[thedoc (OP)]

Android Neox  asked the Naked Scientists:

Why is hot, as in spicy, not a taste? Because there are no taste buds for it?

Michael McGinnis

What do you think?

[chris]

Hi Michael

spiciness is not detected by your taste buds in the same way that salty, bitter or sweet flavours are. These latter taste stimuli are detected by specific classes of nerve fibres carried in two anatomically discrete nerve bundles that terminate in the taste buds. The front two thirds of the tongue is supplied by the chorda tympani nerve and the posterior (back) third of the tongue is supplied by the glossopharyngeal nerve. The taste sensations carried by these nerves are transmitted to a region of the brainstem called the nucleus tractus solitarius, which relays them upwards into the brain.

But if one looks at the nerves that are sensitive to spicy sensations, which are small calibre (C and A delta, to give them their proper terminology) fibres that express receptors for capsaicin (the pungent chilli chemical), these nerves are part of the trigeminal nerve and are found throughout the oral cavity, rather than forming interactions with taste buds. Tracing them into the brain, they connect to a different site called the sensory trigeminal nucleus.

So, in summary, the chilli-sensitive nerves are differently distributed around the mouth and connect to a different target in the brain compared with classical taste nerves; therefore they - and the sensation the subserve, which is detection of heat and cold - are not considered a taste.

[Bored chemist]

If you get chilli powder in your eyes you will realise that it doesn't just affect sensors that respond to taste.

[chris]

If you get chilli powder in your eyes

Good call!

...but not just eyes either! I had a colleague working on capsaicin in the lab. His gloves leaked, as he discovered when he went for a wee, mid-experiment... that certainly did bring tears to his eyes, despite being at the other end of the body... I am also aware of at least one amorous couple who did a similar experiment on each other after earlier cooking curry that involved chopping fresh chillies.

[Bored chemist]

It's an old joke that you can recognise chemists because we are the people who wash our hands before going to the  bathroom.
I gather chefs in Mexican restaurants may be mistaken for chemists.

[thedoc (OP)]

We discussed this question on our  show

Kat put this zinger to Chris...

Chris - 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...

Click to visit the show page for the podcast in which this question is answered. Alternatively, [chapter podcast=1001305 track=16.02.23/Naked_Scientists_Show_16.02.23_1004810.mp3] listen to the answer now[/chapter] or [download as MP3]

[chris]

It's an old joke that you can recognise chemists because we are the people who wash our hands before going to the  bathroom.
I gather chefs in Mexican restaurants may be mistaken for chemists.

Funny, herpes virologists have that reputation too...