Smell and taste

You are born with about 10,000 taste buds, but much of what you call the taste of food actually reaches you through your nose.

Two chemical senses

Hold your nose and eat a piece of apple, then a piece of pear. Without smell, much of the difference between them fades. Taste and smell are the chemical senses: both detect molecules, and both send their information to the brain, where it is combined into flavour. The NIDCD notes that many people who see a doctor because they think they have lost their sense of taste turn out to have lost their sense of smell instead.

How smell works

The olfactory sensory neurons sit in a small patch of tissue high inside the nose. As you breathe in, odour molecules dissolve in the mucus there and bind to receptor proteins on the neurons' dendrites. Each neuron carries one kind of odour receptor. There are more smells in the world than receptor types, so a single odour activates a combination of receptors, and the brain reads that combination as a particular smell. The axons pass through small holes in a skull bone, the cribriform plate, to the olfactory bulb on the underside of the brain.

Smells reach these neurons by two routes. One is through the nostrils. The other is a channel from the back of the mouth up into the nose. When you chew, aromas travel up that back route, which is why food tastes flat when a cold blocks your nose. The 2004 Nobel Prize in Physiology or Medicine went to Richard Axel and Linda Buck for finding the family of odorant receptor genes. In mice they counted about 1,000 such genes; humans have somewhat fewer. Olfactory neurons are also unusual because they are replaced throughout life, and the NIDCD notes that taste cells are the only other sensory cells that are.

How taste works

Taste cells are clustered in taste buds on the tongue, on the roof of the mouth and along the throat. Many of the small bumps on the tongue, called papillae, contain taste buds. Molecules dissolved in saliva reach the taste cells, which send messages through three cranial nerves to the brain: the facial nerve serves the front two thirds of the tongue, the glossopharyngeal nerve the back third, and the vagus nerve the very back near the throat.

Taste cells respond to at least five basic qualities: sweet, salty, sour, bitter and umami. Salty is the detection of sodium ions. Sour is the detection of hydrogen ions, which is why acidic orange juice, with a pH of about 3, tastes sour. Sweet, bitter and umami use receptor proteins that bind particular molecules. Umami, a savoury taste, comes from glutamate, found in broths and some cheeses. A common picture shows a tongue map with sweet at the tip and bitter at the back. The NIDCD calls this a misconception: in humans the different taste cells are scattered across the whole tongue.

Flavour is more than taste and smell. The NIDCD describes a common chemical sense: thousands of nerve endings on the moist surfaces of the eyes, nose, mouth and throat that respond to irritants. They give you the cool feeling of mint and the burn of chilli peppers. Other nerves add heat, cold and texture. All of it is combined with the aroma, and that combination is what lets you tell an apple from a pear.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.