[Microgravity and autonomic nervous system].

How microgravity influences autonomic function is still under investigation. Microgravity induces neuro-vestibular alterations and body fluid shift, and these two changes cause "space motion sickness(SMS)" and cardiovascular deconditioning. "Space motion sickness" is a autonomic syndrome that exhibits nausea, vomiting, headache, anorexia, pallor etc., whose incidence in Space Shuttle mission reaches 67.1%. There are several hypotheses for SMS mechanism: 1) sensory conflict, 2) fluid shift, 3) otolith asymmetry, 4) space orientation readaptation, 5) otolith tilt-translation reinterpretation, and these hypotheses are considered to be combined together to cause SMS. After space flight, 64% of the astronauts suffer from orthostatic intolerance, which is defined as incompletion of 15 min of 70 degrees head-up tilt. Several causes for the deconditioning have been hypothesized, dehydration followed by fluid shift, altered gain for baroreflex sensitivity, decreased venous capacitance, etc. In our previous studies, we recorded muscle sympathetic nerve activity(MSNA) by microneurography under simulated and actual microgravity conditions. Parabolic flight, which induces 20 sec of actual microgravity, suppressed MSNA to 50%. Head-out water immersion suppressed MSNA to 20% while gradual recovery was observed during 3 hours of immersion. Dry immersion for 3 days revealed that MSNA was enhanced after simulated microgravity while the response to orthostasis was unchanged. Bed rest for 6, 14, and 120 days and Neurolab Project clarified the same tendency for longer duration of simulated microgravity. These alterations in MSNA might be attributed to the development of cardiovascular deconditioning after microgravity exposure.