the first connects and starts an impulse in this connecting neuron, the point of stimulation being where the fiber terminations of the first neuron make connection with the dendrites of the second. This impulse in turn stimulates the next neuron, and so on, producing a series of impulses along a given nerve path. [pg 308]In this way the effect of an external stimulus may reach and bring about action in any part of the body. This is in brief the general plan of inducing action in the various organs of the body. This plan, however, is varied according to circumstances, and at least three well-defined forms of action are easily made out. These are known as reflex action, voluntary action, and secondary reflex action.