Reflex contractions of an all-or-none character

Abstract

Sherrington's emphasis on the differences between reflex are conduction and conduction in the nerve trunk has long been familiar to physiologists. With the establishment of the all-or-none law for muscle and for nerve in recent years it was but natural that the query should be raised as to whether reflex action be bound by the limitations of the all-or-none behavior of muscle and of nerve, or whether the central nervous system so alters the final nerve impulse to muscle that it can over-ride these limitations. A single sensory impulse, for example, might always result in a volley of impulses along the final motor neurones, according to Sherrington's suggestion. 1 Variations in the number of impulses comprising this volley might, by summation of contractions, conceivably produce in the muscle degrees of response of almost any grade of fineness, and reflex contractions would not then show an all-or-none character.

The present research was undertaken with the object of comparing minimal, or close to minimal, responses of muscle secured by single shocks to its motor nerve, with similar responses obtained reflexly by stimulation of a sensory nerve.

The muscle employed was the tenuissimus, a long slender, straight-fibered muscle underlying the biceps and containing approximately one thousand fibers. 2 The nerve to this muscle contains about twenty motor neurones, each neurone presumably innervating approximately fifty muscle fibers. If this nerve be stimulated by single shocks gradually increasing in strength the response of the muscle is not gradual, but by step-like increments, there being no gradation between one step and the next. The heights of the first three increments to appear in three of such experiments are shown in Experiments I, II and III of the appended table.