Sympathetic nervous adjustments in man to simulated weightlessness induced by water immersion

To clarify the role of the sympathetic nervous system to adjust the fluid shift under weightlessness, muscle and skin sympathetic activities were recorded microneurographically in human subjects under simulated weightlessness induced by water immersion up to the levels of the knee, the navel, the breast and the neck. The muscle and skin sympathetic activities were reduced in proportion to rise of immersion level up to the neck. These changes of sympathetic activities were almost concomitant with those of simultaneously recorded soleus electromyograms and heart rate. Reductions of the thigh and the leg circumference were also confirmed by strain gauge plethysmogram recorded under the same experimental condition. Based on these findings, it is concluded that the sympathetic nervous system is suppressed under weightlessness simulated by water immersion. This suppression might depend mainly on the activation of intrathoracic low pressure receptors, due to the fluid shift toward the upper part of the body. The suppression of the sympathetic nervous system seems to be important to compensate the fluid shift under weightlessness.