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

The olfactory system and its sensory characteristics The olfactory neuroepithelium serves as the olfactory neuroepithelium, which arises as a protrusion of the cerebral tube and contains olfactory cells - chemoreceptors, which are excited by gaseous substances.

CHARACTERISTIC OF AN ADEQUATE IRRITANT An adequate irritant for the olfactory sensory system is the smell, which is emitted by odorous substances. All odoriferous substances must be volatile in order to enter the nasal cavity with air, and water-soluble in order to penetrate to the receptor cells through the layer of mucus that covers the entire epithelium of the nasal cavities. A huge number of substances satisfy such requirements, and therefore a person is able to distinguish thousands of all kinds of smells. It is important that there is no strict correspondence between the chemical structure of a "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. Tuning the digestive system to process food by the mechanism of unconditioned and conditioned reflexes. 4. Launch of defensive behavior due to the detection of substances harmful to the body or substances associated with danger. 5. Motivation and modulation of sexual behavior through the detection of odorous substances and pheromones.

STRUCTURAL AND FUNCTIONAL CHARACTERISTICS OF THE SMELL ANALYZER. - The peripheral section is formed by the receptors of the upper nasal passage of the mucous membrane of the nasal cavity. The olfactory receptors in the nasal mucosa terminate in olfactory cilia. Gaseous substances dissolve in the mucus surrounding the cilia, then a nerve impulse arises as a result of a chemical reaction. - The conduction department - the olfactory nerve. Through the fibers of the olfactory nerve, impulses enter the olfactory bulb (the structure of the forebrain, in which information is processed) and then follow to the cortical olfactory center. - The central section is the cortical olfactory center located on the lower surface of the temporal and frontal lobes of the cerebral cortex. In the bark, the smell is determined and an adequate reaction of the body to it is formed.

PERIPHERAL DEPARTMENT This department begins with the primary sensory olfactory sensory receptors, which are the ends 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 distant part of the dendrite of such a cell is changed. It expands into a "olfactory club", from which 6–12 cilia extend, while a normal axon extends from the base of the cell. Humans have about 10 million olfactory receptors. In addition, in addition to the olfactory epithelium, additional receptors are also found 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 odorous substances.

The cilia, or olfactory hairs, are immersed in a liquid medium - a layer of mucus produced by the Bowman glands in the nasal cavity. The presence of olfactory hairs significantly increases the area of contact of the receptor with the molecules of odorous substances. The movement of the hairs provides an active process of capturing the molecules of the odorous substance and contact with 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 cells.

The reception of smells is carried out by 3 types of olfactory neurons: 1. Olfactory receptor neurons (ORNs) in the main epithelium. 2. GC-D-neurons in the main epithelium. 3. Vomeronasal neurons (VNNs) in the vomeronasal epithelium. The VNO 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 its smell. Each type of predator has its own special receptor-detector. These three types of neurons differ from each other in the mode of transduction 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 OLFACTORY ANALYZER: A - diagram of the structure of the nasal cavity: 1 - lower nasal passage; 2 - lower, 3 - middle and 4 - upper turbinates; 5 - upper nasal passage; B - diagram of the structure of the olfactory epithelium: 1 - the body of the olfactory cell, 2 - supporting cell; 3 - mace; 4 - microvilli; 5 - olfactory filaments

CONDUCTOR SECTION The same olfactory neurosensory, or neuroreceptor, cell should be considered the first neuron of the olfactory analyzer. 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 are in contact with the main dendrite of the mitral nerve cells of the olfactory bulb, which is the second neuron. The olfactory bulbs lie on the basal (inferior) surface of the frontal lobes. They are referred either to the ancient bark, or isolated in a special part of the olfactory brain. It is important to note that olfactory receptors, unlike receptors of other sensory systems, do not give a topical spatial projection on the bulb, due to their numerous conventional 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. Fibers of the olfactory tract in separate bundles go from the olfactory bulbs to the higher-order olfactory centers, for example, to the anterior nuclei of the thalamus (visual hillock). However, most researchers believe that the processes of the second neuron go directly into the cerebral cortex, bypassing the thalamus. But the olfactory sensory system does not give projections into the new cortex (neocortex), but only into the zones of the arch- and paleocortex: into the hippocampus, limbic cortex, 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 primary and secondary dendrites of 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 vegetative nuclei of the hypothalamus. The connection with the limbic system explains the presence of an emotional component in olfactory perception, for example, the pleasure-producing or hedonic components of the sense of smell.

CENTRAL, OR CORTAL, DEPARTMENT The central department consists of the olfactory bulb connected by branches of the olfactory tract with centers that are located in the paleocortex (the ancient cortex of the cerebral hemispheres) and in the subcortical nuclei, as well as the cortical department, which is localized in the temporal lobes of the brain. gyrus of a sea horse. The central, or cortical, section of the olfactory analysor is localized in the anterior part of the pear-shaped p lobe of the cortex in the area of the gyrus of the sea horse. With

CODING OF SMELLING INFORMATION So, each individual receptor cell is capable of responding to a significant number of different odorous substances. Therefore, different olfactory receptors have overlapping response profiles. Each odorous substance gives a specific combination of olfactory receptors that respond to it and a corresponding pattern (pattern) of excitation in the population of these receptor cells. In this case, the level of arousal depends on the concentration of the odorous irritant substance. Under the action of odorous substances in very low concentrations, the resulting sensation is not specific, but in higher concentrations, the smell is revealed and its identification occurs. Therefore, it is necessary to distinguish between the threshold for the appearance of odor and the threshold for its recognition. In the fibers of the olfactory nerve, constant impulses were found, due to the subthreshold effect of odorous substances. At the threshold and suprathreshold concentrations of various odorous substances, different patterns of electrical impulses appear, which arrive simultaneously in different parts of the olfactory bulb. At the same time, a kind of mosaic of excited and unexcited areas is created in the olfactory bulb. It is believed that this phenomenon underlies the coding of information about the specificity of odors.

OPERATION OF THE OLFACTORY (OLFACTOR) SENSOR 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 sensorineural cell (olfactory sensory receptor ). 2. Transduction of chemical irritation into nervous excitement. Attachment of an irritant molecule (ligand) to a receptor molecule → the conformation of the receptor molecule changes → a cascade of biochemical reactions with the participation of G-protein and adenylate cyclase is launched → c. AMP (cyclic adenosine monophosphate) → protein kinase is activated → it phosphorylates and opens ion channels in the membrane that are permeable to three types of ions: Na +, K +, Ca 2 + →. ... ... → a local electric 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 the afferent olfactory sensory stimulation 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 nervous 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 - the construction of a sensory image of irritation (stimulus) in the form of a sense of smell.

ADAPTATION OF THE Olfactory Analyzer The adaptation of the olfactory analyzer can be observed with prolonged exposure to the olfactory stimulus. Adaptation to the action of an odorous substance occurs rather slowly within 10 seconds or minutes and depends on the duration of the action of the substance, its concentration and the air flow rate (sniffing). In relation to many odorous substances, complete adaptation occurs rather quickly, that is, 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 a short-term action of a weak gustatory or olfactory stimulus: adaptation can manifest itself in an increase in the sensitivity of the corresponding analyzer. It was found that changes in sensitivity and adaptation phenomena mainly occur not in the peripheral, but in the cortical part of the gustatory and olfactory analyzers. Sometimes, especially with the frequent action of the same gustatory or olfactory stimulus, a persistent focus of increased excitability arises in the cerebral cortex. In such cases, the sensation of taste or smell, to which there is an increased excitability, can also appear under the action of various other substances. Moreover, the sensation of the corresponding smell or taste can become annoying, appearing and in the absence of any gustatory or odor stimuli, in other words, illusions and hallucinations arise. If during lunch you say that the dish is rotten or sour, then some people have the corresponding olfactory and taste sensations, as a result of which they refuse to eat. Adaptation to one odor does not reduce the sensitivity to other types of odorants, since different odorous substances act on different receptors.

TYPES OF SENSOR IMPAIRMENT: 1) anosmia - absence; 2) hyposmia - lowering; 3) hyperosmia - increased olfactory sensitivity; 4) parosmia - incorrect perception of odors; 5) violation of differentiation; 5) olfactory hallucinations, when olfactory sensations arise in the absence of odorous substances; 6) olfactory agnosia, when a person smells, but does not recognize it. With age, there is mainly a decrease in olfactory sensitivity, as well as other types of functional disorders of the sense 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 olfactory bulbs), and central, localized in the hippocampus of the cerebral cortex for the main olfactory system.

The 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 region that covers the upper and partly the middle concha of the nasal cavity in humans, as well as the upper part of the nasal septum. Externally, the olfactory area differs from the respiratory part of the mucous membrane in a yellowish color.

The peripheral part of the vomeronasal, or additional, 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 at the border between the septum cartilage and the vomer. In addition to the Jacobsonian 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 reproductive 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- thickening 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, from the elements that make up the walls of the olfactory pits, supporting epithelial cells and neurosensory olfactory cells are formed. 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 terminals of the axons and the dendrites of the mitral neurons of the olfactory bulbs. Some parts 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 multilayered epithelium with a height of 60-90 microns, 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 membrane 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 - a dendrite and a long - central - axon. Their nucleated parts, as a rule, occupy a middle position in the thickness of the olfactory lining.

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

The cytoplasm of peripheral processes contains mitochondria and microtubules elongated along the axis of the process up to 20 nm in diameter. A granular endoplasmic reticulum is clearly visible near the nucleus in these cells. The cilia of the clubs contain longitudinally oriented fibrils: 9 pairs of peripheral and 2 central ones, extending from the basal bodies. The olfactory cilia are mobile and are a kind of antennae for the molecules of odorous substances. The peripheral processes of the olfactory cells can contract under the influence of odorous substances. The nuclei of the olfactory cells are light, with one or two large nucleoli. The nasal portion of the cell continues into a narrow, slightly wriggling axon that runs between the supporting cells. In the connective tissue layer, the central processes make up bundles of the myelin-free olfactory nerve, which are combined into 20-40 olfactory filaments ( filia olfactoria) and through the holes of the ethmoid bone are sent to the olfactory bulbs.

Supporting epithelial cells (epitheliocytus sustentans) form a multi-row epithelial layer, in which the olfactory cells are located. The apical surface of the supporting epithelial cells contains numerous microvilli up to 4 µm in length. Supporting epithelial cells show signs of apocrine secretion and have a high metabolic rate. In their cytoplasm there is an 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 brownish yellow pigment.

Basal epithelial cells (epitheliocytus basales) are located on the basement membrane and are equipped with cytoplasmic outgrowths surrounding the bundles of axons 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 the receptor and respiratory parts. The structure of the receptor part is similar to the olfactory epithelium of the main organ of smell. The main difference is that the olfactory clubs of the receptor cells of the vomeronasal organ carry 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 myelin-free nerve fibers, which are combined into 20-40 filamentous stems ( fila olfactoria) and through the holes of the ethmoid bone are sent to the olfactory bulbs. Each olfactory filament is a myelin-free fiber containing from 20 to 100 or more axial cylinders of receptor cell axons embedded in lemmocytes. The olfactory bulbs contain the second neurons of the olfactory analyzer. These are large nerve cells called mitral, have synaptic contacts with several thousand axons of neurosensory cells of the same, and partially the opposite side. The olfactory bulbs are built like the cortex of the cerebral hemispheres, they have 6 concentrically arranged layers: 1 - the layer of olfactory fibers, 2 - the glomerular layer, 3 - the outer reticular layer, 4 - the layer of mitral cell bodies, 5 - the inner reticular, 6 - granular layer ...

The contact of the axons of neurosensory cells with the mitral dendrites occurs in the glomerular layer, where the excitations of the receptor cells are summed up. Here, the interaction of receptor cells with each other and with small associative cells is carried out. In the olfactory glomeruli, centrifugal efferent influences emanating from the overlying efferent centers (anterior olfactory nucleus, olfactory tubercle, amygdala nucleus, prepiriform cortex) are also realized. The outer reticular layer is formed by the bodies of bundle 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 4-5th layers of the bulbs, and at the exit from them they form olfactory contacts together with the axons of the bundle cells. In the region of the 6th layer, recurrent collaterals depart from the axons of the mitral cells, which are distributed in different layers. The granular layer is formed by an accumulation of grain cells, which are inhibitory in their function. Their dendrites form synapses with recurrent collaterals of mitral cell axons.

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

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

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

It has been established in animals that from the additional 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 premamillary nucleus and the middle amygdala. The connections of the projections of the vomeronasal nerve in humans are still poorly understood.

Olfactory glands... In the underlying loose fibrous tissue of the olfactory region, there are the end sections of the tubular-alveolar glands, which secrete a secret that contains mucoproteins. The terminal sections consist of elements of two kinds: on the outside there are more flattened cells - myoepithelial cells, inside - cells secreting according to the merocrine type. Their transparent, watery secretions, together with the secretions of the supporting epithelial cells, moisturize the surface of the olfactory lining, which is a prerequisite for the functioning of the olfactory cells. In this secret, washing the olfactory cilia, odorous substances dissolve, the presence of which only in this case is perceived by the 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. Vessels of the microcirculatory type resemble the corpora cavernosa. Sinusoidal blood capillaries form plexuses that are capable of depositing blood. Under the action of sharp temperature irritants and molecules of odorous substances, the nasal mucosa can swell strongly 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 (rhinitis) transferred during life, which lead to atrophy of receptor cells and the growth of the respiratory epithelium.

Regeneration... In mammals, in postnatal ontogenesis, the renewal of olfactory receptor cells occurs within 30 days (due to poorly differentiated basal cells). At the end of their life cycle, neurons undergo destruction. Poorly differentiated neurons of the basal layer are capable of mitotic division and are devoid of processes. In the process of their differentiation, the volume of cells increases, a specialized dendrite appears, growing towards the surface, and an axon, growing towards the basement membrane. 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 perform, in fact, a sanitary role, warming and purifying the inhaled air. The main organ of smell, which is the 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 the middle concha of the nasal cavity, as well as the upper part of the nasal septum. Externally, the olfactory area 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 of the participation of this pigment in the reception of odors.

Olfactory epithelium lining the olfactory region of the nose is 100-150 microns thick and contains three types of cells:

1 - olfactory (receptor),

2 - supporting,

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 respect, a person belongs to poorly smelling creatures (microsmatics). For example, in a number of animals - dogs, rats, cats, etc. - the olfactory system is much more developed (macrosmatics).

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

Olfactory receptor cell- bipolar cell, which has a fusiform 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 holes of the ethmoid bone, are sent to the olfactory bulb of the brain.

Like taste cells and outer segments of photoreceptors, olfactory cells are constantly renewed. The life span of the olfactory cell is about 2 months.

Reception mechanisms. The odor molecules come into contact with the olfactory mucosa. It is assumed that the receptor 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 consequence, the generation of a depolarizing receptor potential, which leads to a pulse discharge in the receptor axon (fiber of the olfactory nerve).

Olfactory cells are capable of responding to millions of different spatial configurations of odor molecules. Meanwhile, each receptor cell is capable of responding with physiological excitation to its characteristic, albeit wide, spectrum of odorous substances. It was previously believed that the low selectivity of an individual receptor is due to 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. The rule "One olfactory cell - one olfactory receptor protein" greatly 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.

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

Rice. Scheme of the structure of the olfactory system. (Processes of neurons carrying different receptors go to different glomeruli of the olfactory bulb)

The structure and function of the olfactory bulb. The olfactory pathway is switched for the first time in the olfactory bulb, which belongs to the cerebral cortex. In the paired olfactory bulb of a person, six layers are distinguished, which are arranged concentrically, counting from the surface:

Layer I - fibers of the olfactory nerve;

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

III layer - outer reticular, containing bundle cells; the dendrite of such a cell, as a rule, comes into contact with several glomeruli;

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

V layer - (internal network-like);

VI layer - granular layer. It contains the bodies of the grain cells. The layer of grain cells directly passes into the cell masses of the so-called anterior olfactory nucleus, which is referred to as the third-order olfactory centers.

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

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

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

The structure and function of the central division of the olfactory analyzer.

The fibers of the olfactory tract end in various parts of the forebrain: in the anterior olfactory nucleus, the lateral part of the olfactory tubercle, the prepiriform and periamygdala regions of the cortex, as well as in the adjacent cortico-medial part of the amygdala, including the nucleus of the lateral olfactory tract, which is believed to be , fibers also come from the accessory olfactory bulb. Connections of the olfactory bulb with the hippocampus, and other parts of the olfactory brain in mammals are carried out 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 a direct entrance from the gustatory 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, entering the medial bundle of the forebrain. The ends of the fibers of this bundle are found in the hypothalamus.

Thus, the peculiarity of the olfactory system consists, in particular, in the fact that its afferent fibers on the way to the cortex do not switch in the thalamus and do not pass to the opposite side of the large brain. 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, it becomes clear that the sense of smell is closely related to eating and sexual behavior.

The efferent regulation of the activity of the olfactory bulb has not yet been studied enough, although there are morphological prerequisites that indicate 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, camphor, floral, ethereal, mint, pungent and putrid.

According to the theory of J. Amour and R. Moncrieff (stereochemical theory), the smell of a substance is determined by the shape and size of the odorous molecule, which, in its 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 seven types of receptor sites. The receptive sites are in close contact with the odorant molecules, while the charge of the membrane site 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 under the action of smell depends on the odorous substance: with different odors, the spatial mosaic of excited and inhibited parts of the bulb changes

The sensitivity of the human olfactory system. This sensitivity is quite high: one olfactory receptor can be excited by one molecule of an odorous substance, and the excitation of a small number of receptors leads to sensation. At the same time, the change in the intensity of the action of substances (the discrimination threshold) is estimated by people rather roughly (the smallest perceived difference in the strength of the smell is 30-60% of its initial concentration). In dogs, these indicators are 3-6 times higher.

Olfactory analyzer adaptation can be observed with prolonged exposure to an odorous substance. Adaptation occurs rather slowly within 10 seconds or minutes and depends on the duration of the action of the substance, its concentration and the air flow rate (sniffing). In relation to many odorous substances, complete adaptation occurs rather quickly, that is, 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 a short-term action of a weak gustatory or olfactory stimulus: adaptation can manifest itself in an increase in the sensitivity of the corresponding analyzer. It was found that changes in sensitivity and adaptation phenomena mainly occur not in the peripheral, but in the cortical part of the gustatory and olfactory analyzers.... Sometimes, especially with the frequent action of the same gustatory or olfactory stimulus, a persistent focus of increased excitability arises in the cerebral cortex. In such cases, the sensation of taste or smell, to which there is an increased excitability, can also appear under the action of various other substances. Moreover, the sensation of the corresponding smell or taste can become annoying, appearing and in the absence of any gustatory or odor stimuli, in other words, illusions arise, and hallucinations... If during lunch you say that the dish is rotten or sour, then some people have the corresponding olfactory and taste sensations, as a result of which they refuse to eat. Adaptation to one odor does not reduce the sensitivity to other types of odorants, because different odoriferous substances 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 takes place. It influences eating behavior, takes part in testing food for edibility, in setting up the digestive apparatus for processing food (by the mechanism of a conditioned reflex), and also on defensive behavior, helping to avoid danger due to the ability to distinguish substances harmful to the body. effectively facilitate the extraction of information from memory. Thus, the reaction to smells is not only the work of the organs of smell, but also a social experience. Through smells, we are able to restore the atmosphere of past years or gain memories associated with specific life circumstances. The sense of smell plays a significant role in the emotional sphere of a person.

In addition, the "olfactory memory" has an equally important biological purpose. Despite the fact that the image of the "second half" in humans is built mainly on the basis of information obtained through sight and hearing, individual body odor is also a guideline for recognizing a suitable object for successful procreation. For a more efficient perception of these odors and the corresponding reaction to them, nature has created an "auxiliary" olfactory system vomeronasal system.

The peripheral part of the vomeronasal, or additional, 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 at the border between the septum cartilage and the vomer. In addition to the Jacobsonian 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 reproductive cycle and sexual behavior), and with the emotional sphere.

It was found in animals that from the additional 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 premamillary nucleus and the middle nucleus of the amygdala. The connections of the projections of the vomeronasal nerve in humans are still poorly understood.

Olfactory sensory system (NSS)

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

The value of HSS for humans:

Provides reflex stimulation of the digestive center;

Provides a protective effect with recognition of the chemical composition of the environment in which the body is located;

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

Engages in emotional behavior;

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

Is attracted to the formation of a taste sensation (with a severe cold, food loses its taste)

In animals, it also provides a search for food.

The first classification of odors was made by Eimur, taking into account the source of origin: camphor, floral, musky, mint, ethereal, pungent and putrid. For the perception of odor, an odorant must have two properties: it must 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 normosmia, absence - anosmia, decreased perception of smell - hypoosmia, increased - hyperosmia, disturbances - dysosmia.

It should be emphasized that some substances cause the maximum reaction, others - weak, and the rest - 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 area of the nasal cavity near the nasal septum (Fig. 12.32) in the form of islets 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 airways. Therefore, in order to smell the smell, you need to sniff, that is, take a deep breath. In the case of calm breathing, only 5% of the air passes through the olfactory epithelium.

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

The olfactory cell has a central sprout - an axon and a peripheral one - dendrites. There is a thickening at the end of the dendrite - a mace. On the surface of the club there are microvilli (10-20) up to 0.3 µm in diameter and up to 10 µm in length. It is thanks to them that the surface of the olfactory epithelium is significantly increased and its area can exceed several times the area of the body. The olfactory club is the cytochemical center of the olfactory cell. The 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 (item Trigeminus), which can also react to 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 the section of the olfactory nerve on both sides.

The mechanism of excitation of olfactory receptor cells

Many theories of smell have been created. Among them, the stereochemical theory formulated in 1949 by Moncriff deserves attention. Its meaning lies in the fact that the olfactory system is built of different receptor cells. Each of these cells perceives one smell. It has been proven by testing that musky, camphor, mint, floral, ethereal odors are inherent in substances, the molecules of which, like a "key to a lock", fit 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. Scheme of the olfactory mucosa:

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

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

Olfactory receptors contain about 1000 types of receptor proteins with which odorants interact. Proteins encode about 1000 genes, which is about 3% of the entire gene pool and only emphasizes the importance of the olfactory analyzer. After the odorant molecule binds to the receptor, the system of secondary messengers is activated, in particular the 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 winners G. Excel and L. Buck proved that there are no specific receptors for every single odor. Instead, there is a “receptor alphabet.” A particular smell activates a specific combination of receptors, which, in turn, direct a specific sequence of nerve impulses, then decode by neurons in the brain, such as forming words from letters or music from notes, and a sensation of a specific smell arises ...

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

A person is able to identify only three smells at a time. If there are more than ten odors, she is not able to recognize a single one.

A very close connection between the olfactory apparatus and the reproductive system. The severity of the perception of odors depends on the level of steroid hormones in the body, including sex hormones. This is indicated by the facts, diseases associated with impaired reproductive function, accompanied by a decrease or loss of the ability to perceive odors. With the help of the 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. It is also interesting that the axons of olfactory neurons bypass the thalamus - the collector of all sensory pathways - and go to the olfactory bulbs, which are part of the ancient cortex - the limbic system, which is responsible for memory, emotions, sexual behavior.

Rice. 12.33. The structure of the olfactory epithelium

The unknown meaning of smell is hidden in unsolved riddles. Why is this sensation provided by such a significant number of genes and is closely related to the ancient formations of the brain?

Wired and cerebral divisions of the olfactory sensory system

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

Under the action of various odors in the olfactory bulb, the spatial mosaic of excited and inhibited cells changes. This is reflected in the specificity 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), the amygdala, and the hypothalamus, the olfactory component of emotions is provided. Thus, a large number of centers are involved in the olfactory function.

Olfactory thresholds. adaptation

Distinguish between thresholds for establishing the presence of odor and thresholds for smell recognition. 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 odoriferous substance that allows an odor to be identified. For vanillin, for example, the recognition threshold is 8 × 10-13 mol / l. The thresholds vary depending on a number of factors: physiological state (during the menstrual period - an exacerbation in women), age (in the elderly - an increase), from air humidity (decrease in a humid environment), the speed of air movement through the nasal respiratory tract. The thresholds for the deafblind are significantly reduced. Despite the fact that a person is able to distinguish up to 10,000 different smells, her ability to assess their intensity is very low. An increase in sensation is carried out only if the increase in stimulation occurs by at least 30% compared to the initial value.

The adaptation of the olfactory sensory system is slow and lasts tens of seconds or minutes. It depends on the speed of air movement and the concentration of the odorous substance. Cross-adaptation takes place. With prolonged exposure to any odorant, the threshold increases not only for it, but also for other odorous substances. 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 can accompany epilepsy. Anosmia can be caused by hypogonadism.

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

Sensory taste system

Taste- the sensation arising from the action of the substance on the 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 conjunction 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;

■ start digestive reflexes of juice secretion;

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

Basic tastes: bitter, salty, sour, sweet.

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

Taste receptor cells with microvilli are 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 react only to substances dissolved in water.

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

The largest clusters of receptors are found at the tip, edges, and at the 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 stimulates the receptors of the edges and front of the tongue;

■ sour stimulates the receptors of the lateral edges of the tongue.

The receptor cells are adjacent to the surrounding nerve fibers, 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 the taste buds of substances of the same taste. Adaptation occurs most quickly to salty and sweet substances, slower to sour and bitter ones.

■ Peppers, mustard and similar foods restore taste and stimulate appetite.

Sensory olfactory system

Smell- the body's ability to perceive the smells of various chemicals in the air.

Smell- a sensation arising from the action of a chemical substance in the air on the olfactory (chemical) receptors located in the mucous membrane of the nasal cavity. The number of types of smells perceived by a person is almost endless.

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

■ The molar concentration of a substance, the smell of which can be felt by a person, is about 10 -14 mol / l, ie. just a few molecules per liter of air.

The peripheral section of the olfactory analyzer is represented by olfactory epithelium nasal cavity containing numerous sensitive cells - olfactory chemoreceptors .

Olfactory chemoreceptors are neurons whose dendrites end in the nasal mucosa. The ends of the dendrites have numerous microscopic depressions 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 dimensions of a molecule coincide with the shape and dimensions of any of the depressions on the surface of the receptor (dendrite), then it (the molecule) "lays down" in this depression, causing the appearance of a corresponding nerve impulse. At the same time, the impulses generated by depressions of different shapes, and hence by different molecules, have different characteristics, which makes it possible to distinguish the smells of different substances.

The olfactory receptor cells in the mucosa are among the ciliated supporting cells.

The axons of the olfactory neurons form the olfactory nerve that runs into the cranial cavity. Further, the excitation is carried out to the olfactory centers of the cerebral cortex, in which the recognition of smells is carried out.

Odor adaptation- a decrease in the sensation of the smell of a given substance with its prolonged action on the olfactory receptors. At the same time, the sharpness of perception to other smells remains.