Role of the vestibular system in orthostatic intolerance

站立期间的动脉压（AP）控制如下所述1。在站立开始时重力矢量的方向发生变化，然后流体静压差沿着身体纵轴增大.出现足液移位，静脉回流减少，心输出量减少，AP降低.压力感受器感受到AP的降低，而AP被认为是由负反馈系统-压力反射纠正的.前庭-心血管反射也有助于站立时AP的维持。重力矢量的变化被前庭器官感觉到，并且反射被触发以增加交感神经活动和AP。因此，前庭心血管反射增加AP的基础上的重力变化，而不是基于AP的变化。5.因此，对站立的AP反应是由液体转移诱导的AP降低和前庭心血管反射介导的AP增加的总和确定的。如果前庭系统暴露在不同的重力环境中，前庭系统的敏感性就会改变。本文用超重力环境和限制大鼠后仰行为诱发前庭心血管反射的可塑性改变。因此，减少前庭系统的相位输入是诱导可塑性改变的关键问题。在这种情况下，有趣的是，在老年受试者中观察到前庭心血管反射的敏感性降低，其日常活动减少，然后他们遭受直立性低血压。同样的情况可能发生在太空中，其中头部运动引起的前庭输入是最小的。为了防止这种情况，以及预防前庭系统的可塑性改变，已经提出了弱电流前庭刺激。

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.