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，通常是在冷水浸泡期间。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以下机制和影响：1）寒冷引起的随意肌活动减少和不自主颤抖； 2）体内以及与环境的热量传递。首先，肌肉的磁刺激（MS）和电刺激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 MS（位于颅骨底部）专门刺激脊髓运动神经元。我们还可以在短暂且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，我们可以区分局部神经回路和中枢神经回路以及它们是兴奋性还是抑制性，并确定各自的比例贡献。这些技术还将有助于确定导致次最大颤抖强度的中心机制，即使在核心温度下降时（人们期望最大反应的情况）也是如此。其次，我们将确定部分和全部头部浸没在冷水中夸大核心冷却效应的机制。我们假设深度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。我们将确定头后部、面部或两者短期（屏气）和长期（呼吸）浸入冷水中对心血管和外周血流的影响。我们预计面部浸入与背部浸入对外周血流和核心冷却的影响更大。意义：结果可以指导未来的研究，以最大限度地提高冷水生存情况下的肌肉收缩和颤抖。了解部分或整个头部冷却的夸张的核心冷却效果将有助于设计用于有意（水肺）或意外（生存）冷浸的服装。

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.