Neural Re-adaptation to Earth's Gravity Following Return From Space

摘要

The gravity sensors in the inner ear (the utricle and saccule) no longer receive strong gravitational signals in weightlessness. In an effort to compensate for the reduced input, these sensors may become more sensitive using a process called up-regulation. If this happens, the output of the vestibular nerve from the inner ear would be greatly increased after a space mission, when the sensitized gravity sensors are reexposed to gravity. We believe that information gathered by studying the inner ear of a lower animal, such as a fish, will provide the information needed to prove this. Despite evolution, the balance and equilibrium functions of the inner ear have not appreciably changed since their appearance in the earliest vertebrates. As a result, the fish vestibular system compares favorably in both structure and function with that of mammals. By chronically recording the output of the vestibular system in the fish (specifically the output from the utricle), the question of how microgravity affects the output of the inner ear could be answered precisely. For five days following two NASA Shuttle flights, we recorded from the vestibular nerves supplying the utricle the responses to inertial accelerations (head movements) in four oyster toadfish (Opsanus tau). Within the first day postflight, the magnitude of response to an applied translation (side-to-side) movement was on average three times greater than for controls. The reduced gravitational acceleration in orbit apparently resulted in an up-regulation of the sensitivity of the utricle. By 30 hours postflight, responses were statistically similar to control. The time course of return to normal sensitivity parallels the reported decrease in vestibular disorientation and improvement in balance in astronauts following their return from space.