Gustation

Gustation, or taste, is the sensory modality that detects chemical substances in the mouth. It functions through taste receptor cells organized into taste buds, primarily on the tongue but also in the soft palate, pharynx, and epiglottis. Taste buds are located within structures called papillae; the main types are fungiform on the anterior tongue, circumvallate at the back, and foliate on the lateral edges. Each taste bud contains taste receptor cells that respond to chemical tastants and interface with gustatory nerve fibers to convey information to the brain.

Traditionally, five basic tastes are recognized: sweet, sour, salty, bitter, and umami. Sweet receptors detect sugars and some proteins via a heterodimer of T1R2 and T1R3; umami via T1R1 and T1R3; bitter via diverse T2R receptors; salty primarily through sodium ions acting on epithelial sodium channels; and sour through pH-sensitive mechanisms. Transduction involves ion fluxes or second-messenger cascades, leading to action potentials in afferent nerves. The major cranial nerves carrying gustatory information are the facial nerve (via the chorda tympani for the anterior two-thirds of the tongue), the glossopharyngeal nerve (posterior one-third), and the vagus nerve (pharynx and epiglottis).

Pathways project to the nucleus of the solitary tract, then to the thalamus and the gustatory cortex in the insula and frontal operculum. Gustation interacts with olfaction, texture, and temperature to form flavor. Variation exists due to genetics and age; disorders include ageusia, hypogeusia, and dysgeusia, which can arise from illness or medication.