When a person places a foot under a hot lamp and leaves it there for a short time, local vasodilation and mild local sweating occur. Conversely, placing the foot in cold water causes local vasoconstriction and local cessation of sweating. These reactions are caused by local effects of temperature directly on the blood vessels and also by local cord reflexes conducted from skin receptors to the spinal cord and back to the same skin area and the sweat glands. The intensity of these local effects is, in addition, controlled by the central brain temperature controller, so that their overall effect is proportional to the hypothalamic heat control signal times the local signal. Such reflexes can help prevent excessive heat exchange from locally cooled or heated portions of the body.
Regulation of Internal Body Temperature Is Impaired by Cutting the Spinal Cord. After cutting the spinal cord in the neck above the sympathetic outflow from the cord, regulation of body temperature becomes extremely poor because the hypothalamus can no longer control either skin blood flow or the degree of sweating anywhere in the body. This is true even though the local temperature reflexes originating in the skin, spinal cord, and intraabdominal receptors still exist. These reflexes are extremely weak in comparison with hypothalamic control of body temperature.
In people with this condition, body temperature must be regulated principally by the patient's psychic response to cold and hot sensations in the head region— that is, by behavioral control of clothing and by moving into an appropriate warm or cold environment.
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