A person can navigate the world around him using various types of analyzers. We have the opportunity to sense various phenomena of the external environment with the help of smell, hearing, vision and other senses. Each of us has different analyzers developed to varying degrees. In this article we will try to understand how the olfactory analyzer works, and we will also analyze what functions it performs and what impact it has on health.

Definition of the olfactory organ

It is believed that a person can receive the bulk of information coming from outside through vision, but in the absence of smell, the picture of the world would not be so exciting and bright for us. In general, smell, touch, vision, hearing are what help a person perceive the world around him correctly and fully.

The olfactory system allows you to recognize those substances that have the ability to dissolve and volatility. It helps to perceive images of the world subjectively, through smells. The main purpose of the olfactory organ is to provide the ability to objectively assess the quality of air and food. Why the sense of smell disappears is of interest to many. More on this later.

Basic functions of the olfactory system

Among all the functions of this sense organ, the most significant for human life can be identified:

Evaluating the food consumed for its edibility and quality. It is the sense of smell that allows us to determine how suitable a particular product is for consumption.
Formation of such type of behavior as food.
It is the organ of smell that plays an important role in the preliminary adjustment of such an important system as the digestive system.
Allows you to identify substances that may pose a danger to humans. But this is not all the functions of the olfactory analyzer.
The sense of smell allows you to perceive pheromones, under the influence of which such type of behavior as sexual behavior can be formed and changed.
With the help of the olfactory organ, a person can navigate his environment.

It is worth noting that in people who have lost their vision for one reason or another, the sensitivity of the olfactory analyzer often increases by an order of magnitude. This feature allows them to better navigate the outside world.

The structure of the olfactory organs

This sensory system includes several sections. So, we can highlight:

Peripheral department. It includes receptor-type cells that are located in the nose, in its mucous membrane. These cells have cilia coated in mucus. It is in it that the dissolution of substances with odor occurs. As a result, a chemical reaction occurs, which is then transformed into a nerve impulse. What else does the structure of the olfactory analyzer include?
Wiring department. This section of the olfactory system is represented by the olfactory nerve. It is along it that impulses from the olfactory receptors spread, which then enter the front part of the brain, which contains the so-called olfactory bulb. The primary analysis of data occurs in it, and after this the transmission of nerve impulses occurs to the subsequent section of the olfactory system.
Central department. This section is located in two areas of the cerebral cortex at once - in the frontal and temporal. It is in this section of the brain that the final analysis of incoming information takes place, and it is in this section that the brain forms our body’s reaction to the influence of smell. These are the sections of the olfactory analyzer that exist.

Let's take a closer look at each of them.

Peripheral section of the olfactory system

The process of studying the olfactory system should begin with the first, peripheral section of the odor analyzer. This section is located directly in the nasal cavity. The nasal mucosa in these parts is somewhat thicker and abundantly covered with mucus, which is a protective barrier against drying out and serves as an intermediary in removing the remains of irritants at the end of the process of exposure.

The contact of the odorous substance with the receptor cells occurs here. The epithelium is represented by two types of cells:

Cells of the second type have a pair of processes. The first reaches out to the olfactory bulbs, and the second looks like a stick with a bubble covered with cilia at the end.

Wiring department

The second section conducts nerve impulses and is actually the nerve pathway that forms the olfactory nerve. It is represented by several bundles that pass into the visual thalamus.

This department is interconnected with the limbic system of the body. This is what explains why we experience different emotions when we perceive smells.

Central section of the olfactory analyzer

Conventionally, this section can be divided into two parts - the olfactory bulb and sections in the temporal lobe of the brain.

This department is located in close proximity to the hippocampus, in the frontal part of the piriform lobe.

Mechanism for sensing odor

In order for an odor to be perceived effectively, the molecules must first be dissolved in the mucus that surrounds the receptors. After this, specific proteins built into the membrane of receptor cells interact with mucus.

This contact can occur if there is a correspondence between the shapes of the molecules of the substance and proteins. Mucus performs the function of controlling the availability of receptor cells for irritant molecules.

After the interaction between the receptor and the substance begins, the protein structure changes and sodium ion channels open in cell membranes. After this, sodium ions enter the membranes and excite positive charges, leading to a change in the polarity of the membranes.

Then the mediator is released from the receptor, and this leads to the formation of an impulse in the nerve fibers. Through these impulses, irritation is transmitted to the following parts of the olfactory system. How to restore your sense of smell will be discussed below.

Adaptation of the olfactory system

The human olfactory system has such a feature as the ability to adapt. This occurs if the irritant affects the sense of smell for a long time.

The olfactory analyzer can adapt over varying periods of time. It can take from a few seconds to several minutes. The length of the adaptation period depends on the following factors:

The period of exposure of the odorant to the analyzer.
The level of concentration of the odorous substance.
The speed of movement of air masses.

They sometimes say that their sense of smell has become more acute. What does it mean? The sense of smell adapts to some substances quite quickly. The group of such substances is quite large, and adaptation to their smell occurs very quickly. An example is our addiction to the smell of our own body or clothing.

However, we adapt to another group of substances either slowly or even partially.

What role does the olfactory nerve play in this?

Theory of smell perception

At the moment, scientists claim that there are more than ten thousand distinct odors. However, they can all be divided into seven main categories, so-called primary odors:

Flower group.
Mint group.
Musk group.
Essential group.
Putrid group.
Camphor group.
Caustic group.

They are included in the set of odorant substances for the study of the olfactory analyzer.

If we perceive a mixture of several odors, then our olfactory system is able to perceive them as a single, new smell. Odor molecules of different groups have different shapes and also carry different electrical charges.

Different scientists have different theories explaining the mechanism by which smell perception occurs. But the most common is the one according to which it is believed that membranes have several types of receptors with different structures. They are susceptible to molecules of different shapes. This theory is called stereochemical. Why does the sense of smell disappear?

Types of smell disorders

In addition to the fact that we all have a sense of smell at different levels of development, some may exhibit disturbances in the functioning of the olfactory system:

Anosmia is a disorder in which a person is unable to perceive smells.
Hyposmia is a disorder in which there is a decrease in the sense of smell.
Hyperosmia - characterizes increased sensitivity to odors.
Parosmia is a distorted perception of the smell of substances.
Impaired differentiation.
The presence of olfactory hallucinations.
Olfactory agnosia is a disorder in which a person can smell a smell but is unable to identify it.

It should be noted that over the course of life a person loses sensitivity to different odors, that is, sensitivity decreases. Scientists have found that by the age of 50 a person is able to perceive approximately half as much odors as in his youth.

Olfactory system and age-related changes

During the intrauterine development of the child's olfactory system, the formation of the peripheral part occurs first. This process begins approximately in the second month of development. By the end of the eighth month, the entire olfactory system is already fully formed.

Immediately after birth, you can already observe how the baby perceives smells. The reaction is visible by the movements of facial muscles, heart rate or the position of the child’s body.

It is with the help of the olfactory system that the child is able to recognize the smell of the mother. Also, the olfactory organ serves as an important component in the formation of digestive reflexes. As the child grows, his ability to differentiate odors increases significantly.

If we compare the ability to perceive and differentiate odors in adults and children aged 5-6 years, then in adults this ability is much higher.

In what cases does loss or decrease in sensitivity to odors occur?

As soon as a person loses sensitivity to smells or its level decreases, we immediately begin to wonder why this happened and how to fix it. Among the reasons influencing the acuity of smell perception are:

ARVI.
Damage to the nasal mucosa by bacteria.
Inflammatory processes that occur in the sinuses and nasal passages, caused by the presence of infection.
Allergic reactions.

The loss of smell always depends in a certain way on disturbances in the functioning of the nose. It is the main organ that provides us with the ability to smell. Therefore, the slightest swelling of the nasal mucosa can cause disturbances in the perception of odors. Often, disturbances in the sense of smell indicate that symptoms of rhinitis may soon appear, and in some cases, only upon recovery can one discover that sensitivity to odors has decreased.

How to restore your sense of smell?

If you have lost your sense of smell after suffering from a cold, your doctor will be able to tell you how to get it back. You will most likely be prescribed topical medications, which are vasoconstrictors. For example, “Naphthyzin”, “Farmazolin” and others. However, you should not abuse them.

The use of these drugs for a long time can provoke the opposite effect - swelling of the mucous membrane of the nasopharynx will occur, and this can stop the process of restoring the sense of smell.

It should be noted that even before recovery begins, you can begin to take measures to return your sense of smell to its previous level. It seems possible to do this even at home. For example, you can perform inhalations using a nebulizer or make steam baths. Their goal is to make the mucus in the nasal passages softer, and this can contribute to a faster recovery.

In this case, you can inhale regular steam or steam from an infusion of herbs that have medicinal properties. These procedures should be done at least three times a day, for about 20 minutes. It is important that inhalation of steam should be done through the nose, and exhalation through the mouth. This procedure will be effective throughout the entire period of the disease.

You can also resort to traditional medicine methods. The main way to restore your sense of smell as quickly as possible is inhalation. The most popular recipes include:

Inhaling the vapors of basil essential oil.
Steam inhalation with the addition of eucalyptus oil.
Steam inhalation with the addition of lemon juice and essential oils of lavender and mint.

In addition to inhalations, camphor and menthol oils can be instilled into the nose to restore the sense of smell.

The following may also help restore a lost sense of smell:

The procedure for warming up the sinuses using a blue lamp.
Cyclic tension and weakening of the nasal muscles.
Washing with saline solutions.
Inhaling the aroma of medicinal herbs, such as chamomile, cumin or mint.
The use of medicated tampons that are inserted into the nasal passages. They can be moistened with mint oil mixed with propolis tincture in alcohol.
Taking sage decoction, which is very effective in combating ENT diseases.

If you regularly resort to at least a few of the above preventive measures, the effect will not be long in coming. Using such folk methods, you can return your sense of smell even a couple of years after you lost it, because the receptors of the olfactory analyzer will be restored.

The olfactory sensory system occupies a very important place in the life of animals. It is she who plays a significant role in searching for food, avoiding predators and harmful environmental factors, finding individuals of a different sex or recognizing members of her own species. For example, in some species of butterflies, a male can find a female located at a distance of 8-10 km from him, guided by the smell secreted by her reproductive gland. In addition, the olfactory system is given special importance in the processes of information exchange between individuals of their own species - this is the transmission of alarm and danger signals, marking of territory.

There is no doubt that the sense of smell plays an important role in human life, although this importance is often underestimated. Since humans are significantly inferior to the vast majority of animals in such sensitivity to odors and in the specificity of smell, some researchers believe that the sense of smell is a rudiment, i.e. in the process of evolution, it lost its original meaning. In addition, a person, unlike animals, orients himself in space mainly with the help of vision, and in the social environment with the help of hearing and speech. Meanwhile, olfactory chemoreception plays a much larger role in human life than is usually thought. One of the reasons for such an unobviously great importance of smell is that olfactory signals influence physiological processes and the human psyche, often being unconscious. Thus, the experiment showed that after presenting a person with any volatile substance, the smell of which he was not aware of (he was not aware that the chemical composition of the environment had changed), there was a change in his level of hormones in the blood, a change in emotional colored reactions, physical and mental performance, etc. These and other issues, in particular the connection of the sense of smell with social identification, sexual (choice of a sexual partner) and parental behavior, are discussed very well and quite interestingly in the textbook by D.A. Zhukov. “Biological basis of behavior. Humoral mechanisms".

Just like the gustatory sensory system, the olfactory system increases our chances of survival by informing us about the quality of the environment and food, and the presence of a number of toxic substances. In recent years, aromatherapy, based on the use of odorous substances for health, rehabilitation and medicinal purposes, has been intensively developing.

Peripheral section of the olfactory analyzer. The receptors of the olfactory system are located in olfactory epithelium (olfactory lining), lining the superior nasal concha. The multirow olfactory epithelium contains olfactory receptor cells, basal and supporting cells (Fig. 6.2). The olfactory epithelium lies on the basement membrane, under which are located the olfactory (Bowman's) glands, which produce mucus. The excretory ducts of the glands open on the surface of the olfactory epithelium, providing the release of mucus, which promotes effective olfactory reception (mucus is the medium where odorants dissolve and interact with olfactory receptor cells).

Fig.6.2. Scheme of the structure of the olfactory epithelium

OB – olfactory club; OK – supporting cell; CO – central processes of olfactory cells; BC – basal cell; BM – basement membrane; OL – olfactory hairs; MVR – microvilli of olfactory cells and MVO – microvilli of supporting cells.

Olfactory receptor cells are primary bipolar sensory cells and have two processes - a dendrite (at the top of the cell) and an axon (at the base of the cell). In humans, the number of receptors is 10 million, while, for example, the German Shepherd, which is a macrosmatic, has 224 million. The dendrite on the surface of the olfactory epithelium ends with a special spherical thickening - bulb, or olfactory club. It is an important cytochemical center of the olfactory receptor cell. At the top of the club there are 10-12 very thin cilia (hairs), each of which contains microtubules. The cilia are immersed in the secretion of Bowman's glands. The presence of such hairs increases the area of the receptor membrane with molecules of odorous substances tenfold.

Axons (long central processes) are collected in bundles of 15-40 fibers (olfactory filaments) and, passing through the cribriform plate of the ethmoid bone, are directed to the olfactory bulb of the brain.

Supporting cells separate one receptor cell from another and form the surface of the olfactory epithelium. These cells, glial in origin, have microvilli on their surface. It is believed that supporting cells (like Bowman's glands) take part in the formation of the secretion covering the olfactory epithelium. In addition, they perform a phagocytic function and probably direct the process of growth of processes of receptor cells.

Basal cells located on the basement membrane. They, capable of division, serve as a source of regeneration of receptor cells. As is known, olfactory receptor cells (like taste buds and outer segments of photoreceptors) are constantly renewed - their lifespan is approximately 1.5 months. Basal cells never reach the surface of the olfactory epithelium, i.e. are not directly related to the perception of odorous substances.

Mechanism of olfactory reception. Perception of smell, i.e. The content of one odorous substance or a complex of odorous substances in the analyzed portion of air begins with the process of interaction of the odorous substance with the cilia of the olfactory club of the receptor cell (destruction of the cilia eliminates the chemoreceptor function, which, however, is restored as they are regenerated). To do this, the odorant molecule must be perceived by the corresponding protein receptor located in the membrane of the cilium, i.e. interact with it (when molecules of a chemical substance attach to a receptor protein macromolecule, the conformation of the latter changes). As a result of this interaction, the ionic permeability of the membrane of the dendrite of the receptor cell changes, depolarization occurs, which, when reaching a critical level, causes the generation of an action potential in the cell soma. This potential is sent along the axon to the olfactory bulb.

Let us consider modern ideas about the stages of this process in more detail.

Odorous substances enter the olfactory region when air is inhaled through the nose or through the choanae when air enters through the mouth. During quiet breathing, almost all the air passes through the lower nasal passage and has little contact with the mucous membrane of the olfactory area located in the upper nasal passage. In this case, olfactory sensations are only the result of diffusion between the inhaled air and the air of the olfactory region. Faint odors are not felt during such breathing. In order for odorous substances to reach the olfactory receptors, deeper breathing or several short breaths, quickly following one another, is necessary. This is how animals (humans are no exception) sniff, increasing the air flow in the upper nasal passage. Penetrating into the upper nasal passage, chemicals act on the olfactory cells, which, due to their specificity, allow a person to distinguish one smell from another and even catch a specific smell in a mixture of several odors. It is believed that olfactory cells have a multiplicity of odor perception, but the range of capabilities of each of them is different, i.e. individually, each receptor cell is capable of responding with physiological excitation to its characteristic, albeit wide, range of odorous substances. It is important that these spectra are similar for different cells. As a result, each smell causes an electrical response from many receptor cells of the olfactory lining, in which a certain mosaic (specific pattern) of electrical signals is formed. This mosaic, individual for each scent, is smell code, which, in turn, is deciphered in the higher centers of the olfactory analyzer. The concentration of the odorous substance is reflected in the overall level of cell excitation (increase or decrease in the frequency of impulses).

Conducting information from olfactory receptors. As noted above, the central processes of olfactory receptor cells that perform the functions of an axon, uniting with other similar axons, form olfactory filaments (15-40 pieces), which penetrate into the cranial cavity through the cribriform plate of the same bone and are directed to olfactory bulb. The olfactory bulbs are the first brain center in which impulses received from olfactory receptor cells are processed, and this is the only part of the brain whose bilateral removal always leads to a complete loss of smell. The olfactory bulbs are round or oval-shaped structures that have a cavity or ventricle inside. Histologically, the olfactory bulbs have six concentrically located cell layers and four types of neurons - mitral, fasciculate, granular and periglomerular.

The main features of information processing in the olfactory bulb are: 1) convergence of sensory cells on mitral cells (the axons of approximately 1000 olfactory cells end in the dendrites of one mitral cell), 2) pronounced inhibitory mechanisms and 3) efferent control of impulses entering the bulb. Thus, tuft cells and granule cells of the olfactory bulbs are inhibitory neurons, thanks to which descending control of olfactory afferentation is carried out.

The nasal mucosa also contains free nerve endings. trigeminal nerve (V pair of cranial nerves), some of which are also capable of reacting to odors. In the pharynx area, olfactory stimuli are able to excite fibers glossopharyngeal (IX) And vagus (X) nerves. All of them are involved in the formation of olfactory sensations. Their role, which has nothing to do with the olfactory nerve, persists even when the function of the olfactory epithelium is impaired as a result, for example, of infection (influenza), traumatic brain injury, tumors (and related brain surgeries). In such cases we talk about hyposmia, characterized by a significant increase in the threshold of perception. In pituitary hypogonadism (Kalman syndrome), the sense of smell is provided exclusively by these nerves, since in this case aplasia of the olfactory bulbs occurs.

Central projections of the olfactory sensory system. The axons of mitral cells form olfactory tract, delivering information to various parts of the telencephalon and, first of all, to the neurons of the anterior perforated substance, or anterior olfactory nucleus, and neurons of the septum pellucida. A number of authors call these areas primary projection zones of the olfactory analyzer cortex. In turn, the axons of these neurons form tracts going to other structures of the telencephalon: prepiriform and periamygdala areas of the cortex, nuclei of the amygdala complex, hippocampus, parahippocampal gyrus, uncinate, piriform cortex, temporal gyri (?). In addition, through the amygdala complex (nuclei of the amygdala), communication is also ensured with the vegetative nuclei hypothalamus. Thus, information from olfactory receptor cells reaches almost all structures limbic system and only partially - the structures of the neocortex. This direct connection of the olfactory analyzer with the limbic system explains the presence of a significant emotional component in olfactory perception. For example, a smell can cause a feeling of pleasure or disgust, while changing the functional state of the body. This is what the effect of aromatherapy is based on.

It has been shown that the presence of such a significant number of olfactory brain centers is not necessary for the recognition of odors. It is believed that the above brain structures are associative centers that provide communication between the olfactory sensory system and other sensory systems and the organization on this basis of a number of complex forms of behavior (eating, defensive, sexual, etc.), which are controlled by the limbic system of the brain. In other words, these centers allow you to receive olfactory sensations and at the same time (and this is probably the most important thing in their activity) they make it possible to determine the current need and its awareness, i.e. motivation, as well as behavioral activity associated with the implementation of this need, its vegetative support and assessment of the situation, which is expressed in the formation of a certain emotional state.

It is important to emphasize that the olfactory sensory system is fundamentally different from all other sensory systems in that its afferent fibers do not pass to the opposite side of the cerebrum, do not switch in the thalamus, and, most likely, do not have representation in the structures of the neocortex. Such features of the structural and functional organization are due to the fact that olfactory reception is one of the most ancient types of sensitivity.

In addition, the importance of the sensory olfactory system in the preservation of the species should not be underestimated, since it determines the nature of sexual behavior in animals (and, perhaps, to a certain extent, in humans), the choice of a partner and everything related to the reproductive process, such as protein synthesis -receptors in olfactory receptor cells are strictly controlled by genes. Experiments on animals have shown that the responses of neurons in the olfactory tract can be changed by injections of testosterone, i.e. excitation of olfactory neurons correlates with the content of sex hormones in the body. Undoubtedly, such data should be extrapolated to humans with a certain degree of caution. These issues are discussed in more detail in the textbook by D.A. Zhukov. “Biological foundations of human behavior. Humoral mechanisms".

The sense of smell is the ability to perceive and distinguish odors. According to the development of the ability to smell, all animals are divided into macrosmatics, in which the olfactory analyzer is the leading one (predators, rodents, ungulates, etc.), microsmatics, for which the visual and auditory analyzers are of primary importance (primates, birds) and anosmatics, in which lack a sense of smell (cetaceans). Olfactory receptors are located in the upper part of the nasal cavity. In human microsmatics, the area of the olfactory epithelium supporting them is 10 cm 2, and the total number of olfactory receptors reaches 10 million. But in a macrosmatic German shepherd, the surface of the olfactory epithelium is 200 cm 2, and the total number of olfactory cells is more than 200 million.

The study of the work of smell is complicated by the fact that there is still no generally accepted classification of odors. This is primarily due to the extreme subjectivity of perception of a huge number of olfactory stimuli. The most popular classification is that there are seven main odors: floral, musky, minty, camphorous, ethereal, pungent and putrefactive. Mixing these scents in certain proportions allows you to get any other scent. It has been shown that the molecules of substances that cause certain odors have a similar shape. Thus, the ethereal smell is caused by substances with molecules in the shape of a stick, and the camphor smell is caused by substances in the shape of a ball. However, pungent and putrid odors are associated with the electrical charge of the molecules.

Olfactory epithelium(Fig. 25) contains supporting cells, receptor cells and basal cells. The latter, during their division and growth, can turn into new receptor cells. Thus, the basal cells make up for the constant loss of olfactory receptors that occurs as a result of their death (the lifespan of an olfactory receptor is approximately 60 days).

Olfactory receptors- primary sensory and are part of the nerve cell. These are bipolar neurons, a short, non-branching dendrite of which extends to the surface of the nasal mucosa and carries a bundle of 10-12 motile cilia. The axons of receptor cells are sent to the central nervous system and carry olfactory information. In the mucous membrane of the nasal cavity there are special glands that secrete mucus, which moisturizes the surface of the receptor cells. Mucus also has another function. In mucus, molecules of odorous substances bind to special proteins for a short time. Due to this, hydrophobic odorants are concentrated in this water-saturated layer, which makes them easier to perceive. When you have a runny nose, swelling of the mucous membranes prevents odorous molecules from penetrating the receptor cells, so the threshold of irritation rises sharply and the sense of smell temporarily disappears.

To smell, i.e. excite the olfactory receptors, the molecules of the substances must be volatile and at least slightly soluble in water. The sensitivity of the receptors is very high - it is possible to excite the olfactory cell even with one molecule. Odorous substances brought by inhaled air interact with protein receptors on the membrane of the cilia, causing depolarization (receptor potential). It spreads along the membrane of the receptor cell and leads to the appearance of an action potential that “runs away” along the axon to the brain.

The frequency of action potentials depends on the type and intensity of the odor, but in general a single sensory cell can respond to a range of odors. Usually some of them are preferable, i.e. the threshold for reaction to such odors is lower. Thus, each odorous substance excites many cells, but each of them differently. Most likely, each olfactory receptor is tuned to its own pure odor and transmits information about its modality, encoded by the “channel number” (it has been shown that the receptor for each specific odor substance is localized in a specific area of the olfactory epithelium). The intensity of an odor is encoded by the frequency of action potentials in the olfactory fibers. The creation of a holistic olfactory sensation is a function of the central nervous system.

The axons of olfactory cells assemble into approximately 20-40 olfactory filaments. Actually they are olfactory nerves. The peculiarity of the conducting part of the olfactory system is that its afferent fibers do not cross and do not switch in the thalamus. The olfactory nerves enter the cranial cavity through openings in the ethmoid bone and terminate on the neurons of the olfactory bulbs. Olfactory bulbs located on the lower surface of the frontal lobes of the telencephalon. They are part of the paleocortex (ancient cortex) and, like all cortical structures, have a layered structure. Those. During evolution, the telencephalon (including the cerebral hemispheres) appears primarily to provide olfactory functions . And only later does it increase in size and begin to participate in the processes of memory (old cortex; reptiles), and then in providing motor and various sensory functions (new cortex; birds and mammals). The olfactory bulbs are the only part of the brain whose bilateral removal always results in complete loss of smell.

The most prominent layer in the olfactory bulb is the mitral cells. They receive information from receptors, and the axons of the mitral cells form an olfactory tract that goes to other olfactory centers. Efferent (centrifugal) fibers from other olfactory centers also pass through the olfactory tract. They end on the neurons of the olfactory bulb. The branched endings of the fibers of the olfactory nerves and the branching dendrites of the mitral cells, intertwining and forming synapses with each other, form characteristic formations - glomeruli(balls). They include processes of other cells of the olfactory bulb. It is believed that a summation of excitations occurs in the glomeruli, which is controlled by efferent impulses. Research shows that different neurons of the olfactory bulbs respond differently to different types of odorants, which reflects their specialization in the processes of indicating odorants.

The olfactory analyzer is characterized by rapid adaptation to odors - usually within 1-2 minutes from the onset of action of a substance. The development of this adaptation (habituation) is a function of the olfactory bulb, or rather, the inhibitory interneurons located in it.

So, the axons of the mitral cells form the olfactory tract. Its fibers go to various formations of the forebrain (anterior olfactory nucleus, amygdala, septal nuclei, hypothalamic nuclei, hippocampus, prepiriform cortex, etc.). The right and left olfactory areas are in contact via the anterior commissure.

Most of the areas that receive information from the olfactory tract are considered as association centers. They ensure the connection of the olfactory system with other analyzers and the organization on this basis of many complex forms of behavior - feeding, defensive, sexual, etc. Particularly important in this sense are connections with the hypothalamus and amygdala, through which olfactory signals reach centers that trigger various types of unconditioned (instinctive) reactions.

It is well known that olfactory stimuli have the ability to evoke emotions and retrieve memories. This is due to the fact that almost all olfactory centers are part of the limbic system, which is closely related to the formation and flow of emotions and memory.

Because the activity of the olfactory bulb can be modified due to signals coming to it from other cortical structures; the state of the bulb (and, consequently, the reaction to odors) changes depending on the general level of brain activation, motivations, and needs. This is very important when implementing behavioral programs related, for example, to searching for food, reproduction, territorial behavior .

For a long time, the additional organs of smell were considered vomeronasal or Jacobson's organ (VNO). It was believed that in primates, including humans, VNO is reduced in adults . However, studies in recent years have shown that the VNO is an independent sensory system that has a number of differences from the olfactory system.

VNO receptors are located in the inferomedial wall of the nasal region and differ in structure from the olfactory receptors. An adequate stimulus for these receptors is pheromones– biologically active volatile substances released by animals into the environment and specifically affecting the behavior of individuals of their species. The fundamental difference of this sensory system is that its stimuli are not conscious. Only subcortical centers were found, in particular the hypothalamus, where signals from the VNO are projected, but cortical centers were not found. Pheromones of fear, aggression, sex pheromones, etc. have been described in a number of animals.

In humans, pheromones are secreted by special sweat glands. For humans, only sex pheromones (male and female) have been described so far. And now it becomes clear that a person’s sexual preferences are formed not only on the basis of sociocultural factors, but also as a result of unconscious influences.

Olfactory sensory system (OSS)

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

The value of NSS for humans:

Provides reflex stimulation of the digestive center;

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

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

Involved in emotional behavior;

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

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

In animals, it also ensures the search for food.

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

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

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

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

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

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

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

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

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

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

Mechanism of excitation of olfactory receptor cells

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

Rice. 12.32. Diagram of the olfactory mucosa:

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

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

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

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

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

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

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

Rice. 12.33. The structure of the olfactory epithelium

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

Wire and brain divisions of the olfactory sensory system

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

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

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

Thresholds of olfactory sensation. adaptation

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

Adaptation of the olfactory sensory system occurs slowly and lasts tens of seconds or minutes. It depends on the speed of air movement and the concentration of the odorous substance. Cross adaptation occurs. With prolonged exposure to any odorant, the threshold 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 may accompany epilepsy. Anosmia can be caused by hypogonadism.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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