CORTICAL AND SPINAL MOTOR EXCITABILITY DURING THE TRANSCRANIAL MAGNETIC STIMULATION SILENT PERIOD IN HUMANS

We investigated the electromyographic silent period in abductor pollic is brevis (APB) and flexor carpi radialis muscles following transcrani al magnetic stimulation of human motor cortex. In APB, we measured cor tical stimulation silent period (CSSP) duration as a function of stimu lus intensity, motor-evoked potential (MEP) amplitude and muscle twitc h 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 con trol 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 mu scle twitch force, thus the CSSP is not dependent solely on Renshaw ce ll inhibition or on changes in la and Ib afferent activity following t he cortically induced muscle twitch. At low intensities of stimulation , the interval to resumption of motor unit firing following the peak i n the peri-stimulus-time histogram corresponding to MEP latency someti mes exceeded that which could be accounted for by the motor unit's fir ing rate prior to the stimulus, suggesting that synchronization of mot or unit firing by cortical stimulation cannot account for the CSSP. We found brief inhibition of F-waves during the CSSP in some subjects, r eflecting activation of inhibitory corticospinal projections or segmen tal effects. In contrast, we observed longer inhibition of H-reflexes during the CSSP in all subjects, perhaps resulting from presynaptic in hibition of la afferents. Magnetic MEPs also were inhibited during the CSSP, suggesting inhibition of cortical elements by transcranial magn etic stimulation.