Smell

In the life of terrestrial animals, the sense of smell plays an important role in communicating with the external environment. It serves for the recognition of odors, the determination of gaseous smelling substances contained in the air. In the process of evolution, the olfactory organ, having an ectodermal origin, was first formed near the oral opening, and then combined with the initial section of the upper respiratory tract, separated from the oral cavity. In some mammalian animals, the sense of smell is very well developed (macrosmatics). This group includes insectivores, ruminants, ungulates, predatory animals. Other animals have no sense of smell at all (anasmatics). These include dolphins. The third group consists of animals whose smell is poorly developed (microsmatics). They belong to the primates.

In humans, the organ of smell (organum olfactorium) is located in the upper part of the nasal cavity. The olfactory area of the nasal mucosa (regio olfactoria tunicae mucosae nasi) includes a mucosa covering the upper nasal concha and the upper part of the nasal septum. The receptor layer in the epithelium covering the mucosa includes the olfactory, neurosensory cells (ccllulae neurosensoriae olfactoriae) that perceive the presence of odorous substances. Between the olfactory cells lie supporting epitheliocytes (epitheliocyti sustenans). Supporting cells are capable of apocrine secretion.

The number of olfactory neurosensory cells reaches 6 million (30,000 cells on an area of 1 mm ² ). The distal part of the olfactory cells forms a thickening - the olfactory mace. Each of these thickenings has up to 10-12 olfactory cilia. Cilia are mobile, able to contract under the influence of odorous substances. The nucleus occupies a central position in the cytoplasm. The basal part of the receptor cells continues into a narrow and convoluted axon. On the apical surface of the olfactory cells there are a lot of villi,

The thickness of the loose connective tissue of the olfactory region of olfactory contains (Bowman's) glands (glandulae olfactoriae). They synthesize a watery secret, moisturizing the cover epithelium. In this secret, which is washed by the cilia of olfactory cells, odorous substances dissolve. These substances are perceived by receptor proteins located in the membrane covering the cilia. The central processes of the neurosensory cells form 15-20 olfactory nerves.

Olfactory nerves through the holes of the trellis plate of the eponymous bone penetrate into the cavity of the skull, then into the olfactory bulb. In the olfactory bulb, the axons of the olfactory neurosensory cells in the olfactory glomeruli come into contact with the mitral cells. The processes of mitral cells in the thickness of the olfactory tract are sent to the olfactory triangle, and then enter into the front perforation, in the subculture field (area subcallosa) and the diagonal strip (bandaletta [stria] diagonalis) (the Broca band) in the olfactory strips (intermediate and medial ) . In the lateral stripe, the processes of the mitral cells follow the parahippocampal gyrus and the hook, in which the cortical center of smell is located.

Neurochemical mechanisms of olfaction

In the early 50's. XX century. Earl Sutherland on the example of adrenaline, stimulating the formation of glucose from glycogen, deciphered the principles of signal transmission through the cell membrane, which turned out to be common for a wide range of receptors. Already at the end of the XX century. It was found that the perception of odors is similar, even the details of the structure of the receptor proteins turned out to be similar.

Primary receptor proteins are complex molecules, binding to which ligands causes significant structural changes in them, followed by a cascade of catalytic (enzymatic) reactions. For the odor receptor (odorant), as well as for the visual receptor, this process is terminated by a nerve impulse, perceived by the nerve cells of the corresponding parts of the brain. Segments containing from 20 to 28 residues in each, which is enough to cross a 30 A membrane. These polypeptide regions are folded into a a-helix. Thus, the body of the receptor protein is a compact structure of seven segments that cross the membrane. Such a structure of integral proteins is characteristic of opsin in the retina of the eye, receptors of serotonin, adrenaline and histamine.

To reconstruct the structure of membrane receptors, there is still insufficient X-ray diffraction data. Therefore, in such circuits, analog computer models are now widely used. According to these models, the olfactory receptor is formed by seven hydrophobic domains. Ligand-binding amino acid residues form a "pocket", separated from the cell surface by a distance of 12 A. The pocket is depicted in the form of an outlet constructed in the same manner for different receptor systems.

Binding of the odorant to the receptor leads to the inclusion of one of the two signaling cascades, the opening of the ion channels, and the generation of the receptor potential. Olfactory-specific G-protein can activate adenylate cyclase, which leads to an increase in the concentration of cAMP, the target of which are cation-selective channels. Their discovery leads to the entrance of Na + and Ca2 + into the cell and the depolarization of the membrane.

An increase in the concentration of intracellular calcium causes the opening of Ca-guided Cl channels, which leads to an even greater depolarization and generation of the receptor potential. Signal quenching is due to a decrease in cAMP concentration, due to specific phosphodiesterases, and also due to the fact that Ca2 + binds to ion channels in a complex with calmodulin and reduces their sensitivity to cAMP.

Another way of quenching the signal is associated with the activation of phospholipase C and protein kinase C. As a result of phosphorylation of membrane proteins, cation channels are opened and, as a result, the transmembrane potential changes instantly, as a result of which the action potential is also generated. Thus, protein phosphorylation by protein kinases and dephosphorylation by their corresponding phosphatases turned out to be a universal mechanism of instantaneous cell response to external action. Axons heading into the olfactory bulb are bundled. The mucous membrane of the nose, in addition, contains free ends of the trigeminal nerve, some of which are also capable of reacting to odors. In the pharyngeal region, olfactory stimuli can excite the fibers of the glossopharyngeal (IX) and vagus (X) cerebrospinal nerves. Their role in the perception of odors is not associated with the olfactory nerve and is preserved when the function of the olfactory epithelium is disturbed in diseases and traumas.

The histologically olfactory bulb is divided into several layers, characterized by cells of a specific form, equipped with processes of a certain type with typical types of connection between them.

On the mitral cells there is a convergence of information. In the glomerular (glomerular) layer approximately 1000 olfactory cells terminate on the primary dendrites of one mitral cell. These dendrites also form reciprocal dendrodendritic synapses with periglomerular cells. Contacts between mitral and periglomerular cells are excitatory, and oppositely directed - inhibitory. Axons of periglomerular cells terminate on the dendrites of the mitral cells of the neighboring glomerulus.

Grain cells also form reciprocal dendrodendritic synapses with mitral cells; these contacts affect the generation of impulses by mitral cells. Synapses on mitral cells are also inhibitory. In addition, the grain cells form contacts with collaterals of mitral cells. The axons of mitral cells form the lateral olfactory tract, which leads to the cerebral cortex. Synapses with higher-order neurons provide a link to the hippocampus and (via the amygdala) to the autonomic nuclei of the hypothalamus. Neurons responding to olfactory stimuli are also found in the orbitofrontal cortex and the reticular formation of the midbrain.