Taste System Structure

Evolution equipped living organisms with the ability to detect and respond to chemicals in their environments for the purposes of communication, reproduction, and nutrient detection. Many airborne chemicals are detected by the sense of smell, whereas chemicals in solution that are subject to ingestion are sensed and evaluated by the gustatory (taste) system. The sense of taste acts as a final gating mechanism to the internal milieu: The taste of a substance plays a large role in determining whether or not it will be ingested. Thus, taste critically guides the selection and intake of foods, intimately influencing our health and well-being. This entry describes the structure of the taste system, emphasizing the roles of the various structures in taste processing.

Humans can categorize taste sensations into one of five different general perceptual categories: sweet, salty, sour, bitter, and umami. Umami is a Japanese word that refers to the “savory” or “meaty” taste of certain amino acid stimuli, such as monosodium glutamate (MSG). These different perceptions begin when taste chemicals released from food in the mouth bind with specialized receptors for these chemicals that are linked to the neural pathway underlying taste. Such receptors are known simply as taste receptors. It is noteworthy that when humans describe and talk about the taste of a food they are usually referring to its flavor, a concept that involves smell, tactile (touch and texture) sensations, temperature, and also taste. Yet in precise terms, “taste” originates from the interaction of chemicals dissolved in saliva with taste receptors.

From what researchers currently know, taste receptors are composed of proteins that lie on the surface of “taste cells.” In general, a taste cell is a specialized epithelial (skin) cell in the mouth that harbors taste receptors. These cells become “activated” when a taste chemical binds to a receptor they might express. More specifically, the binding between a taste chemical and its receptor results in a cascade of molecular events inside a taste cell, culminating in a change of the electrical charge across the cell's membrane. This change in electrical potential provides the basis of a signal sent to the brain indicating that a taste stimulus is present and what kind of stimulus it is. In humans and other mammals, taste cells are clumped together in onion-shaped structures called taste buds, each housing 50 to 100 or so cells, including taste cells and supporting cells. Taste buds are found in various regions of the oral cavity, including on the tongue and the soft palate on the roof of the mouth. Taste buds are housed in papilla, which on the tongue are the small protrusions (or bumps) that give this organ its velvety appearance. In most vertebrates, there is continual turnover and replacement of taste cells throughout life. Taste cells have been shown to have a lifespan of 9 to 10 days on average.

It is noteworthy that taste receptors sensitive to different types of taste chemicals (i.e., sweet, salty, bitter) are found in the same taste-cell-containing regions of oral epithelia. Receptors for a single class of taste stimulus are not restricted to specific locales, as has been depicted by the classic “taste map” of the tongue. Although sometimes still finding its way into modern textbook chapters on taste, the tongue map, which is based on old data, has long been considered by taste researchers to be incorrect. It is now known that taste receptors for pleasurable sweet-tasting stimuli are found in taste bud cells located on the front and rear of the tongue, although the classic tongue map would show that sweet can only be detected on the tongue's tip. The outdated tongue map also depicts bitter taste being mediated exclusively by taste receptors on the rear of the tongue. However, the application of bitter stimuli to the front of the tongue produces strong activation of taste circuitry in the brain, indicating that there are indeed bitter taste receptors on the front of the tongue. What is more, it has shown in rodent experiments that disrupting the cranial nerve carrying taste input from the front of the tongue to the brain results in an impairment of a rat's ability to recognize the taste of bitter stimuli in certain behavioral tasks, suggesting that the front of the tongue is critically involved in bitter taste perception.

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