Biology of sensory perception
3 Gustation: the Sense of Taste
Gustation is the chemical sense that helps with the perception of taste. It begins when chemical compounds (tastants), like sugar or salt, dissolve in saliva and activate taste receptors. Saliva and small bumps on the tongue, called filiform papillae, help spread these tastants so they can reach the taste receptors. These receptors are found in taste buds, which are small groups of cells located inside larger bumps on the tongue called papillae. These papillae can be seen on the surface of the tongue.
Five basic tastes
There are five basic tastes: bitter, sweet, salty, sour, bitter, and savoury. Savoury is also known at the fifth taste, called umami.
Bitter, sweet, savoury compounds activate G-protein-coupled receptors. See Table 3 for the different receptors and receptor families involved in taste.
Many different molecules can activate bitter taste receptors, and more than 30 types of bitter receptors have been discovered.
Chemicals that are perceived as sweet, such as sweeteners or sugars, usually have hydroxyl and carbonyl groups in their chemical structure. Also sugar alcohols are perceived as being sweet. Some of them also stimulate bitter receptors.
Savoury receptors are mostly stimulated by the unbound amino acid glutamate. Monosodium glutamate (MSG) is the most common compound known. Addition of MSG to foods enhances the savoury flavour of food by a salty taste, as it also has a sodium molecule.
The perception of salty chemicals is caused by salt ions upon dissolution. A neural response is produced by depolarising the membrane. The most known salty tastant is NaCl (table salt).
Sour taste is perceived due to the release of hydrogen ions (H+) from acidic tastants dissolved in saliva. In contrast to salty taste, the taste is perceived by depolarising the membrane directly via ion channel changes.
Table 3. The 5 basic tastes, examples of tastants, mechanisms of actions and the year of receptor discovery.
| Taste | Tastant | Receptor | Year of receptor discovery |
| Sweet | Sucrose, fructose, mono-, di- and oligosaccharides, sugar alcohols | T1R2, T1R3 | 2001 |
| Sour | acids (H+), e.g. HCl, acetic acid, citric acid | PKD2L1 | 2006 |
| Salt | Most inorganic salts: NaCl, KCl etc. | ENaC | 2010 |
| Umami | monosodium glutamate (MSG), other glutamates, nucleotides (IMP, GMP) | T1R1, T1R3 | 2001 |
| Bitter | caffeine, quinine, saccharine | T2R | 2000 |
Source: Calvo, S. S. C., & Egan, J. M. (2015). The endocrinology of taste receptors. [1].
Is there a sixth basic taste?
In 2015 a new taste receptor was discovered. This receptor, CD36 was discovered as the fat-taste receptor. CD36 gives a neural response when free fatty acids bind to it. However, at this moment fat has not been fully accepted as the sixth basic taste, because it is still unclear whether free fatty acids can solely give a neural response or if more sensory sensations such as aroma or texture are needed to perceive a fatty taste [2].
The Tongue and Taste Buds
When eating, the tastants in the food dissolve in saliva and bind to the taste receptors in the taste buds. These taste buds are nested in taste papillae. These papillae are small bumps on the tongue, that can be seen with the naked eye. Figure 1 shows an illustration of the anatomy of the tongue.
Inside each papilla are taste buds, which are small groups of 30 to 100 taste receptor cells. These cells are also called gustatory receptor cells.
It is a myth that distinct tastes can only be tasted at specific areas of the tongue. For example, sweet on the tip of the tongue and bitter at the back. This “myth of the tongue” is not true. It has been shown that all five basic tastes can be perceived all across the tongue. However, the density of the different taste receptors differs per type of papilla, explaining why specific tastes can be better recognised at certain places on the tongue.
TASTE PAPILAE
There are four types of taste papillae, out of which three have taste buds with taste receptors:
- Circumvallate papillae – These are the biggest papillae. They are found at the back of the tongue in a V-shape. Each one has over 100 taste buds.
- Foliate papillae – These look like small folds on the sides of the tongue. Each one also has over 100 taste buds.
- Fungiform papillae – These are shaped like mushrooms and are found on the front part of the tongue, but not much in the middle. They are the most common type and have between 1 and 8 taste buds each.
- Filiform papillae – These do not have taste buds, and help spread taste molecules across the tongue.
Figure 1. Anatomy of the tongue visually depicted. Bottom right zoomed in on one taste bud. From: “OpenStax AnatPhys fig.14.3 – The Tongue – English labels” by OpenStax and Regents of U-M Medical School, UMich MedSchool [3]. License: CC BY.
GUSTATORY RECEPTOR CELLS
Gustatory receptor cells have tiny, finger-like parts called gustatory hairs that reach into a small opening called the taste pore. In this area, many of the cells have special receptors that can detect different tastants. The number of taste buds varies among individuals, and it can range from about 2,000 to 10,000.
Gustatory receptor cells have a lifespan of 10 to 14 days and are always being replaced. So, every 14 days all taste cells are renewed. It also means that each taste bud contains cells at different stages of growth and development.
Neural processing of taste
When a tastant binds to its specific taste receptor, it triggers a neural signal. The vagus nerve then carries this taste information to a part of the brainstem called the medulla. From there, the information goes to the thalamus, and then it is sent to the primary gustatory cortex.
The gustatory cortex also gets other types of information from the tongue, like temperature and touch. When all these signals are combined, a hedonic value is assigned to the sensory inputs; the taste can be perceived as a pleasant, neutral or unpleasant sensation.
- Calvo, S. S. C., & Egan, J. M. (2015). The endocrinology of taste receptors. Nature Reviews Endocrinology, 11(4), 213-227 ↵
- Hichami, A., Khan, A. S., & Khan, N. A. (2021). Cellular and molecular mechanisms of fat taste perception. In The Pharmacology of Taste (pp. 247-270). Cham: Springer International Publishing. ↵
- 'Anatomy and Physiology', https://openstax.org/details/books/anatomy-and-physiology.] ↵
The act or sensation of tasting
chemical compounds that give taste to food. For example: sucrose, caffeine, MSG, citric acid, NaCl etc.
A nerve ending or other structure in the body, specialized to sense or receive stimuli and to transmit them.
The smallest and most common papillae on the tongue.
Clusters of taste receptor cells, also known as gustatory cells, that allow us to perceive taste.
Structures on the surface of the tongue
The largest papillae located at the back of your tongue. They contain hundreds of taste buds.
Large papillae on the side of the tongue.
The fungiform papillae are one of the three types of papillae involved in gustation, which is the detection of the different tastes.
The gustatory sensory system contributes to the flavor of foods and beverages and communicates information about taste and nutrients.
Small openings in the tongue epithelium that allow tastants to bind on the taste receptor cells.
Also known as Nervus Vagus. One of the cranial nerves that runs from the brainstem to the chest and abdomen. It is responsible for regulating involuntary body functions such as digestion, heart rate and breathing, playing a role in the "rest and digest" response.
The medulla oblongata plays a critical role in transmitting signals between the spinal cord and the higher parts of the brain and in controlling autonomic activities, such as heartbeat and respiration.
The thalamus is the body's information relay station. All information from the senses (except smell) must be processed through the thalamus before being sent to the cerebral cortex for interpretation.
The gustatory cortex, or primary gustatory cortex, is a region of the cerebral cortex responsible for the perception of taste and flavor.
hedonic testing refers to the subjective evaluation of a sensation, for example, liking or palatability.