CORTICAL AND SPINAL MECHANISMS OF FACILITATION TO BRAIN-STIMULATION

Compound muscle action potentials (CMAPs) evoked by magnetic brain sti muli are larger if the subject provides a steady background voluntary contraction of the target muscle. This facilitation could be due eithe r to cortical or spinal mechanisms, or both. Both magnetic and electri cal stimuli given immediately after the onset of a ballistic contracti on also evoke markedly facilitated CMAPs. By contrast, responses some 200 ms after the onset of such a contraction are facilitated if stimul i are magnetic but not if they are electrical. This second phase of fa cilitation is largely cortical in origin. By comparing the size of CMA Ps evoked by magnetic stimuli at two different delays after electromyo gram onset, the total facilitation could be dissected into its spinal and cortical components, The relationship between CMAP area in the fir st dorsal interosseous and stimulus intensity was different in the two phases of facilitation, suggesting a constant background level of spi nal facilitation upon which an increasing descending volley operated, In experiments in which ballistic contractions at increasing force lev els were performed it was found that at low force levels, spinal facil itation predominated, but at forces greater than 10% maximum there wer e roughly equal contributions from increased spinal cord and cortex ex citability. (C) 1996 John Wiley & Sons, Inc.