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

Area of Science:

• Exercise Physiology
• Neuroscience
• Human Physiology

Background:

• Hyperthermia impairs neural drive transmission, affecting cognitive function and muscle activation.
• The protective effects of heat acclimation on these functions during heat stress are not fully understood.

Purpose of the Study:

• To investigate the neural pathways affected by hyperthermia-induced impairments.
• To determine if heat acclimation protects cognitive function and voluntary muscle activation in hyperthermic humans.

Main Methods:

• Participants underwent assessments in control and hyperthermic conditions before and after an 11-day passive heat acclimation protocol.
• Measurements included electrically evoked potentials (H-reflex, M-wave), executive function tests, and maximal voluntary isometric contractions.
• Core body temperature, sweat rate, and heat shock protein 72 levels were also monitored.

Main Results:

• Hyperthermia reduced M-wave and H-reflex amplitudes, indicating impaired peripheral neural drive, an effect not mitigated by heat acclimation.
• Nerve conduction velocity increased in hyperthermia, suggesting axonal conduction, not synaptic failure, is the primary peripheral mechanism.
• Heat acclimation restored executive function and improved the capacity to sustain voluntary activation and torque production during prolonged contractions in hyperthermia.

Conclusions:

• Heat acclimation confers central, but not peripheral, neural adaptations beneficial for cognitive and motor performance under heat stress.
• Peripheral neural drive impairments in hyperthermia are linked to axonal conduction velocity changes.
• Heat acclimation enhances the brain's ability to maintain motor output during sustained effort in the heat.