Magnetic stimulation of the human motor cortex evokes skin sympathetic nerve activity

Single-pulse magnetic coil stimulation (Cadwell MES 10) over the cranium in duces without pain an electric pulse in the underlying cerebral cortex. Sti mulation over the motor cortex can elicit a muscle twitch. In 10 subjects, we tested whether motor cortical stimulation could also elicit skin sympath etic nerve activity (SSNA; n = 8) and muscle sympathetic nerve activity (MS NA; n = 5) in the peroneal nerve. Focal motor cortical stimulation predicta bly elicited bursts of SSNA but not MSNA; with successive stimuli, the SSNA responses did not readily extinguish (94% of discharges to the motor corte x evoked SSNA responses) and had predictable latencies [739 +/- 33 (SE) to 895 +/- 13 ms]. The SSNA responses were similar after stimulation of domina nt and nondominant sides. Focal stimulation posterior to the motor cortex e licited extinguishable SSNA responses. In three of six subjects, anterior c ortical stimulation evoked SSNA responses similar to those seen with motor cortex stimulation but without detectable move ment; in the other subjects, anterior stimulation evoked less SSNA discharge than that seen with motor cortex stimulation. Contrasting with motor cortical stimulation, evoked SSN A responses were more readily extinguished with 1) peripheral stimulation t hat directly elicited forearm muscle activation accompanied by electromyogr ams similar to those with motor cortical stimulation; 2) auditory stimulati on by the click of the energized coil when off the head; and 3) in prelimin ary experiments, finger afferent stimulation sufficient to cause tingling. Our findings are consistent with the hypothesis that motor cortex stimulati on can cause activation of both alpha-motoneurons and SSNA.