Short latency inhibition of human hand motor cortex by somatosensory inputfrom the hand

EMG responses evoked in hand muscles by transcranial stimulation over the m otor cortex were conditioned by a single motor threshold electrical stimulu s to the median nerve at the wrist in a total of ten healthy subjects and i n five patients who had electrodes implanted chronically into the cervical epidural space. The median nerve stimulus suppressed responses evoked by transcranial magne tic stimulation (TMS) in relaxed or active muscle. The minimum interval bet ween the stimuli at which this occurred was 19 ms. A similar effect was see n if electrical stimulation was applied to the digital nerves of the first two fingers. Median or digital nerve stimulation could suppress the responses evoked in active muscle by transcranial electrical stimulation over the motor cortex, but the effect was much less than with magnetic stimulation. During contraction without TRIS, both types of conditioning stimuli evoked a cutaneomuscular reflex that began with a short period of inhibition. This started about 5 ms after the inhibition of responses evoked by: TMS. Recordings in the patients showed that median nerve stimulation reduced the size and number of descending corticospinal volleys evoked by magnetic sti mulation. We conclude that mixed or cutaneous input from the hand can suppress the ex citability of the motor cortex at short latency. This suppression may contr ibute to the initial inhibition of the cutaneomuscular reflex. Reduced spin al excitability in this period could account for the mild inhibition of res ponses to electrical brain stimulation.