Heat acclimation has a protective effect on the central but not peripheral nervous system

This study aimed to clarify the pathway mediating hyperthermia-induced alterations in neural drive transmission and determine if heat acclimation protects voluntary muscle activation and cognitive function in hyperthermic humans. Electrically evoked potentials (H reflex and M wave), executive function (special planning and working memory), and maximal voluntary isometric contractions (120 s) were assessed in 14 participants in control conditions [CON, 24°C, 40% relative humidity (RH)] and in a hyperthermic state (HYP, 44–50°C, 50% RH) on consecutive days in a counterbalanced order. Thereafter, participants were passively heat acclimated for 11 days (1 h per day, 48–50°C, 50% RH) before repeating the initial assessments. Heat acclimation decreased rectal temperature in CON (–0.2°C, P < 0.05), but participants were maintained at ~39°C in HYP. Heat acclimation increased the time required to reach 39°C (+9 min), along with sweat rate (+0.7 l/h), and serum extracellular expression of heat shock protein 72 (eHSP72; +20%) in HYP (P < 0.05). M-wave and H-reflex amplitudes were lower in HYP than CON (P < 0.05) and were not protected by heat acclimation. Nerve conduction velocity was faster in HYP than CON (P < 0.05) without being influenced by heat acclimation. These results suggest that peripheral neural drive transmission in the hyperthermic state is primarily affected by axonal conduction velocity rather than synaptic failure. E...
Source: Journal of Applied Physiology - Category: Physiology Authors: Tags: RESEARCH ARTICLE Source Type: research