**3.6 Alterations in sympathetic neural traffic under microgravity**

Sympathetic neural traffic indirectly measured by the plasma noradrenaline level has been reported to increase during spaceflight from the preflight control level [14, 20], and vagal activity estimated by power spectral analysis of heart rate variability was reduced after long-term spaceflight [21, 22].

Microneurographically recorded neural traffic in humans is known to be muscle and skin sympathetic nerve activity (MSNA and SSNA), and MSNA controls the vasomotor function of the muscular bed, responding to blood pressure changes against gravitational stress [23–25]. MSNA was suppressed during exposure to short-term microgravity induced by parabolic flight [26], mild lower body positive pressure (10–20-mmHg LBPP) [15], and thermoneutral head-out water immersion [27] responding to the loading or unloading of cardiopulmonary receptor-stimulated cephalad fluid shift. On the other hand, MSNA was increased after exposure to long-term microgravity in spaceflight and its simulation induced by dry immersion [28] or 6° head-down tilt bedrest [17] due to different mechanisms, including plasma volume loss, changes in baroreflex, and vascular compliance.
