Transient Receptor Potential Vanilloid 1 cation channel, or TRPV1, plays an important role in the human body in sensing heat and pain. Coincidentally, it is also the prime receptor for capsaicin, an active component found in chili peppers. The binding of capsaicin to the TRPV1 produces the sensations of heat and pain. When this happens in our tongues, it translates into the taste that we know as "spiciness".
So how spicy is capsaicin? It is measured at 16 million SHU, meaning one part of capsaicin needs a 1/16,000,000 dilution in water to lose its spicy taste. Just when you think that's incredible, resiniferatoxin (RTX) a chemical extracted from the Moroccan succulent plant Euphorbia resinifera is currently the spiciest substance in the world--a thousand times hotter than capsaicin. While RTX remains dangerous to be savored by anyone, researchers are attempting to use an iodinated version of the compound as an analgesic agent, given its strong competitive binding to the TRPV1 receptor.
Source: SciShow via Youtube