Sympathetic regulation during thermal stress in human aging and disease

Abstract

Humans control their core temperature within a narrow range via precise adjustments of the autonomic nervous system. In response to changing core and/or skin temperature, several critical thermoregulatory reflex effector responses are initiated and include shivering, sweating, and changes in cutaneous blood flow. Cutaneous vasomotor adjustments, mediated by modulations in sympathetic nerve activity (SNA), aid in the maintenance of thermal homeostasis during cold and heat stress since (1) they serve as the first line of defense of body temperature and are initiated before other thermoregulatory effectors, and (2) they are on the efferent arm of non-thermoregulatory reflex systems, aiding in the maintenance of blood pressure and organ perfusion. This review article highlights the sympathetic responses of humans to thermal stress, with a specific focus on primary aging as well as impairments that occur in both heart disease and type 2 diabetes mellitus. Age- and pathology-related changes in efferent muscle and skin SNA during cold and heat stress, measured directly in humans using microneurography, are discussed.

Keywords: Cold; Diabetes; Heat; Microneurography; Sympathetic nervous system; Thermoregulation.

Figure 1

Muscle sympathetic nerve activity (MSNA) burst frequency was greater in older adults (open symbols) at thermoneutrality [mean skin temperature (T_sk) 34.0°C] and at each 0.5°C decrease in mean T_sk throughout whole-body cooling compared to young adults (filled symbols) (Panel A). Further, MSNA increased during cold stress in older but not young adults, reflected by the increase in MSNA burst frequency from baseline (i.e., mean T_sk 34.0°C) to mean T_sk 30.5°C (Panel B). *P<0.05 vs. young; †P<0.05 vs. mean T_sk 34.0°C. [Figure adapted with permission from (Greaney et al., 2014)].

Figure 2

Whole-body cold stress [(i.e., gradual reduction in mean skin temperature (Tsk) from 34.0°C to 30.5°C] elicited robust and progressive increases in skin sympathetic nerve activity (SSNA; ΔSSNA total activity) in young but not older adults (Panel A). The total increase in SSNA from baseline (i.e., mean T_sk 34.0°C) to mean T_sk 30.5°C is presented in Panel B. *P<0.05 vs. older; †P<0.05 vs. mean T_sk 34.0°C. [Figure adapted with permission from John Wiley and Sons Publishing (Greaney et al., 2015b)].

Figure 3

Whole-body passive heat stress sufficient to elevate core temperature by 0.6°C and 1.2°C increased muscle sympathetic nerve activity [MSNA; burst frequency (left panel) and burst incidence (right panel)] similarly in both healthy young (open symbols) and aged adults (filled symbols. *P<0.05 v. baseline; †P<0.05 vs. aged. [Figure reprinted with permission from John Wiley and Sons Publishing (Gagnon et al., 2015)].

Figure 4

Reflex cutaneous vasodilation during whole-body heat stress (WBH) was blunted in congestive heart failure (CHF; filled bars; upper left panel). The increase in skin sympathetic nerve activity [SSNA; burst frequency (upper right panel) and total activity (lower right panel) during WBH was not different between control subjects (open bars) and those with CHF. There were no group differences in the sweating response to WBH (lower left panel). *P<0.05 vs. baseline; †P<0.05 vs. control. [Figure reprinted with permission from Wolters Kluwer Health, Inc. (Cui et al., 2013)].