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

为探讨微重力条件下流体静压差在急性血流动力学变化中的作用，测定了麻醉大鼠脑灌注压、虹膜毛细血管微循环、大脑皮质和颞肌局部血流量。微重力是由自由下落或抛物线飞行产生的。将大鼠置于30°头高位全身倾斜或水平平卧位。在平坦组中，所测量的变量均未受到微重力的显著影响，而在抬头组中，虹膜毛细血管、颞肌和大脑皮层中的脑灌注压和血流量增加。因此，流体静压差的消失引起头部流体移位，然后增加头部的血流量。另一方面，如果压力反射和前庭交感神经反射不起作用，超重引起的低血压。如果只有前庭交感神经反射起作用，超重力会引起肾交感神经活动增加和巨大的升压反应，而压力感受性反射会减弱这种反应。在此基础上，给出了重力作用下的控制系统框图，并估算了系统的传递函数。在该系统中，重力应力由前庭系统感受，并投射到前庭内侧核和室旁核，从而增加交感神经活动和升压反应。压力感受器感觉到AP的增加，并且这种增加被抑制。因此，用于保护AP免受重力应力的控制系统由两个子系统组成，即，压力反射反馈控制系统和前庭前馈控制系统。前庭系统和压力反射的传递函数分别为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.