2014 年 3 巻 2 号 p. 217-221
In homeothermic animals, metabolic heat production in the brain is higher than in other tissues. However, cerebral tissue is susceptible to heat. Several animals have mechanisms that selectively cool the brain during hyperthermia (i.e., selective brain cooling [SBC]). Carotid retes have been well documented in artiodactyls (hoofed animals) and felids (cats) as mechanisms of SBC. SBC has also been found in some species without carotid retes, such as horses and squirrel monkeys. However, the presence of SBC in humans remains controversial. Brain temperature cannot be directly measured in healthy subjects; therefore, tympanic temperature has been used to estimate brain temperature. However, tympanic temperature is reportedly affected by facial skin temperature. We recently investigated the effect of facial fanning on tympanic and esophageal temperature in normothermic and hyperthermic humans. The results showed that tympanic temperature is not affected by the facial skin temperature under normothermic conditions and that facial fanning may induce SBC under hyperthermic conditions. Regional differences in thermal comfort are present over the body surface in humans, i.e. humans prefer a cool head in the heat and a warm abdomen in the cold. Therefore, preference for a low facial temperature may activate SBC. Several recent studies have demonstrated that whole-brain temperature can be measured noninvasively with proton magnetic resonance spectroscopy. The existence of a thermal gradient within the brain has been suggested. Studies measuring whole-brain temperature will reveal the details of human SBC.