PHYSIOLOGICAL-BASIS OF VOLUNTARY ACTIVITY INHIBITION INDUCED BY TRANSCRANIAL CORTICAL STIMULATION

Apart from exciting corticospinal cells and evoking a motor response, electrical or magnetic transcranial cortical stimulation (TCCS) delive red during voluntary activity produces a prolonged inhibition of activ ity, a silent period (SP), that can last up to 280 msec. The SP durati on after TCCS is directly proportional to stimulus intensity. To deter mine the anatomic substrate and underlying physiological properties of the SP from TCCS, we (1) compared the SP from TCCS with the SP from c utaneous and mixed nerve stimulation, (2) studied the excitability cyc le of spinal motor neurons in 3 double stimulus conditions: (a) after paired peripheral nerve stimulation, (b) after TCCS and peripheral ner ve stimulation, and (c) after paired electrical TCCS, (3) investigated the timing of the excitability of the sensorimotor cortex measuring l ong latency responses from median nerve stimulation delivered inside t he SP from TCCS. We found that after TCCS, spinal motor neuron segment al excitability is reduced for a period up to 100 msec. Inhibition at a suprasegmental, possibly cortical, level was even more prolonged for a period up to 160 msec, suggesting that at least the late part of th e SP after TCCS is due to supraspinal inhibition. Determining the natu re of the supraspinal inhibition such as the possible role of cortical inhibitory mechanisms may be useful in further understanding central mechanisms of voluntary motor control and sensorimotor processing.