Abstract
The information coming from the vestibular otolith organs is important for the brain when reflexively making appropriate visual and spinal corrections to maintain balance. Symptoms related to failed balance control and navigation are commonly observed in astronauts returning from space. To investigate the effect of microgravity exposure on the otoliths, we studied the otolith-mediated responses elicited by centrifugation in a group of 25 astronauts before and after 6 mo of spaceflight. Ocular counterrolling (OCR) is an otolith-driven reflex that is sensitive to head tilt with regard to gravity and tilts of the gravito-inertial acceleration vector during centrifugation. When comparing pre- and postflight OCR, we found a statistically significant decrease of the OCR response upon return. Nine days after return, the OCR was back at preflight level, indicating a full recovery. Our large study sample allows for more general physiological conclusions about the effect of prolonged microgravity on the otolith system. A deconditioned otolith system is thought to be the cause of several of the negative effects seen in returning astronauts, such as spatial disorientation and orthostatic intolerance. This knowledge should be taken into account for future long-term space missions.
Highlights
TO COORDINATE MOVEMENTS, ensure balance, and maintain stable gaze, humans depend on the peripheral vestibular labyrinth, located bilaterally in the inner ear
The vestibular system consists of two main parts: the semicircular canals, which sense rotational movements, and the otolith organs detecting the sum of linear accelerations acting on the head
When orbiting around Earth, the space crew inside the International Space Station (ISS) is in a so-called “free fall,” meaning that instead of the 1-g gravity environment humans experience on Earth, the gravity is reduced to 10Ϫ6 g, i.e., microgravity
Summary
TO COORDINATE MOVEMENTS, ensure balance, and maintain stable gaze, humans depend on the peripheral vestibular labyrinth, located bilaterally in the inner ear. A couple of studies focusing on OCR induced by head tilt, the so-called static torsional otolith-cervical-ocular reflex (OCOR), have been performed by Kornilova and colleagues. They concluded that the otolith function was suppressed early after spaceflight and that it recovered within 8 or 9 days (Kornilova et al 2007a, 2007b, 2011, 2012). The aim of the present study was to investigate whether long-term exposure to microgravity results in changes in the otolith-mediated response OCR in returning astronauts. With a bigger sample size, we were able to look into the more subtle physiological effects, which could be learning effects due to spaceflight experience and adaptation as well as possible differences in OCR response between the two directions of rotation during centrifugation
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