Stimulation of the vestibular system by head movement or changes in gravitational forces is known to induce sympathoexcitation

A postural change from a recumbent to an upright position induces an increase in the hydrostatic pressure gradient, a footward fluid shift, reduced venous return and cardiac output, and reduced arterial pressure (AP). This reduction in AP is sensed by baroreceptors in the blood vessels, and AP is thought to be stabilized by the arterial baroreflex, an important negative feedback process. Alternatively, postural changes might stimulate the vestibular organ, which is also thought to be involved in AP regulation with postural change 1 . AP maintenance during passive postural change was found to be dependent on the vestibular system: if the vestibular system was not functioning properly, AP decreased or fluctuated. Stimulation of the vestibular system by head movement or changes in gravitational forces is known to induce sympathoexcitation 2 . Although sympathoexcitation was observed during postural change, it is not clear whether it is mediated through the vestibular system. Postural change-induced AP decrease might also induce sympathoexcitation through the arterial baroreflex. In this regard, Ray 3 reported that lower body negative pressure, which is a result of postural change-induced footward fluid shift, induces an increase in sympathetic nerve activity (SNA); a further increase in SNA was observed by head-down rotation. This result suggests that the observed sympathoexcitation upon postural change might be a sum of the baroreflex-mediated and the vestibular-mediated sympathoexcitation. Thus it is possible that vestibular-mediated sympathoexcitation modulates arterial baroreflex function during postural change. To examine this, we analyzed the functional interaction between the baroreflex and the vestibular system during postural change.