Intro | Facial Nerve | Glossopharyngeal Nerve | Gustatory Nucleus | Papilla | Primary Gustatory Cortex | Secondary Gustatory Cortex | Taste Buds | Tongue | Vagus Nerve | Ventral Posterior Nucleus
Part 1: Image-Mapped Tutorial
Part 2: Matching Self-Test
Part 3: Multiple-Choice Self-Test
Taste buds are the receptor organs for the sensation of taste. A total of approximately 10,000 are located along the tongue, palate, pharynx, and larynx. The structure of each taste bud resembles that of a citrus fruit with 20-50 receptor cells forming each of the segments of the fruit. Each receptor cell projects cilia onto the surface of the tongue via an opening in the taste bud. Adjacent taste cells form tight junctions along their borders, thus preventing the flow of saliva into the taste bud itself. Because of this arrangement, the dissolved chemicals remain on the surface of the taste bud where the cilia are able to encode their qualities. Taste receptors are modified skin cells. They are, however, similar to neurons and have excitable cell membranes. These excitable membranes release neurotransmitters that in turn excite the adjacent neurons of cranial nerves. The cranial nerves then convey this information toward the brain. Taste receptors are replaced approximately every 10 to 14 days.
The activity of taste receptors during the chemical encoding process has been defined with varying detail. The most basic of the responses belongs to the saltiness receptor cell. Sodium chloride simply breaks down into its component ions, with the "salty" sodium ion directly entering the receptor membrane to induce depolarization (the same basic mechanism underlying the initiation and propagation of an action potential). The higher the concentration of sodium, the greater the receptor response. The saltiness receptor is also responsive to salts containing metallic cations (Na+, K+, Li+) bonded to small anions (Cl-, Br-, SO42-, of NO3-). Molecules that have a common structure stimulate the sweetness receptor cell; a hydrogen ion that is located 0.3 nanometers from a site that seeks to bond with another hydrogen ion. Molecules configured in this manner stimulate an increase in the levels of cyclic AMP within the receptor cell. The cyclic AMP stimulates the opening of calcium channels, and neurotransmitters are then released to stimulate a cranial nerve. Sour receptor cells are stimulated by the hydrogen ions found when acids are in solution. Hydrogen cations bond to the receptor membrane, and along with an undefined mechanism involving the anions that are also in solution, close the potassium channels. The closing of potassium channels prevents potassium efflux, and the build-up of positive charge within the receptor cell induces depolarization and an action potential. Molecules that contain both a positively charged side group and a hydrophobic side group stimulate bitterness receptor cells. These side groups activate an enzyme, phosphodiesterase, which breaks down cyclic AMP within the receptor cell. The decrease in cyclic AMP results in the closing of potassium channels and depolarization of the receptor cell occurs. Finally, some sweetness and bitterness receptor cells function as metabotropic synapses. At these receptors, the stimulating chemical bonds to a membrane site that results in the activation of a G protein (called gustducin). As is characteristic of metabotropic synapses, the G protein induces the release of a second messenger in the receptor cell and an action potential occurs.
Two new taste qualities have been proposed in the literature. Support for the existence of these specialized taste receptors, however, is varied. The first quality is called umani (a Japanese word that means "good taste"). This taste quality is associated with monosodium glutamate (MSG). MSG is used in oriental cooking and is found naturally in meats, cheeses, and several vegetables. A specialized metabotropic glutamate receptor (called mGluR4) may be involved in the taste of umani. The second taste quality to be proposed recently is carbohydrate. This taste quality is associated with complex carbohydrates (vegetables) and has been identified in both monkeys and rodents.