Cortical and spinal motor excitability during the transcranial magnetic stimulation silent period in humans

We investigated the electromyographic silent period in abductor pollicis brevis (APB) and flexor carpi radialis muscles following transcranial magnetic stimulation of human motor cortex. In APB, we measured cortical stimulation silent period (CSSP) duration as a function of stimulus intensity, motor-evoked potential (MEP) amplitude and muscle twitch force. We used peri-stimulus-time histograms to study the effect of cortical stimulation on single-motor unit firing patterns. We compared F-waves, H-reflexes and magnetic MEPs elicited during the CSSP to control responses elicited at rest and during voluntary contraction. CSSP duration depended on the intensity of cortical stimulation. However, we found no relationship between CSSP duration and MEP amplitude or muscle twitch force, thus the CSSP is not dependent solely on Renshaw cell inhibition or on changes in Ia and Ib afferent activity following the cortically induced muscle twitch. At low intensities of stimulation, the interval to resumption of motor unit firing following the peak in the peri-stimulus-time histogram corresponding to MEP latency sometimes exceeded that which could be accounted for by the motor unit's firing rate prior to the stimulus, suggesting that synchronization of motor unit firing by cortical stimulation cannot account for the CSSP. We found brief inhibition of F-waves during the CSSP in some subjects, reflecting activation of inhibitory corticospinal projections or segmental effects. In contrast, we observed longer inhibition of H-reflexes during the CSSP in all subjects, perhaps resulting from presynaptic inhibition of Ia afferents. Magnetic MEPs also were inhibited during the CSSP, suggesting inhibition of cortical elements by transcranial magnetic stimulation.