ABSTRACT
Supraspinal influences on the somatic sensory afferents can be tested in man by the variations of the H reflex facilitation produced by a conditioning sural nerve stimulation. At rest this facilitation remains constant for reflex amplitudes comprised between 15% and 85% of the maximal H response. The time course of this facilitation was studied during 500 msec before the onset of ballistic and tracking movements. Before tracking movements, the H reflex amplitude shows an earlier and stronger facilitation than before ballistic movements. However, in both situations, supraspinal inhibitory control progressively takes place upon the cutaneous afferents. The depression of the cutaneous facilitation becomes statistically significant at the same time the H reflex is enhanced by the descending motor commands. These data fit the suggestion of a differential control over the cutaneous and proprioceptive relays, the latter being facilitated whereas the former are inhibited before a movement.
