The taste receptors are located on the tongue and are housed in particular structures called gustatory buds or gustatory chalices. The taste buds are in turn housed in structures called taste buds which include 30 to 100 buds.
The taste buds are visible on the surface of the tongue as red dots. They are divided into 3 categories:
- fungiform papillae (small and round, on the tip of the tongue)
- foliate papillae (along the sides)
- circumvallate papillae (larger, towards the bottom)
- The taste buds on the other hand are not visible to the naked eye and are made up of different types of cells:
- receptor cells: they perceive different tastes;
- basal cells: they differentiate into new receptor cells in about 10 days;
- support cells: they have a trophic function (nourishment and support of the other cells).
- The taste buds also contain a bundle of afferent taste axons.
The taste buds are located in the oral cavity, at the level of the epiglottis and in the first third of the esophagus. Lingual taste buds are the most numerous (about 4600).
The receptor cells have only a receptor function and are not neurons, so they do not generate action potentials.
The portion of the receptor cells that is sensitive to chemical stimuli is a small membranous region located near the surface of the tongue. This region has microvilli, which extend up to the gustatory pore, a small opening on the lingual surface, where the cell comes into contact with the substances dissolved in saliva.
To ensure that the sensations reach the CNS, at the base of the taste bud the axons of the afferent neurons invade the bud and contract synapses with different receptor cells of the bud.
Furthermore, there are electrical and chemical synapses between receptor cells and some basal cells and between basal cells and sensory axons. In this way there is a kind of simple information processing circuit within each gustatory gem.
Taste receptor cells are subject to a continuous cycle of growth, death and regeneration; their average life is 2 weeks.
The tongue has no regional differences in taste sensations, but most of this organ is sensitive to all basic flavors.
All receptor cells participate to perceive a certain taste (salty, sweet, bitter, sour, umami), but some respond preferentially to one of them. The system is therefore not all-or-nothing, in fact the cells discharge in the presence of different flavors: rapid and continuous discharges indicate the specialized sensitivity for a certain flavor; slow and sporadic discharges indicate that the cell is not specialized for that taste.
The prototype of salty chemicals is table salt (sodium chloride: Na + Cl-). The taste of salt is given by the sodium cation (Na +). The receptor cells have specific Na + channels that are selective for sodium.
When a salty food is eaten, the concentration of the sodium ion increases outside the receptor, resulting in an increase in the sodium concentration gradient with respect to the cytoplasm. The sodium then begins to flow inside the receptor cell, causing a depolarization of the cell membrane, called the receptor potential. This in turn results in the opening of voltage-gated sodium and calcium channels near the synaptic vesicles, causing the release of neurotransmitters on the afferent gustatory axon.
The sour taste is given by the content of acids, which are made up of hydrogen ions (H +), which are responsible for this taste. There are 2 mechanisms of action on the receptors:
1) the hydrogen ions diffuse through the amiloride-sensitive sodium channels (the same ones responsible for the salty taste) creating a current of hydrogen ions towards the cytoplasm, which causes the depolarization of the receptor cell ( i.e. the receptor potential) and the consequent release of neurotransmitters.
2) The hydrogen ions bind, blocking them, to the selective channels for potassium: in this way the permeability of the membrane to the potassium ion is reduced (i.e. fewer potassium ions come out) and the cell is depolarized.
If the only possible mechanism of transduction were the first, the receptor cell could not distinguish a hydrogen ion from a sodium ion.