What Are Taste Buds?
The taste system of humans is generally comprised of taste buds situated within the oropharynx (the middle part of the throat behind the mouth) and larynx (a tube that connects the throat pharynx) to the rest of the respiratory system). Taste buds appear like goblets that span the length of the epithelial surface where they are located. In addition, the human tongue is also associated with certain specialized structures (taste buds) known as papillae, usually confined or more commonly found within the dorsal and lateral tongue borders.
On microscopic examination, each taste bud comprises multiple spindle-shaped epithelial cells that extend from the basement membrane to the superficial surface epithelium. At this junction of the superficial epithelial surface, the taste buds tend to be in contact with the fluid-based environment of the oropharynx and the larynx.
The taste is mediated by three cranial nerves, which include - the facial (VII), glossopharyngeal (IX), and vagus (X) nerves. These three nerves collectively are termed gustatory nerves. These cranial nerves innervate the taste buds in the tongue, palate, epiglottis, and esophagus. The nerve fibers of these nerves play an important part in the distribution of taste sensation amongst the taste cells. Similarly, the sensations of sweet, savory, salty, sour, and bitter are mediated by sensitive receptor channels and neurotransmitters that play an important role in the transduction of taste via the papillae of the tongue.
What Are the Types of Tongue Papillae?
There are four main types of tongue papillae which include-
1. Fungiform Papillae -
The fungiform papilla is located on the anterior two-thirds of the tongue. There are around 200 fungiform papillae present on the tongue surface. The fungiform taste buds have been the most extensively researched, as the anterior tongue is most easily stimulated via neurophysiologic techniques. Moreover, the gustatory sensations via the chorda tympani branch of the facial nerve can also be recorded. The gustatory stimulation or innervation in the papillae is through the glossopharyngeal nerve, which is difficult to record through neurophysiologic techniques.
2. Circumvallate Papillae -
These are associated with the taste buds located in the walls of the papillae or on the sides of the papillae. These are found mainly on the junctions of the oral and pharyngeal tongue regions. The human tongue has been found to have 8 to 12 circumvallate papillae in a chevron arrangement.
3. Foliate Papillae -
These are found on the lateral or posterior tongue borders. These papillae form a series of folds or clefts on the tongue surfaces. The taste buds associated with these papillae are located in the epithelium based within these cleft walls.
4. Filiform Papillae -
These are the most numerous papillae located on the dorsum of the anterior and posterior tongue surfaces. As they do not have taste buds, these papillae do not possess any gustatory functions.
5. Accessory Taste Buds -
Various research suggests that many taste buds are located within the palate, pharynx, and larynx and may be distributed on the surface epithelium rather than contained within the papillae. These taste buds are termed accessory taste buds.
What Are the Types of Taste Cells?
According to taste buds morphology, there are four types of taste cells which include -
Type 1 Cells or Dark Cells -
These are the most frequently observed taste bud cells that comprise 60 % of all taste-relaying cells. These are long and narrow that extend from the base of the taste bud to the taste pore (superficial surface). They are also called dark cells because they are electron-dense and characterized by large and dense cytoplasmic vesicles.
Type 2 Cells or Light Cells -
These taste cells extend from the basement membrane to the taste pore and have an electron-lucent cytoplasm, hence called light cells. These comprise 30 % of the total taste cells.
Type 3 Cells -
These are the cells with apical specialization possessing a similar morphology to type 2 cells and contain dense cored vesicles within the basal portion of the cells.
Type 4 Cells -
These cells are only in contact with the basement membrane and do not extend to the surface of the taste pores. They are also called basal cells. These cells also do not play any role in the transduction of taste.
However, research suggests that the chemoreceptor transduction of taste is observed through synapses within all three types of cells, Type 1, 2, and 3.
Do Taste Buds Have Regenerative Potential?
Human Taste buds also have the dynamic property for regeneration. The gustatory nerves are responsible for the regeneration and promotion of the relay of sensation via the taste buds. Various studies show that when a gustatory nerve is transected, then the taste buds also do not relay the appropriate taste stimulus. The glossopharyngeal nerve (cranial nerve IX) is majorly responsible for the innervation of the posterior surface of the tongue. At the same time, the anterior taste buds are innervated by the chorda tympani nerve.
The carotid body comprises sensitive chemoreceptors, which are sensitive to blood gas levels and involved in the reinnervation of taste buds. However, irrespective of the source of reinnervation of the taste buds, these taste buds in the human tongue are highly functional and exhibit neural responses to the receptive fields upon chemical stimulation of the specific region of the tongue. Hence, taste bud cells possess this dynamic property of regeneration, and like any other epithelial cells, they can have a rapid turnover or replacement.
To conclude, the physiology of human taste involves complex and dynamic gustatory interaction between the taste buds and the nerves and has the potential for regeneration and relay response.