Gustatory system
Encyclopedia
The gustatory system is the sensory system
Sensory system
A sensory system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory receptors, neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, somatic...

 for the sense of taste
Taste
Taste is one of the traditional five senses. It refers to the ability to detect the flavor of substances such as food, certain minerals, and poisons, etc....

.

Importance

The gustatory system allows humans to distinguish between safe and harmful food. Bitter and sour foods we find unpleasant, while salty, sweet, and meaty tasting foods generally provide a pleasurable sensation. The five specific tastes received by gustatory receptors are salty, sweet, bitter, sour, and umami
Umami
Umami , popularly referred to as savoriness, is one of the five basic tastes together with sweet, sour, bitter, and salty.-Etymology:Umami is a loanword from the Japanese meaning "pleasant savory taste". This particular writing was chosen by Professor Kikunae Ikeda from umai "delicious" and mi ...

, which means “savory” or “meaty” in Japanese.

According to Lindemann, both salt and sour taste mechanisms detect, in different ways, the presence of sodium chloride in the mouth. The detection of salt is important to many organisms, but specifically mammals, as it serves a critical role in ion and water homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...

 in the body. It is specifically needed in the mammalian kidney
Kidney
The kidneys, organs with several functions, serve essential regulatory roles in most animals, including vertebrates and some invertebrates. They are essential in the urinary system and also serve homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and...

 as an osmotically active compound which facilitates passive re-uptake of water into the blood. Because of this, salt elicits a pleasant taste in most humans.

Sour taste can be mildly pleasant in small quantities, as it is linked to the salt flavour, but in larger quantities it becomes more and more unpleasant to taste. This is because the sour taste can signal over-ripe fruit, rotten meat, and other spoiled foods, which can be dangerous to the body because of bacteria which grow in such mediums. As well, sour taste signals acids ( ions), which can cause serious tissue damage.

The bitter taste is almost completely unpleasant to humans. This is because many nitrogenous organic molecules which have a pharmacological effect on humans taste bitter. These include caffeine
Caffeine
Caffeine is a bitter, white crystalline xanthine alkaloid that acts as a stimulant drug. Caffeine is found in varying quantities in the seeds, leaves, and fruit of some plants, where it acts as a natural pesticide that paralyzes and kills certain insects feeding on the plants...

, nicotine
Nicotine
Nicotine is an alkaloid found in the nightshade family of plants that constitutes approximately 0.6–3.0% of the dry weight of tobacco, with biosynthesis taking place in the roots and accumulation occurring in the leaves...

, and strychnine
Strychnine
Strychnine is a highly toxic , colorless crystalline alkaloid used as a pesticide, particularly for killing small vertebrates such as birds and rodents. Strychnine causes muscular convulsions and eventually death through asphyxia or sheer exhaustion...

, which respectively compose the stimulant in coffee
Coffee
Coffee is a brewed beverage with a dark,init brooo acidic flavor prepared from the roasted seeds of the coffee plant, colloquially called coffee beans. The beans are found in coffee cherries, which grow on trees cultivated in over 70 countries, primarily in equatorial Latin America, Southeast Asia,...

, addictive agent in cigarettes, and active compound in many pesticides. It appears that some psychological process allows humans to overcome their innate aversion to bitter taste, as caffeinated drinks are widely consumed and enjoyed around the world. It is also interesting to note that many common medicines have a bitter taste if chewed; the gustatory system apparently interprets these compounds as poisons. In this manner, the unpleasant reaction to the bitter taste is a last-line warning system before the compound is ingested and can do damage.

Sweet taste signals the presence of carbohydrates in solution. Since carbohydrates have a very high calorie count (saccharides have many bonds, therefore much energy), they are desirable to the human body, which has evolved to seek out the highest calorie intake foods, as the human body in the distant past has never known when its next meal will occur. They are used as direct energy (sugars) and storage of energy (glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...

). However, there are many non-carbohydrate molecules that trigger a sweet response, leading to the development of many artificial sweeteners, including saccharin
Saccharin
Saccharin is an artificial sweetener. The basic substance, benzoic sulfilimine, has effectively no food energy and is much sweeter than sucrose, but has a bitter or metallic aftertaste, especially at high concentrations...

, sucralose
Sucralose
Sucralose is an artificial sweetener. The majority of ingested sucralose is not broken down by the body and therefore it is non-caloric. In the European Union, it is also known under the E number E955. Sucralose is approximately 600 times as sweet as sucrose , twice as sweet as saccharin, and 3.3...

, and aspartame
Aspartame
Aspartame is an artificial, non-saccharide sweetener used as a sugar substitute in some foods and beverages. In the European Union, it is codified as E951. Aspartame is a methyl ester of the aspartic acid/phenylalanine dipeptide. It was first sold under the brand name NutraSweet; since 2009 it...

. It is still unclear how these substances activate the sweet receptors and what evolutionary significance this has.

The umami
Umami
Umami , popularly referred to as savoriness, is one of the five basic tastes together with sweet, sour, bitter, and salty.-Etymology:Umami is a loanword from the Japanese meaning "pleasant savory taste". This particular writing was chosen by Professor Kikunae Ikeda from umai "delicious" and mi ...

 taste, which signals the presence of the amino acid
Amino acid
Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 L-glutamate, triggers a pleasurable response and thus encourages the intake of peptides and proteins. The amino acids in proteins are used in the body to build muscles and organs, transport molecules (hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...

), antibodies, and the organic catalysts known as enzymes. These are all critical molecules, and as such it is important to have a steady supply of amino acids, hence the pleasurable response to their presence in the mouth.

Function

In the human body a stimulus
Stimulus (physiology)
In physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity....

 refers to a form of energy which elicits a physiological or psychological action or response. Sensory receptors are the structures in the body which change the stimulus from one form of energy to another. This can mean changing the presence of a chemical, sound wave, source of heat, or touch to the skin into an electrical action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

 which can be understood by the brain, the body’s control center. Sensory receptors are modified ends of sensory neurons; modified to deal with specific types of stimulus, thus there are many different types of sensory receptors in the body. The neuron is the primary component of the nervous system, which transmits messages from sensory receptors all over the body.

Taste as a form of Chemoreception

Taste is a form of chemoreception which occurs in specialized receptors in the mouth. These receptors are known as taste cells, and they are contained in bundles called taste buds, which are contained in raised areas known as papillae that are found across the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

. To date, there are five different types of taste receptors known: salt, sweet, sour, bitter, and umami. Each receptor has a different manner of sensory transduction: that is, detecting the presence of a certain compound and starting an action potential which ultimately alerts the brain. It is a matter of debate whether each taste cell is tuned to one specific tastant or to several; Smith and Margolskee claim that “gustatory neurons typically respond to more than one kind of stimulus, [a]lthough each neuron responds most strongly to one tastant” (35). Researchers believe that the brain interprets complex tastes by examining patterns from a large set of neuron responses. This enables the body to make “keep or spit out” decisions when there is more than one tastant present. “No single neuron type alone is capable of discriminating among stimuli or different qualities, because a given cell can respond the same way to disparate stimuli” (39). As well, serotonin
Serotonin
Serotonin or 5-hydroxytryptamine is a monoamine neurotransmitter. Biochemically derived from tryptophan, serotonin is primarily found in the gastrointestinal tract, platelets, and in the central nervous system of animals including humans...

 is thought to act as an intermediary hormone which communicates with taste cells within a taste bud, mediating the signals being sent to the brain. With that in mind, specific types of taste receptors will now be discussed.
Receptor molecules are found on the apical (on top) microvilli of the taste cells.

Salt

Arguably the simplest receptor found in the mouth is the salt (NaCl) receptor. An ion channel
Ion channel
Ion channels are pore-forming proteins that help establish and control the small voltage gradient across the plasma membrane of cells by allowing the flow of ions down their electrochemical gradient. They are present in the membranes that surround all biological cells...

 in the taste cell wall allows ions to enter the cell. This on its own depolarizes the cell, and opens voltage-regulated gates, flooding the cell with ions and leading to neurotransmitter
Neurotransmitter
Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 release. This sodium channel is known as ENaC and is composed of three subunits. ENaC can be blocked by the drug amiloride
Amiloride
Amiloride is a potassium-sparing diuretic, first approved for use in 1967 , used in the management of hypertension and congestive heart failure. Amiloride was also tested as treatment of cystic fibrosis, but it was revealed inefficient in vivo due to it's short time of action, therefore...

 in many mammals, especially rats. The sensitivity of the salt taste to amiloride in humans, however, is much less pronounced, leading to conjecture that there may be additional receptor proteins besides ENaC that may not have been discovered yet.

Sour

Sour taste signals the presence of acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...

ic compounds ( ions in solution). There are three different receptor proteins at work in sour taste. The first is a simple ion channel which allows hydrogen ions to flow directly into the cell. The protein for this is ENaC, the same protein involved in the distinction of salt taste (this implies a relationship between salt and sour receptors and could explain why salty taste is reduced when a sour taste is present). There are also gated channels present. The first is a channel, which ordinarily allows ions to escape from the cell. ions block these, trapping the potassium ions inside the cell (this receptor is classified as MDEG1 of the EnAC/Deg Family). A third protein opens to ions when a hydrogen ion attaches to it, allowing the sodium ions to flow down the concentration gradient into the cell. The influx of ions leads to the opening of a voltage regulated gate. These receptors work together and lead to depolarization of the cell and neurotransmitter release.

Bitter

There are many different classes of bitter compounds which can be chemically very different. It is interesting that the human body has evolved a very sophisticated sense for bitter substances: we can distinguish between the many radically different compounds which produce a generally “bitter” response. This may be because the sense of bitter taste is so important to survival, as ingesting a bitter compound may lead to injury or death. Bitter compounds act through structures in the taste cell walls called G protein-coupled receptors (GPCR’s). Recently, a new group of GPCR’s was discovered, known as the T2R’s, which it is thought respond to only bitter stimuli. When the bitter compound activates the GPCR, it in turn releases gustducin
Gustducin
Gustducin is a G protein associated with basic taste and the gustatory system. Due to its relatively recent discovery and isolation, not all is known about its nature and its associated pathways. It is known that it plays a large role in the transduction of bitter, sweet and umami stimuli and...

, the G-protein it was coupled to. Gustducin is made of three subunits. When it is activated by the GPCR, its subunits break apart and activate phosphodiesterase, a nearby enzyme, which in turn converts a precursor within the cell into a secondary messenger, which closes potassium ion channels. As well, this secondary messenger can stimulate the endoplasmic reticulum
Endoplasmic reticulum
The endoplasmic reticulum is an organelle of cells in eukaryotic organisms that forms an interconnected network of tubules, vesicles, and cisternae...

 to release , which contributes to depolarization. This leads to a build-up of potassium ions in the cell, depolarization, and neurotransmitter release. It is also possible for some bitter tastants to interact directly with the G protein, because of a structural similarity to the relevant GPCR.

Sweet

Like bitter tastes, sweet taste transduction involves GPCR’s. The specific mechanism depends on the specific molecule. “Natural” sweeteners such as saccharides activate the GPCR, which releases gustducin. The gustducin then activates the molecule adenylate cyclase, which is already inside of the molecule cAMP, or adenosine 3', 5'-cyclic monophosphate. This protein will either directly or indirectly close potassium ion channels, leading to depolarization and neurotransmitter release. Synthetic sweeteners such as saccharin activate different GPCR’s, initiating a similar process of protein transitions, starting with the protein Kinase A(PKA), which ultimately leads to the blocking of potassium ion channels.

Umami (Savory)

It is thought that umami receptors act much the same way as bitter and sweet receptors (they involve GPCR’s), but not much is known about their specific function. It is thought that the amino acid L-glutamate bonds to a type of GPCR known as a metabotropic glutamate receptor (mGluR4
Metabotropic glutamate receptor 4
Metabotropic glutamate receptor 4 is a protein that in humans is encoded by the GRM4 gene.Together with GRM6, GRM7 and GRM8 it belongs to group III of the metabotropic glutamate receptor family. Group III receptors are linked to the inhibition of the cyclic AMP cascade.Activation of GRM4 has...

). This causes the G-protein complex to activate a secondary receptor, which ultimately leads to neurotransmitter release. The intermediate steps are not known.

Transmission to Brain

In humans, the sense of taste is conveyed via three of the twelve cranial nerves. The facial nerve
Facial nerve
The facial nerve is the seventh of twelve paired cranial nerves. It emerges from the brainstem between the pons and the medulla, and controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity...

 (VII) carries taste sensations from the anterior two thirds of the tongue
Tongue
The tongue is a muscular hydrostat on the floors of the mouths of most vertebrates which manipulates food for mastication. It is the primary organ of taste , as much of the upper surface of the tongue is covered in papillae and taste buds. It is sensitive and kept moist by saliva, and is richly...

, the glossopharyngeal nerve
Glossopharyngeal nerve
The glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...

 (IX) carries taste sensations from the posterior one third of the tongue while a branch of the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...

(X) carries some taste sensations from the back of the oral cavity.
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