OLfactory SYSTEM AND ITS SENSORY CHARACTERISTICS Olfaction is the ability to distinguish in sensations and perceptions the chemical composition of various substances and their compounds with the help of appropriate receptors. With the participation of the olfactory receptor, orientation in the surrounding space occurs and the process of cognition of the external world occurs.

OLfactory SYSTEM AND ITS SENSORY CHARACTERISTICS The organ of smell is the olfactory neuroepithelium, which appears as a protrusion of the brain tube and contains olfactory cells - chemoreceptors, which are excited by gaseous substances.

CHARACTERISTICS OF AN ADEQUATE STIMULAR An adequate stimulus for the olfactory sensory system is the smell emitted by odorous substances. All odorous substances that have an odor must be volatile in order to enter the nasal cavity with the air, and water-soluble in order to penetrate to the receptor cells through the layer of mucus covering the entire epithelium of the nasal cavities. A huge number of substances satisfy these requirements, and therefore a person is able to distinguish thousands of different odors. It is important that there is no strict correspondence between the chemical structure of the “fragrant” molecule and its smell.

FUNCTIONS OF THE OLfactory SYSTEM (OSS) With the participation of the olfactory analyzer, the following is carried out: 1. Detection of food for attractiveness, edibility and inedibility. 2. Motivation and modulation of eating behavior. 3. Setting up the digestive system to process food according to the mechanism of unconditioned and conditioned reflexes. 4. Triggering defensive behavior due to the detection of substances harmful to the body or substances associated with danger. 5. Motivation and modulation of sexual behavior due to the detection of odorants and pheromones.

STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF THE OLfactory ANALYZER. - The peripheral section is formed by the receptors of the upper nasal passage of the mucous membrane of the nasal cavity. Olfactory receptors in the nasal mucosa end in olfactory cilia. The gaseous substances dissolve in the mucus surrounding the cilia, then a chemical reaction produces a nerve impulse. - Conductor section - olfactory nerve. Along the fibers of the olfactory nerve, impulses arrive at the olfactory bulb (the structure of the forebrain in which information is processed) and then travel to the cortical olfactory center. - Central department - cortical olfactory center, located on the lower surface of the temporal and frontal lobes of the cerebral cortex. In the cortex, the smell is detected and the body’s adequate response to it is formed.

PERIPHERAL DIVISION This section begins with the primary sensory olfactory sensory receptors, which are the endings of the dendrite of the so-called neurosensory cell. By their origin and structure, olfactory receptors are typical neurons capable of generating and transmitting nerve impulses. But the far part of the dendrite of such a cell is changed. It is expanded into an “olfactory club”, from which 6–12 cilia extend, while a regular axon extends from the base of the cell. Humans have about 10 million olfactory receptors. In addition, additional receptors are located in addition to the olfactory epithelium also in the respiratory region of the nose. These are free nerve endings of the sensory afferent fibers of the trigeminal nerve, which also respond to odorants.

Cilia, or olfactory hairs, are immersed in a liquid medium - a layer of mucus produced by the Bowman's glands of the nasal cavity. The presence of olfactory hairs significantly increases the area of contact of the receptor with molecules of odorant substances. The movement of hairs ensures the active process of capturing molecules of an odorous substance and contacting it, which underlies the targeted perception of odors. The receptor cells of the olfactory analyzer are immersed in the olfactory epithelium lining the nasal cavity, in which, in addition to them, there are supporting cells that perform a mechanical function and are actively involved in the metabolism of the olfactory epithelium. Some of the supporting cells located near the basement membrane are called basal.

Odor reception is carried out by 3 types of olfactory neurons: 1. Olfactory receptor neurons (ORNs) mainly in the epithelium. 2. GC-D neurons in the main epithelium. 3. Vomeronasal neurons (VNNs) in the vomeronasal epithelium. The vomeronasal organ is thought to be responsible for the perception of pheromones, volatile substances that mediate social contact and sexual behavior. Recently, it was found that the receptor cells of the vomeronasal organ also perform the function of detecting predators by their smell. Each type of predator has its own special receptor-detector. These three types of neurons differ from each other in their transduction method and working proteins, as well as in their sensory pathways. Molecular geneticists have discovered about 330 genes that control olfactory receptors. They encode about 1000 receptors in the main olfactory epithelium and 100 receptors in the vomeronasal epithelium, which are sensitive to pheromones

PERIPHERAL DEPARTMENT OF THE OLFACTURAL ANALYZER: A - diagram of the structure of the nasal cavity: 1 - lower nasal passage; 2 - lower, 3 - middle and 4 - upper nasal concha; 5 - upper nasal passage; B - diagram of the structure of the olfactory epithelium: 1 - body of the olfactory cell, 2 - supporting cell; 3 - mace; 4 - microvilli; 5 - olfactory filaments

CONDUCTION DIVISION The first neuron of the olfactory analyzer should be considered the same olfactory neurosensory, or neuroreceptor, cell. The axons of these cells are collected in bundles, penetrate the basement membrane of the olfactory epithelium and are part of the unmyelized olfactory nerves. They form synapses at their ends called glomeruli. In the glomeruli, the axons of the receptor cells contact the main dendrite of the mitral nerve cells of the olfactory bulb, which represent the second neuron. The olfactory bulbs lie on the basal (lower) surface of the frontal lobes. They are classified either as an ancient cortex or as a special part of the olfactory brain. It is important to note that olfactory receptors, unlike receptors of other sensory systems, do not provide a topical spatial projection on the bulb due to their numerous convergent and divergent connections.

The axons of the mitral cells of the olfactory bulbs form the olfactory tract, which has a triangular extension (olfactory triangle) and consists of several bundles. The fibers of the olfactory tract go in separate bundles from the olfactory bulbs to the olfactory centers of a higher order, for example, to the anterior nuclei of the thalamus (visual thalamus). However, most researchers believe that the processes of the second neuron go directly to the cerebral cortex, bypassing the thalamus. But the olfactory sensory system does not provide projections to the new cortex (neocortex), but only to areas of the archi- and paleocortex: the hippocampus, limbic cortex, and amygdala complex. Efferent control is carried out with the participation of periglomerular cells and cells of the granular layer located in the olfactory bulb, which form efferent synapses with the primary and secondary dendrites of the mitral cells. In this case, there may be an effect of excitation or inhibition of afferent transmission. Some efferent fibers come from the contralateral bulb through the anterior commissure. Neurons responding to olfactory stimuli are found in the reticular formation; there is a connection with the hippocampus and the autonomic nuclei of the hypothalamus. The connection with the limbic system explains the presence of an emotional component in olfactory perception, for example, the pleasurable, or hedonic, components of the sensation of smells.

CENTRAL OR CORTICAL DEPARTMENT The central section consists of the olfactory bulb, connected by branches of the olfactory tract with centers located in the paleocortex (ancient cortex of the cerebral hemispheres) and in the subcortical nuclei, as well as the cortical section, which is localized in the temporal lobes of the brain, gyrus of the sea horse. The central, or cortical, section of the olfactory analyzer is localized in the anterior part of the piriform lobe of the cortex in the area of the seahorse gyrus. With

CODING OF OLfactory INFORMATION So, each individual receptor cell is capable of responding to a significant number of different odorous substances. Due to this, different olfactory receptors have overlapping response profiles. Each odorant produces a specific combination of olfactory receptors that respond to it and a corresponding pattern of excitation in the population of these receptor cells. In this case, the level of excitation depends on the concentration of the odorous irritant substance. When exposed to odorous substances in very small concentrations, the resulting sensation is not specific, but in higher concentrations the smell is detected and identified. Therefore, it is necessary to distinguish between the threshold for the appearance of an odor and the threshold for its recognition. Constant impulses due to subthreshold exposure to odorous substances were found in the fibers of the olfactory nerve. At threshold and above-threshold concentrations of various odorous substances, different patterns of electrical impulses arise, which arrive simultaneously in different parts of the olfactory bulb. At the same time, a kind of mosaic of excited and non-excited areas is created in the olfactory bulb. It is believed that this phenomenon underlies the encoding of information about the specificity of odors.

WORK OF THE OLFACTORY (OLFACTORY) SENSORY SYSTEM 1. Movement of chemical irritation (irritant) to sensory receptors. An irritant substance in the air enters the nasal cavity through the airways → reaches the olfactory epithelium → dissolves in the mucus surrounding the cilia of receptor cells → one of its active centers binds to a molecular receptor (protein) built into the membrane of the olfactory neurosensory cell (olfactory sensory receptor ). 2. Transduction of chemical stimulation into nervous excitation. Attachment of an irritant molecule (ligand) to a receptor molecule → the conformation of the receptor molecule changes → a cascade of biochemical reactions is launched with the participation of G-protein and adenylate cyclase → c is produced. AMP (cyclic adenosine monophosphate)→protein kinase is activated→it phosphorylates and opens ion channels in the membrane, permeable to three types of ions: Na+, K+, Ca 2+→. . . →a local electrical potential (receptor) arises→the receptor potential reaches a threshold value (critical level of depolarization)→an action potential and a nerve impulse are generated (generated).

3. Movement of afferent olfactory sensory excitation to the lower nerve center. The nerve impulse, resulting from transduction in the neurosensory olfactory cell, runs along its axon as part of the olfactory nerve into the olfactory bulb (olfactory lower nerve center). 4. Transformation in the lower nerve center of afferent (incoming) olfactory excitation into efferent (outgoing) excitation. 5. Movement of efferent olfactory excitation from the lower nerve center to the higher nerve centers. 6. Perception - construction of a sensory image of irritation (irritant) in the form of a sensation of smell.

ADAPTATION OF THE OLfactory ANALYZER Adaptation of the olfactory analyzer can be observed during prolonged exposure to an odor stimulus. Adaptation to the action of an odorous substance occurs rather slowly within 10 seconds or minutes and depends on the duration of action of the substance, its concentration and the speed of air flow (sniffing). In relation to many odorous substances, complete adaptation occurs quite quickly, i.e. their smell ceases to be felt. A person ceases to notice such continuously acting stimuli as the smell of his body, clothes, room, etc. In relation to a number of substances, adaptation occurs slowly and only partially. With short-term exposure to a weak taste or olfactory stimulus: adaptation can manifest itself in an increase in the sensitivity of the corresponding analyzer. It has been established that changes in sensitivity and adaptation phenomena mainly occur not in the peripheral, but in the cortical part of the taste and olfactory analyzers. Sometimes, especially with frequent exposure to the same taste or olfactory stimulus, a persistent focus of increased excitability appears in the cerebral cortex. In such cases, the sensation of taste or smell to which increased excitability has arisen may also appear under the influence of various other substances. Moreover, the sensation of a corresponding smell or taste can become intrusive, appearing even in the absence of any taste or odor stimuli, in other words, illusions and hallucinations arise. If you say during lunch that a dish is rotten or sour, then some people develop corresponding olfactory and gustatory sensations, as a result of which they refuse to eat. Adaptation to one odor does not reduce sensitivity to odorants of another type, since different odorants act on different receptors.

TYPES OF OLfactory impairment: 1) anosmia – absence; 2) hyposmia – decrease; 3) hyperosmia – increased olfactory sensitivity; 4) parosmia – incorrect perception of odors; 5) impaired differentiation; 5) olfactory hallucinations, when olfactory sensations occur in the absence of odorous substances; 6) olfactory agnosia, when a person smells a smell, but does not recognize it. With age, there is mainly a decrease in olfactory sensitivity, as well as other types of functional disorders of smell.

The olfactory analyzer is represented by two systems - the main and vomeronasal, each of which has three parts: peripheral (olfactory organs), intermediate, consisting of conductors (axons of neurosensory olfactory cells and nerve cells of the olfactory bulbs), and central, localized in the hippocampus of the cerebral cortex for main olfactory system.

Main organ of smell ( organum olfactus), which is a peripheral part of the sensory system, is represented by a limited area of the nasal mucosa - the olfactory area, covering in humans the upper and partly middle concha of the nasal cavity, as well as the upper part of the nasal septum. Externally, the olfactory region differs from the respiratory part of the mucous membrane in a yellowish color.

The peripheral part of the vomeronasal, or accessory, olfactory system is the vomeronasal (Jacobson) organ ( organum vomeronasale Jacobsoni). It looks like paired epithelial tubes, closed at one end and opening at the other end into the nasal cavity. In humans, the vomeronasal organ is located in the connective tissue of the base of the anterior third of the nasal septum on both sides of it at the border between the septal cartilage and the vomer. In addition to the Jacobson's organ, the vomeronasal system includes the vomeronasal nerve, the terminal nerve and its own representation in the forebrain - the accessory olfactory bulb.

The functions of the vomeronasal system are associated with the functions of the genital organs (regulation of the sexual cycle and sexual behavior), and are also associated with the emotional sphere.

Development. The olfactory organs are of ectodermal origin. The main organ develops from placode- thickenings of the anterior part of the ectoderm of the head. The olfactory pits are formed from the placodes. In human embryos at the 4th month of development, supporting epithelial cells and neurosensory olfactory cells are formed from the elements that make up the walls of the olfactory pits. The axons of the olfactory cells, united with each other, form a total of 20-40 nerve bundles (olfactory pathways - fila olfactoria), rushing through the holes in the cartilaginous anlage of the future ethmoid bone to the olfactory bulbs of the brain. Here synaptic contact is made between the axon terminals and the dendrites of the mitral neurons of the olfactory bulbs. Some areas of the embryonic olfactory lining, plunging into the underlying connective tissue, form the olfactory glands.

The vomeronasal (Jacobson) organ is formed in the form of a paired anlage at the 6th week of development from the epithelium of the lower part of the nasal septum. By the 7th week of development, the formation of the cavity of the vomeronasal organ is completed, and the vomeronasal nerve connects it with the accessory olfactory bulb. In the vomeronasal organ of the fetus of the 21st week of development there are supporting cells with cilia and microvilli and receptor cells with microvilli. The structural features of the vomeronasal organ indicate its functional activity already in the perinatal period.

Structure. The main organ of smell - the peripheral part of the olfactory analyzer - consists of a layer of multirow epithelium 60-90 μm high, in which three types of cells are distinguished: olfactory neurosensory cells, supporting and basal epithelial cells. They are separated from the underlying connective tissue by a well-defined basement membrane. The surface of the olfactory lining facing the nasal cavity is covered with a layer of mucus.

Receptor, or neurosensory, olfactory cells (cellulae neurosensoriae olfactoriae) are located between the supporting epithelial cells and have a short peripheral process - the dendrite and a long central one - the axon. Their nuclear-containing parts, as a rule, occupy a middle position in the thickness of the olfactory lining.

In dogs, which have a well-developed olfactory organ, there are about 225 million olfactory cells; in humans, their number is much smaller, but still reaches 6 million (30 thousand per 1 mm2). The distal parts of the dendrites of the olfactory cells end with characteristic thickenings - olfactory clubs (clava olfactoria). The olfactory clubs of cells on their rounded apex bear up to 10-12 mobile olfactory cilia.

The cytoplasm of the peripheral processes contains mitochondria and microtubules with a diameter of up to 20 nm elongated along the axis of the process. Near the nucleus in these cells, a granular endoplasmic reticulum is clearly visible. The club cilia contain longitudinally oriented fibrils: 9 pairs of peripheral and 2 central, extending from the basal bodies. Olfactory cilia are mobile and act as antennas for molecules of odorous substances. The peripheral processes of olfactory cells can contract under the influence of odorous substances. The nuclei of olfactory cells are light, with one or two large nucleoli. The nasal part of the cell continues into a narrow, slightly winding axon that passes between the supporting cells. In the connective tissue layer, the central processes form bundles of the unmyelinated olfactory nerve, which are combined into 20-40 olfactory filaments ( filia olfactoria) and through the openings of the ethmoid bone are directed to the olfactory bulbs.

Supporting epithelial cells (epitheliocytus sustentans) form a multirow epithelial layer in which the olfactory cells are located. On the apical surface of the supporting epithelial cells there are numerous microvilli up to 4 µm long. Supporting epithelial cells show signs of apocrine secretion and have a high metabolic rate. Their cytoplasm contains the endoplasmic reticulum. Mitochondria mostly accumulate in the apical part, where there are also a large number of granules and vacuoles. The Golgi apparatus is located above the nucleus. The cytoplasm of the supporting cells contains a brown-yellow pigment.

Basal epithelial cells (epitheliocytus basales) are located on the basement membrane and are equipped with cytoplasmic projections surrounding the axon bundles of olfactory cells. Their cytoplasm is filled with ribosomes and does not contain tonofibrils. There is an opinion that basal epithelial cells serve as a source of regeneration of receptor cells.

The epithelium of the vomeronasal organ consists of receptor and respiratory parts. The receptor part is similar in structure to the olfactory epithelium of the main olfactory organ. The main difference is that the olfactory clubs of the receptor cells of the vomeronasal organ bear on their surface not cilia capable of active movement, but immobile microvilli.

The intermediate, or conductive, part of the main olfactory sensory system begins with olfactory unmyelinated nerve fibers, which are united into 20-40 thread-like trunks ( fila olfactoria) and through the openings of the ethmoid bone are directed to the olfactory bulbs. Each olfactory filament is an unmyelinated fiber containing from 20 to 100 or more axial cylinders of receptor cell axons embedded in lemmocytes. The second neurons of the olfactory analyzer are located in the olfactory bulbs. These are large nerve cells called mitral, have synaptic contacts with several thousand axons of neurosensory cells of the same, and partly the opposite, side. The olfactory bulbs are built like the cerebral cortex, have 6 concentrically located layers: 1 - layer of olfactory fibers, 2 - glomerular layer, 3 - outer reticular layer, 4 - layer of mitral cell bodies, 5 - internal reticulate, 6 - granular layer .

Contact of the axons of neurosensory cells with the dendrites of the mitral cells occurs in the glomerular layer, where the excitations of the receptor cells are summed up. This is where receptor cells interact with each other and with small associative cells. In the olfactory glomeruli, centrifugal efferent influences emanating from overlying efferent centers (anterior olfactory nucleus, olfactory tubercle, nuclei of the amygdala complex, prepiriform cortex) are also realized. The outer reticular layer is formed by the bodies of tufted cells and numerous synapses with additional dendrites of mitral cells, axons of interglomerular cells and dendro-dendritic synapses of mitral cells. The 4th layer contains the bodies of mitral cells. Their axons pass through the 4th-5th layers of the bulbs, and at the exit from them they form olfactory contacts together with the axons of tufted cells. In the region of the 6th layer, recurrent collaterals depart from the axons of the mitral cells and are distributed in different layers. The granular layer is formed by an accumulation of granule cells, which in their function are inhibitory. Their dendrites form synapses with recurrent collaterals of the axons of mitral cells.

The intermediate, or conductive, part of the vomeronasal system is represented by unmyelinated fibers of the vomeronasal nerve, which, like the main olfactory fibers, unite into nerve trunks, pass through the openings of the ethmoid bone and connect to the accessory olfactory bulb, which is located in the dorsomedial part of the main olfactory bulb and has a similar structure .

The central section of the olfactory sensory system is localized in the ancient cortex - in the hippocampus and in the new - hippocampal gyrus, where the axons of the mitral cells (olfactory tract) are sent. This is where the final analysis of the olfactory information takes place.

The sensory olfactory system is connected through the reticular formation to the autonomic centers, which explains the reflexes from the olfactory receptors to the digestive and respiratory systems.

It has been established in animals that from the accessory olfactory bulb the axons of the second neurons of the vomeronasal system are directed to the medial preoptic nucleus and the hypothalamus, as well as to the ventral region of the premammillary nucleus and the middle amygdala nucleus. The connections between the projections of the vomeronasal nerve in humans have so far been little studied.

Olfactory glands. In the underlying loose fibrous tissue of the olfactory region there are the terminal sections of the tubular-alveolar glands, which secrete a secretion that contains mucoproteins. The terminal sections consist of two types of elements: on the outside there are more flattened cells - myoepithelial ones, on the inside there are cells secreting the merocrine type. Their clear, watery secretion, together with the secretion of supporting epithelial cells, moisturizes the surface of the olfactory lining, which is a necessary condition for the functioning of olfactory cells. In this secretion, washing the olfactory cilia, odorous substances dissolve, the presence of which only in this case is perceived by receptor proteins embedded in the membrane of the cilia of the olfactory cells.

Vascularization. The mucous membrane of the nasal cavity is abundantly supplied with blood and lymphatic vessels. Microcirculatory vessels resemble corpora cavernosa. Blood capillaries of the sinusoidal type form plexuses that are capable of depositing blood. Under the influence of sharp temperature stimuli and molecules of odorous substances, the nasal mucosa can swell greatly and become covered with a significant layer of mucus, which complicates nasal breathing and olfactory reception.

Age-related changes. Most often they are caused by inflammatory processes suffered during life (rhinitis), which lead to atrophy of receptor cells and proliferation of the respiratory epithelium.

Regeneration. In mammals during postnatal ontogenesis, renewal of olfactory receptor cells occurs within 30 days (due to poorly differentiated basal cells). At the end of the life cycle, neurons undergo destruction. Poorly differentiated neurons of the basal layer are capable of mitotic division and lack processes. During their differentiation, the volume of cells increases, a specialized dendrite appears, growing towards the surface, and an axon grows towards the basement membrane. The cells gradually move to the surface, replacing dead neurons. Specialized structures (microvilli and cilia) are formed on the dendrite.

The olfactory analyzer is represented by two systems - the main and vomeronasal, each of which has three parts:

Peripheral (olfactory organs - nasal neuroepithelium);

Intermediate, consisting of conductors (axons of neurosensory olfactory cells and nerve cells of the olfactory bulbs);

Central (paleocortical, thalamic, hypothalamic and neocortical projections).

The human hoc has three chambers: lower, middle and upper. The lower and middle chambers essentially perform a sanitary role, warming and purifying the inhaled air. The main organ of smell, which is a peripheral part of the sensory system, is represented by a limited area of the nasal mucosa - olfactory area, covering in humans the upper and partly middle concha of the nasal cavity, as well as the upper part of the nasal septum. Externally, the olfactory region differs from the respiratory part of the mucous membrane in a yellowish color, due to the presence of pigment in the cells. There is no convincing evidence for the participation of this pigment in the reception of odors.

Olfactory epithelium, lining the olfactory region of the nose, has a thickness of 100-150 microns and contains three types of cells:

1 – olfactory (receptive),

2 – support,

3 – basal (regenerative).

In the connective tissue layer of the olfactory lining in terrestrial vertebrates there are the terminal sections of the Bowman's glands, the secretion of which covers the surface of the olfactory epithelium.

The number of olfactory receptors is very large and is largely determined by the area occupied by the olfactory epithelium and the density of receptors in it. In general, in this regard, humans are classified as poorly smelling creatures (microsmatics). For example, in a number of animals - dogs, rats, cats, etc. - the olfactory system is much more developed (macromatic).

Rice. Scheme of the structure of the olfactory epithelium: OB - olfactory club; OK - supporting cell; CO - central processes of olfactory cells; BC - basal cell; BM - basement membrane; OL – olfactory hairs; MVR – olfactory microvilli; MVO – microvilli of supporting cells

Olfactory receptor cell- a bipolar cell with a spindle shape. On the surface of the receptor layer, it thickens in the form of an olfactory club, from which hairs (cilia) extend; each hair contains microtubules (9+2). The central processes of the olfactory receptors are unmyelinated nerve fibers that are collected in bundles of 10-15 fibers (olfactory filaments) and, passing through the openings of the ethmoid bone, are directed to the olfactory bulb of the brain.

Like taste cells and photoreceptor outer segments, olfactory cells are constantly renewed. The lifespan of an olfactory cell is about 2 months.

Reception mechanisms. Odor molecules come into contact with the olfactory mucosa. It is assumed that the receiver of odor molecules are protein macromolecules, which change their conformation when odor molecules are attached to them. This causes the opening of sodium channels in the plasma membrane of the receptor cell and, as a result, the generation of a depolarizing receptor potential, which leads to a pulse discharge in the receptor axon (olfactory nerve fiber).

Olfactory cells are capable of responding to millions of different spatial configurations of odorant molecules. Meanwhile, each receptor cell is capable of responding with physiological excitation to its characteristic, albeit wide, range of odorous substances. Previously, it was believed that the low selectivity of an individual receptor is explained by the presence of many types of olfactory receptor proteins in it, but it has recently been found that each olfactory cell has only one type of membrane receptor protein. This protein itself is capable of binding many odorous molecules of various spatial configurations. Rule “one olfactory cell - one olfactory receptor protein” significantly simplifies the transmission and processing of information about odors in the olfactory bulb - the first nerve center for switching and processing chemosensory information in the brain.

When odorous substances act on the olfactory epithelium, a multicomponent electrical potential is recorded from it. Electrical processes in the olfactory mucosa can be divided into slow potentials, reflecting excitation of the receptor membrane, and fast (spike) activity belonging to single receptors and their axons. The slow total potential includes three components: positive potential, negative potential to turn on (it is called electroophthalmogram, EOG) and negative potential to turn off. Most researchers believe that EOG is the generator potential of olfactory receptors.

Rice. Diagram of the structure of the olfactory system. (The processes of neurons carrying different receptors go to different glomeruli of the olfactory bulb)

Structure and function of the olfactory bulb. The olfactory pathway first switches in the olfactory bulb, which belongs to the cerebral cortex. In the paired human olfactory bulb, six layers are distinguished, which are located concentrically, counting from the surface:

Layer I - fibers of the olfactory nerve;

Layer II is a layer of olfactory glomeruli (glomeruli), which are spherical formations with a diameter of 100-200 microns, in which the first synaptic switching of olfactory nerve fibers to neurons of the olfactory bulb occurs;

Layer III - outer reticular, containing tuft cells; The dendrite of such a cell, as a rule, comes into contact with several glomeruli;

Layer IV is a layer of mitral cell bodies containing the largest cells of the olfactory bulb - mitral cells. These are large neurons (soma diameter at least 30 μm) with a well-developed apical dendrite of large diameter, which is connected to only one glomerulus. The axons of mitral cells form olfactory tract, which also includes the axons of tuft cells. Within the olfactory bulb, the axons of the mitral cells give off numerous collaterals that form synaptic contacts in various layers of the olfactory bulb;

V layer - (internal reticular);

Layer VI is the granular layer. The bodies of granule cells are contained here. The layer of granule cells directly passes into the cell masses of the so-called anterior olfactory nucleus, which is classified as a third-order olfactory center.

In response to adequate stimulation, a slow long-term potential is recorded in the olfactory bulb, at the ascending front and apex of which evoked waves are recorded. They arise in the olfactory bulb of all vertebrates, but their frequency varies. The role of this phenomenon in odor recognition is not clear, but it is believed that the rhythm of electrical oscillations is formed by postsynaptic potentials in the bulb.

Mitral cells unite their axons into bundles of the olfactory tract, which from the bulb goes to the structures of the olfactory brain .

Olfactory tract forms the olfactory triangle, where the fibers are divided into separate bundles. Some of the fibers go to the uncus of the hippocampus, the other part passes through the anterior commissure to the opposite side, the third group of fibers goes to the septum pellucida, the fourth group to the anterior perforated substance. In the hippocampal hook there is the cortical end of the olfactory analyzer, which is connected with the thalamus, hypothalamic nuclei, and structures of the limbic system.

Structure and function of the central section of the olfactory analyzer.

The fibers of the olfactory tract terminate in various parts of the forebrain: in the anterior olfactory nucleus, the lateral part of the olfactory tubercle, the prepiriform and periamygdala areas of the cortex, as well as in the adjacent corticomedial part of the amygdala complex, including the nucleus of the lateral olfactory tract, which is believed to , fibers also come from the accessory olfactory bulb. Connections between the olfactory bulb and the hippocampus and other parts of the olfactory brain in mammals occur through one or more switches. From the primary olfactory cortex, nerve fibers are directed to the medioventral nucleus of the thalamus, to which there is also direct input from the taste system. The fibers of the medioventral nucleus of the thalamus, in turn, are directed to the frontal region of the neocortex, which is considered as the highest integrative center of the olfactory system. Fibers from the prepiriform cortex and the olfactory tubercle go in the caudal direction, becoming part of the medial forebrain bundle. The endings of the fibers of this bundle are found in the hypothalamus.

Thus, the peculiarity of the olfactory system is, in particular, that its afferent fibers on the way to the cortex do not switch in the thalamus and do not move to the opposite side of the cerebrum. It has been shown that the presence of a significant number of centers of the olfactory brain is not necessary for the recognition of odors, therefore, most of the nerve centers into which the olfactory tract is projected can be considered as associative centers that ensure the connection of the olfactory sensory system with other sensory systems and the organization on this basis of a number of complex forms behavior - food, defensive, sexual. From the description of these centers, the close connection of the sense of smell with eating and sexual behavior becomes clear.

The efferent regulation of the activity of the olfactory bulb has not yet been sufficiently studied, although there are morphological prerequisites indicating the possibility of such influences.

Coding of olfactory information. Based on some psychophysiological observations of human perception of odors There are 7 primary odors: musky, camphorous, floral, ethereal, minty, pungent and putrefactive.

According to the theory of J. Eymour and R. Moncrieff (stereochemical theory), the smell of a substance is determined by the shape and size of the odorous molecule, which in configuration fits the receptor site of the membrane “like a key to a lock.” The concept of different types of receptor sites interacting with specific odorant molecules suggests the presence of seven types of receptive sites. The receptive sites are in close contact with the odorant molecules, and the charge of the membrane region changes and a potential arises in the cell.

As studies using microelectrodes show, single receptors respond to stimulation by increasing the frequency of impulses, which depends on the quality and intensity of the stimulus. Each olfactory receptor responds not to one, but to many odorous substances, giving “preference” to some of them. It is believed that the coding of odors and their recognition in the centers of the olfactory sensory system can be based on these properties of receptors, which differ in their tuning to different groups of substances. Electrophysiological studies of the olfactory bulb revealed that the electrical response recorded in it during the action of an odor depends on the odorous substance: with different odors, the spatial mosaic of excited and inhibited areas of the bulb changes

Sensitivity of the human olfactory system. This sensitivity is quite large: one olfactory receptor can be excited by one molecule of an odorant, and the stimulation of a small number of receptors leads to a sensation. At the same time, the change in the intensity of the action of substances (discrimination threshold) is assessed by people quite roughly (the smallest perceived difference in the strength of the odor is 30-60% of its initial concentration). In dogs, these figures are 3-6 times higher.

Adaptation of the olfactory analyzer can be observed with prolonged exposure to an odorous substance. Adaptation occurs rather slowly over 10 seconds or minutes and depends on the duration of action of the substance, its concentration and the speed of air flow (sniffing). In relation to many odorous substances, complete adaptation occurs quite quickly, i.e. their smell ceases to be felt. A person ceases to notice such continuously acting stimuli as the smell of his body, clothes, room, etc. In relation to a number of substances, adaptation occurs slowly and only partially. With short-term exposure to a weak taste or olfactory stimulus: adaptation can manifest itself in an increase in the sensitivity of the corresponding analyzer. It has been established that changes in sensitivity and adaptation phenomena mainly occur not in the peripheral, but in the cortical part of the taste and olfactory analyzers. Sometimes, especially with frequent exposure to the same taste or olfactory stimulus, a persistent focus of increased excitability appears in the cerebral cortex. In such cases, the sensation of taste or smell to which increased excitability has arisen may also appear under the influence of various other substances. Moreover, the sensation of a corresponding smell or taste can become intrusive, appearing even in the absence of any gustatory or odor stimuli, in other words, illusions arise, and hallucinations. If you say during lunch that a dish is rotten or sour, then some people develop corresponding olfactory and gustatory sensations, as a result of which they refuse to eat. Adaptation to one odor does not reduce sensitivity to odorants of another type, because Different odorants act on different receptors.

Functions of the olfactory analyzer. With the participation of the olfactory analyzer, orientation in the surrounding space is carried out and the process of cognition of the external world occurs. It influences eating behavior, takes part in testing food for edibility, in setting up the digestive apparatus to process food (through the mechanism of a conditioned reflex), and also in defensive behavior, helping to avoid danger due to the ability to distinguish substances harmful to the body. Humans have a sense of smell. effectively contribute to the retrieval of information from memory. Thus, the reaction to odors is not only the work of the senses, but also a social experience. Through smells we are able to restore the atmosphere of past years or regain memories associated with specific life circumstances. The sense of smell plays a significant role in the emotional sphere of a person.

In addition, “olfactory memory” has an equally important biological purpose. Despite the fact that a person’s image of a “second half” is built mainly on the basis of information obtained through vision and hearing, individual body odor is also a guideline for recognizing a suitable object for successful procreation. To more effectively perceive these odors and react accordingly to them, nature has created an “auxiliary” olfactory system vomeronasal system.

The peripheral part of the vomeronasal, or accessory, olfactory system is vomeronasal (Jacobson) organ. It looks like paired epithelial tubes, closed at one end and opening at the other end into the nasal cavity. In humans, the vomeronasal organ is located in the connective tissue of the base of the anterior third of the nasal septum on both sides of it at the border between the septal cartilage and the vomer. In addition to the Jacobson's organ, the vomeronasal system includes the vomeronasal nerve, the terminal nerve and its own representation in the forebrain - the accessory olfactory bulb.

The functions of the vomeronasal system are associated with the functions of the genital organs (regulation of the sexual cycle and sexual behavior), and with the emotional sphere.

It has been established in animals that from the accessory olfactory bulb the axons of the second neurons of the vomeronasal system are directed to the medial preoptic nucleus and the hypothalamus, as well as to the ventral region of the premammillary nucleus and the middle nucleus of the amygdala. The connections between the projections of the vomeronasal nerve in humans have so far been little studied.

Olfactory sensory system (OSS)

The olfactory sensory system (OSS) is a structural and functional complex that provides the perception and analysis of odors

The value of NSS for humans:

Provides reflex stimulation of the digestive center;

Provides a protective effect by recognizing the chemical composition of the environment in which the body is located;

Increases the overall tone of the nervous system (especially pleasant smells)

Involved in emotional behavior;

Plays a protective role, including sneezing, coughing and breath-holding reflexes (when inhaling ammonia vapor);

Involved in the formation of the sense of taste (with a severe runny nose, food loses its taste)

In animals, it also ensures the search for food.

The first classification of odors was made by Eymur, taking into account the source of origin: camphor, floral, musky, minty, ethereal, acrid and putrefactive. To perceive odor, an odorous substance must have two properties: be soluble and volatile. This is probably why odors are better perceived in humid air and when it moves (before rain).

Normal perception of smell is called normoosmia, absence - anosmia, decreased perception of smell - hypoosmia, increased perception - hyperosmia, disturbances - dysosmia.

It should be emphasized that some substances cause a maximum reaction, others - a weak one, and others - inhibition of receptor cells.

Structural and functional characteristics of the peripheral part of the olfactory sensory system

Olfactory receptors are exteroceptive, chemoreceptive, primary sensitive, they are characterized by spontaneous activity and the ability to adapt.

The olfactory epithelium is “hidden” in the nasal mucosa, covering 10 cm2 of the roof of the nasal cavity at the nasal septum (Fig. 12.32) in the form of islands with an area of about 240 mm2.

The olfactory epithelium contains approximately 10-20 million receptor cells.

The olfactory epithelium is located away from the respiratory tract. Therefore, to feel the smell, you need to sniff, that is, take a deep breath. In the case of quiet breathing, only 5% of the air passes through the olfactory epithelium.

The surface of the epithelium is covered with mucus, which controls the accessibility of odorant substances - odorants - to the receptor surface.

The olfactory cell has a central sprout - the axon and a peripheral sprout - dendrites. At the end of the dendrite there is a thickening - a club. On the surface of the club there are microvilli (10-20) with a diameter of up to 0.3 microns and a length of up to 10 microns. It is thanks to them that the surface of the olfactory epithelium increases significantly and its area can exceed several times the area of the body. The olfactory club is the cytochemical center of the olfactory cell. Olfactory cells are constantly renewed. their life lasts two months. Olfactory cells are characterized by constant spontaneous activity, which is modulated by the action of odorants. In addition to receptor cells, the olfactory epithelium contains supporting and basal cells (Fig. 12.33). The respiratory part of the nose, where there are no olfactory cells, receives the endings of the trigeminal nerve (p. Trigeminus), which can also react to the smell (ammonia). The glossopharyngeal nerve is also involved in the perception of some odors. (n. Glossopharyngeus). Therefore, the sense of smell does not completely disappear even after sectioning the olfactory nerve on both sides.

Mechanism of excitation of olfactory receptor cells

Many theories of smell have been created. Among them, the stereochemical theory formulated in 1949 by Moncrieff deserves attention. Its meaning lies in the fact that the olfactory system is built from different receptor cells. Each of these cells perceives one odor. The test proved that musky, camphor, mint, floral, ethereal odors are inherent in substances whose molecules, like a “key to a lock,” fit into the chemoreceptor substances of the olfactory cells. According to the stereochemical theory, from primary odors all others can be formed according to the type of three primary co-

Rice. 12.32. Diagram of the olfactory mucosa:

V - trigeminal nerve, IX - glossopharyngeal nerve, X - vagus nerve

Lera (red - blue - green), from which all the others are formed.

Olfactory receptors contain about 1000 types of receptor proteins with which odorants interact. Proteins encode about 1000 genes, which is approximately 3% of the entire gene pool and only emphasizes the importance of the olfactory analyzer. After the odorant molecule binds to the receptor, a system of second messengers is activated, in particular G-protein, which activates adenylate cyclase, and adenosine triphosphate is converted to cAMP. This leads to the opening of ion channels, the entry of positively charged ions and the occurrence of depolarization, that is, a nerve impulse.

2004 Nobel Prize laureates G. Excel and L. Buck proved that there are no specific receptors for each individual smell. Instead, there is a "receptor alphabet." A particular smell activates a specific combination of receptors, which in turn send a specific sequence of nerve impulses, then is decoded by neurons in the brain, such as forming words from letters or music from notes, and the sensation of a particular smell is created. .

In this sense, an allegorical expression even appeared: we smell not with our nose, but with our brain.

A person can simultaneously identify only three odors. If there are more than ten odors, she is not able to recognize any of them.

A very close connection between the olfactory apparatus and the reproductive system. The acuity of smell perception depends on the level of steroid hormones in the body, including sex hormones. This is indicated by facts, diseases associated with reproductive dysfunction, accompanied by a decrease or loss of the ability to perceive odors. With the help of an olfactory analyzer, pheromones influence our body. There is an opinion that we like the smell of those people who are very different from us genetically. Also interesting is the fact that the axons of the olfactory neurons bypass the thalamus - the collector of all sensory pathways - and are directed to the olfactory bulbs, which are part of the ancient cortex - the limbic system, which is responsible for memory, emotions, and sexual behavior.

Rice. 12.33. The structure of the olfactory epithelium

Unsolved mysteries hide the meaning of smell unknown to us. Why is this sensation provided by such a significant number of genes and has a close connection with the ancient formations of the brain?

Wire and brain divisions of the olfactory sensory system

The olfactory sensory system pathways, unlike the others, do not pass through the thalamus. The body of the first neuron is represented by the olfactory receptor cell as the primary sensitive receptor. The axons of these cells form groups of 20-100 fibers. They make up the olfactory nerve, which goes to the olfactory bulb. The body of the second neuron, the mitral cell, is located there. In the olfactory bulb there is a topical localization of the olfactory epithelium. As part of the axons of the mitral cells, impulses are sent to the hook, that is, to the pyriform or periamygdala cortex. Some fibers reach the anterior hypothalamus and amygdala and other parts.

When exposed to different odors, the spatial mosaic of excited and inhibited cells changes in the olfactory bulb. This is reflected in the specifics of electrical activity. Thus, the nature of electrical activity depends on the characteristics of the odorous substance.

It is believed that the olfactory bulbs are sufficient to preserve the olfactory function. An essential role of the anterior hypothalamus, its irritation causes sniffing. Thanks to the connections of the olfactory brain with the limbic cortex (hippocampus), amygdala, and hypothalamus, the olfactory component of emotions is provided. Thus, a large number of centers are involved in the olfactory function.

Thresholds of olfactory sensation. adaptation

There are thresholds for identifying the presence of an odor and thresholds for recognizing an odor. The threshold of smell (the appearance of sensation) is determined by the minimum amount of an odorous substance, which makes it possible to establish its presence. The recognition threshold is the minimum amount of an odorous substance that allows the smell to be identified. For vanillin, for example, the recognition threshold is 8 × 10-13 mol/l. Thresholds vary depending on a number of factors: physiological state (during the menstrual period - exacerbation in women), age (in older people - increase), air humidity (decreased in a humid environment), the speed of air movement through the nasal respiratory tract. Thresholds in deaf-blind people are significantly reduced. Despite the fact that a person is able to distinguish up to 10,000 different odors, her ability to assess their intensity is very low. The sensation is enhanced only if the irritation increases by at least 30% compared to the initial value.

Adaptation of the olfactory sensory system occurs slowly and lasts tens of seconds or minutes. It depends on the speed of air movement and the concentration of the odorous substance. Cross adaptation occurs. With prolonged exposure to any odorant, the threshold not only for it, but also for other odorous substances increases. The sensitivity of the olfactory sensory system is regulated by the sympathetic nervous system.

Hyperosmia is sometimes observed with hypothalamic syndrome, hypoosmia - under the influence of radiation. Olfactory hallucinations may accompany epilepsy. Anosmia can be caused by hypogonadism.

Organoleptic method— a method for controlling the quality of drinks and food products, based on testing their properties for taste and smell; used in food production and perfumery. Smell and taste are essential chemical characteristics of a substance.

Sensory taste system

Taste- a sensation that occurs when a substance acts on taste buds located on the surface of the tongue and in the mucous membrane of the oral cavity. Taste sensations are perceived by a person in combination with sensations of heat, cold, pressure and smell of substances entering the oral cavity.

❖ The role of taste. They allow:

■ determine the quality of food;

■ trigger digestive juice secretion reflexes;

■ stimulate the absorption of those substances that are necessary for the body, but are rarely found.

Basic tastes: bitter, salty, sour, sweet.

Taste sensory system carries out the perception and analysis of chemical stimuli acting on the taste organs.

Taste receptor cells with microvilli located inside taste buds . Receptor cells come into contact with food, the molecules of which cause the formation of corresponding nerve impulses in the receptors.

■ Taste buds respond only to substances dissolved in water.

Taste buds located in taste buds, which are outgrowths (folds) of the mucous membrane of the tongue.

The largest clusters of receptors are at the tip, edges, and root (back) of the tongue.

❖ Sensitive areas of the tongue:

■ sweet stimulates the receptors of the tip of the tongue;

■ bitter stimulates the receptors of the root of the tongue;

■ salty excites receptors on the edges and front of the tongue;

■ sour excites receptors on the lateral edges of the tongue.

Adjacent to the receptor cells are the nerve fibers covering them, which enter the brain as part of the cranial nerves. Through them, nerve impulses enter the posterior central gyrus of the cerebral cortex, where taste sensations are formed.

Adaptation to taste- a decrease in taste sensations with prolonged action on taste buds of substances of the same taste. Adaptation occurs most quickly to salty and sweet substances, and slower to sour and bitter ones.

■ Pepper, mustard and similar products restore the sense of taste and stimulate appetite.

Sensory olfactory system

Smell- the body’s ability to perceive the odors of various chemicals in the air.

Smell- a sensation that occurs when a chemical substance in the air acts on the olfactory (chemical) receptors located in the mucous membrane of the nasal cavity. The number of types of odors perceived by humans is almost endless.

Olfactory sensory system carries out the perception and analysis of chemical stimuli (odors) located in the external environment and acting on the olfactory organs.

■ The molar concentration of a substance that can be smelled by a person is about 10 -14 mol/l, i.e. just a few molecules per liter of air.

The peripheral section of the olfactory analyzer is presented olfactory epithelium nasal cavity containing numerous sensory cells - olfactory chemoreceptors .

Olfactory chemoreceptors are neurons whose dendrites end in the mucous membrane of the nasal cavity. The ends of the dendrites have numerous microscopic cavities of various shapes. Molecules of volatile substances that enter the nasal cavity along with the inhaled air come into contact with the ends of the dendrites. If the shape and size of the molecule coincide with the shape and size of one of the depressions on the surface of the receptor (dendrite), then it (the molecule) “fits” into this depression, causing the appearance of a corresponding nerve impulse. At the same time, pulses generated by cavities of different shapes, and therefore different molecules, have different characteristics, which makes it possible to distinguish the odors of different substances.

Olfactory receptor cells in the mucosa are found among the ciliated supporting cells.

The axons of the olfactory neurons form the olfactory nerve, which passes into the cranial cavity. Next, excitation is carried out to the olfactory centers of the cerebral cortex, in which smell recognition is carried out.

Adaptation to smell- reduction in the sensation of smell of a given substance due to its prolonged action on the olfactory receptors. At the same time, the acuteness of perception to other odors is preserved.