Animals use their tongues for a range of purposes — to taste food, to clean themselves, or to capture or manipulate their prey. In humans — and every other species — the tongue is also used to shape complex sounds. New research out this week adds a new role to the tongue's repertoire — smell receptors were discovered on the human tongue, implying that we might be able to use them to smell too.
To date, the general consensus has been that you use your nose to smell and your tongue to taste. Taste and smell were considered to be independent sensory systems that did not interact until their respective information reached the brain.
While many people associate the flavour sensations we get from food and drink with taste, the distinctive flavours of most foods, in fact, come more from their smell than their taste. The flavour that we perceive our food to have comes from our brain combining information from taste, smell, and other senses to create the multi-modal sensation of flavour.
The tongue helps us to detect whether something tastes sweet, sour, salty, buttery or umami through its cells, which carry taste receptor proteins. These proteins interact with molecules in the food we eat and trigger a signalling cascade that leads to the release of chemicals — known as neurotransmitters — which activate specific regions of the brain where taste is perceived and processed.
Previous research on genetically modified mice found that as well as being found in the nose, receptors responsible for detecting odours were also located in the taste cells of the tongue. Curious as to whether or not human tongues had these cells, the researchers went on a taste bud-exploring mission. Using a method known as calcium imaging, which causes specific cells to fluoresce, researchers were able to show not only that the human tongue contains odour-detecting proteins in the taste cells, but that they also respond to scent molecules in a manner similar to odour receptor cells.
The research, published in the journal Chemical Senses, went on to run smell tests on these human taste cells. Using eugenol, a scent extracted from clove oil, they found that the smell-sensing proteins on the tongue were able to trigger a detection of the smell at eugenol concentrations that were below the level needed to trigger a taste response on the same tongue.
This new discovery might help to explain earlier research which found humans could discriminate between foods that only differed in their smell, even when the nose smelling function of the taster was blocked.
It also suggests the interactions between the senses of smell and taste are likely to be more complex than scientists had previously thought, and questions whether the perceived flavour of food actually begins in the brain or on the tongue.
The ability of the tongue to smell could imply that the tongue is vital in detecting delicate aromas which might tweak subtle flavours in different foods.
Discovering that our tongue now has smell receptors doesn't mean that we all need to go around flicking them in and out like snakes trying to detect our environment (everyone I have spoken to about this topic in the last days has started doing just that!). Instead, it opens up interesting possibilities — for instance, of being able to create low-sugar foods for weight loss which through their odour could make people perceive that the food contains more sugar that it does. It also opens up new areas of exploration for chefs who experiment with aromas to see if they can tickle not only people's taste buds but their olfactory proteins too.
• Dr Michelle Dickinson, creator of Nanogirl, is a nanotechnologist who is passionate about getting Kiwis hooked on science and engineering. Tweet her your science questions @medickinson