Background: Human aging is characterized by a marked increase in muscle sympathetic nerve traffic (MSNA). No information exists, however, on the effects of aging on skin sympathetic nerve traffic (SSNA) and on its reflex modulation by thermoregulatory mechanisms.
Methods and results: In 13 young, 11 middle-aged, and 12 elderly healthy subjects, we measured arterial blood pressure (Finapres), skin temperature (thermocouples), and resting MSNA and SSNA (microneurography). Measurements also included the SSNA responses to (1) an acute increase and reduction (+/-8 degrees C) in room temperature, each lasting 45 minutes and (2) an acoustic stimulus capable to trigger an emotional arousal. Although resting MSNA was progressively and significantly (P<0.05) increased from young to middle-aged and elderly groups, SSNA was significantly (P<0.05) reduced in the latter compared with the former 2 groups. Cold exposure induced a SSNA increase that was significantly (P<0.01) smaller in the elderly than in young and middle-aged subjects. Conversely, heat exposure induced a SSNA reduction that was significantly (P<0.05) smaller in elderly than in young and middle-aged subjects. Compared with SSNA in young individuals, the SSNA change from cold to warm temperature was reduced by 61% in the elderly group. This was not the case, however, for the SSNA responses to the arousal stimulus, which were superimposable in the 3 groups.
Conclusions: These data provide the first demonstration of a dichotomy in the MSNA and SSNA responses to aging. They also show that aging markedly impairs thermoregulatory control of SSNA and that this impairment might participate at the age-related SSNA decrease.