Cold-induced decrement in voluntary and involuntary muscle activity, and influence of the head on thermoregulation and heat transfer

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Hundreds of Canadians die annually from cold exposure, often during cold-water immersion. Survival and recovery can be improved by understanding physiological responses to cold. We intend to further study mechanisms for, and implications of: 1) cold-induced reduction in voluntary muscle activity and involuntary shivering; and 2) the transfer of heat within the body and with the environment. First, magnetic stimulation (MS) and electric stimulation of muscles, will identify effects of cooling on excitatory and inhibitory neural pathways. Transcranial MS stimulates the entire motor pathway from corticospinal motor neuron to muscle. Cervico-medullary MS (at the base of the skull) specifically stimulates spinal motor neurons. We can also specifically cool the body core and/or the local muscle during brief and fatiguing voluntary muscle contractions. Therefore we can differentiate between local and central neural circuits and whether they are excitatory or inhibitory, and determine the proportional contribution of each. These techniques will also help determine central mechanisms that account for submaximal shivering intensity, even when core temperature is decreasing (a condition when one would expect a maximal response). Second, we will determine mechanisms for exaggerated core cooling effects of partial and total head submersion in cold water. We hypothesize that profound vasoconstriction reduces the amount of perfused tissue (thermal core), thus head cooling exerts a large affect on a smaller core. We will determine cardiovascular and peripheral blood flow effects of short term (breath holding) and long-term (breathing) immersion of the back of the head, face or both, in cold water. We expect a greater effect on peripheral blood flow and core cooling with facial vs dorsum immersion. SIGNIFICANCE: Results could direct future research to maximize muscle contraction and shivering during cold-water survival situations. Understanding the exaggerated core cooling effect of partial or whole head cooling will aid design of garments for intentional (SCUBA) or accidental (survival) cold immersion.