Is spicy a taste?
Why is hot, as in spicy, not a taste? Because there are no taste buds for it?
Kat put this zinger to Chris...
Chris - Well actually, someone wrote on the Naked Scientist web forum, which you can go to: nakedscientist.com/forum. They said - rub some chilli in your eye and then you'll understand it's not about taste - it's about pain. And the answer to this question is, actually, an anatomical one and a functional one. We tend to talk about five different tastes or taste sensations: bitter, sweet, salty, and sour. And they are recorded or detected on your tongue by what we call taste buds, which are specialised clusters of nerve endings which are covered in receptors - these are molecules that can lock onto various things in food and they then signal to the nerve and tell the nerve to fire off impulses to your brain stem, which then tells your brain "this is what I am tasting." And that's confined to the tongue and there are two nerves that do that job; there's a nerve at the anterior (the front) two thirds of the tongue called the chorda tympani, which supplies some of those taste sensations; there's another nerve called the glossopharyngeal nerve, which innervates the back third of the tongue.
Now pain, on the other hand, is not just restricted just to the tongue. You can sense those sensations all around your mouth cavity, all over your tongue, and in your throat. And chilli is capsaicin (that's the molecule), and it binds onto a totally different class of nerve fibres called C-fibres. They have on their surfaces a certain molecular docking station or receptor called TRPV1. When the capsaicin binds onto them it triggers a burning - or heat - sensation; those same nerve fibres also tell you whether something is hot or cold. So, in other words, what we call taste is detected in a very special way in a very special place by a very special cluster of nerves. Those pain sensations are detected alongside all the other sensations you can feel in your mouth by the trigeminal nerve, so it's anatomically different and it's functionally different. Therefore we don't regard the chilli pepper taste as a taste, we regard it as a sensation.
Kat - And presumably this is why the saying goes - a really really hot curry or a chilli "will burn on the way out as well as the way in".
Chris - It does. You don't have very many of those pain fibres innervating the rest of your GI tract. But your mouth and your bottom end, those actually are made by the outer layer of your body embryologically, the outer coating of your developing embryo, growing in and uniting with your gut tube and taking with it the kinds of nerves that are there. And your trigeminal nerve is the nerve concerned, and that's the one that's got the receptors for chilli. Which is why, yes, you're very sensitive on the way in, and very sensitive on the way out, if it's a hot one...