Form of the Reflex Response in Relation to the Pattern of Afferent Stimulation

Abstract

The contraction curve and the after discharge of the flexor reflex response to selective ipsilateral afferent stimulation were studied on 52 cats. Action potentials of the afferent impulses in the popliteal or saphenous nerve, tibialis anticus muscle reflex in spinal cats were recorded. In this group of experiments, discharges from a small condenser (.001 mf) furnished the stimulus. Larger condensers (.02 mf) were used in a second group of experiments which included also decerebrate animals. It was established that with the larger condensers repetitive firing of groups of fibers of the afferent nerve followed a single shock; only the lower threshold fibers fired twice with small voltages. As the strength of stimulus was increased, more of the fibers were repeatedly stimulated, while the higher threshold fibers were fired once. Large voltages repeated the entire action potential with each shock. With the small condenser discharges, a single stimulus fired only once all fibers within the threshold of that stimulus. The rate of stimulation varied from 1 to 15 per second, and in most instances periods of stimulation did not exceed 10 seconds.

Results. Voltages within the threshold range of the large fibers of the A group, including the alpha, beta and gamma fibers, produced excitatory responses in the contraction curve which persisted during the period of stimulation at a constant level. Cessation of stimulation resulted in prompt relaxation of the muscle, without indication of an after discharge. Action potentials indicated that alpha fiber threshold coincided with 1.5 v at .001 mf. The rest of the alpha, beta, gamma fibers of the A group were fired with shocks of 3–4.5 v.

Stronger voltages (6 v) produced a slightly stronger contraction, which decreased after an initial maximum. An after discharge was manifest in the delay in relaxation of the muscle at the conclusion of stimulation. Progressively stronger voltages produced initially stronger contractions which, however, fell off more rapidly during stimulation, and were followed, after stimulation stopped, by after discharges, occurring as separate slow contractions, a latent period intervening. With 6 volts the lower threshold fibers