Sensory Systems/Gustatory System

Tongue and Taste Buds
Taste cells are epithelial and are clustered in taste buds located in the tongue, soft palate, epiglottis, pharynx and the esophagus the tongue being the primary organ of the Gustatory System. Taste buds are located in papillae along the surface of the tongue. There are three types of papillae in human: fungiform located in the anterior part containing approximately five taste buds, circumvallate papillae which are bigger and more posterior than the previous ones and the foliate papillae that are in the posterior edge of the tongue. Circumvallate and foliate papillae contain hundreds of taste buds. In each taste bud there are different types of cells: basal, dark, intermediate and light cells. Basal cells are believed to be the stem cells that give rise to the other types. It is thought that the rest of the cells correspond to different stages of differentiation where the light cells are the most mature type of cells. An alternative idea is that dark, intermediate and light cells correspond to different cellular lineages. Taste cells are short lived and are continuously regenerated. They contain a taste pore at the surface of the epithelium where they extend microvilli, the site where sensory transduction takes place. Taste cells are innervated by fibers of primary gustatory neurons. They contact sensory fibers and these connections resemble chemical synapses, they are excitable with voltage-gated channels: K+, Na+ and Ca+ channels capable of generating action potentials. Although the reaction from different tastants varies, in general tastants interact with receptors or ion channels in the membrane of a taste cells. These interactions depolarize the cell directly or via second messengers and in this way the receptor potential generates action potentials within the taste cells, which lead to Ca2+ influx through Ca2+ voltage-gated channels followed by the release of neurotransmitters at the synapses with the sensory fibers.

Tongue map
The idea that the tongue is most sensitive to certain tastes in different regions was a long time misconception, which has now been proved to be wrong. All sensations come from all regions of the tongue.

Supertasters
An average person has about 5'000 taste buds. A "supertaster" is a person whose sense of taste is significantly more sensitive than average. The increase in the response is thought to be because they have more than 20’000 taste buds, or due to an increased number of fungiform papillae.

Transduction of Taste
As mentioned before we distinguish between 5 types of basic tastes: bitter, salty, sour, sweet and umami. There is one type of taste receptor for each flavor known and each type of taste stimulus is transduced by a different mechanisms. In general bitter, sweet and umami are detected by G protein-coupled receptors and salty and sour are detected via ion channels.

Bitter
Bitter compounds act through G protein coupled receptors (GPCR’s) also known as a seven-transmembrane domains, which are located in the walls of the taste cells. Taste receptors of type 2 (T2Rs) which is a group of GPCR’s is thought respond to bitter stimuli. When the bitter-tasting ligand binds to the GPCR it releases the G protein gustducin, its 3 subunits break apart and activate phosphodiesterase, which in turn converts a precursor within the cell into a secondary messenger, closing the K+ channels. This secondary messenger stimulates the release of Ca2+, contributing to depolarization followed by neurotransmitter release. It is possible that bitter substances that are permeable to the membrane are sensed by mechanisms not involving G proteins.

Salt
The amiloride-sensitive epithelial sodium channel (ENaC), a type of ion channel in the taste cell wall, allows Na+ ions to enter the cell down an electrochemical gradient, altering the membrane potential of the taste cells by depolarizing the cell. This leads to an opening of voltage-gated Ca2+ channels, followed by neurotransmitter release.

Sour
The sour taste signals the presence of acidic compounds (H+ ions) and there are three receptors: 1) The ENaC, (the same protein involved in salty taste). 2) There are also H+ gated channels; one is the K+ channel, which allows K+ outflux of the cell. H+ ions block these so the K+ stays inside the cell. 3) A third channel undergoes a configuration change when a H+ attaches to it leading to an opening of the channel and allowing an influx of Na+ down the concentration gradient into the cell, leading to the opening of a voltage gated Ca2+ channels. These three receptors work in parallel and lead to depolarization of the cell followed by neurotransmitter release.

Sweet
Sweet transduction is mediated by the binding of a sweet tastant to GPCR’s located in the apical membrane of the taste cell. Saccharide activates the GPCR, which releases gustducin and this in turn activates cAMP (cyclic adenylate monophosphate). cAMP will activate the cAMP kinase that will phosphorylate the K+ channels and eventually inactivate them, leading to depolarization of the cell and followed by neurotransmitter release.

Umami (Savory)
Umami receptors involve also GPCR’s, the same way as bitter and sweet receptors. Glutamate binds a type of the metabotropic glutamate receptor mGlurR4 causing a G-protein complex to activate a secondary receptor, which ultimately leads to neurotransmitter release. In particular how the intermediate steps work, is currently unknown.



Signal Processing
In humans, the sense of taste is transmitted to the brain via three cranial nerves. The VII facial nerve carries information from the anterior 2/3 part of the tongue and soft palate. The IX nerve or glossopharyngeal nerve carries taste sensations from the posterior 1/3 part of the tongue and the X nerve or vagus nerve carries information from the back of the oral cavity and the epiglottis.

The gustatory cortex is the brain structure responsible for the perception of taste. It consists of the anterior insula on the insular lobe and the frontal operculum on the inferior frontal gyrus of the frontal lobe. Neurons in the gustatory cortex respond to the five main tastes.

Taste cells synapse with primary sensory axons of the mentioned cranial nerves. The central axons of these neurons in the respective cranial nerve ganglia project to rostral and lateral regions of the nucleus of the solitary tract in the medulla. Axons from the rostral (gustatory) part of the solitary nucleus project to the ventral posterior complex of the thalamus, where they terminate in the medial half of the ventral posterior medial nucleus. This nucleus projects to several regions of the neocortex, which include the gustatory cortex.

Gustatory cortex neurons exhibit complex responses to changes in concentration of tastant. For one tastant, the same neuron might increase its firing and for an other tastant, it may only respond to an intermediate concentration.

Taste and Other Senses
In general the Gustatory Systems does not work alone. While eating, consistency and texture are sensed by the mechanoreceptors from the somatosensory system. The sense of taste is also correlated with the olfactory system because if we lack the sense of smell it makes it difficult to distinguish the flavor.

Spicy food
(black peppers, chili peppers, etc.)

It is not a basic taste because this sensation does not arise from taste buds. Capsaicin is the active ingredient in spicy food and causes “hotness” or “spiciness” when eaten. It stimulates temperature fibers and also nociceptors (pain) in the tongue. In the nociceptors it stimulates the release of substance P, which causes vasodilatation and release of histamine causing hiperalgesia (increased sensitivity to pain).

In general basic tastes can be appetitive or aversive depending on the effect that the food has on us but also essential to the taste experience are the presentation of food, color, texture, smell, previous experiences, expectations, temperature and satiety.

Ageusia (complete loss of taste)
Ageusia is a partial or complete loss in the sense of taste and sometimes it can be accompanied by the loss of smell.

Dysgeusia (abnormal taste)
Is an alteration in the perception associated with the sense of taste. Tastes of food and drinks vary radically and sometimes the taste is perceived as repulsive. The causes of dysgeusia can be associated with neurologic disorders.