University of Virginia Library

Summation

In the discharge of energy, summation plays an important rôle. Summation is attained by the repetition of stimuli at such a rate that each succeeding stimulus is applied before

the nerve-cells have returned to the resting stage from the preceding stimulus. If drops of water fall upon the skin from a sufficient height to cause the slightest unpleasant sensation, and at such a rate that before the effect of the stimulus of one drop has passed another drop falls in precisely the same spot, there will be felt a gradually increasing painful sensation which finally becomes unbearable. This is summation of stimuli. When, for a long time, a patient requires frequent painful wound dressings, there is a gradual increase in the acuteness of the pain of the receptors. This is caused by summation. In a larger sense, the entire behavior of the individual gives considerable evidence of summation, e. g., in the training of athletes, the rhythmic discharge of muscular energy at such intervals that the resting stage is not reached before a new exercise is given results in a gradual ascent in efficiency until the maximum is reached. This is summation, and summation plays a large rôle in the development of both normal and pathologic phenomena.

We have now pointed out the manner in which at least a part of the nervous energy of man may be discharged. The integrative action of the nervous system and the discharge of nervous energy by phylogenetic association may be illustrated by their analogy to the action of an electric automobile. The electric automobile is composed of four principal parts: The motor and the wheels (the muscular system and the skeleton); the cells of the battery containing stored electricity (brain-cells, nervous energy); the controller, which is connected with the cells by wiring (the receptors and the nerve-fibers); and an accelerator for increasing the electric discharge (thyroid gland?). The machine

is so constructed that it acts as a whole for the accomplishment of a single purpose. When the controller is adjusted for going ahead (adequate stimulus of a receptor), then the conducting paths (the final common path) for the accomplishment of that purpose are all open to the flow of the current from the battery, and the vehicle is integrated to go ahead. It spends its energy to that end and is closed to all other impulses. When the controller is set for reverse, by this adequate stimulus the machine is integrated to back, and the battery is closed to all other impulses. Whether integrated for going forward or backward, if the battery be discharged at a proper rate until exhausted, the cells, though possessing no more power (fatigue), have sustained no further impairment of their elements than that of normal wear and tear. Furthermore, they may be restored to normal activity by recharging (rest). If the vehicle be placed against a stone wall, and the controller be placed at high-speed (trauma and fear), and if the accelerator be used as well (thyroid secretion?), though the machine will not move, not only will the battery soon be exhausted, but the battery elements themselves will be seriously damaged (exhaustion—surgical shock).

We have now presented some evidence that nervous energy is discharged by the adequate stimulation of one or more of the various receptors that have been developed in the course of evolution. In response to an adequate stimulus, the nervous system is integrated for a specific purpose by the stimulated receptor, and but one stimulus at a time has possession of the final common path—the nerve mechanisms for action. The most numerous receptors are those for harmful contact; these are the nociceptors. The effect

of the adequate stimulus of a nociceptor is like that of pressing an electric button that sets great machinery in motion.

With this conception, the human body may be likened to a musical instrument—an organ—the keyboard of which is composed of the various receptors, upon which environment plays the many tunes of life; and written within ourselves in symbolic language is the history of our evolution. The skin may be the "Rosetta Stone" which furnishes the key.