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MRC Human Movement and Balance Unit, Institute of Neurology, London.

1. The threshold for obtaining EMG responses after transcranial magnetic stimulation of the brain is reduced by voluntary contraction of the target muscle. The present experiments tested whether some of this effect is due to increased cortical, as opposed to spinal, excitability during the contraction. 2. Magnetic stimulation was delivered with a figure-of-eight coil oriented with the junction region along the interaural line and also (in 4 of 7 subjects) with a circular coil centred at the vertex. The intensity of the conditioning stimulus was subthreshold for evoking a motor response in the relaxed wrist flexor muscles of the forearm. The presence of a small descending corticospinal volley in both the relaxed and active conditions was detected by measuring the facilitation of test H reflexes elicited in the flexor muscles of the forearm. 3. In all subjects, magnetic stimulation with either coil facilitated the H reflex at conditioning-test intervals of -1 to -3 ms (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 facilitation at a conditioning-test interval of -3 to -5 ms. 4. In all subjects, the threshold for obtaining facilitation of the H reflex using a conditioning-test interval of -1 to -3 ms was reduced, and the amount of facilitation was larger, if subjects performed a weak tonic voluntary 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 H reflex facilitation at the conditioning-test interval of -1 to -3 ms was produced by indirect activation of corticospinal neurones by the magnetic stimulus, whereas at -3 to -5 ms, the facilitation was produced by direct activation of corticospinal axons. It is concluded that tonic voluntary contraction of a target muscle decreases the threshold for indirect activation of corticospinal neurones but not for direct stimulation of their axons.

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