Temperature Regulation

The other major way to increase body temperature is by heat conservation. Particularly in larger mammals, such as adult humans, the body makes sufficient heat from its internal metabolism so that body temperature can be increased merely by shunting blood flow away from the skin to deep vascular beds. In animals with fur, piloerection, another sympathetic response, increases the thickness of the fur coat and thus conserves heat. Humans also have piloerection called gooseflesh, but this is not nearly as effective in heat conservation. The thermogenic (brown adipose) and heat-conserving mechanisms are coordinated by medullary raphe neurons that activate both pathways.

A third mechanism for generating heat is by increased muscle activity or shivering. Less is known about this pathway, but it is presumed that hypothalamic neurons activate motor pattern generators that cause increased muscle activity, which is thermogenic. All three mechanisms require energy, and so the heat production system also activates the cardiovascular system to increase cardiac output and the respiratory system to maintain blood oxygenation.

Anterior pituitary hormones do not seem to play much of a role in the regulation of body temperature over a period of minutes or even hours, although in the absence of thyroid hormone, body temperature falls. Body temperature also rises (during the active cycle) and falls (during the sleep cycle) daily, and this typically occurs before the onset of motor activity or rest, and so is not due to a simple change in muscle activity. There are also changes in body temperature during the menstrual cycle, which may reflect the fact that the preoptic area is also involved in reproductive function.

In addition to inhibiting the heat production and conservation systems, warm-sensitive neurons in the preoptic area also increase blood flow to the skin as well as sweating, to permit heat loss, and increase vasopressin secretion, which permits conservation of fluids that are necessary to support increased sweating. Sweating is mediated by two sets of sympathetic nerves, one of which is noradrenergic and the other cholinergic. The cholinergic sympathetic input appears to be of primary importance for thermoregulatory sweating, whereas the noradrenergic axons may be more important for emotional sweating.

Paroxysmal hypothermia is a rare neurologic disorder, most often seen in individuals who have agenesis of the corpus callosum (due to a failure of the anterior wall of the third ventricle to develop properly) or a congenital tumor or other lesion affecting the preoptic area. Such individuals have periods of several days at a time during which their body temperature drops to about 30° C, and they lapse into a stuporous state. Presumably this represents an unusual hypothalamic response, similar to that seen in hibernation states, but there have been too few patients with this syndrome to study it closely.

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