Humans have approximately 10,000 taste buds, each of which contains 50 to 100 taste cells that come into contact with the substrate to be tasted via tiny taste buds and then report their information to the central nervous system (CNS) via afferent nerve fibers. About 75% of the buds are integrated into the mucosa of the tongue, with the remainder distributed among the soft palate, nasopharynx, larynx, and upper part of the esophagus.
What are taste buds?
Taste buds (caliculi gustatorii) are small cup-like structures in the mucosa of the tongue. Each taste bud contains, among other things, up to 100 taste cells that come into contact with the substrate (food) on the tongue via tiny taste buds (microvilli) in the taste pore (porus gustatorius). They transmit their “impressions” as an electrical impulse via afferent nerve fibers to the responsible nerve switching sites in the central nervous system. The taste bud cells can be divided into type I, II and III cells. Taste buds are grouped on the mucous membrane of the tongue into so-called papillae, which are differentiated according to their appearance as Wall papillae, leaf papillae and fungal papillae. While wall papillae contain several hundred taste buds, mushroom papillae contain only 3 to 5 each. The taste cells can only distinguish between the tastes sweet, sour, bitter salty and umami. The term “umami” is a Japanese expression and, as the fifth flavor, can be roughly described as meaty, savory and tasty. Each taste bud contains sensory cells for all five tastes. The sense of taste is strongly interconnected with the sense of smell. An impaired sense of smell, for example due to a cold, also impairs the sense of taste.
Anatomy and structure
The taste buds, 20 to 40 µm in diameter, are integrated into the epithelium of the oral mucosa. The taste buds have a cup-like shape and taper upward to form the taste pore, which is 4 to 10 µm in diameter. Short sensory stalks (microvilli) protrude from the taste pore, each of which is connected to “its” taste bud at the other end. At the membrane surface of the microvilli are the actual taste receptors, which can be excited depending on the nature of the food. Each taste bud contains up to about 100 taste sensory cells, which are connected by afferent nerve fibers to the central nervous system for reporting their impulses. New taste cells continuously develop from the undifferentiated basal cells that each taste bud contains at its base, as these are relatively short-lived and must be continually replaced. The classification of taste cells into the three cell types I, II and III is based on morphological and immunohistochemical distinguishing features. A distinction according to function and tasks could not (yet) be made because differentiated knowledge about this is not available.
Function and tasks
The main function of the taste buds is to perform, together with the sense of smell, a preliminary examination of food for the criteria toxic/dangerous, edible, or inedible. The protective function of protecting the body from toxins or otherwise dangerous substances is based in part on genetic preprogramming, but for the most part on acquired experience stored in taste and smell memory. Another important task of the taste buds is to pre-screen food for contained sugars. On the one hand, the body demands energy in the form of sugar; on the other hand, too much rapidly bioavailable sugar (glucose) can drive blood sugar levels to dangerous heights. To prevent this from happening, the taste buds trigger a cascade of physiological reactions with their accumulated “strong sweet” messages. Above all, the pancreas is trimmed to insulin production in order to be able to process the expected sugar quickly and transfer it to some kind of suitable intermediate storage. If the “sweet message” was a false one because the taste buds fell for sweetener, this upsets the metabolism. Too high an insulin level causes the glucose level in the blood to drop sharply within 10 to 15 minutes, which can lead to drastic hypoglycemia.A fascinating task of the taste buds ensures that naturally left foods taste particularly good to us if they contain minerals, enzymes and vitamins that the body needs at the moment. The criteria by which this works are not known.
Diseases and ailments
A disturbance of the sense of taste can be caused by pathologically altered taste buds, for example by inflammation in the mucous membrane of the tongue or by a disturbance in the nervous system. The excitations reported by the taste bud cannot be correctly transmitted or processed in the central nervous system. Disturbances in the sensation of taste are referred to as dysgeusia. A distinction can be made between qualitative and quantitative dysgeusia. A complete loss of taste sensation is called ageusia. Qualitative dysgeusia is manifested by an altered taste sensation; under certain circumstances, a taste sensation is even virtually created, quasi hallucinated (phantogeusia). A rather unpleasant dysgeusia is cacogeusia, in which all taste stimuli are perceived as unpleasantly nauseating. Quantitative dysgeusia usually occurs in conjunction with impaired olfactory sensation. Inflammation in the oral mucosa or in the mucosa of the tongue can lead to temporary impairment of taste sensation and cause quantitative dysgeusia. Nerve inflammation (neuritis) can cause dysgeusia if the neuritis interferes with or completely stops the transmission of taste impulses. Dysfunctions in the processing of nervous impulses in the central nervous system, caused for example by tumors, neurotoxins or alcohol and other drugs, can also lead to dysgeusia. Most dysgeusia associated with secondary diseases, such as mucosal inflammation or neuritis, is temporary in nature and disappears once the secondary disease has been cured. Permanent total loss of taste sensation is very rare.
