Muscle sympathetic nerve activity (MSNA) can be directly recorded from huma
n peripheral nerves in situ using microneurography. MSNA plays an essential
role to control systemic blood pressure against gravitational stress. MSNA
was enhanced by changing posture against terrestrial gravity from lying to
sitting, and from sitting to standing. This activity was enhanced by head-
up tilt depending on the gravitational input from the head to the leg (+Gz)
in the human body. Orthostatic hypotension occurred when MSNA response to
gravitational stress was impaired both in high and low responders of this s
ympathetic outflow. Syncope was preceded and/or associated by a withdrawal
of MSNA. MSNA was suppressed by short-term exposure to microgravity but was
enhanced after longterm exposure to microgravity. Orthostatic intolerance
after exposure to prolonged microgravity was associated with a reduction of
increased MSNA response to gravitational stress. Aging influenced gravity-
related responses of MSNA.