Olfactory analyzer: structure and functions. How to restore your sense of smell

Definition of the concept

Olfactory (olfactory) sensory system , or olfactory analyzer, is a neural system for recognizing volatile and water-soluble substances by the configuration of their molecules, creating subjective sensory images in the form of odors.

Just like the gustatory sensory system, the olfactory system is a chemical sensitivity system.

Functions of the olfactory sensory system(OSS)
1. Detection of food for attractiveness, edibility and inedibility.
2. Motivation and modulation of eating behavior.
3. Setup 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.

Characteristics of an adequate stimulus

An adequate stimulus for the olfactory sensory system is smell, which is emitted by odorous substances.

All odorous substances that have an odor 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 covering the entire epithelium of the nasal cavities. Satisfies these requirements great amount substances, and therefore a person is able to distinguish thousands of different odors. It is important that there is no strict correspondence between chemical structure"fragrant" molecule and its smell.
Most existing theories of odors are based on the subjective identification of several typical odors as the main ones (by analogy with the four taste modalities) and the explanation of all other odors by their various combinations. And only the stereochemical theory of odors is based on identifying an objective correspondence between the geometric similarity of molecules of odorous substances and their inherent odor.
Construction of three-dimensional models of odorous molecules based on their preliminary study using diffraction x-rays and infrared stereoscopy showed that not only natural, but also artificially synthesized molecules have an odor corresponding to a certain form of molecules and different from the odor inherent in another form of molecules. In this regard, there is a hypothesis about the presence of seven types of olfactory molecular chemoreceptors capable of attaching substances that stereochemically correspond to them. Among several hundred experimentally studied odorous molecules, it was possible to identify seven classes in which substances with a similar stereochemical configuration of molecules and a similar odor were located: 1) camphor, 2) ethereal, 3) floral, 4) musky, 5) peppermint, 9) caustic, 7) putrid. These seven odors are considered primary, and all other odors are explained various combinations primary odors.

Classification of odorous substances and odors
Odorous substances can be divided into two large groups:
1. Olfactive (odorous) substances that irritate only the olfactory cells. These include the smell of cloves, lavender, anise, benzene, xylene, etc.
2. “Caustic” substances that, simultaneously with the olfactory cells, irritate the free endings of the trigeminal nerves in the nasal mucosa. This group includes the smell of camphor, ether, chloroform, etc.
United and generally accepted classification there are no smells. It is impossible to characterize a smell without naming the substance or object to which it is characteristic. So, we talk about the smell of camphor, rose, onion, in some cases we generalize the smells of related substances or objects, for example, a floral smell, fruity, etc. It is believed that the resulting variety of different odors is the result of a mixture of “primary odors.” The acuity of smell is influenced by many factors, in particular hunger, which increases the acuity of smell; pregnancy, when not only exacerbation is possible olfactory sensitivity, but also its perversion.

In the odor classification system widely used today,proposed by the Dutch otolaryngologist Hendrik Zwaardemaker in 1895, all the smellsgrouped into 9 classes:

I. Essential odors (fruit and wine). These include the smells of fruit essences used in perfumery: apple, pear, etc., as well as beeswax and ethers.
II. Aromatic smells(spices, camphor)- the smell of camphor, bitter almonds, lemon.
III. Balsamic scents(floral scents; vanilla)- the smell of flowers (jasmine, lily of the valley, etc.), vanillin, etc.
IV. Ambro-musky scents(musk, sandalwood)- the smell of musk, amber. This also includes many odors of animals and some mushrooms.
V. Garlic smells(garlic, chlorine) - the smell of ichthyol, vulcanized rubber, smelly resin, chlorine, bromine, iodine, etc.
VI. Burnt smells(roasted coffee, creosote)- the smell of roasted coffee, tobacco smoke, pyridine, benzene, phenol (carbolic acid), naphthalene.
VII. Caprylic, or canines (cheese, rancid fat)- s the smell of cheese, sweat, rancid fat, cat urine, vaginal secretions, semen.
VIII. Nasty or repulsive(bugs, belladonna)- some smells narcotic substances, obtained from nightshade plants (the smell of henbane): the smell of bedbugs belongs to the same group of odors.
IX. Nauseating(feces, cadaverous odor)- corpse smell, feces smell.

From this list it is clear that odors can be of plant, animal and mineral origin. Plants are characterized by incense, while animals are characterized by persistence.

Crocker-Henderson system includes only four basic odors: aromatic, sour, burnt and caprylic (or goat).

In the stereochemical model Eimura 7 main smells: camphor, ethereal, floral, musky, peppermint, acrid and putrid.

"Prism of smells" Henning identifies six main types of odors: aromatic, ethereal, spicy, resinous, burnt and putrid - one at each vertex of a triangular prism.

True, so far none of the existing classifications of odors have received universal recognition.

The most famous and widespread classification in perfumery was proposed in 1990 by the French Perfumery Committee Comite Francais De Parfum. According to this classification, all fragrances are combined into 7 main groups (families).

Aromatherapy uses a system of subjective description of the aromas used using concepts from other sensory modalities .

Structure of the olfactory analyzer

Peripheral department
This section 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 far part of the dendrite of such a cell is changed. It is expanded into an “olfactory club”, from which 6–12 (1–20 according to other sources) cilia extend, while a regular axon extends from the base of the cell (see figure). 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 sensory afferent fibers trigeminal nerve, which also react to odorous substances.

The outstanding American wine critic and taster Robert Parker has a unique sense of smell and the ability to distinguish tastes, and in addition - a well-trained sensory memory - he forever remembers the taste of wine once tasted.
He tasted 220,000 wines - up to 10,000 wines a year - and reviewed them all in his famous newsletter The Wine Advocate.
Robert Parker has developed the most famous and sought-after 100-point scale for assessing the quality of wines in the world - by vintage (harvest year) - the so-called Robert Parker scale - which all world wine markets are measured against. And this success was ensured by two well-developed sensory systems: olfactory and gustatory! ...Well, and of course, the highest nervous activity It also turned out to be not superfluous! ;)

Sources:

Smirnov V.M., Budylina S.M. Physiology of sensory systems and higher nervous activity: Textbook. aid for students higher schools, institutions. M.: "Academy", 2003. 304 p. ISBN 5-7695-0786-1
Lupandin V.I., Surnina O.E. Fundamentals of sensory physiology: Textbook. M.: Sfera, 2006. 288 p. ISBN 5-89144-670-7

Axons receptor cells, having united in a bundle, they go to the olfactory bulb, where the second neurons are located. The fibers of the cells of the olfactory bulb form the olfactory tract, which has a triangular extension and consists of several bundles. The olfactory bulb generates rhythmic impulses, the frequency of which changes when various odorous substances are blown into the nose. The olfactory tract bundles pass into various brain structures: the amygdala, hypothalamus (responsible for the emotional component of olfactory sensations), reticular formation, orbitofrontal cortex, preperiform cortex and periform lobe, into the olfactory bulb of the opposite side. The central section of the olfactory analyzer is located in the anterior part of the pyriform lobe in the region of the seahorse gyrus (hippocampus). Odorous substances are also perceived by the free endings of the trigeminal nerve fibers (V pair of cranial nerves), located in the nasal mucosa. So, substances with pungent odor(ammonia) are sensed by the endings of the trigeminal nerve and can cause respiratory arrest or protective reflexes (sneezing). These reflexes close at the level medulla oblongata.

A person is able to distinguish a variety of smells. There is a classification (J. Eimour, 1962) of odors that serves practical purposes. It identifies seven main, or primary, odors: 1) camphor-like, 2) floral, 3) musky, 4) minty, 5) ethereal, 6) putrid, 7) pungent. The variety of odors is associated with a mixture of primary odors. In addition, there are so-called olfactory substances that irritate only the olfactory receptors. These include: the smell of cloves, lavender, anise, benzene, xylene, etc. - these are substances of the first group.

The second group includes mixed substances that irritate not only the olfactory cells, but also the endings of the trigeminal nerve. This is the smell of camphor, ether, chloroform, etc.

Adaptation to the action of an odorous substance 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).

Olfactory acuity determined threshold of olfactory sensitivity - this is the minimum amount of an odorous substance that is perceived as the corresponding odor. Determination of olfactory sensitivity thresholds is carried out using olfactometry.

The acuity of smell is affected by air humidity and temperature, and the state of the peripheral part of the analyzer. Swelling of the nasal mucosa during a runny nose causes a decrease in the acuity of smell - hypoosmia or complete loss of olfactory sensitivity - anosmia, which is observed either with atrophy of the receptor apparatus, or with a violation of the cortical part of the analyzer, with which it can also be associated hyperosmia-increased sense of smell, as well as parosmia - incorrect perception of odors, olfactory hallucinations in the absence of odorous substances - olfactory agnosia. With age, a decrease in olfactory sensitivity has been noted.

Taste analyzer

Taste is a contact type of sensitivity and is a multimodal sensation, since chemical stimuli are perceived in combination with thermal, mechanical and olfactory ones.

There are four “primary” taste sensations: sweet, sour, salty, bitter. The tip of the tongue perceives mainly sweet taste, the root - bitter, the middle part - sour, the side parts of the tongue - salty and sour. The lowest thresholds of taste sensitivity are for bitter taste and are determined by the concentration of substances acting on the receptors. The long-term effect of a substance on taste buds leads to adaptation to this type of taste. So, if a person often consumes sour and salty food(spicy), then the thresholds for these types of taste increase. Adaptation to sweet and salty foods develops faster than to bitter and sour foods.

Taste receptors - taste cells are located in taste buds or bulbs. The latter are localized in the taste buds of the tongue and in the form of separate inclusions on the back wall of the pharynx, soft palate, tonsils, larynx, epiglottis. They are divided into three types: 1) mushroom-shaped (on the entire surface of the tongue), 2) grooved - across the wall of the tongue, at its root, 3) leaf-shaped - along the posterior edges of the tongue.^ In humans, there are 2000 taste buds, each of which contains 40 - 60 receptor cells.

Mechanism of taste perception is as follows. The flavoring substance, broken down by saliva into molecules, enters the pores of the taste buds, interacts with the glycocalyx and is adsorbed on the cell membrane of the microvillus, coming into contact with the receptor protein. It is assumed that in the area of ​​the microvillus there are stereospecific receptor sites that perceive only their own substance molecules. As a result, the membrane is depolarized and a receptor potential is generated. The mediator (acetylcholine, serotonin, etc.) formed in the receptor cell at the receptor-afferent synapse leads to the appearance of EPSP, and then AP, which is transmitted along the fibers of the tympanic chord - branches of the facial (VII pair), glossopharyngeal (IX pair) and upper laryngeal (X pair) cranially -cerebral nerves into the medulla oblongata, into the nucleus of the solitary nerve in the form of patterned neural activity that determines different taste sensations. From the medulla oblongata, nerve fibers in the medial lemniscus are directed to the ventral nuclei of the thalamus visualis and further to the cerebral cortex - the lateral part of the postcentral gyrus and the hippocampus.

Taste sensitivity can change depending on the state of the body (during fasting, pregnancy). Alcohol and nicotine increase taste thresholds. Complete loss of taste is called ageusia, reduced

naya - spogevsia, increased taste sensitivity -gi- pergeusia, perversion of taste - parageusia.

The sense of smell has only attracted the attention of researchers over the last forty years - until then it received very little attention.

The reason for little interest in the issue of smell is that smell in human life does not play such an important role as vision and hearing.

Olfaction is phylogenetically one of the most ancient sense organs, and its study is extremely necessary for both physiology and clinical medicine, especially neuropathology.

Clinicians are interested in the possibility of determining the area of ​​damage to the olfactory analyzer based on the nature of the disorder olfactory function.

Studying olfactory disturbances in the tumor clinic big brain, we are convinced that data from a thorough study of olfactory function have great diagnostic value.

As you know, the olfactory region is located in the upper part of the nasal cavity, the so-called olfactory fissure. The space delimiting this area is the septum, the superior and middle conchae, and the cribriform plate. The mucous membrane covering this area is different from the rest of the nasal mucosa brown spots, receiving their color from the pigment contained in the olfactory cells: the indicated spots or islands generally occupy 250 mm2 of area and have irregular shape. Accurate definition there is no area of ​​distribution of the olfactory part of the nasal mucosa containing pigment; this area varies among individuals, sometimes occupying part of the superior nasal turbinate and nasal septum, sometimes moving to the middle turbinate. The olfactory pigment is apparently similar to the pigment of the retina, and its disappearance leads to a loss of smell, which is observed in old people, in people with a disease of the epithelium of the olfactory fissure itself.

The olfactory epithelium consists of three types of cells:

1) the olfactory cells themselves;

2) cylindrical olfactory cells;

3) small basal cells.

Sensitive cells of the olfactory epithelium are bipolar. One free end of such a cell faces the olfactory cavity and has hairs at the end, which together form a fringed tissue called the border olfactory septum.

But unlike other receptors, olfactory cells, like the cells of the retina, are parts of the central nervous system, brought to the periphery. The process of the olfactory cell protrudes through the hole in the marginal olfactory septum and here expands into a vesicle from which the cilia extend. These ciliated olfactory vesicles are the true receptors of the olfactory sense. Embryologically, they originate from centrosomes and their surrounding centrospheres.

The olfactory vesicles are immersed in a semi-liquid outer membrane secreted by supporting cells (membrana limitans). The other end of the sensitive cell is directed into the cranial cavity and, connecting with other similar processes of sensitive cells, forms olfactory fibers. These latter, passing through the cribriform plate into the cranial cavity, are immersed in the olfactory bulb.

Olfactory fibers are accompanied by fibers of the trigeminal nerve. Having plunged into the olfactory bulb, the fibers of the sensory cells branch in a tree-like manner and, intertwining with the same branches of the mitral cells, form the olfactory glomeruli. Olfactory glomeruli, the so-called glomeruli, are spherical particles sitting on a layer of olfactory fibers. These spherical formations essentially represent a ball of entangled inseparable two bundles of fibers going one to another. One of these bundles, the ascending one, is a cylindrical process of the bipolar cell of the olfactory epithelium branched into a bouquet; the descending bundle coming towards it is also a branched protoplasmic main process of the mitral cell. In humans, each glomerulus receives branching from only one mitral cell and the cylindrical processes of many bipolar cells of the olfactory epithelium.

The microscopic structure of the olfactory bulbs consists of five layers:

1) layer nerve fibers;

2) layer of glomeruli;

3) molecular layer with brush cells;

4) a layer of mitral cells, which serve for further transmission of olfactory impulses to the brain;

5) granular layer, poorly developed in humans, consisting of granule cells and Golgi cells.

Thus, the olfactory bulb is like an intercalary ganglion. This is where the peripheral olfactory pathway ends and the central olfactory pathway begins.

The first neuron of the central olfactory pathway there will be an olfactory tract. The olfactory tract consists of ganglion cells, nerve fibers, remnants of the ventricular ependyma, cells and blood vessels. All these elements form the olfactory tubercle, which is a pyramidal eminence on the lower edge of the olfactory sulcus. The base of this pyramid is the olfactory tubercle. In more detail, the human olfactory tract, together with the bulb, represents the underdeveloped olfactory gyrus of macrosmatic animals. The olfactory tract consists of three layers:

1) a layer of olfactory fibers, the most superficial to the thinnest, covering the bulb with a very thin cingulate layer (described above as a layer of nerve fibers);

2) a layer of mitral fibers, consisting of three zones: a) superficial, b) deep, formed by a layer of cells called mitral, and c) lower, formed by a layer of simple or double glomeruli;

3) layer of central fibers.

The cells, called mitral cells, are shaped like a pyramid or miter. The top of the pyramid faces upward. A long thin axon departs from it, which penetrates the layer of central fibers, bends and goes along the tract to the olfactory triangle. Throughout its path, this axon releases collaterals. Some of them descend between the mitral cells, others approach the cells of the central layer or go to the cells of the cortex. The lateral angles of the mitral cells give rise to protoplasmic processes, generously branching in the plane of the parent cell, except for one, called the main one, which extends from the base of the mitral cell. This most powerful process of all descends in a straight line down to the glomerulus.

Everywhere in the deep zone of the second layer there are small cells scattered near the mitral and having the same significance as the mitral, giving processes to the glomeruli and into the layer of central fibers.

The layer of central fibers is very dense and consists of centronetal and centrifugal fibers: the first are the axons of mitral cells and their equivalents, the second are fibers coming from the anterior commissure of the brain, and corticofugal fibers penetrating into the deep zone, the significance of which is currently still unknown .

The fibers of the tract go in four directions:

1) through the lateral olfactory bundle - into the hook of its side; these fibers end in the ammon's horn, in its tonsil nucleus;

2) through the anterior commissure - into the tract of the opposite side and ends in its cortical layer;

3) from the olfactory triangle - to the gray matter of the transparent septum (septum pellucidum);

4) finally, from the olfactory triangle - to the anterior perforated substance.

The anterior part of the perforated space in macrosmatic animals is highly developed and is designated as the olfactory tubercle.

The paths of the second central neuroma are as follows:

1) from gray matter transparent septum as part of the vault to the horn of Ammon;

2) from the anterior perforated space through a semicircular strap around the caudate nucleus, separating it from the visual thalamus, among the terminal stripes and further along the bottom lateral ventricle into the horn of Ammon and to the hook;

3) from the olfactory triangle in the Wallenberg bundle to the mammillary body.

The third central neuron consists of the following formations and pathways coming from the mamillary body as part of bundles.

The olfactory system also includes fiber systems that go:

1) from the anterior nucleus of the visual thalamus and the gray matter of the transparent septum, the so-called terminal stripes of the visual thalamus, and reach the leash node;

2) from the leash knot, in the form of a Meynert bundle, to the interpeduncular nucleus;

3) from the interpeduncular nuclei to the deep dorsal tegmental ganglion.

Along with the systems just mentioned, there are the following formations classified as part of the olfactory sphere:

1) paths from the nucleus of the amygdala that go along the fornix to reverse side into the mamillary body;

2) a bundle from the posterior deep node of the tegmentum, running along the back of the bottom of the Sylvian aqueduct and the tegmentum of the medulla oblongata, the so-called longitudinal dorsal fasciculus of Schütz, which ends in all nuclei of the tegmentum of the pons and medulla oblongata.

There is a close connection between the primary olfactory centers (olfactory triangle, olfactory bulb) and the nuclei of the trigeminal nerve. This close anatomical connection of the olfactory centers with the trigeminal and other cranial nerves(vagus, vestibule) probably explains many phenomena caused by the olfactory act, in addition to the purely olfactory sensation - changes in the rhythm of breathing and pulse with pleasant and unpleasant olfactory sensations, a decrease and increase in muscle tone, the appearance of dizziness in connection with the perception of certain odors.

Thus, we distinguish the paths and centers of the primary order - the first olfactory neuron (olfactory cells located in the olfactory fissure, the central processes of olfactory cells in the form of filaments, penetrating through the perforated plate of the ethmoid bone and ending in the area of ​​the olfactory bulbs).

Pathways and centers of secondary order - II neuron olfactory system- fibers from the olfactory bulbs go in the olfactory tracts and end in an extension - the olfactory triangle. The third neuron of the olfactory analyzer begins here.

The anterior commissure connects the primary olfactory centers. The secondary olfactory formations are connected by the hypocampal commissure or commissure of the lyre of David and back the anterior commissure, which also connects the gynocampal gyri.

All third-order neurons are projection, association and commissural fibers.

The olfactory pathways are mostly uncrossed. In the area of ​​the anterior commissure there is an anastomosis of the olfactory tracts, in the area of ​​the middle commissure there is an anastomosis of the fibers entering the ammonian horn.

The cortical ends of the olfactory analyzer are also connected to each other by a large white commissure.

The olfactory pathways have connections with various departments brain From the olfactory triangles there are paths to the papillary bodies at the base of the brain. These formations are involved in the regulation of autonomic functions. From here the vegetotropic effect of smell (vasodilation, increased heart rate, etc.) becomes clear.

Through the mamillary bodies, the olfactory pathways are connected to the visual thalamus. In the area of ​​the visual thalamus there is a connection between the olfactory and vestibular analyzers. Clinically, this connection is confirmed by the influence of olfactory stimulation on vestibular chronaxy and other observations.

Olfactory connections with the visual thalamus and mamillary bodies have a double direction (in one direction or the other), that is, impulses can be conducted in both directions.

The connections between the olfactory formations and the tegmentum of the brainstem and the varoli are described. pons and medulla oblongata (via descending paths posterior longitudinal fasciculus).

Motor movements are carried out along these pathways unconditioned reflexes to olfactory stimulation (facial movements, as well as general motor reaction, etc.).

There is a rich anatomical and physiological connection between the I and V cranial nerves, as well as with the autonomic nervous system.

By many authors confirms the anatomical connection between the sense of smell and the trigeminal systems both on the periphery and in the center. The centers of smell in the visual thalamus are connected with the nuclei of the trigeminal nerve by the Gudden tract. The anterior perforated space receives bilateral fibers from the olfactory tracts and fibers from the pons, possibly from the sensory nuclei of the trigeminal nerve, also come here. In the optic thalamus, the nucleus of the olfactory nerve lies next to the nucleus of the V nerve. While studying the phenomenon of olfactory fatigue, I passed a stream of odorous air through the nose under a certain pressure for a long time and received, in addition to the sensation of smell, also a sensation of pain.

Olfactory nerve(I pair) begins from the olfactory cells located in the mucous membrane of the upper part of the nasal cavity, the dendrites of which perceive aromatic substances. The axons of the olfactory cells in the form of 15-20 olfactory filaments form the olfactory nerve and pass through the openings in the ethmoid bone into the cranial cavity, where they end in the olfactory bulb. Here are the second neurons of the olfactory analyzer, the fibers of which are directed posteriorly, forming the right and left olfactory pathways (tractus olfactorius dexter et sinister), which are located in the olfactory grooves on the base frontal lobes brain The fibers of the olfactory pathways follow to the subcortical olfactory centers: mainly to the olfactory triangle, as well as to the anterior perforated substance and the septum pellucidum, where they switch to third neurons. These neurons conduct olfactory stimuli from the primary olfactory centers to the cortical section of the olfactory analyzer on their own and the opposite side. Cortical center sense of smell is located on inner surface the temporal lobe in the anterior parts of the gyrus near the seahorse (parahippocampal), mainly in its uncus. The fibers of the third neurons, having made a partial decussation, reach the cortical olfactory centers in three ways: some of them pass over the corpus callosum, another part under the corpus callosum, the third directly through the uncinate fasciculus (fasciculus uncinatu).

1 - olfactory threads; 2 - olfactory bulb; 3 - olfactory pathway; 4 - subcortical olfactory centers; 5 - olfactory fibers over the corpus callosum; 6 - olfactory fibers under the corpus callosum; 7 - cingulate gyrus; 8 - parahippocampal gyrus; 9 - cortical section of the olfactory analyzer.

Olfactory research. The patient is allowed to smell a weakly aromatic substance with each half of the nose separately. Strong irritating odors (vinegar, ammonia) should not be used, since the irritations they cause are perceived mainly by the receptors of the trigeminal nerve. It is necessary to find out whether the patient feels and recognizes the smell, whether the sensation is the same on both sides, whether he has olfactory hallucinations.

Smell disorders can be in the form of decreased perception (hyposmia), complete loss of it (anosmia), exacerbation (hyperosmia), distortion of smell (parosmia), as well as olfactory hallucinations, when the patient perceives odors without a corresponding stimulus.

Bilateral impairment of smell is observed more often with inflammatory pathological processes in the nasal cavity that are not related to neurological pathology. Unilateral hypo- or anosmia occurs when the olfactory bulb, olfactory pathway and olfactory triangle reach the intersection of fibers heading to the cortical olfactory projection area. This pathology occurs with a tumor or abscess in the anterior cranial fossa damaging the olfactory bulb or olfactory pathway. In this case, hypo- or anosmia occurs on the affected side. Unilateral damage to the fibers of the olfactory analyzer above the subcortical olfactory centers does not lead to loss of smell, since each of the subcortical centers and, accordingly, each half of the nose is connected to both cortical departments of smell. Irritation of the cortical areas of the olfactory analyzer in the temporal lobe leads to the appearance of olfactory hallucinations, often the aura of an epileptic seizure.

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

Smell is perception the body with the help of the olfactory organs of certain properties (odors) of various substances.

Olfactory organs in humans are presented olfactory epitelium, located in the superoposterior nasal cavity and cover the areas of the superior lateral concha and nasal septum on each side. The olfactory epithelium is covered with a layer of olfactory mucus and consists of olfactory receptors (specialized chemoreceptors), supporting and basal cells. The respiratory region (that part of the nasal mucosa in which there are no olfactory cells) contains free endings of sensory fibers of the trigeminal nerve (V), which also react to odorous substances. This partially explains the preservation of the sense of smell in the event of a complete interruption of the olfactory fibers.

A person can smell thousands of different substances, but no clear chemical difference has been found between substances corresponding to different odors. Designed for practical purposes odor classifications(or primary odors) indicate that chemically similar substances often appear in different odor classes, and substances of the same odor class differ significantly in their chemical structure.

The diverse possibilities of smell are described by the following basic odors::

1. camphor,
2. floral,
3. musky,
4. mint,
5. ethereal,
6. caustic,
7. putrefactive.

IN natural conditions As a rule, there are mixtures of odors in which certain components predominate. Distinction based on their quality is possible only to a certain extent, and only in conditions of very high concentrations some substances. The similarity and difference of odors is associated with the structure and (or) vibrational properties of odorous molecules. It is believed that the key to five of the seven basic odors is stereochemistry odor substances, i.e. spatial correspondence of the configuration of odorous molecules to the shape of receptor sites on the surface membrane of olfactory microvilli. For the perception of caustic and putrid smell It is not the shape of the molecules that is considered important, but the charge density on them. There is a point of view that the specificity of odor is associated with the correspondence of the resonant vibrational frequencies of the stimulus and receptor molecules.

Since at low concentrations of an odorous substance a person only perceives the odor, but cannot determine its quality, the properties of the sense of smell describe the detection thresholds and odor recognition thresholds. With suprathreshold stimulation of the sense of smell, as the concentration of the odorous substance increases, the sensation intensifies. Olfactory sensations change with changes chemical properties stimulus is relatively slow, i.e. olfactory system inertial. As a result of prolonged action of the stimulus, the sense of smell and its changes weakens, the person adapts to the presence of environment odoriferous substance. In cases of intense and prolonged stimulation of the sense of smell, even complete adaptation occurs, that is, complete loss of sensation.

Peripheral part of the olfactory system

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The implementation of the functions of the sensitive olfactory epithelium is ensured by the receptor cells located in it, the number of which in humans reaches 10 million (in a shepherd dog - over 200 million). In addition to receptor (olfactory) cells, the epithelium contains supporting and basal cells. The latter have the ability to develop into olfactory cells and, therefore, represent immature sensory cells. Unlike taste cells, olfactory cells are primary sensory cells and send axons to the brain from their basal pole. These fibers form thick bundles under the sensory epithelium (olfactoryfibers), which go to the olfactory bulb.

The upper part of the olfactory cell extends into the mucus layer, where it ends in a bundle of 6-12 olfactory hairs (cilia) on each cell, with a diameter of 0.2-0.3 microns. Odor molecules diffuse through slime layer and reach the membrane of the olfactory hairs. Sources of mucus are Bowman's glands, goblet cells of the respiratory region and supporting cells of the olfactory epithelium, which therefore perform a dual function. The flow of mucus is regulated by kinocilia of cells in the respiratory region.

Molecules of odorous substances interact with special molecules in the membranes of olfactory cells. However, the existence large number effective odorous substances does not allow us to talk about the content of individual receptor molecules for each substance in the sensory membrane. It is obvious that several closely related odorants react with the same receptor molecule. Olfactory cells have characteristic responses, the features of which depend on chemical composition irritant. Excitation of individual cells occurs under the influence of many stimuli, but the relative sensitivity of olfactory cells to different active substances at certain concentrations is not the same. At a given concentration, each odorous substance causes a specific spatiotemporal distribution of impulses in the afferent fibers, characteristic only of this substance. Since many sensory cells are involved in the reaction, the receptor space for a particular substance has real geometric dimensions in the sensory epithelium. An increase in the concentration of an odorous substance leads to an increase in the frequency of impulses in most nerve fibers. Some odorants inhibit the spontaneous activity of sensory nerve cells.

Between the olfactory hair, immersed in mucus, and the base of the axon of the sensory cell, under the action of odorous substances, a potential difference and an electric current of a certain direction arises, called generator It causes depolarization of the most excitable zone of the axon. Inhibition and enhancement of spontaneous activity depends on the direction of the current. Excitatory - depolarizing - potentials in olfactory cells are always greater in amplitude on average than inhibitory - hyperpolarizing ones.

The total electrical activity of the olfactory epithelium is called electroolfactogram. This is a negative electrical oscillation with an amplitude of 12 mV and a duration exceeding the duration of odor exposure. The electroolfactogram consists of three waves - for turning on a stimulus, for a continuing stimulus, and for turning it off. The electronegativity of the surface of the olfactory epithelium reflects the fact that the number of excited receptors is always greater than inhibited ones.

Central division of the olfactory system

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The axons of the olfactory cells united in a bundle go to the olfactory bulb - the primary central department olfactory system (Fig. 16.16), in which the primary processing of sensory information coming from olfactory receptor cells occurs. The cellular elements in the olfactory bulb are arranged in layers. Large mitral cells are second-order neurons of the olfactory pathway. These cells have one main dendrite, the distal branches of which form synapses with the fibers of the olfactory cells (glomeruli). About 1000 fibers converge on each mitral cell. The axons of the olfactory cells also make synaptic contact with the periglomerular cells, which form lateral connections between the glomeruli. The nature of the connections provides the basis for the process associated with coding - lateral inhibition.

The olfactory bulb generates rhythmic potentials that change when odorous substances are blown into the nose. There is no connection between these potentials and the encoding of odor information. It is believed that from the point of view of distinguishing odors, it is not the absolute frequency values ​​that are significant, but their change relative to the resting rhythm. Electrical stimulation of the olfactory bulb in humans causes the sensation of smell.

The axons of the mitral cells make up the olfactory tract, which directly or indirectly through its connections with other tracts, transmits olfactory signals to many areas of the brain, including the olfactory bulb of the opposite side, to structures located in the paleocortex and subcortical nuclei forebrain, to the structures of the limbic system, through the amygdala complex to the autonomic nuclei of the hypothalamus.

The output of excitation signals from the olfactory bulb is under efferent control, which occurs at the peripheral level (Fig. 16.16).

The sense of smell provides protective reflexes such as sneezing and holding your breath; substances with a pungent odor (ammonia) lead to a reflexive cessation of breathing. Reflex reactions of this type are associated with irritation of the fibers of the trigeminal nerve. These reflexes close at the level of the medulla oblongata. At the same time, the sense of smell has functional influences to a variety of emotions, to general mood. The likelihood of such an influence is determined by the connections between the olfactory organ and the limbic system.