Evidence favouring presynaptic inhibition between antagonist muscle afferents in the human forearm.

1. A single motor-threshold electrical stimulus to the radial nerve in the spiral groove produced three phases of inhibition in the H reflex of relaxed wrist and finger flexor muscles. The first phase of inhibition occurred with a conditioning-test interval of -1 to 3 ms and represents disynaptic Ia reciprocal inhibition of flexor motoneurones by afferent input from extensor muscles. The second phase occurred with a conditioning-test interval of 5-40 ms. The characteristics of the second inhibitory phase have been investigated in the present study. 2. The threshold intensity of radial nerve stimulation required to produce the second phase of inhibition was 0.7 X motor threshold and was the same as that required to produce the first (disynaptic) phase of inhibition. 3. The time course of the effect of a radial nerve stimulus on the size of e.m.g. response to transcranial electrical stimulation of the motor cortex also was studied. The results were compared to those obtained with H reflex testing. The time course and depth of the first phase of inhibition were identical with both test inputs. The second phase of inhibition, however, was seen only with the H reflex test stimulus. When the corticospinal volley was timed to arrive at the cervical cord 30 ms after the radial nerve afferent volley, the flexor muscle response was facilitated rather than inhibited. 4. A single stimulus (2-3 X sensory threshold) to the superficial (cutaneous) branch of the radial nerve at the wrist failed to produce inhibition of the flexor H reflex. Instead, the H reflex was facilitated when the median nerve Ia volley (for the H reflex) was timed to arrive at the cervical cord between 30-40 ms after the radial nerve cutaneous afferent volley. In contrast to results using mixed nerve stimulation in the spiral groove, there was no difference between the effect of this cutaneous input on the cortically evoked e.m.g. response and the H reflex. 5. We conclude that the second phase of flexor H reflex inhibition is produced by activity in large-diameter (group I) extensor muscle afferents which acts on the H reflex pathway at a premotoneuronal site.(ABSTRACT TRUNCATED AT 400 WORDS)