The Brain and Cold
As the temperature of a human body drops below the level required for health, various reactions occur to protect the core--that is, the abdominal organs and brain--from damage, even at the expense of the extremities. The brain is literally insulated against extreme cold, and it will begin to suffer symptoms of damage only at the point of advanced hypothermia, long after muscular symptoms such as shivering first appear. This assumes that the core temperature is not dropping very rapidly, as when a person is submerged in cold water
Hypothermia
The average healthy temperature of a human body is 36.1 to 37.5° C, or 96.9 to 99.5° F. Hypothermia is defined as any temperature below 35° C or 96° F. This is the beginning of stage 1 hypothermia. Brain function becomes noticeably impaired only during stage 2, which begins at around 33°C or 93°F. The person may exhibit confusion. Many of the symptoms which may appear potentially neurological, including lack of coordination, stumbling and labored speech, are in fact muscular in nature. As part of the body's effort to protect the internal organs, the vasomotor response constricts blood vessels in and near the skin. Consequently, the hands, face and legs lose function, although the brain is largely unaffected.
Stage 3 Hypothermia
In stage 3 hypothermia (beginning around 32° C or 89.6° F), the cold begins to penetrate the body's defenses and reach the brain. The body will continue to send warm blood to the brain even at the expense of other core organs; however, at stage 3, there is simply not enough warm blood to go around. Thinking slows, speech and walking become very difficult, and, most tellingly, irrational behaviors begin to emerge, such as combativeness, disorientation, paradoxical undressing (the removal of items of clothing despite the cold) and terminal burrowing (seeking enclosed spaces or digging under nearby snow, leaves or other debris). Torpor sets in at some point during stage 3.
The cold begins to shut down cellular activity, which means that although the body has by now suffered enormous damage from hypothermia and very little blood is flowing, true brain death is delayed.
Paradoxical Undressing
Paradoxical undressing is potentially non-neurological in nature. Although the irrational act of removing one's clothes when one is freezing may be the result of cold damage to the hypothalamus (the part of the brain responsible for temperature regulation), it may also be the result of exhaustion of the vasomotor muscles that had been keeping blood away from the surface of the skin. If this is the case, the freezing person removes his clothes not because of brain damage but because of the sudden rush of warm blood to the surface of the skin, which leads the already confused individual to believe that he is warm.
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