OBSERVATIONS ON INTERACTION OF NERVE IMPULSES IN THE GRAY MATTER AND ON THE NATURE OF CENTRAL INHIBITION

Abstract

Motoneurons supplying the quadriceps muscle in anesthetized cats discharge 1.0-1.1 msec. after a sixth lumbar dorsal root (L6DR) volley arrives at the spinal cord. Detectable inhibition of the discharge is apparent if an L7DR volley arrives at the cord 1.0-1.1 msec. before the motoneurons discharge. Both intervals are briefer by ca. 0.3 msec. if estimated from the time of arrival of the dorsal root impulses in the vicinity of the quadriceps motor nucleus. The inhibition rapidly increases to become complete as the interval at which the L7DR volley precedes the L6DR volley increases. The po-tential changes recorded in the motor nucleus during the synaptic delay associated with the excitation of the motoneurons by an L6DR volley are modified by a simultaneously arriving or slightly antecedent L7DR volley. This fact may be interpreted as evidence that during the synaptic delay interaction occurs between impulses in the terminal branches of the 2 groups of dorsal root fibers. A strong presumption that at least part of the interaction is in fact between impulses in the 2 groups of premotor fibers, and that this interaction underlies the "direct" inhibition of the quadriceps moneurons by an L7DR volley, is created by the finding that the responsiveness of the motoneurons as tested by the size of their antidromically evoked somatic action potential is not altered at intervals following an L7DR volley at which the reflex motor discharge evoked by an L6DR volley is virtually abolished.