Topic 28. ORGANS OF TASTE AND SMELL

The olfactory analyzer consists, like any other, of the central and peripheral sections.

The peripheral part of the olfactory analyzer is represented by the olfactory field - the olfactory lining, which is located on the middle part of the superior nasal concha and the corresponding section of the mucous membrane of the nasal septum.

The olfactory epithelium contains receptor cells. Their central processes - axons - transmit information to the olfactory bulb. The olfactory receptors are the first neuron of the olfactory pathway and are surrounded by supporting cells.

The body of the olfactory cell contains numerous mitochondria, cisterns of the endoplasmic reticulum with ribosomes, elements of the Golgi complex, and lysosomes. Olfactory cells, in addition to the central one, also have a short peripheral process - a dendrite, ending on the surface of the olfactory epithelium with a spherical thickening - an olfactory club with a diameter of 1 - 2 mm. It contains mitochondria, small vacuoles and basal bodies, several olfactory hairs up to 10 mm long extending from the top of the club, having the structure of typical cilia.

The subepithelial connective tissue contains the terminal sections of the Bowman glands, blood vessels, and bundles of unmyelinated nerve fibers of the olfactory nerve. The mucus that is secreted by the Bowman glands covers the surface of the olfactory lining.

Olfactory cilia immersed in mucus are involved in the process of chemosensing.

The olfactory nerve is a collection of thin olfactory filaments that pass through a hole in the ethmoid bone into the brain to the olfactory bulbs. In addition to non-myelinated fibers, separate myelinated fibers of the trigeminal nerve pass through the connective tissue layer of the olfactory lining.

The receptor cells of the olfactory lining register 25 - 35 odors.

Their combinations form many millions of perceived odors. Olfactory receptor neurons depolarize in response to adequate stimulation. The cAMP-dependent gate ion channels are built into the plasmolemma of the olfactory cilia, which open when interacting with cAMP.

cAMP-dependent gate channels are activated as a result of a sequence of events - interaction with the receptor protein in the olfactory cilia plasmolemma, G-protein activation, increased adenylate cyclase activity, and increased cAMP levels.

The inositol triphosphate system is also related to the mechanism of chemosensing in the olfactory organ. Under the action of certain odorous substances, the level of inositol triphosphate rapidly increases, which interacts with calcium channels in the plasmolemma of olfactory receptor neurons. Thus, the cAMP and inositol triphosphate second messenger systems interact with each other, providing a better perception of various odors.

Through cAMP-dependent gate ion channels, not only monovalent cations pass into the cell, but also calcium ions, which binds to calmodulin. The resulting calcium-calmodulin complex interacts with the channel, which prevents cAMP activation, as a result of which the receptor cell becomes insensitive to the action of odorous irritants.

The lifespan of olfactory cells is about 30 - 35 days. Olfactory receptors are an exception among all other neurons; they are updated by precursor cells - the basal cells of the epithelium of the olfactory lining.

Support cells. Among them, tall cylindrical and smaller cells that do not reach the surface of the receptor layer are distinguished. Cylindrical cells on the apical surface contain microvilli 3–5 µm long. In addition to well-developed organelles of general importance, supporting cells in the apical part contain many secretory granules.

The taste analyzer, as well as the olfactory one, consists of a central and peripheral sections. The peripheral part of the taste analyzer is represented by taste buds, which are found in the epithelium of the oral cavity, anterior pharynx, esophagus, and larynx. Their main localization is the chemosensitive papillae of the tongue (mushroom-shaped, trough-shaped and foliate). In children, taste buds are also found in the epithelium of the mucous membrane of the lips, epiglottis, and vocal cords.

The taste bud is elliptical in shape, 27-115 µm high and 16-70 µm wide. In their apical region there is a taste canal filled with an amorphous substance, which opens on the surface of the epithelium with a taste pore.

The kidney is formed by 30 - 80 elongated cells, closely adjacent to one another. Most of these cells come into contact with nerve fibers penetrating the kidney from the subepithelial nerve plexus, which contains myelinated and unmyelinated nerve fibers. All cell types of the taste bud form afferent synapses with nerve terminals.

The development of the taste buds of the tongue proceeds in parallel with the germination of nerve fibers in the epithelium. The differentiation of the kidneys begins simultaneously with the appearance of clusters of unmyelinated nerve fibers directly under the location of the future kidney.

Taste bud cells are morphologically heterogeneous. There are four types of cells.

Type I cells in the apical part have up to 40 microvilli protruding into the cavity of the taste canal. The apical part of the cells contains a large number of electron-dense granules. The cytoskeleton is represented by well-defined bundles of microfilaments and microtubules. Some of these structures form a compact bundle, the narrowed end of which is connected to a pair of centrioles. The Golgi complex, which is related to the formation of electron-dense granules, is located above the nucleus. In the basal part of the cell there are small dense mitochondria. A well-developed granular endoplasmic reticulum is concentrated in the same area.

Type II cells have lighter colored cytoplasm. Along with vacuoles varying in size, it contains expanded cisterns of a smooth endoplasmic reticulum. The apical part of the cell contains sparse and small microvilli. There are multivesicular bodies, lysosomes.

Type III cells contain low microvilli, centrioles, and a small amount of vesicles up to 120 nm in diameter in the apical part. The granular endoplasmic reticulum is poorly developed. Numerous flattened cisterns and vesicles form a well-defined smooth endoplasmic reticulum. A characteristic feature of cells is the presence in the cytoplasm of granular vesicles with a diameter of 80 - 150 nm, as well as light vesicles with a diameter of 30 - 60 nm. These vesicles, primarily light ones, are related to afferent synapses. Granular vesicles are located in other parts of the cell, but are always present in the area of ​​synapses.

Type IV cells are located in the basal part of the taste bud and do not reach the taste duct. They contain a large nucleus and bundles of microfilaments. The function of these cells remains unclear. It is possible that type IV cells are precursors for all types of taste bud cells.

chemoreceptor cells. Although contacts with afferent fibers form all types of cells, the function of chemosensing is associated mainly with type III cells. In the presynaptic region of taste cells, granular vesicles contain serotonin, a mediator of the afferent synapse. Sweet stimuli activate adenylate cyclase in taste receptor cells, which leads to an increase in cAMP levels. Bitters act through a G-protein called gastducin, which through an increase in phosphodiesterase activity leads to a decrease in cAMP levels.

In the taste bud, there is a constant renewal of cells. From the peripheral region of the taste bud, cells move to its central part at a rate of 0,06 µm/h. The average lifespan of the cells of the taste organ is 250 ± 50 hours. After damage to the nerves that innervate the taste buds, the latter degenerate, and when the nerves regenerate, they are restored. The results of these studies suggest that taste buds are under neurotrophic control.

Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu.

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