Taste Receptors
To risk stating the obvious, taste receptors (a.k.a. chemoreceptors) facilitate a sense of taste. At a basic level, they provide us with the ability to decide if something is safe to swallow. On an exquisite level, taste receptors give us the ability to differentiate from the merely palatable to the most delectable of flavors, with each person having his or her own taste preferences. These chemoreceptors are distributed throughout the head and neck, with their greatest concentrations in the oral cavity, epiglottis, and along the laryngeal rim.
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Super tasters have a stronger sense of taste. This is likely genetic, as well as related to the number of fungiform papillae (Bartoshuk, Duffy, & Miller, 1994).
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Spicy is not a taste; it is mediated via pain and thermal receptors.
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Taste can be impacted by other senses. Using olfaction, a lemon “taste” can be stimulated from lemon aroma compounds (Lawless & Heymann, 2010). Through audition, foods can taste “crunchy” or “bubbly” (Zampini & Spence, 2010).
In the oral cavity, chemoreceptors are found on the tongue, soft palate and faucial pillars. The taste receptors on the tongue are located within papillae. In clusters, these receptor cells, or taste buds, respond (depolarize) to specific molecular concentrations and send that information to the brain stem. That is, each taste receptor is responsive to a specific chemical solution. The average adult tongue contains between 2000 and 8000 papillae. There are four types of papillae on the tongue, but only three contain taste buds. The fungiform papillae are mushroom shaped (when viewed in a longitudinal section). They are located in the tongue tip and dorsal surface of the tongue body. They are innervated by CN VII. The foliate papillae are ridges and grooves along the lateral and posterior aspect of the tongue body. They are innervated by CN VII and IX. The circumvallate papillae are large, but fewer in number— approximately 10-14 for most adults. They are arranged in a “v” or semi-circular shaped row and located on the posterior surface of the tongue body (oral part of the tongue) just anterior to the sulcus terminalis. They are innervated by CN IX. The filiform papillae are numerous, but do not contain taste buds. They are mechanical receptors. Chemoreceptors have been identified for five different chemical taste signatures and include bitter, salty, sour (acid), sweet, and savory (umami, glutamate). Taste researchers continue to seek evidence of other taste-specific cells. Some probable culprits include metallic taste, fatty taste, or alkaline taste (the opposite of sour).
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Taste buds provide a vital function for nutrient selection. They are also important in gut function. Collectively, stimulation of taste receptors, along with sight and smell, initiate reflexes involved in readying digestive enzymes in the gut and alerting the system to the oncoming need for peristalsis (Chaudhari & Roper, 2010).
The long held myth that specific locations of the tongue were responsible for specific taste is now well known to be false. In general, all five tastes can be perceived in multiple locations across the tongue body. However, there is a greater predominance of sweet receptors in the front of the tongue and sour and bitter receptors in the back of the tongue. The assumption around the predominance of posterior bitter receptors is that they provide final attention to alert the system to bad food or toxic substances. This will allow expulsion before being swallowed.
Oral taste information is sent via CN VII and/or IX to the Nucleus Tractus Solitarius (NTS) in the medulla. Cortical lesions, such as a unilateral stroke, will not typically alter oral taste function. When individuals report loss of taste, it is often related to a peripheral injury with ipsilateral loss of taste.
Taste receptors are also located on the palate, in the larynx, and in the upper esophagus region. Laryngeal receptors are located mostly on the undersurface of the epiglottis and along the laryngeal rim. These are innervated by CN X. They behave differently from oral taste receptors in that they are not sensitive to the five different chemical taste signatures; instead they are responsive to non-saline, so that all foreign chemical signatures will elicit a response. This provides protection from foreign material entering into the airway.