Smell

In the life of land animals, the sense of smell plays an important role in communication with the external environment. It serves to recognize odors, determine gaseous odorous substances contained in the air. In the process of evolution, the olfactory organ, which has an ectodermal origin, was initially formed near the mouth opening, and then combined with the initial section of the upper respiratory tract, separated from the oral cavity. Some mammals have a very well-developed sense of smell (macrosmatics). This group includes insectivores, ruminants, ungulates, and predatory animals. Other animals have no sense of smell at all (anasmatics). These include dolphins. The third group consists of animals whose sense of smell is poorly developed (microsmatics). These include primates.

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

The number of olfactory neurosensory cells reaches 6 million (30,000 cells per 1 mm2 ⁾. The distal part of the olfactory cells forms a thickening - the olfactory club. Each of these thickenings has up to 10-12 olfactory cilia. The cilia are mobile and can 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 many villi,

The olfactory glands ( glandulae olfactoriae) are located in the thickness of the loose connective tissue of the olfactory region. They synthesize a watery secretion that moistens the integumentary epithelium. In this secretion, which washes the cilia of the olfactory cells, odorous substances are dissolved. 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.

The olfactory nerves penetrate the cranial cavity through the openings of the cribriform plate of the olfactory bone, 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 the mitral cells in the thickness of the olfactory tract are directed to the olfactory triangle, and then, as part of the olfactory stripes (intermediate and medial), they enter the anterior perforated substance, the subcallosal area (area subcallosa) and the diagonal strip (bandaletta [stria] diagonalis) (Broca's strip). As part of the lateral strip, the processes of the mitral cells follow into the parahippocampal gyrus and into the hook, which contains the cortical olfactory center.

Neurochemical mechanisms of olfactory perception

In the early 1950s, Earl Sutherland used the example of adrenaline, which stimulates the formation of glucose from glycogen, to decipher the principles of signal transmission through the cell membrane, which turned out to be common to a wide range of receptors. Already at the end of the 20th century, it was discovered that the perception of odors is carried out in a similar way, even the details of the structure of the receptor proteins turned out to be similar.

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

There is not enough X-ray structural data to reconstruct the structure of membrane receptors. Therefore, analog computer models are currently widely used in such schemes. According to these models, the olfactory receptor is formed by seven hydrophobic domains. Ligand-binding amino acid residues form a "pocket" located 12 A from the cell surface. The pocket is depicted as a rosette, constructed in the same way for different receptor systems.

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

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

Another signal quenching pathway involves activation of phospholipase C and protein kinase C. Phosphorylation of membrane proteins opens cation channels and, as a consequence, instantly changes the transmembrane potential, which also generates an action potential. Thus, phosphorylation of proteins by protein kinases and dephosphorylation by corresponding phosphatases turned out to be a universal mechanism for the instantaneous response of a cell to an external stimulus. Axons directed to the olfactory bulb are combined into bundles. The nasal mucosa also contains free endings of the trigeminal nerve, some of which are also capable of responding to odors. In the pharynx, olfactory stimuli can excite the fibers of the glossopharyngeal (IX) and vagus (X) cranial nerves. Their role in the perception of odors is not associated with the olfactory nerve and is preserved in case of dysfunction of the olfactory epithelium due to diseases and injuries.

Histologically, the olfactory bulb is divided into several layers, characterized by cells of a specific shape, equipped with processes of a certain type with typical types of connections between them.

The convergence of information occurs on the mitral cells. In the glomerular layer, approximately 1,000 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, while contacts in the opposite direction are inhibitory. Axons of periglomerular cells terminate on the dendrites of mitral cells of the adjacent glomerulus.

Granule cells also form reciprocal dendrodendritic synapses with mitral cells; these contacts influence the generation of impulses by mitral cells. Synapses on mitral cells are also inhibitory. Granule cells also form contacts with collaterals of mitral cells. Axons of mitral cells form the lateral olfactory tract, which is directed to the cerebral cortex. Synapses with neurons of higher orders provide connections with the hippocampus and (via the amygdala) with 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.