"The answer hinges on the fact that spicy foods excite the receptors in the skin that normally respond to heat. Those receptors are pain fibers, technically known as polymodal nociceptors. They respond to temperature extremes and to intense mechanical stimulation, such as pinching and cutting; they also respond to certain chemical influences. The central nervous system can be confused or fooled when these pain fibers are stimulated by a chemical, like that in chile peppers, which triggers an ambiguous neural response.
"So how does the brain decide whether the mouth is being pinched, cut, burned or affected by chemical? Scientists are not certain how the process works, but probably the brain makes a judgment based on the type and variety of stimuli being received. Stimulus to the nociceptors alone might indicate dangerous, extreme temperature. But capsaicin, the active ingredient in chile peppers, also stimulates the nerves that respond only to mild increases in temperature--the ones that give the sensation of moderate warmth. So capsaicin sends two messages to the brain: 'I am an intense stimulus,' and 'I am warmth.' Together these stimuli define the sensation of a burn, rather than a pinch or cut.
"The central nervous system reacts to whatever the sensory system tells it is going on. Therefore, the pattern of activity from pain and warm nerve fibers triggers both the sensations and the physical reactions of heat, including vasodilation, sweating and flushing.
"Most people think of the 'burn' of spicy food as a form of taste. In fact, the two sensory experiences are related but are very distinct. They innervate the tongue the same way, but the pain system that is triggered by capsaicin is everywhere on the body, so one can get thermal effects everywhere. Some liniments contain compounds that produce similar temperature stimuli to the nerves in the skin. Menthol acts in much the same way as capsaicin, but in this case, it stimulates the fibers that register cold temperatures, not those that respond to warmth. This is why products containing menthol have names like 'Icy Hot'--menthol stimulates both the hot (pain) and cold receptors, sending the brain a really ambiguous signal. That difference explains why there is no confusing menthol and capsaicin: one gives rise to a cool burn, the other to a hot burn.
"The sensations produced by menthol and capsaicin are accidents of human physiology--we obviously did not evolve receptors to react to these compounds. The chemicals fool pain receptors whose real purpose is to register critical events, like damage to the skin and the inflamation that often results. The tenderness around an injury is caused in part by the response of these same nerves to chemicals released in the skin. We humans are peculiar creatures--we've taken a nerve response that normally signals danger and turned it into something pleasurable."