[The afferent control of the cortically evoked reactions of the interneurons in the spinal cord ventral horn]

Abstract

Responses of spinal interneurons of the ventral horn (L6, 7) to stimulation of motor cortex and hindlimbs' afferents, were studied in cats. The cortically induced responses were shown to be modulated by a preceding afferent wave. Three types of effects were revealed: facilitation, depression, and biphasic changes. The facilitation was rather short (not over 30-35 msec) whereas the depression varied from 60-100 to 300-400 msec. The facilitation was commonly observed for the short-latency responses whereas the long-latency ones were rather depressed. The depression was more obvious for ipsilateral responses, when conditioning contralateral afferents rather than ipsilateral ones, high-threshold afferents rather than low-threshold ones. The data obtained suggest the mechanism of presynaptic inhibition to play a major part in the depression of descending activity, as well as the defacilitation in the interneurons themselves due to a blocking of the background afferent inflow from other inputs.