NORMAL THERMOREGULATION

Thermoregulation is similar to many other physiologic control systems in that the brain uses negative and positive feedback to minimize perturbations from preset, "normal" values. Since 1912, animals have been known to regulate body temperature poorly when the hypothalamus is destroyed. The importance of thermal input from the skin surface was recognized in the late 1950s, when it was observed that mice placed in a cold environment shivered before decreasing their hypothalamic temperature.

In the early 1960s, physiologists reported active thermoregulation in response to isolated warming and cooling at sites other than the hypothalamus or skin surface, including extrahypothalamic portions of the brain, deep abdominal tissues, and the spinal cord.^[1] Thus, thermoregulation is based on multiple, redundant signals from nearly every type of tissue. The processing of thermoregulatory information occurs in three phases: afferent thermal sensing, central regulation, and efferent responses.

Afferent Input

Temperature information is obtained from thermally sensitive cells throughout the body. Cold-sensitive cells are anatomically and physiologically distinct from those that detect warmth. Warm receptors increase their firing rates when temperature increases, whereas cold receptors do so when temperature decreases. Cutaneous warm receptors rarely depolarize at normal skin temperatures and are probably important only during heat stress. Cold signals travel primarily by means of Aδ nerve fibers and warm information by unmyelinated C fibers, although some overlap occurs. ^[2] C fibers also detect and convey pain sensation, which is why intense heat cannot be distinguished from sharp pain. Most ascending thermal information traverses the spinothalamic tracts in the anterior spinal cord, but no single spinal tract is critical for conveying thermal information. Consequently, the entire anterior cord must be destroyed to ablate thermoregulatory responses. The hypothalamus, other parts of the brain, the spinal cord, deep abdominal and thoracic tissues, and the skin surface each contribute roughly 20% of the total thermal input to the central regulatory system.^{[3] [4]}