What is Gustation? 

Gustation is the perception of the taste or flavors of the food ingested, by the gustatory system. The process of taste occurs when the chemicals present in food or other substances react with the taste receptor cells present in the taste buds. The taste buds are located in the oral cavity, primarily on the tongue. The flavors of food and other substances are determined by the process of taste, along with the olfaction and trigeminal nerve stimulation. Human beings have taste receptor cells on the taste buds, which are located throughout the upper surface of the tongue and also on the epiglottis.  

Tongue and its Structure 

Gustation is associated mainly with the tongue. The major portion of taste determination is performed by the tongue. The tongue, along with other parts of the oral cavity, is covered by a stratified squamous epithelium (flattened epithelial cells present in layers upon a basal membrane). The tongue is covered by thousands of small bumps, known as papillae. The papillae are visible to the naked eye and contain hundreds of taste buds within them. There are three types of papillae, divided based on their appearance. They are: 

• Vallate 
• Foliate 
• Fungiform 

The number of taste buds present on each papilla varies. Each of these taste buds encloses several specialized gustatory receptor cells for the transduction of taste stimuli.  

Gustational Signal Transduction

As soon as the food comes in contact with the taste buds and the molecules of the food interact with the receptor cells, these cells release neurotransmitters onto the dendrites of the sensory neurons. The neurons on the receiving end are a component of the facial and glossopharyngeal nerves and also a part within the valgus nerve dedicated to the gag or pharyngeal reflex (reflex contraction of the back of the throat).The taste buds are connected to these nerves at various positions throughout the tongue. The facial nerve connects to the taste buds at the anterior third of the tongue, the glossopharyngeal nerve connects at the posterior two-thirds of the tongue and the valgus nerve connects at the extreme posterior of the tongue, bordering on the pharynx. 

The taste signals gathered by the three cranial nerves travel to the medulla of the brain. From here, most of the information is carried forward towards the primary gustatory cortex, located near the inferior margin of the post-central gyrus at the lateral parietal lobe of the brain. The primary gustatory cortex is principally responsible for the perception and sensation of taste. Although this may be the only function of the primary gustatory cortex, it is believed that this particular area of the brain also receives information regarding the smell and texture of the food, thereby contributing to the overall experience of taste.  

Types of Primary Taste Sensations 

The five primary taste sensations that are detected by the taste buds are saltiness, sweetness, bitterness, sourness, and savoriness, also known by the Japanese term, “umami”. Though it has always existed, the taste sensation of umami or savoriness is a recent concept, with it being discovered only recently by scientists. The following content provides a detailed account of the various types of basic tastes. 

Sweetness 

The sensation of sweetness is often regarded as a pleasing one and is detected by the gustatory sensors in response to sugars or substances that mimic sugar. Sugars may contain aldehydes and ketones, which incorporate a carbonyl functional group. Sweetness is sensed by an assortment of G-protein coupled receptors (GPCR) attached to the G-protein gustducin found on the taste buds. For the brain to perceive sweetness, at least two different variants of the sweetness receptors must be activated. These receptors are namely T1R2+3 (heterodimer) and T1R3 (homodimer), which are responsible for detecting sweet sensations in humans and animals. The compounds that can bind with these receptors are the ones that the brain counts as sweet. To understand the process that happens further into the taste sensory system, the example of natural sweeteners such as saccharides could be considered. When the saccharides come in contact with the taste buds, they activate GPCR, which releases gustducin. The protein gustducin then activates the enzyme adenylate cyclase (regulatory enzyme present in cells), which catalyzes the production of adenosine 3’, 5’- cyclic monophosphate or cAMP. The cAMP closes the potassium ion channels in the receptor cells, which causes depolarization and the release of neurotransmitters. Another example is synthetic sweeteners such as saccharin, which activates different GCPR and stimulates depolarization and neurotransmitter release using a different pathway. 

Saltiness 

Salt is nothing but chemically sodium chloride. The simplest taste receptors found in the oral cavity are that of salt receptors. Saltiness is a taste that occurs mainly due to the presence of sodium ions. It is a characteristic of the metals present in the alkali group. However, not all of the metals are as salty as sodium, and the farther from it, the less salty the sensation is. The taste receptor cells contain a sodium channel that allows the sodium cations to enter. The influx of sodium ions results in the depolarization of the cell and opens up voltage-dependent calcium channels, thereby creating a flood of positive calcium ions into the cell. As a result, the cell releases a neurotransmitter that activates the neurons associated with the salty taste. 

Bitterness  

Bitterness could be considered as one of the most sensitive of tastes and to many, it represents unpleasantness or distaste. The portion of the tongue present in the extreme posterior area contains sensors for bitterness. These sensors contain a type of receptor known as TAS2R (taste receptor type 2, also known as T2R) such as TAS2R38, which coupled with the G protein, gustducin, are responsible for the perception of a bitter taste. These receptors can be identified not just by their ability to detect bitterness but also by their morphology. The TAS2R family of receptors in humans is thought to include about twenty-five different taste receptors, few of which can distinguish a wide variety of bitter-tasting compounds. 

The protein gustducin comprises three parts, which when activated by the GPCR, break apart and further activates an enzyme known as phosphodiesterase. The enzyme then converts a precursor within the cell into a secondary messenger that seals potassium ion channels of the cell. The precursor also stimulates the endoplasmic reticulum of the cells to release positive calcium ions, which leads to depolarization and the release of neurotransmitters.  

Umami (savory) 

Umami is a taste sensation that adds flavor to any food such as cheese, soy sauce. The taste receptor cells that respond to umami interact specifically with glutamate, an important composition causing savoriness. This happens in a way similar to how sweet receptors respond to sugar. Glutamate binds to a variant of G protein-coupled glutamate receptors. A type of glutamate known as L-glutamate may bond with metabotropic glutamate receptor, a type of GPCR. The binding causes the G protein complex to activate the sensation of umami.  

In addition to the basic tastes, the tongue can also perceive different sensations, which are largely processed by the somatosensory system of the brain. These extra sensations may include pungency or spiciness, coolness, numbness, astringency. 

Gustatory Receptors and Biosensors: an Analogy 

The gustatory receptor cells of the oral cavity function in a way that is similar to that of a biosensor. Biosensors are analytical devices that allow the performance of chemical analysis of substances, through a combination of biological recognition elements and physicochemical transducer. The biosensors called electronic tongues are man-made tools that are designed based on the science behind the working of a taste receptor. They are chiefly used in the food industry, to analyze the quality of food products. 

Pitfalls and Common Mistakes 

The gustatory receptors consist of very specific enzymes (phosphodiesterase, adenylate cyclase) and proteins (G proteins) that are responsible for taste sensations exclusive to their functioning. It may be possible for the students to get confused with the taste sensation and its appropriate enzymes or proteins. Hence, it is important to concentrate and learn the names properly. 

Context and Applications 

This topic is significant in the professional exams for both Bachelors and Master courses related to biology. Some of the courses are listed below:  

• Bachelors in Anatomy and Physiology 
• Bachelors in Biochemistry  
• Bachelors in Zoology 
• Masters in Anatomy and Physiology