The mouth houses the sensory chemoreceptors for taste—or gustation—called taste buds. Taste buds are located on the tongue, soft palate, cheeks, pharynx, and epiglottis, with the majority concentrated on the tongue. Each visible bump on the surface of the tongue is called a lingual papilla, and the taste buds are found deep within the vertical surfaces of the papillae. The specialized cells that detect taste are called gustatory cells and are found within the stratified squamous epithelial tissue of the tongue. The gustatory epithelial cells contain hair-like projections, which are long microvilli called gustatory hairs. These are in close contact with the sensory neurons of the glossopharyngeal nerve, or the facial nerve.
Three cranial nerves carry information about taste:
1. Facial—It transmits sensory information from the anterior (front) two-thirds of the tongue.
2. Glossopharyngeal—It transmits sensory information from the posterior (back) one-third of the tongue.
3. Vagus—It transmits sensory information from the few tastes buds in the epiglottis and throat. It carries taste information at the back of mouth.When food chemicals dissolve in saliva, they bind to specific receptors on the gustatory cells. This reaction initiates a G-protein response, in which proteins transmit signals within cells, and ultimately opens ion channels to start an action potential. The stimulated gustatory cells signal the sensory neurons to relay the information to the brain. Gustation processing occurs in various locations of the brain, including the medulla, thalamus, cerebral cortex, hypothalamus, and limbic system. The five recognized discrete taste sensations are sweet, sour, salty, bitter, and umami. Overall taste interpretation is based on the combinations of taste receptors activated as well as the information coming from the olfactory system. The tastes of food can also be influenced by other sensory receptors in the mouth. The temperature of food, detected by thermoreceptors, can affect the taste. Mechanoreceptors detect the textures, which can affect the affinity to the food for some people. Nociceptors are triggered by spicy foods and can enhance flavors for some and deter others. When cells are damaged by heat, spicy foods, or abrasion, they are replaced by basal epithelial cells, stem cells found in the basal layer of the epithelium.