The Tongue and its Taste Buds
The tongue is more complex then it may appear to a naked eye. It is a common misconception that the small red dots that are observed on our tongue are called taste buds. When in reality, these bumps are known as papillae, and the taste buds themselves are found as collections of cells on the papillae or tucked inside. The papillae are red due to the abundance of blood vessels found within them. Papillae gather in clumps on the tip, along the side and on the surface of the tongue. More specifically, there are three different types of papillae; circumvallate, foliate and fungiform. Circumvallate papillae are most commonly found at the very back of the tongue, and consist of a small bump surrounded by a hollow ditch. Foliate papillae on the other hand are found in the back corners of the tongue and similar to circumvallate papillae, have a hollow ditch surrounding an even narrower bump. Both circumvallate and foliate papillae contain taste buds on the sides of the bump, within the small ditch. Last but not least, fungiform papillae are elevated bumps containing the taste buds directly on the surface. each human being has approximately two hundred papillae.
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The epithelial cells are what make up the outermost protective layer of the papillae. Within each papilla itself, the taste buds are buried with only the outer taste pore revealed to the external environment of the mouth. The taste bud is comprised of three principal cells, including the supporting cells, the sensory receptor cells and the basal cells. The supporting cells are responsible for upholding the structure of each individual bud, and also contain microvilli. The purpose of the microvilli are to increase the surface area for reception. Sensory receptor cells on the other hand, are the sites of sensory transduction. Finally, basal cells derive from new epithelium approximately every ten days and are the growing new receptor cells. See the diagram to the left of the taste sensory cell (sensory receptor cells).
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There are five different types of taste receptors, one for each flavor, in which the molecules/chemicals specific to each taste fit into on the microvilli. As a huge generalization, sugars are sweet, salts or ionic molecules are salty, alkaloids are bitter, amino acids are umami and acids are typically sour. Nonetheless, the taste receptors are not all physically built in a similar manner. Due to the fact that compounds resulting in a sour or salty taste are either ionic molecules or acids, they have the capability to ionize or dissociate. Therefore, rather there being a molecular pocket for a chemical complex to fit into, salt and sour receptors are composed of an ion channel. As exemplified in the diagram below, sodium ions would enter through the salt receptor channel and hydrogen ions would enter through the sour receptor channel. As a result, a potential difference would be created across the membrane due to opposite charges, furthermore creating an electrical signal. On the other hand, the receptor cells for sweet and bitter tasting chemicals are similar to molecular pockets that accommodate only specific structural molecules. In other words, a molecular complex fits into the receptor like two puzzle pieces and a “secondary messenger system” commences to produce a similar cell depolarization for a different channel.
In 2008, scientists were able to conclude that sweet, bitter and umami converge on the same transduction channel, known as TRPM5. In other words, receptors known as T1R and T2R are shaped to be activated by bitter, umami and sweet molecules. Once activated, they intern stimulate G proteins, followed by the stimulation of phospholipase C. After the generation of phospholipase C, two intracellular messengers are generated; Inositol trisphosphate (IP3) and diacylglycerol. The TRPM5 channel as a result becomes open and the depolarization which is brought on by the arrival of positive ions creates a depolarization. Overall, it is the PLC (phospholipase C) that activates the TRPM5. Additionally, due to the fact that TRMP5 channels are calcium sensitive, The IP3 molecules would release them from internal storage area in the process, and the channel would become depolarized.
Generally, each taste is activated differently from the others. This is exemplified in the figure to the left, as each of the five tastes, excluding fat, has its own receptors or ion channels.
Watch this video to solidify your general understanding before further proceeding through the website.
Generally, each taste is activated differently from the others. This is exemplified in the figure to the left, as each of the five tastes, excluding fat, has its own receptors or ion channels.
Watch this video to solidify your general understanding before further proceeding through the website.