EFFECT OF TONIC VOLUNTARY ACTIVITY ON THE EXCITABILITY OF HUMAN MOTORCORTEX

1. The threshold for obtaining EMG responses after transcranial magnet ic stimulation of the brain is reduced by voluntary contraction of the target muscle. The present experiments tested whether some of this ef fect is due to increased cortical, as opposed to spinal, excitability during the contraction. 2. Magnetic stimulation was delivered with a f igure-of-eight coil oriented with the junction region along the intera ural line and also (in 4 of 7 subjects) with a circular coil centred a t the vertex. The intensity of the conditioning stimulus was subthresh old for evoking a motor response in the relaxed wrist flexor muscles o f the forearm. The presence of a small descending corticospinal volley in both the relaxed and active conditions was detected by measuring t he facilitation of test H reflexes elicited in the flexor muscles of t he forearm. 3. In all subjects, magnetic stimulation with either coil facilitated the H reflex at conditioning-test intervals of -1, to -3 m s (median nerve stimulus before magnetic). This was followed by a long -lasting facilitation. In three of the seven subjects stimulation with the figure-of-eight coil elicited an additional, earlier peak of faci litation at a conditioning-test interval of -3 to -5 ms. 4. In all sub jects, the threshold for obtaining facilitation of the K reflex using a conditioning-test interval of -1 to -3 ms was reduced, and the amoun t of facilitation was larger, if subjects performed a weak tonic volun tary contraction. In contrast, with a conditioning-test interval of -3 to -5 ms voluntary contraction had no effect on the threshold. 5. It is suggested that K reflex facilitation at the conditioning-test inter val of -1 to -3 ms was produced by indirect activation of corticospina l neurones by the magnetic stimulus, whereas at -3 to -5 ms, the facil itation was produced by direct activation of corticospinal axons. It i s concluded that tonic voluntary contraction of a target muscle decrea ses the threshold for indirect activation of corticospinal neurones bu t not for direct stimulation of their axons.