Electrical stimulation over muscle tendons in humans - Evidence favouring presynaptic inhibition of Ia fibres due to the activation of group III tendon afferents

Electrical stimulation over muscle tendons produces a transient suppression 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 from II, afferent fibres. To clarify its origin we conducted several experiments in IO nor-mal healthy subjects. The EMG silence after tendon stimulation appeared at relatively high stimulus intensities (>50 mA); conditioning cutaneous stimulation left it unchanged, and the inhibition had a short recovery cycle (50 ms). Tendon stimulation still evoked EMG suppression during an ischaemic block of fast-conducting afferents. The motor potentials evoked bq, transcranial magnetic stimulation of the motor cortex during the EMG silence remained almost unchanged, whereas the H reflex was strongly inhibited Hence we conclude that tendon stimulation activates slow-conducting tendon afferents, possibly group III fibres, connected not through a polysynaptic pathway originating from Ib afferents but through nit oligo-or disynaptic inhibitory circuit. The EMG suppression after tendon stimulation probably represents a dysfacilitation of the alpha-motor neurons due to presynaptic inhibition of In fibres produced by tendon afferent input to the spinal cord.