Facilitation and Inhibition of the Soleus H Reflex Related to Movements of the Upper Limb in Humans

Abstract

In studies on healthy volunteers, we recorded an EMG discharge from the m. soleus corresponding to the H reflex evoked by transcutaneous stimulation of the n. tibialis comm. Changes in the magnitude of this reflex related to realization of brief voluntary movements of the ipsilateral upper limb were examined. The subjects were in a prone position. Fast flexion-extension of the forearm resulted first in 100- to 200-msec-long facilitation of the H reflex begun 30-40 msec before the appearance of EMG activity in the m. biceps brachii; this feature is indicative of the central nature of this effect related to the action of motor programs initiating the forearm movement. Facilitation of the H reflex was followed by its inhibition lasting several seconds. Within an interval corresponding to the maximum suppression of the H response, we tested the effect of additional conditioning stimulation of the n. peroneus comm. Occlusion of the inhibitory effects indicates that the same inhibitory neurons mediate the influences from both the peroneal input and the pathways transmitting inhibitory influences from the neuronal systems controlling upper limb muscles. Contractions of the ipsilateral m. biceps brachii evoked by direct electrical stimulation of the latter also resulted in inhibition of the soleus H reflex, which was rather similar in its time course to the above-mentioned inhibitory effects. There was no inhibition of the reflex after stimulations of the cutaneous receptors and n. medianus. These findings allow us to suppose that long-lasting inhibition of the H reflex induced by voluntary movements of the upper limb results from afferent influences from the receptors of contracting muscles. Such effects can be realized via the propriospinal pathways or long reflex arcs.