When HMS Beagle docked at the southern tip of Tierra del Fuego, Charles Darwin remarked on the capacity of the locals to deal with cold: A woman, who was suckling a recently born child, came one day alongside the vessel and remained there out of mere curiosity, whilst the sleet fell and thawed on her naked bosom, and on the skin of her naked baby. Japanese pearl divers dive for long periods in cold water without the comfort of wetsuits, whereas many of us whimper as the waters of the relatively warm Pacific or Indian Oceans reach our midriff. Why is there such variation in our reaction to cold? The perception of cold begins when nerves in the skin send impulses to the brain about skin temperature. These impulses respond not only to the temperature of the skin, but also to the rate of change in skin temperature. So we feel much colder jumping into cold water, when skin temperature drops rapidly, than after we have stayed there for a while, when our skin temperature is low but constant. The burst of nerve impulses generated by falling skin temperature provides early warning of an event likely to cause body core temperature (the temperature of the internal organs) to fall. If unchecked, a fall in body core temperature can result in lethal hypothermia. The perception of cold begins when nerves in the skin send impulses to the brain about skin temperature. In healthy people, physiological systems prevent hypothermia from occurring. Impulses from the skin arrive at the hypothalamus, a brain area responsible for controlling the internal environment of the body, which generates instructions in the nervous system that prevent a drop in body core temperature. Nervous impulses sent to muscles generate extra metabolic heat through shivering. Blood vessels that would otherwise transport warm blood from the internal organs to the cold skin, where the blood would lose heat, constrict, constraining most blood, and its heat, to the internal organs. Impulses arriving at the cerebral cortex, the part of the brain where reasoning occurs, generate information about how cold we feel. These combine with impulses arriving from the limbic system, responsible for our emotional state, to determine how miserably cold we feel. These feelings motivate us to perform certain behaviours, such as curling up or putting on more clothes, and to complain. Feeling cold is not the same as being cold. Jumping into a cool swimming pool feels cold, but it can cause body core temperature to rise because of the warm blood retained in the core. Body temperature can stay elevated for up to an hour. Many of us also have felt cold at the beginning of a fever, when the body core temperature starts to rise. During a fever, the nerve circuits that control body temperature are reset to a higher level, so the body responds as if it is cold until its temperature stabilises around that higher level. While fever indicates a problem, is there anything wrong with feeling excessively cold rather than actually being cold? Some of us have the misfortune to suffer from Raynaud’s phenomenon, a condition in which the blood flow is too low to keep the fingers and toes warm. Feeling excessively cold during pregnancy, when the foetus acts as a small furnace, may be a symptom of low thyroid hormone activity, needing hormone supplementation. But some healthy people can feel colder than do others in the same environment. Women often report that they feel colder than men in the same environment. This is probably because they have a lower skin temperature, a consequence of more subcutaneous fat and the hormone oestrogen. Feeling cold is not the same as being cold. Some of us may inherit feeling excessively cold. A study of twins found that the prevalence of the feeling of cold hands and feet is highly heritable, implying a genetic basis for exaggerated temperature perception. Some of us also may feel cold simply because of how others close to us look, a phenomenon called “cold contagion”. In one study, healthy volunteers contribute to obesity. We’d probably all be much better off if we spent more time being cold. Source
