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

每年有数百名加拿大人死于寒冷的暴露，通常是在冷水浸泡期间。了解对寒冷的生理反应可以提高存活率和恢复能力。我们打算进一步研究以下方面的机制和影响：1)寒冷导致的自主肌肉活动减少和不由自主的颤抖；2)体内和与环境的热量传递。首先，磁刺激(MS)和电刺激肌肉，将确定冷却对兴奋性和抑制性神经通路的影响。经颅多发性硬化症刺激从皮质脊髓运动神经元到肌肉的整个运动通路。颈髓多发性硬化症(位于颅底)特异性地刺激脊髓运动神经元。在短暂和疲劳的随意肌肉收缩期间，我们还可以特别冷却身体核心和/或局部肌肉。因此，我们可以区分局部神经回路和中枢神经回路，以及它们是兴奋的还是抑制的，并确定每个回路的比例贡献。这些技术还将有助于确定即使在核心温度下降(人们预计会有最大反应的情况下)时，导致次极大抖动强度的中央机制。其次，我们将确定部分和全部头部淹没在冷水中的夸大核心冷却效应的机制。我们假设，严重的血管收缩减少了组织(热核)的灌注量，因此头部冷却对较小的组织(热核)有很大影响。我们将确定短期(屏气)和长期(呼吸)头后、面部或两者同时浸泡在冷水中对心血管和外周血流的影响。我们预计面部和背部浸泡对外周血流量和核心冷却的影响更大。意义：结果可能会指导未来的研究，最大限度地使肌肉收缩和在冷水生存情况下颤抖。了解部分或整个头部冷却夸大的核心冷却效果将有助于服装设计，以应对故意(潜水)或意外(生存)冷浸泡。

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.