MODELS FOR CHRONIC RESPONSES IN CARDIOVASCULAR, AUTONOMIC, AND CENTRAL NERVOUS SYSTEM TO GRAVITATIONAL STRESS

为探讨静水压差在微重力急性血流动力学改变中的作用，对麻醉大鼠脑灌注压、虹膜毛细血管微循环、大脑皮层和颞肌局部血流量进行了测定。微重力是通过自由落体或抛物线飞行产生的。将大鼠置于30°仰卧位、全身倾斜或水平平卧位。在平坦组，所有测量变量均未受到微重力的显著影响，而在抬头组，虹膜毛细血管、颞肌和大脑皮层的脑灌注压和血流量增加。因此，静水压差的消失会导致头侧液体移动，进而增加头部的血流量。另一方面，如果压力感受器反射和前庭交感神经反射不起作用，超重力会引起低血压。如果只有前庭交感神经反射起作用，超重力会导致肾交感神经活动增加和巨大的升压反应，而压力感受器反射会减弱这些反应。在这些实验数据的基础上，绘制了控制系统在重力应力作用下的框图，并估计了系统的传递函数。在这个系统中，重力压力被前庭系统感知，并投射到前庭内侧核和下丘脑室旁核，然后增加交感神经活动，引起升压反应。压力感受器可感受到AP的增加，并抑制AP的增加。因此，保护AP免受重力应力影响的控制系统由两个子系统组成，即压力感受器反馈控制系统和前庭前馈控制系统。前庭系统和压力感受器反射的估计传递函数分别为16和8。

To examine the role of hydrostatic pressure difference in acute hemodynamic change during microgravity, cerebral perfusion pressure, capillary microcirculation of the iris, local blood flow of the cerebral cortex and the temporal muscle were measured in anesthetized rats. Microgravity was produced by free drop or parabolic flight. Rats were placed in 30° head-up whole body-tilt or horizontal flat position. In the flat group, none of the measured variables was significantly affected by microgravity, whereas in the head-up group, cerebral perfusion pressure and blood flow in the iris capillary, the temporal muscle, and the cerebral cortex were increased. Thus disappearance of the hydrostatic pressure difference induces cephalad fluid shift, and then increases blood flow of the head. On the other hand, hypergravity induced hypotension, if the baroreflex and vestibulosympathetic reflex did not operate. If the only vestibulosympathetic reflex operated, hypergravity induced an increase in renal sympathetic nerve activity and a huge pressor response, which were attenuated by the baroreflex. Based on these experimental data, block diagram of the control system during gravitational stress was depicted and the transfer function of the system was estimated. In this system, gravitational stress is sensed by the vestibular system, and projects to the medial vestibular nucleus and paraventricular hypothalamic nucleus, and then increases sympathetic nerve activity and elicits pressor response. The increase in AP is sensed by baroreceptor and the increase is suppressed. Thus the control system for protecting the AP from gravitational stress is composed of two subsystems, i.e., the baroreflex feedback control system and the vestibular feedforward control system. The estimated transfer functions of vestibular system and baroreflex were 16 and 8, respectively.