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) induc ed by transcranial magnetic stimulation (TMS) of the motor cortex, we inves tigated motor thresholds as parameters of the excitability of the cortico-m uscular pathway after a suprathreshold conditioning stimulus in the abducto r digiti minimi muscle (ADM) of normal humans. Since the unconditioned moto r 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 activ e and resting motor threshold indicates the magnitude of the voluntary driv e on the cortico-muscular pathway. Therefore, we compared conditioned resti ng 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 an d approximately 17% of the maximum stimulator output above the unconditione d threshold, due to decreased excitability of the cortico-muscular pathway after the conditioning stimulus. Some 30-40 ms after the conditioning stimu lus, cAMT approximated cRMT, indicating complete suppression of the volunta ry motor drive. This suppression did not start directly after the condition ing stimulus since cAMT was still significantly lower than the cRMT within the first 30-40 ms. Threshold elevation was significantly longer than the S P (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 thre sholds were still significantly elevated. We conclude that the SP is largel y due to a suppression of voluntary motor drive, while the threshold elevat ion 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 pat hway of fast cortico-spinal fibers activated by TMS is partially different from the pathway involved in the maintenance of tonic voluntary muscle acti vation.