ELECTRICAL-STIMULATION OVER MUSCLE TENDONS IN HUMANS - EVIDENCE FAVORING PRESYNAPTIC INHIBITION OF IA FIBERS DUE TO THE ACTIVATION OF GROUP-III TENDON AFFERENTS

Electrical stimulation over muscle tendons produces a transient suppre ssion of voluntary EMG activity; its onset latency is similar to 55 ms in the forearm extensor muscles. This phenomenon has been attributed to the activation of a polysynaptic inhibitory pathway originating fro m II, afferent fibres. To clarify its origin we conducted several expe riments in IO nor-mal healthy subjects. The EMG silence after tendon s timulation appeared at relatively high stimulus intensities (>50 mA); conditioning cutaneous stimulation left it unchanged, and the inhibiti on had a short recovery cycle (50 ms). Tendon stimulation still evoked EMG suppression during an ischaemic block of fast-conducting afferent s. The motor potentials evoked bq, transcranial magnetic stimulation o f the motor cortex during the EMG silence remained almost unchanged, w hereas the H reflex was strongly inhibited Hence we conclude that tend on stimulation activates slow-conducting tendon afferents, possibly gr oup III fibres, connected not through a polysynaptic pathway originati ng from Ib afferents but through nit oligo-or disynaptic inhibitory ci rcuit. The EMG suppression after tendon stimulation probably represent s a dysfacilitation of the alpha-motor neurons due to presynaptic inhi bition of In fibres produced by tendon afferent input to the spinal co rd.