Cutaneous withdrawal reflexes of the upper extremity.

We characterized reflexes of the upper limb elicited by electrical stimulation of the fingers. Surface electromyogram (EMG) was recorded from 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 effective stimulus for evoking EMG activity in relaxed arm muscles. Habituation was prominent. Latencies of EMG activity were <100 ms for most proximal and forearm muscles, and at least 40 ms prior to voluntary withdrawal movements. The timing of EMG activity in arm muscles was similar 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 the hand at the same time the grasp is released.