Role of the vestibular system in orthostatic intolerance

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The arterial pressure (AP) control during stand-up is described as follows1. The direction of the gravitational vector changes at the onset of stand-up, and then hydrostatic pressure difference along with the longitudinal body axis increases.2. Footward fluid shift occurs, and venous return and cardiac output decrease, and then AP decreases.3. The decrease in AP is sensed by baroreceptor, and AP is thought to be corrected by negative feedback system, baroreflex.4. Vestibulo-cardiovascular reflex also contribute to the AP maintenance at the onset of stand-up. Changes in gravitational vector are sensed by the vestibular organ, and the reflex is triggered to increase sympathetic nerve activity and AP. Thus, the vestibulo-cardiovascular reflex increases AP based on changes in gravity not based on AP changes.5. Accordingly, AP response to stand-up is determined the sum of the fluid shift-induced AP decrease and the vestibulo cardiovascular reflex-mediated AP increase.However, the vestibular system is known to be highly plastic, i.e., if the vestibular system is exposed to a different gravitational environment, the sensitivity of the vestibular system is altered. Plastic alteration of the vestibulo-cardiovascular reflex was induced by hypergravity environment and restriction of the rear-up behavior in rats. Thus, reduced phasic input to the vestibular system is a key issue to induce the plastic alteration. In this context, it is interesting that the reduced sensitivity of vestibulo-cardiovascular reflex was observed in aged subjects, whose daily activity reduced, and then they suffer orthostatic hypotension. The same situation may occur in space, where the head movement-induced vestibular input is minimal. To prevent this situation, and a prophylaxis for the plastic alteration of the vestibular system, a weak galvanic vestibular stimulation has been proposed.