Why do we feel false sensations of hot and cold on our tongues and in our mouth

Q: Why do curries made of chillies taste very hot ?

Krishna: Yes, chillies are hot while mint is cold. 

Our nervous system is packed with receptors, which are proteins embedded in cell membranes. Ions generally cannot move freely across these cell membranes, but must enter or exit a cell through ion channels (which you can think of as tiny gates) which convert chemical messages into electrical signals, which your brain interprets as a sensation. Under certain conditions, ion channels open and allow the ions to move in or out of the cell, creating an electrical signal. For the most part, the receptors detect the correct stimulus, be it chemical, light, or vibration. But sometimes these ion channels can be fooled.

Transient receptor potential channels  (or TRPCs) are a type of ion channel which behave like mini thermometers. An example of which, called TRPM8, responds to lower temperatures (below 26°C) and to the menthol molecule found in minty flavors. So when you eat something minty, this ion channel opens and your senses are fooled. The cooling sensation it produces means menthol is found in many applications such as lip balms, decongestants, cough mixtures, mouthwashes, toothpastes and treatments for aches and pains. Some athletes say their performance improves when inhaling minty flavors, and that's why they chew gum.

Another example of this type of ion channel is TRPV1. These channels respond to heat (above 43°C) and  capsaicin  (a molecule found in chili peppers) also activates them. The burning sensation in your mouth when chewing chillies is due to capsaicin. Your brain is convinced you're eating something hot, despite no actual change in physical temperature. These receptors are clustered on our tongue, in our mouth and on our skin because their aim is to prevent us from sensations that will burn us but capsaicin also triggers it too, giving a false response.

Have you ever put your hand in really hot water and it has initially felt cold? Some cold receptors can be activated by both low temperatures and extremely high temperatures and is a phenomenon known as paradoxical cold sensation.  It's usually corrected within a matter of seconds.

Have you ever jumped into really cold water and noticed the initial shock wears off pretty quickly? The proteins in the ion cells become desensitised. So it's not quite as bracing as your body gets used to it. This is highlighted by frequent cold water swimmers,  who generally don't have the extreme response to cold as novices do.

You can demonstrate really easily how our heat sensors can be fooled. Get three containers, large enough so you can submerge your hand. Fill one with cold water, one with hot water (usual bath temperature is fine) and one with warm water. Place the container filled with warm water in the middle of the three. Place one hand in the cold water and one in the hot water for one minute. Then simultaneously put your hands into the middle container containing the warm water. You'll be amazed at the perceived difference in temperature and the ease at which we can fool certain ion channels.

This video explains things clearly ...