thermoreception

Thermoreception is your body's sense of warmth and cold — the way you can feel that a mug of cocoa is hot or that a snowball is freezing, even with your eyes closed. Tucked just under your skin (and inside your mouth and other surfaces) are tiny nerve endings that act like little thermometers. Some of them wake up when things get warm, others wake up when things get cool, and together they keep a running report on the temperature touching you and the temperature inside you.

The trick happens at the level of single molecules. Embedded in the membrane of these nerve endings are special proteins called temperature-sensitive ion channels — think of them as tiny gates that pop open at certain temperatures. When a gate opens, electrically charged particles (ions) rush through it, and that flow sparks a nerve signal that travels to the brain, where it is read as warmth or cold. One famous gate, called TRPV1, opens in real heat — and, curiously, also when chili pepper's capsaicin touches it, which is why spicy food literally feels hot. Another, TRPM8, opens in the cold and is also tripped by menthol, which is why mint feels cool. This sense protects you: it pulls your hand back from a stove before you are burned and makes you shiver or seek shelter before you freeze.