The chemoreceptors for taste are packaged in taste buds (~10,000)
Taste buds include about 50 taste receptor cells. Each taste bud has a taste pore from which fluid in the mouth can contact the receptor cells.
The receptor cells are modified epithelium with many surface folds (microvilli) to increase surface area. Epithelial cells differentiate into supporting cells and then into receptor cells. Taste receptors have a life span of about ten days
The microvilli plasma membranes contain receptor sites that bind selective chemical molecules – tastants. Binding of a tastant with the receptor site produces a depolarizing receptor potential
Terminal afferent endings of several cranial nerves synapse with taste buds in various regions of the mouth. The nerves relay to the brain stem and thalamus and on the cortical gustatory area.
Taste discrimination is base on varying combination of four primary tastes: salt, sour, sweet, bitter and umami. Receptor cells use different pathways to bring about depolarization
Salt – direct entry of Na+ is responsible for
Sour – H+ ion block K+ channels, the decrease in K+ produces depolarizaition
Sweet – sugars –glucose and others bind with a taste receptor that activates a G protein which turns on cAMP second messenger pathway which ultimately blocks K+ channels
Bitter – alkaloids, poisons also activate a G protein (gastducin) which sets off a second messenger pathway similar to the visual G protein
Umami – triggered by amino acids that activate a G protein pathway but is yet unknown