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Capsaicin is the oil-soluble chemical that is found in hot chili peppers (it’s what makes them hot!!).

In the body, there are two types of somatosensory signals that are transduced by free nerve endings (pain and temperature). These two modalities use thermoreceptors and nociceptors to transduce temperature and pain stimuli, respectively. Nociception is the sensation of potentially damaging stimuli. Nociceptive pain is mediated by cutaneous (essentially, skin) receptors that detect heat, cold, severe force, and chemical insult.

So, when someone eats spicy foods (containing capsaicin), the capsaicin molecules activate polymodal nociceptors that respond to heat and capsaicin, effectively creating a pain sensation. Most nociceptors respond to noxious mechanical stimuli (painful pressure, squeezing or cutting the tissue), noxious thermal stimuli (heat or extreme cold), and chemical stimuli and are therefore called polymodal.

The dynamics of capsaicin are unusual in that the molecule remains bound for a long time. Because of this, capsaicin decreases the ability of other stimuli to elicit pain sensations through the activated nociceptor. For this reason, capsaicin can be used as a topical analgesic, such as in products such as Icy Hot™.

To learn more, watch this Ted Talk linked here and included below.