Autonomic responses to environmental stimuli in human body.

The author reviewed in this paper current microneurographic findings on the responses of muscle sympathetic nerve activity (MSNA) and skin sympathetic nerve activity (SSNA) to the environment in humans with special reference to vibration-induced white finger (VWF). 1) MSNA was enhanced by +Gz gravitational input, while being suppressed by simulated weightlessness through the baroreflex mechanism to maintain hemodynamic homeostasis. 2) MSNA was enhanced by hypobaric hypoxia through the chemoreflex mechanism. 3) SSNA was lowest under thermoneutral ambient temperature condition. Sudomotor component of SSNA increased under hot ambient temperature, while vasomotor component of SSNA increased under cold ambient temperature. 4) MSNA and vasomotor component of SSNA increased by local cold stimuli such as when a hand was immersed into cold water. 5) SSNA was enhanced by local vibration of the human body. The vibratory frequency of 60 Hz was the most effective for vibration-induced SSNA response. With a constant vibratory frequency of 60 Hz, SSNA increased depending on the vibratory acceleration. MSNA was not enhanced by local vibration of the body. 6) SSNA was markedly enhanced by combined stimuli of local vibration and noise. 7) MSNA increased during handgrip exercise, presumably depending on afferent inputs from muscle metaboreceptors. 8) The sympathetic response to environmental stress was markedly influenced by aging. The basal level of MSNA increased with aging, while the MSNA responsiveness to gravitational stress became reduced by aging. MSNA responsiveness to simulated weightlessness was also reduced by aging. 9) Vibration-induced white finger may be related to complex autonomic dysfunctions including excessive somato-sympathetic reflex induced by local vibration, cold stimuli and handgrip exercise. Gravity-dependent sympathetic nerve responses and the influence of aging may also contribute to the underlying mechanisms of VWF.