Gustatory perception

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Schematic representation of a taste bud

As gustatory perception (from Latin gustare , taste, taste ”) is the subjectively experienced experience of sensations of taste that are caused by stimulating specific sensory organs of taste ( Latin  gustus ) such as the taste buds .

The sense of taste , like the sense of smell , is addressed by chemical stimuli, but it is a near sense with which the ingested food can be checked before it is actually consumed. In adults, the sensory cells of the organ of taste are located in the lining of the tongue and throat and convey five (or six) basic qualities. Sour and bitter tastes can indicate immature, fermented, or toxic foods. The taste qualities salty , sweet , umami (and fatty ) characterize a food roughly according to its content of minerals and nutritional substances such as carbohydrates, proteins and fats.

The sensory impression, which is called “ taste ” in everyday language , is an interplay of the sense of taste and smell together with tactile and temperature sensations from the oral cavity. In terms of the physiology of the senses, however, the human sense of taste comprises only the basic taste qualities mentioned; they are perceived with taste receptors that are predominantly on the tongue .

As dysgeusia disorders are referred to the taste perception. Ageusia is called the failure of the sense of taste.

Location of the sensory cells

In mammals, the receptor cells for different taste qualities are arranged in taste buds ( caliculi gustatorii ), which are located on the tongue in the taste papillae ( papillae gustatoriae ), as well as in the mucous membranes of the oral cavity, pharynx and throat. About 25% of the taste buds are located on the front two thirds of the tongue, and another 50% on the back third. The rest are distributed over the soft palate , nasopharynx , larynx and the upper esophagus . Each taste bud can contain 50 to 150 sensory cells, depending also on the species of a mammal, and a taste bud can contain several to numerous taste buds.

The papillae of the tongue are divided according to their shape into wall, leaf, mushroom and thread papillae. Wall papillae ( papillae vallatae ) are located in the rear third of the back of the tongue in a V-shaped arrangement near the base of the tongue. Everyone has around seven to twelve of these papillae, each with several thousand taste buds. The leaf papillae ( papillae foliatae ) are also located in the rear third of the tongue, but on its edge, and contain several hundred taste buds. The up to four hundred fungal papillae ( papillae fungiformes ) are found distributed over the entire surface of the tongue, primarily on the front two thirds of the tongue and each contain three to five taste buds in humans. Filiform papillae do not contain any taste buds, but are used to assess the mechanical properties of the food consumed .

Human infants and toddlers not only have more taste buds in number, but also some on the hard palate, in the middle of the tongue and in the mucous membrane of the lips and cheeks. With increasing age, their number is thinned out and concentrated on specific locations.

The taste qualities

Currently, at least five - possibly six - basic taste qualities are assumed:

Umami ( Japanese for 'tasty, spicy') is a generally less well-known quality of taste that was first described in 1909 by the Japanese researcher Kikunae Ikeda . Ikeda succeeded in isolating glutamic acid from the seaweed, which is a main ingredient in dashi , a Japanese fish sauce , and identified it as the decisive component of dashi in terms of taste. He gave this quality its name as a compound of umai ('spicy') and mi ('taste'). A strong umami taste indicates protein and amino acid-rich foods, but can also be caused solely by a high concentration of glutamic acid or the flavor enhancer monosodium glutamate . Receptors from the CaSR group bind calcium ions and intensify the sensory impressions umami, sweet and salty.

It has been known since the beginning of the 20th century that the aforementioned taste qualities can be triggered to different degrees on the tongue in different regions, but are basically perceived by all taste-sensitive areas. Although the differences between the tongue areas with regard to the sensitivity for individual qualities in humans are rather small, a division of the tongue into “taste zones” can be found in many textbooks.

More taste qualities

At the end of 2005, a group of scientists led by Philippe Besnard identified a possible taste receptor for fat : the glycoprotein CD36 , which has been detected in the taste sensory cells of the tongue and can bind fatty acids with high affinity . Until then, it was debatable whether there was a sixth basic quality that is triggered by fat in food. It was generally assumed that the preference for fatty foods stems from their smell and consistency alone . In order to clarify the question of a possible further basic taste for fat, the researchers carried out experiments with normal ( wild type ) and with genetically modified mice without the CD36 receptor ( knockout mice ). The mice were given the choice of two types of food, one of which contained fat and the other only a substance that mimicked the consistency of the fat. It was found that the normal mice with CD36 had a strong preference for the fatty food, but not the knockout mice without CD36. In addition, only the common mice responded to fatty foods by producing fat-specific digestive juices . From these results it can be concluded that the CD36 is involved in the perception of fat in the food of rodents.

In the meantime, scientists from the same group have also shown that the stimulation of the taste sensory cells of the mouse that express CD36 with linoleic acid leads to an activation of intracellular signal cascades and the release of neurotransmitters.

Linoleic acid is a component of many vegetable fats that occur in food and is released in the oral cavity by special enzymes ( lipases ). The release of neurotransmitters by taste sensory cells is necessary for the information to be passed on to the brain, where it is processed.

The existence of such a further taste quality was supported in 2010 by a smaller study with 30 test persons. The test subjects were able to differentiate between different fatty acids in otherwise tasteless solutions. Furthermore, a connection between BMI and the sensitivity of this taste quality could be shown. Accordingly, subjects with a more sensitive sense of taste for fatty acids consumed less fat than those with a less sensitive one.

In addition, other taste qualities are discussed again and again, such as alkaline , metallic and water-like.

The sense of smell , which is responsible for all other “taste impressions”, plays an essential role for complex taste impressions . This becomes clear in the case of severe colds, if one can no longer perceive any taste impressions beyond the basic categories with a blocked nose. In many animal species there is also no separation between taste and smell perception.

Hot ” is qualified as a taste sensation, but strictly speaking it is a pain signal from the nerves in dishes that are seasoned with chilli , for example , then caused by the alkaloid capsaicin .

Taste receptors

The taste qualities bitter, sweet and umami are mediated by G-protein-coupled receptors and the signal transduction is now quite well characterized. The details of the perception of sour and salty, on the other hand, are still largely unclear. The chemical structure of the salty and sour tasting substances suggests that ion channels play a crucial role in perception.

Sweet, bitter and umami

Postulated mechanism of action of sweet substances on the protein receptor : The better the molecule fits, the greater the interaction and the sweetening power , which partially explains the increased values ​​of sweeteners compared to glucose , but is not understood in detail.

A heterodimeric receptor is responsible for the perception of the sweet taste , which is composed of the two G-protein-coupled receptors T1R2 and T1R3 . This heterodimer imparts the sweet taste of all substances that taste sweet to humans, although they have very different molecular structures. The ability to detect a large number of different substances is brought about by the particularly long extracellular N-terminus of the two receptor subunits. Different parts of the N-terminus are required to bind the individual substances. All species of the cat family have a mutation in the T1R2 gene, which is why they have no sweetness perception. The sweet taste can be suppressed with certain substances (for example gymnemic acids , lactisol , Hodulcine , Gurmarin and Ziziphin ).

The umami taste receptor has a very similar structure. It is also a heterodimer, but it is composed of one T1R1 and one T1R3 subunit. It is able to recognize different L - amino acids and shows a high specificity for the amino acids glutamic and aspartic acid in humans. The presence of purine nucleotides, such as inosine monophosphate and guanosine monophosphate , increases the receptor activation and thus the umami taste.

In contrast to the other taste qualities, a large number of receptors are responsible for the perception of the bitter taste. They form the gene family of the T2Rs , which has about 25-30 members in humans. The individual T2R types are - in different combinations - expressed in the same receptor cells . This means that although the individual receptors are sometimes very specific for one or a few bitter substances, mammals cannot differentiate between different bitter substances by taste. Ultimately, all bitter substances activate the same receptor cells and transmit the same information to the brain. Some bitter substances are also able to directly influence signal transduction by inhibiting or activating the enzymes involved . Receptors for bitter substances have also been found on smooth muscle cells of the bronchial system. There their activation causes bronchodilation.

Even if the receptors for sweet, umami and bitter are different, the intracellular signal cascade that they trigger is the same: The heterotrimeric G protein gustducin , which is structurally closely related to the, is bound to the G protein-coupled receptors Transducin is from the rods of the retina. The α-subunit of Gustducin has bound a guanosine diphosphate molecule (GDP) in the resting state . The binding of the flavors to the G-protein-coupled receptors leads to the exchange of the GDP by a guanosine triphosphate (GTP) and to the dissociation of the gustducin into the α-subunit and a βγ-dimer. In the following, the phospholipase C β2 (PLCβ2) is activated , which splits phosphatidylinositol-4,5-bisphosphate (PIP 2 ) in the membrane into the two second messengers inositol trisphosphate (IP 3 ) and diacylglycerol (DAG). By opening IP 3 -controlled calcium channels in the endoplasmic reticulum (ER), IP 3 leads to an increase in the intracellular Ca 2+ concentration. This results in the opening of TRPM5 channels and the depolarization of the taste sensory cell .

Salty and sour

For a long time, the epithelial sodium channel was considered the most important candidate for the salty taste receptor in humans. Today it is known that although rodents play a major role in the perception of salty taste, it only plays a subordinate role in humans. It is assumed that in addition to the cations , such as Na + , the anions of the salts, such as Cl - , also have an influence.

Contrary to long-standing assumptions, the extracellular pH value in the taste receptor cells appears to be less important for the detection of sour taste than the intracellular pH value . This would also explain why organic acids such as acetic acid or citric acid taste significantly more acidic than inorganic acids such as hydrochloric acid at the same pH value . The organic acids in the undissociated state are much less polar than the inorganic acids and are therefore more able to cross the cell membrane . In the cells, they then dissociate into protons and anionic acid residues and thus lower the intracellular pH. The inorganic acids, on the other hand, cannot penetrate the cell membrane without dissociating. Only at correspondingly high concentrations do the protons (or their hydrated forms) created by extracellular dissociation reach the receptor cells via ion channels. Only significantly higher concentrations of inorganic acids in the oral cavity lead to the same lowering of the pH value in the sensory cells. It is assumed that the low pH value leads to changes in the intracellular components of membrane proteins and ultimately to activation of the receptor cells.

Nevertheless, the search for the actual receptor for the “sour” taste quality is slow. After a number of theories had proposed various ion channels and transporters as acid receptors in the last few decades, a particularly interesting candidate was identified in 2006 with the transmembrane protein PKD2L1 (short for "Polycystic kidney disease 2-like 1"). It has been shown that in mice in which the PKD2L1-expressing cells were selectively killed, the corresponding nerves were no longer activated by Sauer stimuli. The other taste qualities were apparently not affected.

Through a series of experiments we now know that every taste cell only contains receptors for a certain taste quality, i.e. that detection takes place separately at the level of the sensory cells. However, a taste bud houses the sensory cells of several qualities. And also in the associated afferent nerves, each fiber codes for more than one taste quality.

Calcium / magnesium ions

The results of investigations at the Monell Chemical Senses Center by Tordoff suggest that there may be a taste quality for calcium / magnesium ions. In these studies, receptors were found on the tongue of mice that respond specifically to calcium / magnesium ions.

Since a strain of mice preferred calcium-containing liquid in the comparison test (presumably because of the taste), its genetic makeup was examined more closely. Two genes were identified that seem to be involved in the formation of calcium / magnesium-specific taste receptors. One of the genes is also involved in the sweet and umami receptor. These two receptors are also built up as heterodimers by combining two different gene products (see above). In addition to the gene TAS1R3 to or for the calcium / magnesium taste in mice CaSR be required. The responsible genes are also present in the human genome, but products of the second gene in humans could only be assigned to structures in the brain and digestive system.

Neural processing

The transfer of information from the (secondary) taste sensory cells to the afferent neurons , which are responsible for transmission to the brain, is still unclear. It is known that taste receptor cells, a number of neurotransmitters and neuropeptides , such as serotonin , norepinephrine , γ-aminobutyric acid , cholecystokinin and neuropeptide Y can distribute. There are also indications that adenosine triphosphate plays an important role in signal transmission from the sensory cell to the nerve cell.

In mammals, the taste information is transmitted to the brain via the three cranial nerves, the facial nerve (VII), the glossopharyngeal nerve (IX) and the vagus nerve (X). There the first interconnection takes place in the rostral part of the nucleus tractus solitarii . From there, the taste information goes on to the nucleus ventralis posteromedialis , pars parvocellularis (VPMpc) of the thalamus . In primates this is done by direct projection, in rodents, on the other hand, there is an intermediate station on the way to the thalamus with the nucleus parabrachialis . The VPMpc of the thalamus, in turn, projects into the islet cortex , which is where the primary gustatory cortex is located. Integration with other sensory impressions, primarily tactile and temperature information from the oral cavity, already takes place here. The secondary gustatory cortex , the next higher station in taste processing, is located in the orbitofrontal cortex and partially overlaps with the secondary olfactory cortex . In addition to the “main route” described here, there are multiple branches at every processing level. These lead to the hypothalamus and the limbic system , among other things . There are also numerous interconnections from higher back to lower levels.

Sensory processing

The complexity of gustatory perception is achieved through a combinatorial system of representations in the brain, which allows a detailed analysis of the subtleties of a sensory impression. This system of our nervous system, the vector coding , can be understood as a representation in a feature space (with six basic tastes a six-dimensional space). A certain taste is represented in this room by an activation pattern of all six receptor types. If the tongue could only distinguish ten intensity levels per basic taste, the total number of distinguishable activation patterns would be 10 6 = 1,000,000. With just six different types of receptors, one could differentiate a million different flavors. A multitude of differentiation and perception possibilities arise from simple basics .

"Taste" as a word in the sense of "smell"

The word “taste” comes from Middle High German smecken , meaning “smell” and “stink”. The German dictionary of the Brothers Grimm indicates a double relationship in Old High German and Middle High German for the meaning of the word taste : B. meaning. The verb refers in the older language to the sensations of smell and taste. the developed nhd. written language has given up the first of the two ways of using it, but this has been preserved in the Upper German dialects, in part even excluding the second. "

Thus, at least in the Alemannic (Baden-Württemberg, Switzerland) and Bavarian (Bavaria, Austria) dialects of German, the term “taste” can occasionally lead to confusion, as the speakers of these dialects mean by “taste” a term that also includes or only includes the meaning 'smell' (“taste through the nose”), in contrast to New High German. An older example of a resulting misunderstanding can be found at the beginning of the second part of the novel Die Günderode by Bettina von Arnim (1840), which tells of a Mr. Arenswald who ate a number of smelly snails that had been touted as snails , "Which taste very good".

Regarding the noun form taste and the semantic field denoted by this word, there are comparable relationships - outside of the technical language use - that make misunderstandings possible. The odor of a food usually contributes significantly to the complex sensory impression when eating, also known as “taste” .

See also


Web links

Wiktionary: taste  - explanations of meanings, word origins, synonyms, translations

Individual evidence

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