Magnetic brain stimulation: The silent period after the motor evoked potential

In 25 normal subjects, we studied the EMG silent period following the magnetic motor evoked potential (MEP) when the target muscle was tonically contracted (post–MEP silent period [PMSPI). In the first dorsal interosseus muscle (FDI), PMSP duration increased in linear proportion to stimulus intensity, but not to the size of the preceding MEP. The PMSP was longer in hand and forearm muscles than in upper arm muscles. In the FDI, PMSP was longer than the peripheral silent period (PSP) even when multiple peripheral stimuli were used to get M responses whose twitch force was equivalent to that of MEPs. Weak magnetic stimuli evoked silent periods preceded by no MEP in several subjects. Spinal α–motoneurons (α–MNs) were partially inhibited during the first PMSP portion, but later this effect recovered. MEPs due to weak electrical stimuli to motor cortex were only slightly inhibited during the late PMSP. Segmental inhibitory loops evoked by the muscle twitch and inhibitory projections descending to α–MNs from the cortex predominantly underlie earlier PMSP portions, but recurrent intracortical inhibition may also contribute. Later portions are predominantly due to other stimulus–related cerebral inhibitory or suppressing phenomena.