Gustation is a chemical sense that, along with olfaction (smell), contributes to our perception of taste. It starts with the activation of receptors by chemical compounds (tastants) dissolved in the saliva. The saliva and filiform papillae on the tongue distribute the tastants and increase their exposure to the taste receptors.
Taste receptors are found on the surface of the tongue as well as on the soft palate, the pharynx, and the upper esophagus. On the tongue, taste receptors are contained within structures called taste buds. The taste buds are embedded within papillae, which are visible on the tongue surface. There are three types of papillae that contain taste buds and their receptors. Circumvallate papillae are the largest papillae and are located near the back of the tongue. Foliate papillae resemble folds on the side of the tongue. Fungiform papillae are found across the front three-quarters of the tongue but are less concentrated in the middle of the tongue.
There are five basic tastes: salty, sour, sweet, bitter, and savory (or umami). The perception of salty taste is caused by tastants that release sodium ions upon dissolution. Sour taste, by contrast, is produced by the release of hydrogen ions from dissolved acidic tastants. Salty and sour tastants produce a neural response by depolarizing the membrane directly (salty tastants) or via ion channel changes (sour tastants).
Sweet, bitter, and savory compounds activate G-protein-coupled receptors. Chemicals perceived as sweet, like glucose and fructose, typically have hydroxyl and carbonyl groups. Numerous diverse molecules stimulate bitter receptors, and over 30 receptors for bitter taste have been identified. The amino acid glutamate activates savory receptors. The addition of monosodium glutamate, or MSG, to foods enhances the savory flavor.
Cranial nerves carry taste information from receptors in taste buds to the medulla in the brainstem. From there, the information travels to the thalamus and is then relayed to the primary gustatory cortex. This gustatory cortex also receives other sensory information from the tongue, like thermal and mechanical stimulation. The incorporation of the different sensory inputs provides a hedonic quality to flavor, allowing it to be perceived as pleasant or unpleasant.