Taste
The tongue is covered with thousands of small bumps called papillae, which are visible to the naked eye. Within each papilla are hundreds of taste buds. The exceptions to this is the filiform papillae that do not contain taste buds. There are between 2000 and 5000 taste buds that are located on the back and front of the tongue. Others are located on the roof, sides and back of the mouth, and in the throat. Each taste bud contains 50 to 100 taste receptor cells. These receptors sit on microvilli at the top of the cell. They react when a substance in the mouth dissolves into base chemicals. Digestive enzymes in saliva begin to dissolve food before it reaches these structures. This chemical reaction triggers an electrical signal sent to the brain.
Taste receptors in the mouth sense five basic tastes: sweetness, sourness, saltiness, bitterness, and savoriness. Savoriness is also known as savory or umami. Sweetness signals the presence of carbohydrates in solution. Sourness describes acidity found in substances like lemon or vinegar. Saltiness seems to have two components involving sodium chloride detection. Bitterness warns people of poisons found in plants. Umami signals the presence of amino acid L-glutamate. Scientists identified these distinct categories through experiments showing they exist separately from one another. Western physiologists believed there were only four basic tastes until the early 20th century. Japanese research postulated the concept of a savory taste during that same period.
As the gustatory system senses both harmful and beneficial things, all basic tastes bring either caution or craving depending upon the effect the things they sense have on the body. Sweetness helps to identify energy-rich foods while bitterness warns people of poisons. Cats cannot taste sweetness and several other carnivores including hyenas and dolphins have lost the ability to sense up to four of their ancestral five basic tastes. Some rodents can taste starch which humans cannot. The ability to detect bitter-tasting toxic compounds at low thresholds provides an important protective function for many species. Plant leaves often contain toxic compounds so leaf-eating primates tend to prefer immature leaves. These differences show how evolution shaped taste perception across different animal groups over time.
In humans, the sense of taste is conveyed via three of the twelve cranial nerves. The facial nerve carries taste sensations from the anterior two thirds of the tongue. The glossopharyngeal nerve carries taste sensations from the posterior one third of the tongue. A branch of the vagus nerve carries some taste sensations from the back of the oral cavity. Signals travel through these nerves to the nucleus of the solitary tract in the brainstem. This region processes gustatory and sensory information about temperature and texture. The signal then moves to the thalamus before reaching the insula cortex. The frontal operculum acts as a memory and association hub for taste. Serotonin released during meals suppresses appetite while salivary nuclei decrease saliva secretion.
Aristotle postulated in 350 BC that the two most basic tastes were sweet and bitter. He was one of the first persons to develop a list of basic tastes. In 1907 Japanese chemist Kikunae Ikeda isolated dashi taste which he identified as monosodium glutamate. Western physiologists believed there were four basic tastes until the early 20th century. The concept of a savory taste was not present in Western science at that time. Researchers have since identified receptors for bitter, sweet and savory tastes as G protein-coupled receptors. As of 2019 molecular mechanisms for each taste appear different though all perception relies on activation of P2X purinoreceptors. Recent research suggests humans can taste starch independently of other tastes without suggesting an associated chemical receptor.
Research has shown that TAS2Rs such as TAS2R38 are responsible for the human ability to taste bitter substances. Among tasters some are so-called supertasters to whom certain compounds are extremely bitter. The variation in sensitivity is determined by two common alleles at the TAS2R38 locus. Studies have shown that supertasters require less fat and sugar in their food to get the same satisfying effects. These people tend to consume more salt than others because their heightened sense of bitterness makes them avoid bitter foods. Geneticists discovered a calcium receptor on the tongues of mice in 2008. Whether this perception exists or not in humans remains unknown. About 50% of patients with SARS-CoV-2 experience disorders associated with their sense of smell or taste including complete loss of taste.
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Common questions
What is the structure of taste buds on the tongue?
The tongue contains between 2000 and 5000 taste buds located within papillae. Each taste bud houses 50 to 100 receptor cells that sit on microvilli at the top of the cell.
How many basic tastes does the human sense detect according to modern science?
Taste receptors in the mouth sense five basic tastes including sweetness, sourness, saltiness, bitterness, and savoriness. Savoriness is also known as savory or umami and signals the presence of amino acid L-glutamate.
Which cranial nerves convey taste sensations from different parts of the tongue?
The facial nerve carries taste sensations from the anterior two thirds of the tongue while the glossopharyngeal nerve handles the posterior one third. A branch of the vagus nerve carries some taste sensations from the back of the oral cavity.
When did Western physiologists first acknowledge a fifth basic taste?
Western physiologists believed there were only four basic tastes until the early 20th century. Japanese chemist Kikunae Ikeda isolated dashi taste which he identified as monosodium glutamate during that same period.
Why can cats not taste sweetness compared to other animals?
Cats cannot taste sweetness and several other carnivores including hyenas and dolphins have lost the ability to sense up to four of their ancestral five basic tastes. This evolutionary difference shows how species adapted to specific dietary needs over time.