The sensation of taste is received by specialized receptor organs called taste buds. These are intraepithelial, oval shaped collections of cells that measure approximately 70 to 80 μm in length and 30 to 40 μm in width. They extend the full length of the epithelium from the basement membrane to small surface openings called taste pores. Numbering in the thousands, taste buds are found in the epithelium of the circumvallate, foliate, and the fungiform papillae, as well as scattered less densely around the soft palate, epiglottis, and the upper pharynx. They are not found in the filiform papillae. The greatest concentration of taste buds is found embedded in the circumvallate papillae which sit at the same level of the tongue but have deep troughs surrounding them. Underneath these troughs are serous glands, called the glands of von Ebner, that secrete a fluid that will serve as a solvent for taste provoking substances. These fluids also serve to cleanse the area to receive a new taste. The foliate papillae also have these underlying glands.
Each taste bud is composed of around 50 to 80 spindle shaped cells, of which there are 4 different cell types. These cell types are:
• Type 1 – Dark cells or sustentacular cells. Represent 60% of total.
• Type 2 – Light cells or gustatory cells. Represent 30% of total.
• Type 3 – Intermediate cells. Represent 7% of total.
• Type 4 – Basal cells. Represent 3% of total.
While the relationship between different cell types is not fully understood, it is commonly believed that basal cells are reserve cells that replace cells of the taste bud, which have a life span of approximately 10 days. Additionally, research has suggested the following progression: Basal cells give rise to dark cells, which mature into light cells, which become intermediate cells and die.
Type 1, 2, and 3 cells have microvilli that extend into the taste pore. On these microvilli are receptors that interact with tastants, which are the taste provoking chemicals dissolved in saliva. There are 5 taste modalities, and they have different types of receptors. Salty and sweet are received by ion channel receptors while sour, bitter, and umami, which is a receptor specific for glutamate, (also called savory) are received by G protein-coupled receptors. While each taste bud can interpret all 5 modalities, a greater concentration of a receptor type make individual taste buds more sensitive to a specific modality.
After the tastants have interacted with their receptor, an action potential is generated within that cell of the taste bud. Nerve fibers form synapses with Type 1, 2, and 3 cells which can then carry the nerve impulse to the brain where it is interpreted as a taste sensation. Type 2 light cells show a more intimate relationship with these nerve fibers, suggesting that they are the stronger taste receptors. Taste sensation from the anterior two thirds of the tongue is carried through the chorda tympani branch of the facial nerve, from the posterior one third of the tongue through the glossopharyngeal nerve, and from the epiglottis, soft palate, and upper pharynx through the vagus nerve.
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