Complete suppression of voluntary motor drive during the silent period after transcranial magnetic stimulation

To evaluate changes in the motor system during the silent period (SP) induced by transcranial magnetic stimulation (TMS) of the motor cortex, we investigated motor thresholds as parameters of the excitability of the cortico-muscular pathway after a suprathreshold conditioning stimulus in the abductor digiti minimi muscle (ADM) of normal humans. Since the unconditioned motor threshold was lower during voluntary tonic contraction than at rest (31.9+/-5.4% vs. 45.6+/-7.5%), it is suggested that the difference between active and resting motor threshold indicates the magnitude of the voluntary drive on the cortico-muscular pathway. Therefore, we compared conditioned resting and active motor threshold (cRMT and cAMT) during the SP. cRMT showed an intensity-dependent period of elevation of more than 200 ms in duration and approximately 17% of the maximum stimulator output above the unconditioned threshold, due to decreased excitability of the cortico-muscular pathway after the conditioning stimulus. Some 30-40 ms after the conditioning stimulus, cAMT approximated cRMT, indicating complete suppression of the voluntary motor drive. This suppression did not start directly after the conditioning stimulus since cAMT was still significantly lower than the cRMT within the first 30-40 ms. Threshold elevation was significantly longer than the SP (220+/-41 vs. 151+/-28 ms). Recovery of the voluntary motor drive started late in the SP and was nearly complete at the end of the SP, although thresholds were still significantly elevated. We conclude that the SP is largely due to a suppression of voluntary motor drive, while the threshold elevation is a different inhibitory phenomenon that is of less importance for the generation of the SP, at least in its late part. It is argued that the pathway of fast cortico-spinal fibers activated by TMS is partially different from the pathway involved in the maintenance of tonic voluntary muscle activation.