Electrical stimulation over muscle tendons: evidences favouring a group iii afferent mediated disfacilitation of alfamotoneurones

Electrical stimulation over the muscle tendons produce 60 ms after a transient suppression of the voluntary EMG activity in the forearm extensor muscles. To clarify its origin we conducted several experiments with this technique in eleven normal subjects. We have seen that the EMG silence appears 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 afférents. The motor potentials evoked by transcranial magnetic stimulation remained almost unchanged. We conclude that tendon stimulation activates slow-conducting tendon afférents, likely group III fibres, connected through a disynaptic inhibitory circuit. The EMG suppression might originate from an indirect disfacilitation of the alfamotoneurones following the inhibition of gamma-motoneurones.