SUPPRESSION OF VOLUNTARY MOTOR-ACTIVITY REVEALED USING TRANSCRANIAL MAGNETIC STIMULATION OF THE MOTOR CORTEX IN MAN

1. Suppression of voluntary muscle activity of hand and arm muscles in response to transcranial magnetic stimulation (TMS) of the motor cort ex has been investigated in man. 2. Suppression could be elicited by l ow levels of TMS without any prior excitatory response. The latency of the suppression was 3-8 ms longer than the excitation observed at a h igher stimulus intensity. The duration of the suppression ranged from 8 to 26 ms. 3. A circular stimulating coil was used to determine thres hold intensity for excitation and suppression of contraction of thenar muscles in response to TMS at different locations over the motor cort ex. The locations for lowest threshold excitation coincided with those for lowest threshold suppression. Suppression was elicited at a lower threshold than excitation at all locations. 4. A figure-of-eight stim ulating coil was positioned over the left motor cortex at the lowest t hreshold point for excitation of the right thenar muscles. The orienta tion for the lowest threshold excitatory and inhibitory responses was the same for all subjects. That orientation induced a stimulating curr ent travelling in an antero-medial direction. Suppression was invariab ly elicited at lower thresholds than excitation. 5. When antagonistic muscles (second and third dorsal interosseus) were co-contracted, TMX evoked coincident suppression of voluntary EMG in the two muscles with out prior excitation of either muscle. This suggests that the suppress ion is not mediated via corticospinal activation of spinal interneuron es. 6. Test responses to electrical stimulation of the cervical spinal cord were evoked in both relaxed and activated thenar muscles. In the relaxed muscle, prior TMS at an intensity that would suppress volunta ry activity failed to influence the test responses, suggesting absence of inhibition at a spinal level. However, in the activated muscle, pr ior TMS could reduce the test response. This may be explained by disfa cilitation of motoneurones due to inhibition of corticospinal output. 7. We propose that suppression of voluntary muscle activity by TMS is due in large part to activation of a mechanism within the motor cortex that reduces the corticospinal output to the muscle. It is concluded that TMS evokes excitation and inhibition via neuronal structures lyin g close to one another and having similar orientations.