Voluntary activity (behavioral thermoregulation)
Unpleasant cold sensations result in behavioral responses (increased motor activity, curling up and searching for warmer places and clothing).
The hypothalamic temperature center receives inputs from skin cold receptors and projects them to the motor cortex and finally to the motor nerves. This leads to increased muscle tone and to oscillating contractions of muscles, shivering, that occurs mostly in trunk muscles. Shivering increases metabolic rates by 2-5 times the resting value. Due to the increased metabolic rate shivering should be avoided in winter sports.
Chemical or non-shivering thermogenesis Chemical thermogenesis is well established in rodents and newborn humans and closely related to uncoupling protein 1 (UCP 1) in brown fat. Cold exposure elicits the release of norepinephrine and thyroid hormones and activates sympathetic nerves that stimulate the expression of UCP 1. It uncouples the normal ox-idative phosphorylation in the mitochondria and the production of protons is decreased. Less ATP is formed and more heat is generated (Fig. 2.3.5). The role of UCP 1 in adults is not well established, but recent studies have shown the presence of homologues of UCP 1. UCP 2 is widely expressed in fat, muscles and viscera and stimulated by starving and fatty acids. UCP 3 is abundantly expressed in skeletal muscles and is stimulated by cold. UCP 2 and 3 also regulate the production of ATP and their roles in heat production are under research .
Meals increase heat production by a mechanism formerly called specific dynamic action, now diet-induced thermogenesis (DIT). The resting metabolic rate is increased about 10% for 1-2 h after a meal.
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