MODULATION OF MOTOR-ACTIVITY BY CUTANEOUS INPUT - INHIBITION OF THE MAGNETIC MOTOR EVOKED-POTENTIAL BY DIGITAL ELECTRICAL-STIMULATION

We examined the inhibitory effect of a brief train of digital (D2) ele ctrical stimuli at 4 times perception threshold on transcranial magnet ic motor evoked potentials (MEPs) recorded from abductor pollicis brev is (APE) and flexor carpi radialis (FCR) muscles ipsilateral to the si de of D2 stimulation. We compared this to the inhibitory effect of ips ilateral D2 stimulation on averaged rectified EMG recorded at 10% maxi mum voluntary contraction and on F-responses and H-reflexes recorded f rom these same muscles. We also compared MEPs recorded following D2 st imulation just above perception threshold to MEPs following higher int ensity D2 stimulation. As well, we assessed the effect of preceding D2 stimulation on MEPs recorded from a relaxed versus tonically contract ed hand muscle. D2 stimulation elicited a triphasic response of modest MEP facilitation followed by inhibition and further facilitation. The duration and onset of MEP inhibition correlated with those of the ini tial period of rectified EMG inhibition, however, the magnitude of MEP inhibition was generally less than the magnitude of EMG inhibition, c onsistent with a greater inhibitory effect of digital afferents on sma ller motor neurons. MEPs were not facilitated during the rebound of EM G activity (the E2 period) that usually followed the initial period of EMG inhibition (I1 period). The behavior of H-reflexes and F-response s following ipsilateral D2 stimulation suggested that inhibition of bo th EMG and MEPs is not mediated via presynaptic inhibition of Ia affer ents, and that inhibition is augmented by descending rather than segme ntal input to spinal motor neurons. Tonic contraction of the target mu scle during D2 stimulation decreased the inhibitory effect of the prec eding digital stimulus possibly due to recruitment of larger spinal mo tor neurons less likely to be inhibited by cutaneous input.