Inflight Centrifugation as a Countermeasure for Deconditioning of Otolith-based Reflexes

INTRODUCTION The human balance system comprises of the semi-circular canals, which sense rotation of the head, and the otoliths, which act as linear accelerometers. On Earth, the otoliths sense the constant linear acceleration of gravity, and this information is used by the brain to determine the spatial vertical, and the orientation of the head with respect to the vertical. This information is critical in controlling our posture and eye movements during everyday activities such as walking and driving an automobile. In addition, recent studies have suggested that the otoliths play a role in the activation of sympathetic outflow in response to changes in posture, triggering a vestibulo-sympathetic reflex which produces changes in heart rate and vascular tone that contributes to maintain blood flow to the brain during orthostatic stress. During our 1998 Neurolab (STS-90) experiment, four payload crewmembers were exposed to artificial gravity (a 1-g or 0.5-g centripetal acceleration) generated by in-flight centrifugation. In contrast to previous post-flight studies, both in-flight and post-flight measures of otolith-ocular function were unimpaired. Post-flight tests also indicated no symptoms of orthostatic intolerance (an inability to maintain blood flow to the brain) in all four payload crew. This is an unlikely occurrence if the finding that 64% of astronauts experience profound symptoms of post-flight orthostatic intolerance (Buckey et al. J Appl Physiol 1996; Fritsch Yelle et al. J Appl Physiol 1996) is a general phenomenon. In addition, preliminary data suggests that sympathetically-mediated vasoconstriction was better maintained in the payload crew compared to two other crewmembers not exposed to in-flight centrifugation. A possible explanation for these results is that intermittent exposure to artificial gravity during the 16-day mission had prevented deconditioning of otolith-ocular and vestibulo-sympathetic reflexes in the microgravity environment. The aim of the current proposal is to obtain control measures of otolith and orthostatic function following short duration missions, utilizing techniques developed for the Neurolab flight, from astronauts who have not been exposed to in-flight centrifugation. This will enable a direct comparison with data obtained from the Neurolab crew. Deficits in otolith-mediated responses, specifically ocular counter-rolling and spatial orientation of the angular vestibulo-ocular reflex, would support the hypothesis that intermittent exposure to in-flight centripetal acceleration is a countermeasure for otolith deconditioning. Furthermore, a correlation between post-flight otolith deconditioning and orthostatic intolerance would establish an otolithic basis for this condition.