The tongue is mainly a set of muscle fibers covered with moist epithelial cells that form the gustatory papillae. These structures are responsible for detecting flavours. There are four types of papillae.
The gustatory papillae (filiform, fungiform, foliate, circumvallate) contain multiple taste buds which are the structures that allow flavours to be transformed into nerve impulses. Taste buds are also found on the soft palate, the inner surface of the cheeks, the pharynx and the epiglottis.
Taste and smell are closely related. For example, when someone has a cold with a congested nose, they can hardly taste anything. In fact, about 80% of the sensations attributed to taste are actually related to smell. Flavours, detected by the tongue, are combined with scents, detected by the nose. When food is chewed, it releases odorous molecules which reach the olfactory epithelium in the nose.
Flavours are divided into five groups: sweet, salty, sour, bitter and umami. Umami was discovered recently and corresponds to foods rich in protein and glutamate.
For a flavour to be perceived by the tongue, the substance in question must meet 4 conditions:
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Be sapid, meaning that the substance must have a flavour.
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Be soluble in saliva, in other words, dissolve in saliva.
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Have a sufficient concentration to stimulate the cells of the taste buds.
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Be in contact with the taste buds.
Contrary to popular belief, specific areas on the tongue are not designated to detect each taste. In fact, the receptors often detect several flavours, although their sensitivity to different substances can vary. However, it’s often believed that sweetness is perceived on the tip of the tongue, saltiness and acidity on the sides of the tongue, bitterness on the back of the tongue (aftertaste) and umami in the pharynx.
The sapid molecules (which have a flavour) first come into contact with the tongue and stimulate the taste buds inside the gustatory papillae. There, the flavours are transformed into nerve impulses which propagate through the cranial nerves and the brain stem before they are finally analyzed in the gustatory area of the cerebrum.
Apart from detecting taste, the tongue has other functions. The tongue provides a lot of information about the food in the mouth: its texture, temperature, spiciness, amount of fat, etc. In fact, this function is performed by sensory receptors similar to those found in the skin (pressure, temperature and pain).
The tongue is also involved in the first stage of digestion, which takes place in the mouth. It helps with chewing and saliva production. It also helps with swallowing by pushing the food bolus towards the pharynx.
Finally, the tongue helps form words by enabling several different sounds to be produced depending on its position in the mouth.