Olfactory system (olfactory analyzer). Olfactory nerve

Center located on the inferior surface of the temporal and frontal lobes of the cortex cerebral hemispheres. The olfactory cortex is located at the base of the brain, in the region of the parahippocampal gyrus, mainly in the ncus. Some authors attribute the ammon's horn and gyrus dentatus to the cortical representation of the olfactory center.

What all these brain formations have in common is the presence of a close relationship with the limbic system (cingulate gyrus, hippocampus, amygdala, septal area). They are involved in maintaining the constancy of the internal environment of the body, regulating vegetative functions and the formation of emotions and motivations. This system is otherwise called the “visceral brain”, since this part telencephalon can be considered as a cortical representation of interoreceptors. Information comes here from internal organs about the state of the internal environment of the body.

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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 the air, and water-soluble in order to penetrate to the receptor cells through the layer of mucus covering the entire epithelium of the nasal cavities. 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 esters.
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 them existing classifications smells have never 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: Proc. 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

Smell- this is the ability to sense and identify odors that are a specific irritant of the olfactory analyzer. The olfactory analyzer consists of a peripheral section, pathways and a cortical olfactory center. The peripheral section is represented by the olfactory epithelium, located in the nasal cavity in upper sections the middle turbinate, the superior turbinate and the upper part of the nasal septum. The perception of odors is carried out by sensitive neuroreceptor cells of the olfactory epithelium, which, by origin and physiological characteristics close to nerve cells brain. The sensitive part is a peripheral process, at the top of which there is a bundle of 5-20 modified flagella. Along with flagellar olfactory cells, receptor cells bearing microvilli at the apex are described. These morphological differences reflect the functional specialization of olfactory cells. It is currently assumed that the membrane of olfactory flagella and microvilli is apparently the site of interaction of the cell with molecules of odorant substances. The central processes form the olfactory nerves, passing in the form of 15-20 thin filaments through the lamina cribrosa into the cranial cavity. The primary central olfactory formations, located in the mediobasal regions of the anterior cranial fossa, are represented by the olfactory bulbs (bulbus olfactorius), olfactory tracts (tractus olfactorius), and olfactory triangles. The processes of the olfactory cells as part of the olfactory stripes enter the area subcallosa, Broca's strip (stria Broca). The cortical olfactory center (secondary central olfactory formations) is localized in the mediobasal regions temporal lobe brain, in the hippocampus (gyrus hippocampi). Throughout their entire length, the olfactory fibers run homolaterally. Their interaction with each other is ensured by the neural and trophic connections between them.

It is known that when individual structures of the olfactory analyzer are damaged, all its components are involved in the process, providing a single holistic response to the introduction of an infectious agent or traumatic injury. Thus, the ability of neurotropic viruses, in particular the influenza virus, to move from the nasal cavity along axonal and perineural pathways into the cranial cavity has been established. Damage to the olfactory receptor layer in the nasal cavity inevitably leads to degenerative changes in the olfactory bulbs, and vice versa. Thanks to the extensive connections of the olfactory analyzer with the reticular formation, hypothalamus, limbic system, vestibular analyzer, olfactory function related to frequency breathing movements and heartbeats, blood pressure, body temperature, muscle tone, state of statics and coordination.

The human olfactory function includes two complementary components: perception and differentiation of odors. Olfactory signals play an important biological role: they provide information about the presence of environment certain chemical compounds, perform a signaling function (food, sexual, protective, orientation). Based on their effect on the olfactory, trigeminal, and glossopharyngeal nerves, odorous substances are distinguished as olfactory and mixed (olfactotrigeminal, olfactoglossopharyngeal). Substances that are an adequate irritant of the olfactory nerve or odorous substances with olfactory action include valerian officinalis, rose oil, tar, turpentine, vanillin, honey, tobacco, coffee, etc. Iodine, menthol, acetone, and formaldehyde have an olfactotrigeminal effect. Iodoform, chloroform, and acetic acid.

Olfactory disorders are polyetiological. Classification clinical forms Olfactory disorders developed by Academician of the Russian Academy of Medical Sciences Yuri Mikhailovich Ovchinnikov et al. distinguishes three forms of dysosmia: perceptual, conductive and mixed. Most common species dysosmia - respiratory or conductive hypo- and anosmia, which is caused by rhinogenic causes, i.e. changes in the nasal cavity, mechanically complicating or preventing the access of odorous substances to the olfactory area. Impaired sense of smell during sinusitis, in addition to the conductive component, is also caused by a change in the pH of the secretion of the Bowman's glands, which is a solvent of odorous substances. For chronic inflammatory diseases The nasal cavity and paranasal sinuses also show metaplasia of the epithelium, which leads to damage to the olfactory receptor apparatus. With sinusitis that occurs with the formation of purulent-putrefactive contents, objective cacosmia may appear. With atrophic and subatrophic changes in the mucous membrane of the nasal cavity, both a conductive component and damage to the olfactory neuroepithelium occur. There are also hereditary dysosmia: for example, in Kallmann syndrome, which is transmitted in an autosomal dominant manner with varying degrees of penetrance, hypogonadotropic eunuchoidism and anosmia occur. This syndrome shows possible connection between the sense of smell and sexual development. With Kallmann syndrome, underdevelopment of the hypothalamus or absence of the olfactory epithelium, kidney abnormalities, cryptorchidism, deafness, diabetes, deformities may be observed. facial skeleton. Perceptual (neurosensory or essential) olfactory disorders occur with peripheral damage to neuroepithelial cells and/or olfactory nerves, as well as in the case of central disorders olfactory formations of the anterior or middle cranial fossae.

Common reasons olfactory disorders“receptor level” - injuries of the olfactory zone and cribriform plate, inflammatory process, traumatic brain injury, drug intoxication, allergic reaction, genetic mutation, deficiency of vitamins A and B12, intoxication with salts of heavy metals (cadmium, lead, mercury), inhalation of vapors of irritating substances (formaldehyde), viral infection. In this case, the interaction of the receptor cell with G-protein molecules is disrupted, and the production of peptides is noted that inhibit the activity of olfactory receptor cells. Damage to G-protein was noted by a number of authors against the background endocrine pathology(pseudohypoparathyroidism, Addison's disease, Cushing's syndrome), including during treatment with antithyroid drugs, when prescribing radioactive iodine. At the same time, estrogens taken orally play a protective role for the olfactory neuroepithelium in relation to toxic substances in postmenopausal women. Olfactory disorders can also be caused by the following factors: exposure to a neurotropic virus, mainly the influenza virus, impaired Zn metabolism, ionizing radiation.

Pathological changes at the level of the olfactory nerve are most often caused by infectious diseases, metabolic disorders, toxic effects medicines, demyelinating processes, damage due to surgical interventions, tumors (in particular, meningioma of the olfactory nerve). Central olfactory disorders are varied and, according to the classification of O.G. Ageeva-Maikova, are divided into lesions of primary olfactory formations in the mediobasal sections of the anterior cranial fossa, which is manifested by hypo- and anosmia on the side of the pathological process, and lesions of secondary olfactory formations in the temporobasal sections of the middle cranial fossa, which manifests itself in impaired recognition of odors, hyperosmia or olfactory hallucinations. The causes of central olfactory disorders may be traumatic brain injury, impairment cerebral circulation, brain tumors, demyelinating processes, metabolic disorders, genetic and infectious diseases, sarcoidosis, Parkinson's disease, Alzheimer's disease. Cases of dysosmia with dysmenorrhea have been described. There are disturbances of the sense of smell with syphilis, scleroma and tuberculosis treated with streptomycin, with basal and optochiasmal arachnoiditis, allergic rhinosinusopathy, after rhinosurgical interventions, with pathology of the digestive organs, congenital hereditary anosmia.

It should be noted that impairment of olfactory acuity is possible in all three forms of dysosmia, either by the type of anosmia (lack of perception and recognition of odors) or by the type of hyposmia (decreased ability to perceive and adequately recognize odorous substances). Impairments in the differentiation of odors are possible in perceptual and mixed forms dysosmia and manifest themselves as aliosmia, when odorous substances are perceived as one of the odors of the environment, including cacosmia (putrid, fecal odor), torcosmia (chemical, bitter odor, burning, metal odor), parosmia - a specific transformation in the recognition of odors. Phantosmia is manifested by olfactory hallucinations. We should not forget about the possibility of objective cacosmia, in particular, with purulent lesions of the sphenoid sinus. If a patient has both conductive and perceptual components of olfactory disorders, perceptual-conductive (mixed) dysosmia is distinguished. The inability to describe a smell in words, even if it is familiar, is called olfactory agnosia.

Source: “Differential diagnosis of nervous diseases” edited by G.A. Akimova and M.M. Same; Saint Petersburg; publishing house "HIPPOCRATES", 2001 (pp. 31 - 33).

Olfactory disorders include hyposmia and anosmia, which can be unilateral or bilateral, as well as hyperosmia, parosmia, olfactory illusions and olfactory hallucinations, which are not characterized by lateralization. Special diagnostic value have unilateral smell disorders, since bilateral ones are more often the result of various diseases of the nasal cavity. Therefore, in case of bilateral olfactory disorders highest value acquire examination of the nasal mucosa, as well as reliable indications of the absence of smell disorders in the period preceding the disease nervous system.

Unilateral hyposmia or anosmia, indicating damage to the peripheral part of the olfactory analyzer, can be observed with unilateral pathological processes in the area of ​​the olfactory fossa - fractures of the base of the skull with damage to the cribriform plate of the ethmoid bone, with traumatic hematomas anterior cranial fossa, tumors located on the base of the skull in the area of ​​the olfactory fossa, platform, small wings of the sphenoid bone, tubercle of the sella tuprec and spreading anteriorly. All of these processes can lead to bilateral anosmia (or hyposmia), however, as noted above, bilateral olfactory disorders require careful evaluation. In the majority listed cases The sense of smell itself suffers while the trigeminal component of the sensitivity of the nasal mucosa is preserved. Unilateral hyperosmia and parosmia with damage to the peripheral part of the olfactory analyzer is extremely rare.

Bilateral hyposmia and anosmia may be associated with compression of the olfactory bulbs, olfactory tracts and primary olfactory centers by the cerebral ventricles sharply stretched due to hydrocephalus, with gross disturbances venous outflow from the sinuses with some tumors of the chiasmatic-sellar region, with acute and chronic inflammatory processes V meninges anterior cranial fossa (purulent and serous meningitis, basal arachnoiditis) With inflammatory lesions in the process of restoring a reduced sense of smell, a stage of parosmia is possible - the appearance unusual sensations under the influence of ordinary olfactory stimuli. It should be noted that hyposmia or anosmia occurs only when the olfactory pathways are damaged up to the olfactory triangle, that is, at the level of the first and second neurons. Due to the fact that third neurons have cortical representation on both their own and the opposite side, damage to the cortex in the olfactory projection field does not cause loss of smell. However, if the cortex of this area is irritated, olfactory illusions and hallucinations may occur (see below).

Olfactory illusions and hallucinations (feeling unpleasant odor mold, rotting, sour products, etc.) indicate irritation by the pathological process of the cortical olfactory projection zone, primarily the uncus of the parakippocampal gyrus. Olfactory hallucinations can be a manifestation of simple partial epileptic seizures, which in some cases transform into complex partial and generalized ones seizures. Such disturbances can occur with tumors of the corresponding localization or be a manifestation of epilepsy. Olfactory agnosia - impaired recognition of a previously known smell - is associated with focal, usually bilateral processes in the hippocampus. It should be borne in mind that disturbances in the sense of smell often occur when various diseases, not associated with damage to the nervous system ( diabetes, hypothyroidism, scleroderma, Paget's disease, etc.).

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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 under conditions of very high concentrations of certain 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, and a person adapts to the presence of an odorous substance in the environment. 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. Molecules of the odorous substance diffuse through the mucous 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 various active substances at certain concentrations varies. 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 pulse frequency 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 influence of odorous substances, a potential difference arises and electricity a certain direction, 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 the olfactory receptor cells. 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 pungent odor(ammonia) lead to a reflex 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 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.

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 the sense of smell is located on the inner surface of the temporal lobe in the anterior parts of the gyrus near the seahorse (parahippocampal), mainly in its hook (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 senses and recognizes the smell, whether the sensation is the same on both sides, and 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.