This constancy implies a very precise control of heat balance and given rise to the concept of a set-point for temperature regulation. The core temperature is that of the main internal organs and often measured in the rectum. Temperatures obtained in the mouth or ear are about 0.5 o C lower and show greater variation. There is also a diurnal variation in core temperature. The highest occurs in the evening and the lowest in the early morning.
Females also have core temperature that varies during the menstrual cycle.
Brain temperature is typical of core and maintained by the thermal regulatory center in the hypothalamus . Neurons that are highly temperature sensitive are located in the anterior part of the hypothalamus. Other neurons show a decrease in firing frequency with increasing hypothalamic temperature. Still other neurons begin to fire when the brain temperature drops below the set point temperature. Below this is a certain lower critical temperature where the metabolic rate rises linearly as temperature is lowered further.