Abstract
Gravity acts on the circulatory system to alter hydrostatic pressure gradient and then causes fluid shift; this could alter venous return and cardiac output, followed by an alteration of arterial pressure (AP). These hemodynamic changes have been thought to be mainly corrected by the baroreflex. However, recently we have demonstrated that the vestibular system has a significant role in maintaining AP during hypergravity. In conscious rats, AP decreases during hypergravity, if both the vestibular system and baroreflex are absent, probably due to blood redistribution. If the vestibular system but not the baroreflex operates, AP increases markedly, suggesting that the hypergravity is detected by the vestibular organ, which reflexively increases AP. This increase is suppressed, if the baroreflex operates. Thus, the vestibular system acts as a feedforward AP controller against hypergravity. The AP increase observed in the intact rats indicates that AP is overcompensated rather than compensated by the vestibular system. While feedforward control has the advantage of a short response delay, the major disadvantage is the instability of the response and the large correct error. Comparison of the AP responses between intact and baroreflex lesioned rats suggests that the overcompensated AP is compensated by the negative feedback control system, the baroreflex. The vestibular system is known to be highly plastic, i.e., the sensitivity of the vestibular system alters if subjects are maintained in a different gravitational environment. The plasticity of this system and the role of this system upon posture change in human will be discussed. [J Physiol Sci. 2007;57 Suppl:S6]
