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，通常是在冷水中浸泡。通过了解对寒冷的生理反应，可以改善生存和恢复。我们打算进一步研究的机制和影响：1)冷诱导的随意肌活动减少和不自主颤抖；2)身体内部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MS刺激从皮质脊髓运动神经元到肌肉的整个运动通路。颈髓质多发性硬化症（位于颅骨底部）特别刺激脊髓运动神经元。我们也可以在短暂和疲劳的随意肌肉收缩期间专门冷却身体核心和/或局部肌肉。因此，我们可以区分局部和中枢神经回路，以及它们是兴奋性的还是抑制性的，并确定各自的比例贡献。这些技术也将有助于确定解释亚最大颤抖强度的核心机制，即使当核心温度下降时（当人们期望最大反应时）。其次，我们将确定在冷水中部分浸没和全部浸没的堆芯冷却效应被夸大的机制。我们假设严重的血管收缩减少了灌注组织（热核心）的数量，因此头部冷却对较小的核心产生了很大的影响。我们将确定短期（屏气）和长期（呼吸）将后脑勺、脸或两者浸泡在冷水中对心血管和外周血流量的影响。我们预计面部浸泡比背部浸泡对外周血流量和核心冷却有更大的影响。意义：结果可以指导未来的研究在冷水生存情况下最大化肌肉收缩和颤抖。了解部分或整个头部冷却的夸张核心冷却效果将有助于设计有意（水肺）或意外（生存）的冷浸服装。

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.