Why do my teeth chatter when I get cold?
First off, teeth chattering is a localized manifestation of shivering, so explaining the one explains the other.
Shivering is one of many responses to environment of which the human body is capable because it is Homoiothermic, or warm-blooded. Homoiothermy is the ability to maintain a relatively constant internal temperature (about 37°C [99° F]), regardless of the environmental temperature. The ability to maintain an internal temperature distinguishes homoiotherms from cold-blooded, or poikilothermic, animals, which usually have about the same temperature as their environment.
Body temperatures of homoiotherms are kept at a constant value by regulatory mechanisms that counteract the effects of the external environment. In cold environments, regulatory mechanisms maintain body temperature by increasing heat production and decreasing heat loss. Shivering, a regulatory mechanism of many warm-blooded animals, increases heat production. I could go into all of the body’s techniques for maintaining a constant temperature, but that would be too obnoxious, even for mrlucky. Suffice it to say that shivering (and teeth chattering) is the result of involuntary muscular contraction and relaxation, mediated by the thermostatic control center in the hypothalamus.
The immediate mechanism of muscular force generation is a chemical reaction, induced by the electrical stimulation of the brain. This chemical reaction is accompanied by a release of heat, which is used to maintain body temperature.
Shivering plays a rather important role in identifying just how dangerous is the cold to which your body is reacting. The shiver response ceases when the body’s core temperature reaches around 86-90°. That’s stage III of hypothermia, and one of the frightening things about it is that it can appear to an observer that the hypothermic is warming (after all, the shivering has stopped!). In fact, at that point, the sufferer is about halfway to death.
Brrr! Makes my teeth chatter just thinking about it.