Chili peppers can feel hot because capsaicin activates TRPV1 receptors on sensory nerve endings. Those receptors normally respond to potentially damaging heat and related chemical conditions, so the nervous system receives a heat-like warning signal even when the food itself is not at a high temperature.
A chili pepper can be room temperature and still produce a strong burning sensation. The heat is neurological rather than a thermometer reading.
Capsaicin presses a heat-sensing button
Capsaicin, a compound found in chili peppers, binds to a protein called TRPV1 on certain sensory nerve cells. TRPV1 is involved in detecting noxious heat and other potentially harmful conditions. When capsaicin activates it, the nerve sends signals that the brain interprets in much the same sensory category as physical heat.
That is why the mouth may feel as though it is burning even though the tissue has not suddenly reached the temperature of hot soup. The sensation is real; the source of the signal is chemical activation of a receptor rather than a large rise in temperature.
Spiciness is not one of the basic tastes
Sweet, salty, sour, bitter and umami are detected through taste systems. Chili burn belongs largely to chemesthesis—the detection of chemical irritation through sensory nerves. Menthol produces a related trick in the opposite direction by activating receptors associated with cooling sensations.
This distinction explains why “spicy” does not behave exactly like sweetness or saltiness. It can be felt on tissues beyond the tongue wherever the relevant sensory nerves are exposed.
Why reactions differ
People vary in their sensitivity and experience with spicy foods. Repeated exposure can change perceived intensity, and the amount and type of capsaicinoids differ among peppers and foods. Other ingredients, food temperature and the area of tissue exposed also affect the experience.
The important point is that the brain is responding to a receptor signal designed to warn about heat and irritation. Capsaicin effectively borrows that alarm pathway. The food does not need to be physically hot for the nervous system to report a convincing sensation of heat.
This also explains why the sensation can linger briefly after the bite is swallowed: capsaicin may remain in contact with receptors until it is diluted, displaced or cleared. The persistence is another clue that the nervous system is responding to a chemical trigger rather than measuring the food’s actual temperature.