Living beings sense various activities happening around them. They receive stimuli from the environment in and around them. This receiving of the stimulations is termed as ‘sensing‘. The senses are of two different types: General senses and Special senses.
• General senses:
The general senses do not have special structures to carry out the functions. These senses have their receptors spread throughout the body, usually associated with skin, muscles, joints and viscera.
• Special senses:
The special senses have special organs housing the sensory receptors. The special senses are taste, vision, hearing, (& equilibrium) and smell.
In this post, we discuss about the special sense ‘Taste’.
The sense ‘taste’ in itself involves 5 basic stimuli namely- sweet, sour, salty, bitter and savoury. The mix of these five basic stimuli give the complex foods their respective tastes. It is, however, also affected by other senses, like smell, texture (touch) and temperature.
The organ used for the sense taste is the ‘tongue‘. Tongue is a muscular organ present in the mouth. The outer layer is moist and pink tissue called mucosa. Tongue has many tiny structures called papillae.
The papillae are the structures which house the taste buds (except filliform papillae). They are spread throughout the surface of the tongue. There are 4 different types of papillae in humans, they are;
1. Foliate papillae:
As the name suggests, these papillae look like leaf (edge on). These are present in groups on each side of the tongue. These papillae have taste buds scattered over the surface. The taste buds are present on the lateral wall of the foliate papillae (see fig 2).
2. Vallate papillae:
Vallate literally means been bordered by a raised edge, and that’s how is the structure of this type of papillae. These papillae are round in shape and are surrounded by a thick wall. They are present in set of seven to twelve papillae near the back of the tongue, arranged in a V-shaped formation (see fig 2). Large numbers of taste buds are present on the walls of the round structure. Secretory gland cells are present at the base of this papillae which produce watery mucus at the base of the wall that envelops the taste buds. These taste buds are sensitive to bitter flavors.
3. Filiform / Conical papillae:
This type of papillae is present in higher number than other papillae. They are arranged in a slightly regular rows running parallel to the central groove of the tongue. These papillae are either simple conical in shape or may have frilled tips (fig 2). These papillae do not have taste buds but give the tongue a rough texture. This roughness allows the tongue to act as an abrasive layer and help in cleaning and rasping (licking of bodies by the animals, licking of ice cream). This function of tongue when complemented by the antibacterial action saliva helps animals nurse their wounds.
4. Fungiform papillae:
These papillae are mushroom-shaped (the fungus, hence the name) and are specifically concentrated at the tip and along the sides. The taste buds are present all over the surface of fungiform papillae (see fig 2). These are sensitive to sweet and sour tastes.
~ Taste buds
- The taste buds are found on tongue and also in the palate, pharynx and upper portion of the oesophagus. They are embedded in the epithelial layer of the tongue along the walls of the papillae.
A taste bud is a bud-shaped structure, with a narrow apical opening called taste pore facing on the outer side of the papillae. On the opposite interior side are the afferent nerve fibres transmitting the information to the brain.
The solutes in the oral cavity enters the taste bud through the taste pore. Each taste bud contains around 50 to 100 taste cells and basal cells. The microvilli from the taste cells extend into the taste pore, which contain the taste cell receptors. There is a great amount of lateral connection between the taste cells within a bud, through electrical as well as chemical synapses.
The taste buds transmit the information about the taste compounds (in oral cavity) to the nerves (and then the brain).
• Taste cells:
Taste cells are electrically excitable and capable of generating action potentials due to the presence of the voltage-dependent channels for Na+, Ca2+ and K+. The difference in distribution of these ion channels on the surface of the cells may reflect differences in their functional activities. Sweet, bitter, and savory (umami) work with a signal through a G protein-coupled receptor. Salty and sour work with ion channels.
Taste cells are of three major types: Type I, Type II (receptor), and Type III (presynaptic) taste cells.
- – Type I cells/ Glial-like cells
- These cells degrade or absorb neurotransmitters. Basically they clear extracellular K+ accumulated after the action potentials in receptor and presynaptic cells. Hence these support the functioning of the receptor cells like glial cells in the nervous system.
- – Type II/ Receptor cells
- These cells have receptors that bind sweet, bitter or umami compounds. The receptors are G protein–coupled receptors with seven transmembrane domains. The membranes of these cells express voltage-gated Na- and K- channels essential for producing action potentials. Hence these cells respond to the different taste compounds in the oral cavity and get excited.
- – Type III/ Presynaptic) cells
- These cells form the synaptic junctions with nerve terminals. They are excitable and express voltage-gated Na- and K- channels to support action potentials and release neurotransmitters. They receive signals generated by receptor cells and transmit the information to the sensory afferent nerve fibers, thus acting as a link between the receptor cells and nerve fibers. These cells respond broadly to sweet, bitter, salty, sour, and umami compounds
• Basal cells
The basal cells in the taste bud are spherical or ovoid and do not extend processes into the taste pore. They are thought to be undifferentiated or immature taste cells. Exact significance of basal cells are still to be discovered.
~ Mechanism of detecting different gustatory (taste) information:
Taste receptor cells utilize several molecular mechanisms to detect and distinguish among these compounds. Two different models have been proposed to account for information coding in the gustatory system: labeled line and across-fiber pattern coding model.
1. Labeled-line model:
- This model assumes that one taste receptor cell responds to single taste quality and then sends information by separate afferent pathways to the gustatory cortex via the medulla and the thalamus.
- (One cell: one taste)
- 2. Across-fiber pattern-coding model: This model assumes that individual taste cells respond to different taste qualities. The information is transmitted to the brain by afferent fibers that have broadly overlapping response spectral. Thus, particular quality is determined by the pattern of activity across all of the afferent nerve fibers, rather than by activity in any single nerve fiber.
- (One cell: Different tastes: pattern is determined)
Since the mechanism is still unclear, it is suggested that the gustatory information coding may utilize either of the two or both the mechanisms.
~ Perception of taste by the brain:
The taste cells synapse with the sensory afferents within three cranial nerves innervate the taste buds. These three cranial nerves are the cranial nerves VII (facial nerve), IX (glossopharyngeal nerve) and X (vagus).
These cranial nerve then synapse in medulla (solitary tract). The axons from medulla terminate in the ventral posterior medial nucleus (VPMpc) of thalamus. This nucleus projects to the gustatory cortex.
The gustatory cortex is the region where the taste is perceived.
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Fig 1 & 4: Gravina, Stephen & L Yep, Gregory & Khan, Mehmood. (2013). Human Biology of Taste. Annals of Saudi medicine. 33. 217-22. 10.5144/0256-4947.2013.217.
Fig 3: modified from Anthony L. Mescher: Junqueira’s Basic Histology, 14th Edition. http://www.accessmedicine.com:
Fig 5: Sharvit, Gil. (2016). Expectancy Effects of Pain and Disgust in Perceptual and Moral Decisions. 10.13097/archive-ouverte/unige:91624.
Fig 6: https://www.alliedacademies.org/articles/ageusia-hyposmia-dizziness-and-tinnitus-as-presenting-symptoms-of-multiple-sclerosis-11060.html)