Sympathetic nerve activity during natural stimulation of horizontal semicircular canals in humans.

We have shown that static head-down neck flexion elicits increases in muscle (MSNA) but not skin sympathetic nerve activity (SSNA) in humans. These findings suggest that stimulation of the otolith organs causes differential sympathetic outflow to vascular beds. The purpose of the present study was to determine whether yaw head rotation (YHR), which stimulates the horizontal semicircular canals, elicits sympathetic nerve responses. To test this question, we recorded MSNA (n = 33) and SSNA (n = 25) before and during 3 min of sinusoidal YHR performed at 0.1, 0.6, and 1.0 Hz. At all frequencies, YHR elicited no significant changes in heart rate and mean arterial pressure. Likewise, YHR did not significantly change either MSNA or SSNA at all frequencies. Our results indicate that stimulation of the horizontal semicircular canals by YHR does not alter SNA to either muscle or skin. Moreover, these results provide evidence to support the concept that the otolith organs but not the horizontal semicircular canals participate in the regulation of SNA in humans.