Vestibulosympathetic reflexes induced by 0.08 Hz sinusoidal linear acceleration at 5 mG

![CDATA[We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), a means of selectively modulating vestibular afferent input without affecting other inputs, can cause partial entrainment of muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA) in human subjects. Given that slow movements of the body, such as those experienced during wind-induced sway of tall buildings, can cause motion sickness and that some of the symptoms of motion sickness are linked to increases in skin sympathetic nerve activity, we tested the hypothesis that physiological activation of the vestibular system at low frequencies entrains SSNA and also entrains MSNA. Skin sympathetic nerve activity was recorded via tungsten microelectrodes inserted into the common peroneal nerve in 10 awake subjects; MSNA was recorded in five. Subjects were seated (head vertical, eyes closed) on a motorised platform and slow sinusoidal accelerationsdecelerations (~5 mG) were applied in the X (antero-posterior) or Y (medio-lateral) direction at 0.08 Hz; composite movements in both directions were also applied at 0.16 Hz. Subjects either reported feeling a vague sense of movement (with no sense of direction), or no movement at all, consistent with the non-palpable motion of a swaying building. Nevertheless, cross-correlation analysis revealed a marked entrainment of SSNA and MSNA for all types of movements, as measured by the modulation index. Vestibular modulation for movements in the X axis was 96.7±2.5 % for SSNA and 85.5±6.6 % for MSNA; there were no significant differences for movements in the Y direction or composite movements in the XY plane. For each sinusoidal cycle there were two major peaks of modulation – one associated with acceleration as the platform moves forward or to the side and one in the opposite direction. We interpret these observations as reflecting robust vestibulosympathetic reflexes mediated by afferents within the utricle, and suggest that these reflexes may contribute to the signs of motion sickness.]]