Regulation of Body Temperature
Rodbard[36] suggested that the neural mechanisms governing homeostatic temperature control developed in the hypothalamus from circulatory control neurons in the course of evolution from reptile to mammal. Body temperature is regulated when changes in blood temperature stimulate heat-sensitive receptors in the hypothalamus. Nerve impulses from cold receptors in the skin can also activate heat-producing neurons. The most important heat producers are the skeletal muscles, brain, liver, and heart. The greatest heat radiator is the skin, especially that on the hands. Local electrical stimulation of the heat-producing center induces shivering and constriction of blood vessels in the skin, activating vasomotor nerves and decreasing blood flow. Warming of the heat-loss center suppresses vasomotor nerve activity and in this way increases blood flow through the skin.
In brain death, the neural connection between the temperature-regulating center and peripheral body tissues is lost, and the patient becomes poikilothermic. When criteria from the National Institute of Neurological Diseases and Stroke (NINDS) were applied to establish brain death, such patients showed only a “tendency for the temperatures to be subnormal.”[17] In contrast, when brain death was established using the criterion of cessation of all brainstem functions, “poikilothermia was found in all patients later than 24 hours after brain death.”[37] Even if infection occurs, fever should not develop in cases of brain death, because the temperature-regulating centers no longer function. After brain death, body temperature tends to be hypothermic, even with vigorous application of external heat.
