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Sensory-Motor Function in Microgravity   G. Clément, CNRS-Université Paul Sabatier, Toulouse, France

   The human sensory-motor system allows us to ascertain the status of our body, sense our environment, and make relevant adjustments in relation to this environment to achieve various goals. The sensory part, relying on our body’s physiological sensors, detects the motion or position of body parts relative to each other or to the environment. The motor part refers to our movement within and relative to our environment.

   Spaceflight creates a challenge for those sensory-motor functions that depend on gravity, which include postural balance, locomotion, and eye-head or hand coordination. The sensory system, and in particular the vestibular system, must adapt to microgravity when entering orbit and normal gravity upon re-entry to Earth. The motor system is also affected: the mass and strength of the “antigravity” muscles decrease because they are less used in microgravity. Until adaptation is complete, which takes much longer than the actual g-transition itself, these systems can be considered maladjusted, resulting in disturbed sensory-motor functioning.

   The vestibular system also plays a role in spatial cognition and navigation, i.e., the knowledge of directional heading and place in the environment. Recent research has demonstrated that this system also adjusts heart rate, blood pressure, immune responses, and arousal. Astronauts experience dizziness and disorientation during their first days in weightlessness. Upon returning to Earth after spaceflight, they frequently have difficulties standing upright, stabilizing their gaze, and walking or turning corners in a coordinated manner. Their sense of balance and spatial orientation take time to re-adapt to Earth-normal conditions. Something about the vestibular system obviously adapts to changing conditions, but what? Why? How? Might a better understanding of this microgravity-induced vestibular function help people back on Earth prevent the dizziness, disorientation, and susceptibility to falling that some patients and older people experience? Answers to these important and interesting questions require us to know more about the physiology of the human vestibular system on Earth as well as in space.