INHIBITION OF HAND MUSCLE MOTONEURONS BY PERIPHERAL-NERVE STIMULATIONIN THE RELAXED HUMAN SUBJECT - ANTIDROMIC VERSUS ORTHODROMIC INPUT

In active muscle, a supramaximal conditioning stimulus to peripheral n erve produces a classic silent period in the EMG. The present experime nts examined the effect of this type of conditioning stimulus on moton eurone excitability in relaxed muscle. EMG responses evoked by transcr anial magnetic stimulation of the brain were recorded from the first d orsal interosseus muscle (FDI) in 10 healthy subjects and 5 patients w ith sensory neuropathy. These responses (motor evoked potentials) were conditioned by supramaximal peripheral nerve stimuli given 0-150 msec beforehand. In the normal subjects, the classic silent period in the FDI lasted about 100 msec. The same conditioning stimulus only abolish ed motor evoked potentials when the conditioning-test interval was so short that the antidromic peripheral nerve volley collided with the or thodromic volley set up by magnetic brain stimulation. At longer condi tioning-test intervals, although remarkably inhibited (65% mean suppre ssion between 10 and 40 msec), the test motor potential was never comp letely abolished and gradually recovered by 100 msec. Inhibition of co rtically evoked motor potentials did not depend upon activity set up b y the conditioning stimulus in peripheral nerve sensory fibres. The pa tients with complete peripheral sensory neuropathy had the same extent and time-course of inhibition as the normal subjects. We conclude tha t in relaxed subjects the inhibitory effect of peripheral conditioning results almost exclusively from the motoneuronal inhibitory mechanism s consequent to antidromic invasion.