Flavor

The organ of taste (organum giistus) develops from the ectoderm. In fish, taste buds (bulbs) that perceive the "sense of taste" are present not only in the epithelial lining of the oral cavity, but also in the skin (cutaneous chemical sense). Taste buds in terrestrial vertebrates are located only in the initial section of the digestive tract, reaching a high level of development in higher mammals. In humans, taste buds (caliculi gustatorii) in the amount of about 2000 are located mainly in the mucous membrane of the tongue, as well as the palate, pharynx, and epiglottis. The greatest number of taste buds are concentrated in the grooved papillae (papillae vallatae) and foliate papillae (papillae foliatae), there are fewer of them in the fungiform papillae (papillae fungiformes) of the mucous membrane of the back of the tongue. They are not found in filiform papillae. Each taste bud consists of taste cells and supporting cells. At the top of the bud is a taste pore (opening) (porus gustatorius), which opens onto the surface of the mucous membrane.

On the surface of the taste cells are the endings of the nerve fibers that perceive taste sensitivity. In the area of the anterior 2/3 of the tongue, the sense of taste is perceived by the fibers of the chorda tympani of the facial nerve, in the posterior third of the tongue and in the area of the circumvallate papillae - by the endings of the glossopharyngeal nerve. This nerve also provides taste innervation of the mucous membranes of the soft palate and palatine arches. From sparsely located taste buds in the mucous membrane of the epiglottis and the inner surface of the arytenoid cartilages, taste impulses come through the superior laryngeal nerve - a branch of the vagus nerve. The central processes of the neurons that provide taste innervation in the oral cavity are directed as part of the corresponding cranial nerves (VII, IX, X) to their common sensory nucleus of the solitary tract (nucleus solitarius), lying in the form of a longitudinal cellular cord in the posterior part of the medulla oblongata. The axons of the cells of this nucleus are directed to the thalamus, where the impulse is transmitted to the following neurons, the central processes of which end in the cortex of the cerebrum, the hook of the parahippocampal gyrus. The cortical end of the taste analyzer is located in this gyrus.

Mechanisms of taste buds

The mechanisms of taste and smell perception are largely analogous, since both sensations are activated by chemical stimuli coming from the outside world. Indeed, taste stimuli generally act on G-protein-coupled receptors in ways very similar to those described above for olfaction. At the same time, some taste stimuli (mainly salts and acids) act directly on the membrane conductivity of receptor cells.

Taste receptors are located on neuroepithelial hair cells located in taste buds on the surface of the tongue. Unlike olfactory receptors, they do not have axons, but form chemical synapses with afferent neurons in the taste buds. Microvilli extend from the apical pole of the taste cell into the open pore of the taste bud, where they come into contact with taste stimuli (substances dissolved in saliva on the surface of the tongue).

The initial stages of chemosensory perception occur in taste cells, which have receptors on the apical portion, located near the opening of the taste pore. Like olfactory receptor cells, taste cells die off every two weeks and new cells are regenerated from the basal cells. There are separate types of receptors for each of the five tastes perceived.

Taste of salt or acid

It is created by the direct action of sodium ions or protons on specific channels - amiloride-sensitive Na-channels, which perceive saltiness, and H-sensitive channels, which perceive sourness. Penetration of the corresponding charges into the taste cell leads to depolarization of its membrane. This initial depolarization activates potential-controlled Na- and Ca-channels in the basolateral part of the taste cell, which leads to the release of neurotransmitter in the basal part of the taste cell and the generation of an action potential in the ganglion cell.

In humans and other mammals, the receptors that perceive the taste of sweet and amino acids consist of seven transmembrane domains and are associated with a G protein. The perception of sweet is carried out by a pair of receptors T1R3 and T1R2, and amino acids - T1R3 and TR1. The TR2 and TR1 receptors are found in different parts of the receptor cell. When bound to sugars or other sweet stimuli, the T1R2/T1R3 receptor initiates a cascade of reactions mediated by a G protein, which leads to the activation of phospholipase C (isoform PLCb2) and, accordingly, to an increase in the concentration of IP3 and the opening of the so-called TRP-Ca channels (specific TRPM5 channels), due to the work of which: depolarization of the taste cell occurs due to an increase in the intracellular concentration of Ca2+. The T1R1/T1R3 receptor is adapted to perceive twenty b-amino acids that are part of proteins, but cannot perceive D-amino acids. Transduction of the amino acid signal through this receptor is carried out using the same signaling cascade as for sugars.

Another family of G protein-coupled receptors, known as T2Rs, is responsible for the perception of bitter taste. There are about 30 subtypes of these receptors, encoded by 30 different genes. These receptors are absent from cells that have TR1, TR2, or TR3 receptors. Thus, bitter receptors are receptors of a special class. Bitter taste signaling has a signaling mechanism similar to sweet and amino acid tastes, involving a taste cell-specific G protein, gustducin. Structurally, this protein is 90% homologous to transducin, a G protein of photoreceptors. The same level of similarity is observed between transducins functioning in rods and cones. The sequences of the 38 C-terminal amino acids of a-transducin and a-gustducin were found to be identical.

Free glutamate is found in many foods, including meat, cheese, and some vegetables. In the form of monosodium glutamate, it is used as a food seasoning. The taste of glutamate is transmitted by the G protein-coupled metabotropic glutamate receptor, which is specifically expressed in taste buds. Using the conditioned taste aversion method, it was shown that both monosodium glutamate and the specific mGluR4 (metabotropic glutamate receptor type 4) agonist L-AP4 evoke similar taste sensations in rats.

"Hot" taste of some products

Another example of the multifunctionality of molecular receptors. The taste of pepper is perceived not by the taste cells themselves, but by pain fibers in the tongue, which are activated by capsaicin compounds. The capsaicin receptor has been cloned and shown to be a calcium-selective cation channel. It is formed by small fibers (C-fibers) coming from the cells of the spinal ganglia and signaling pain. Thus, nature has provided peppers with a chemical targeting of this receptor, possibly to repel herbivores by activating pain fibers.

Taste cells are capable of generating a receptor potential when stimulated. By means of synaptic transmission, this excitation is transmitted to the afferent fibers of the cranial nerves, through which it enters the brain as impulses. The chorda tympani, a branch of the facial nerve (VII), innervates the anterior and lateral parts of the tongue, and the glossopharyngeal nerve (IX) - its posterior part. The taste buds of the epiglottis and esophagus are innervated by the superior laryngeal branch of the vagus (X) nerve. Branching, each fiber receives signals from receptors of different taste buds. The amplitude of the receptor potential increases with the concentration of the stimulating substance. Depolarization of receptor cells has an excitatory effect, and hyperpolarization - an inhibitory effect on afferent fibers. The fibers of the IX pair of cranial nerves react especially strongly to substances with a bitter taste, and the VII pair react more strongly to the action of salty, sweet and sour, and each fiber reacts to a greater extent to one specific stimulus.

The taste fibers of these cranial nerves terminate within or near the nucleus of the solitary tract of the medulla oblongata, associated with the ventral posteromedial nucleus of the thalamus. The axons of the third-order neurons terminate in the postcentral gyrus of the cerebral cortex. Some cortical cells respond only to substances with one taste quality, others also to temperature and mechanical stimuli.

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