Anatomy of Taste Buds and Their Receptors

Each taste bud consists of up to 100 polarized neuroepithelial cells that form “islands” of columnar pseudostratified cells embedded in the epithelium. Each bud has a central taste pore through which microvilli extend from the receptor cells. Just below their apical ends, the cells are joined by desmosomes, which seal off the intracellular spaces from the taste pore. There are three types of receptor cells (types I, II, and III) and basal cells that make up the taste bud. Taste buds are constantly being renewed.

Taste buds are innervated by both large and small fibers, which emerge from a subepithelial nerve plexus and enter the bud at its base. The larger fibers run in clefts between taste cells, while the smaller fibers (possibly terminal branches derived from large fibers) tend to run in invaginations found in the basal parts of taste cells.

The sensation of taste can be divided into five primary qualities: sweet (sucrose), sour (hydrochloric acid), salty (sodium chloride), bitter (quinine), and umami (L-glutamate and other L-amino acids). Sweet foods signal the presence of carbohydrates, which supply energy. Sour foods signal dietary acids and are frequently aversive. Salty taste sensation helps to regulate body water balance and blood pressure. Bitter taste is aversive and guards against poison consumption. Umami reflects food’s protein content. The tip of the tongue is sensitive to all five stimuli but especially to sweet and salty substances, the sides of the tongue to sour substances, and the base of the tongue to bitter substances.

Water-soluble compounds evoke taste sensations by binding to the apical parts (microvilli) of the taste cells. Type I cells are the most abundant and function primarily by terminating synaptic transmission and regulating neurotransmitters. In type II cells, sweet, bitter, and umami ligands bind to taste receptors, resulting in an increase of cytoplasmic calcium and depolarization of the cell membrane, ultimately resulting in adenosine triphosphate (ATP) release. Sour taste excites type III presynaptic cells. The presynaptic type III cells also form synaptic junctions with nerve terminals and also express proteins involved in synapses. These cells release both serotonin and norepinephrine. Salty taste is detected by direct permeation of sodium through membrane ion channels. The cell type underlying salty taste has not been identified, although type I cells have been implicated.

Only gold members can continue reading. Log In or Register to continue