Abstract

The H-reflex elicited in triceps surae by percutaneous stimulation of the posterior tibial nerve was conditioned by stimuli applied through the same electrode. The differential sensitivity of motor and sensory fibres to duration of the stimulus pulse made it possible to condition the H-reflex with either a motor or a sensory stimulus. With both types of conditioning, the H-reflex was inhibited at conditioning-test intervals of 2-3 msec and was then facilitated, the peak of facilitation occurring at 5-8 msec with motor conditioning and 6-10 msec with sensory conditioning. The phase of facilitation was followed by further inhibition. We have concluded (1) that the effects of motor conditioning on the H-reflex result from the discharge of Renshaw cells activated by the antidromic volley in the motor axons, and (2) that the effects of sensory conditioning (at the times used in these experiments) are largely due to the activation of Renshaw cells secondary to the discharge of alpha motoneurones by the conditioning volley.