CUTANEOUS WITHDRAWAL REFLEXES OF THE UPPER EXTREMITY

We characterized reflexes of the upper limb elicited by electrical sti mulation of the fingers. Surface electromyogram (EMG) was recorded fro m several upper extremity muscles, and a finger was stimulated through paired ring electrodes. A train of 4-10 shocks at a frequency of 300 Hz and an intensity 4-6 times the perceptual threshold was the most ef fective stimulus for evoking EMG activity in relaxed arm muscles. Habi tuation was prominent. Latencies of EMG activity were <100 ms for most proximal and forearm muscles, and at least 40 ms prior to voluntary w ithdrawal movements. The timing of EMC; activity in arm muscles was si milar to that of the E2 component of the cutaneomuscular reflex evoked in these same muscles during contraction, and coincided with a silent period in active hand muscles. We conclude that cutaneous stimuli to the fingers activate a complex motor pattern that tends to withdraw th e hand at the same time the grasp is released. (C) 1998 John Wiley & S ons, Inc.