Brain, Vol 121, Issue 2 373-380, Copyright © 1998 by Oxford University Press
A Priori, A Berardelli, M Inghilleri, F Pedace, M Giovannelli and M Manfredi
Electrical stimulation over muscle tendons produces a transient suppression
of voluntary EMG activity; its onset latency is approximately 55 ms in the
forearm extensor muscles. This phenomenon has been attributed to the
activation of a polysynaptic inhibitory pathway originating from Ib
afferent fibres. To clarify its origin we conducted several experiments in
10 normal 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 by 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 an
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 Ia fibres produced by tendon
afferent input to the spinal cord.
ARTICLES
Electrical stimulation over muscle tendons in humans. Evidence favouring presynaptic inhibition of Ia fibres due to the activation of group III tendon afferents
Department of Neurological Science, Universita La Sapienza, Roma, Italy.
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