THE ROLE OF CUTANEOUS INPUTS DURING MAGNETIC TRANSCRANIAL STIMULATION

Latency and amplitude characteristics of motor evoked potentials (MEPs ) from abductor digiti minimi (ADM) and first dorsal interosseus (FDI) muscles were evaluated in 7 healthy volunteers via magnetic transcran ial stimulation of the hemiscalp overlying contralateral motor areas, MEPs in complete relaxation and during contraction were recorded in tw o different experimental conditions: before and following anesthesia o f median (sensory + motor) and radial (sensory) nerve fibers at wrist. This procedure induced a complete loss of skin sensation from dorsal and palmar aspects of the hand area ''enveloping'' the FDI muscle, On the other hand, the skin overlying the ADM muscle, as well as the stre ngth of ulnar nerve supplied muscles were spared, This selective senso ry deprivation lead to the following short-term changes: the physiolog ical latency ''jump'' toward shorter values in contracted MEPs vs. rel axation was partially lost in the FDI (3.0 +/- 1.4 ms in basal conditi on, 1.8 +/- 1.1 ms after anesthesia, P = 0.028), while it was still cl early evident in the ADM (3.7 +/- 0.9 ms and 3.3 +/- 1/0 ms, respectiv ely). Moreover, minor amplitude changes of MEPs during active contract ion in the two muscles were detected: MEPs recorded from the FDI muscl e were less potentiated during voluntary contraction than those record ed from the ADM muscle, The role of the cutaneous input in governing l atency/amplitude characteristics of MEPs is discussed. (C) 1996 John W iley & Sons, Inc.