SPINAL AND SUPRASPINAL MECHANISMS CONTRIBUTE TO THE SILENT PERIOD IN THE CONTRACTING SOLEUS MUSCLE AFTER TRANSCRANIAL MAGNETIC STIMULATION OF HUMAN MOTOR CORTEX

In the voluntarily activated muscle, transcranial magnetic stimulation (TMS) of motor cortex produces subsequently to the motor evoked poten tial (MEP) a silent period (SP) in the electromyogram. We studied the time course of soleus motoneuron (MN) pool excitability after conditio ning TMS by Hoffmann reflex (HR) testing, to determine whether inacces sibility of MNs after corticospinal input contributes to the SP. Coinc idently with the early part of the SP, and only in the contracting sol eus, MN depression was obtained that covaried with the degree of prein nervation, and with the size of the preceding MN discharge. However, M N excitability recovered significantly prior to the end of the SP. It is concluded that in the contracting soleus spinal mechanisms (most li kely Renshaw inhibition and MN afterhyperpolarization) contribute to t he early part of the SP, while the late part of the SP is supraspinal (probably cortical) in origin.