Taste

The organ of taste (organum giistus) develops from the ectoderm. In fish perceiving the "sense of taste" taste buds (bulbs) are not only in the epithelial cover of the mouth, but also in the skin (dermal chemical sensation). Flavored kidneys in terrestrial vertebrates are located only in the initial section of the digestive tube, reaching a high development in higher mammals. In humans, taste buds (caliculi gustatorii) in an amount of about 2000 are found mainly in the mucous membrane of the tongue, as well as the palate, throat, epiglottis. The largest number of taste buds is concentrated in the papillae vallatae and papillae foliatae, less in the mushroom papillae fungiformes of the tongue mucosa. In threadlike papillae they do not exist. Each taste bud consists of taste and supporting cells. On the top of the kidney there is a taste pore (hole) (porus gustatorius) opening on the surface of the mucous membrane.

On the surface of the taste cells are the endings of nerve fibers that perceive the taste sensitivity. In the region of the front 2/3 of the tongue, the sense of taste is perceived by the fibers of the tympanic strings of the facial nerve, in the posterior third of the tongue and in the region of the grooved papillae - by the endings of the glossopharyngeal nerve. This nerve carries the taste innervation of the mucous membranes of the soft palate and palatine arches. From rarely located taste buds in the mucosa of the epiglottis and the inner surface of the arytenoid cartilages, taste impulses come through the upper laryngeal nerve, a branch of the vagus nerve. The central processes of neurons performing taste innervation in the oral cavity are sent in the composition of the corresponding cranial nerves (VII, IX, X) to the common sensitive nucleus of the single path (nucleus solitarius), which lies in the form of a longitudinal cellular strand in the posterior part of the medulla oblongata. The axons of the cells of this nucleus are sent to the thalamus, where the impulse is transferred to the following neurons, the central processes of which terminate in the cerebral cortex, the hook of the para-hippocampal gyrus. In this gyrus is the cortical end of the taste analyzer.

Mechanisms of taste receptors

The mechanisms of perception of taste and smell are similar in many respects, since both sensations are activated by chemical stimuli coming from the outside world. Indeed, taste stimuli tend to act on receptors associated with G-proteins, 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 the receptor cells.

The taste receptors are localized on the hair neuroepithelial cells located in the taste buds on the surface of the tongue. Unlike olfactory receptors, they do not have axons, but they form chemical synapses with afferent neurons in the taste buds. Microvilli are sent from the apical pole of the taste cell to 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 pass in taste cells that have receptors on the apical part located near the opening of the taste pore. Like the olfactory receptor cells, the taste cells die off every two weeks and new cells regenerate from the basal cells. For each of the five perceived flavors, there are separate types of receptors.

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 take saline, and H-sensitive channels that perceive acidic channels. Penetration of the corresponding charges inside the taste cell leads to the depolarization of its membrane. This initial depolarization activates the potential-directed Na and Ca channels in the basolateral part of the taste cell, which leads to the release of the neurotransmitter at the basal site of the taste cell and the generation of the action potential in the ganglionic cell.

In humans and other mammals, receptors that perceive the taste of sweet and amino acids, consist of seven transmembrane domains and are associated with G-protein. The perception of sweet is due to a pair of T1R3 and T1R2 receptors, and the amino acids are T1R3 and TR1. Receptors TR2 and TR1 are detected in different parts of the receptor cell. When bound to sugars or other stimuli, the sweet T1R2 / T1R3 receptor initiates a G-protein mediated cascade of reactions that leads to the activation of phospholipase C (the isoform of PLCb2) and, correspondingly, an increase in the concentration of IP3 and the opening of so-called TPR-Ca channels (specific TPRM5 channels), due to the work of which: the depolarization of the taste cell occurs due to an increase in the intracellular Ca2 + concentration. The T1R1 / T1R3 receptor is adapted to perceive the twenty b-amino acids that make up proteins, but can not perceive D-amino acids. The transduction of the amino acid signal through this receptor is accomplished using the same signaling cascade as for sugars.

Another family of G protein-coupled receptors, known as T2R, 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 in those cells in which there are TR1-, TR2- or TR3-receptors. Thus, the receptors of the bitter are receptors of a special class. The transmission of the bitter taste signal has a signal-like mechanism similar to the sweet and taste of amino acids, including the G-protein-specific custucine, which is specific for taste cells. Structurally this protein is 90% homologous to transducin - G-protein photoreceptors. The same level of similarity is observed between the transducins, functioning in sticks and cones. In a-transducin and a-gustucine, sequences of 38 C-terminal amino acids proved to be identical.

In many foods, including meat, cheese and some vegetables, free glutamate is found. In the form of sodium glutamate it is used as a food seasoning. The taste of glutamate is transferred by the metabolic G-protein-metabolized glutamate receptor, which is specifically expressed in the taste buds. By the method of conditional taste, it was shown that both glutamate sodium and a specific agonist of mGluR4 receptors (metabotropic glutamate receptors of the 4th type) of L-AP4 cause similar taste sensations in rats.

"Burning" taste of a number of products

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

Taste cells can stimulate receptors to generate receptor potential. With the help of a synaptic transmission, this excitation is transmitted to the afferent fibers of the cerebrospinal nerves, through which it enters in the form of impulses to the brain. The drum string - the 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 upper larynx of the vagus (X) nerve. Branched, each fiber receives signals from the receptors of different taste buds. The amplitude of the receptor potential increases together with the concentration of the stimulating substance. Depolarization of receptor cells exerts an exciting, and hyperpolarization - inhibitory effect on the afferent fibers. The fibers of the IX pair of cranial nerves react very strongly to substances with a bitter taste, and the VII pairs - stronger to the action of salty, sweet and sour, with each fiber reacting more to a certain stimulus.

The taste fibers of these cranial nerves terminate within or near the nucleus of a single path of the medulla oblongata associated with the ventral post-medodial thalamus nucleus. Axons of neurons of the third order terminate in the postcentral gyrus of the cerebral cortex. A number of cortical cells react only to substances with one taste quality, others - also to temperature and mechanical stimuli.

[1], [2]