Olfactory sensory system. Olfactory analyzer, its structure and functions. Modern theories of odor perception. Adaptation and sensitivity of the olfactory sensory system The meaning of the olfactory sensory system

Odor perception cannot be measured directly. Instead, indirect methods are used, such as assessing intensity (how strong is the odor?), determining the threshold of perception (i.e., at what strength does the odor become noticeable), and comparing it with other odors (what is the odor like?). There is usually a direct relationship between the perception threshold and sensitivity.

There is a large group of disorders of the olfactory analyzer, as well as individual reduced sensitivity to odors, sometimes reaching anosmia.

For more information, see the article Smell and Smell Disorders

American scientists Richard Axel and Linda Buck received the Nobel Prize in 2004 for their research into the human sense of smell.

Odor appellants, attractants, odorous baits are substances that attract animals with their smell. Telergones and pheromones are chemical substances released by animals into the environment to influence other organisms. Musks conventionally called the secretions of specific skin glands, usually having a strong odor. For brevity, the latter were sometimes called odorous glands. Excretion products may include saliva, musks, etc.; as well as urine (urine) and excrement. Marking activity refers to the behavior of animals associated with leaving odorous marks by excretion products, musk, etc.

Evolution of smell

From an evolutionary point of view, smell is one of the most ancient and most important senses, with the help of which animals navigate their environment. This analyzer is one of the main ones in many animals. “It preceded all other senses with which an animal could sense at a distance the presence of food, individuals of the opposite sex, or the approach of danger” (Milne L., Milne M., 1966). There are three main aspects of the olfactory behavior of animals: orientation (how animals search for odors), reaction (how they react to their sources and relate to them) and signaling (how they use odors to communicate with each other). In phylogenesis, the human sense of smell deteriorates.

The connection between a person’s sense of smell and gender

The sense of smell varies by gender, and women generally outperform men in sensitivity, recognition, and discrimination of odors. Very few studies have noted male superiority. A study by Toulouse and Wahid found that women were better than men at identifying the odors of camphor, citral, rose water, cherry water, mint and anethole. Similar results were obtained in a number of subsequent studies. LeMagnin found that women were more sensitive to the smell of testosterone, but found no differences to the smells of safrole, guaiacol, amyl salicylate and eucalyptus. More recent studies have found differences in the odors of many substances including citral, amyl acetate, androstenone derivatives, exaltolide, phenylethyl alcohol, m-xylene and pyridine. Kolega and Koster conducted experiments with several hundred substances. Nine substances had lower odor thresholds in women. They also found that girls outperformed boys on a range of odor discrimination tests.

It is known that the sense of smell of women not taking hormonal contraceptives changes during the menstrual cycle. The sense of smell becomes most acute in the period shortly before and after ovulation, for example, sensitivity to male pheromones increases thousands of times. In women taking birth control pills, their sense of smell remains constant throughout the entire cycle. The study involved women from 18 to 40 years old who were asked to distinguish the smells of anise, musk, cloves, ammonia and citrus.

Relationship between a person’s sense of smell and age

In newborn babies, the sense of smell is highly developed, but in one year of life it is lost by 40-50%. A study based on a survey of 10.7 million people showed a decrease in the sensitivity of smell with age for all 6 odors studied. The ability to distinguish odors also decreased. The effect of age was more significant than the effect of gender, with women retaining their sense of smell to an older age than men.

It has been shown that with age, atrophy of the olfactory fibers occurs and their number in the olfactory nerve steadily decreases (table).

Lateralization of smell

Primary processing of signals from the stimulated nostril occurs on the same side of the body (ipsilateral), with areas associated with smell in the cortex being a direct projection of areas of the olfactory epithelium.

Absolute sensitivity

Absolute sensitivity studies have found conflicting results in many cases. When determining the threshold of perception, the left nostril was more sensitive in left-handed subjects, while the right nostril was more sensitive in right-handed subjects. Kane and Ghent found greater sensitivity in the right nostril regardless of handedness, but other authors found no difference. In the last two studies, the authors used phenylethyl alcohol, which is characterized by weak activity against the trigeminal nerve. The results of the experiments may also be influenced by switching the dominance of the nostrils during the day every 1.5-2 hours. It can be concluded that the right nostril is somewhat more sensitive, at least in right-handed people.

Odor discrimination

The results for odor discrimination, as well as for absolute sensitivity, are ambiguous, but indicate some superiority of the right nostril. A number of authors have found an advantage for the right nostril regardless of handedness. However, other authors have found a left nostril advantage in left-handed subjects. In the work of Savik and Berglund, the advantage of the right nostril was established only for familiar odors, while Broman showed its advantage also for unfamiliar odors. A right nostril advantage has been shown in studies of intensity categorization of odors, although these results were only significant for women.

Memory for smells

Hemispheric differences in odor recognition were more consistent. Thus, patients with lesions of the right hemisphere recognized odors worse than patients with lesions of the left hemisphere, which may indicate the superiority of the right hemisphere. In tests of verbal and visual odor recognition on healthy subjects, when the first stimulus (odor) was presented to both sides, reaction times were faster when the second stimulus (word or picture) was presented to the right hemisphere compared to the left. Olson and Kane found only a shorter response from the right nostril to odor cues and no difference in memory performance. Other authors have found no differences in odor recognition.

Odor identification

Patients with dissociated hemispheres were able to verbally recognize odors presented only to the left nostril and were able to recognize odors presented non-verbally to the right nostril. At the same time, the left hemisphere had an advantage in both verbal and non-verbal recognition of odors.

Notes

The mystery of the smell
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Olfactory receptors, unlike taste receptors, are excited by gaseous substances, while taste receptors are only stimulated by substances dissolved in water or saliva. Substances perceived through the sense of smell cannot be divided into groups according to their chemical structure or the nature of the responses evoked by receptor cells: they differ in great diversity. Therefore, it is customary to distinguish a fairly large number of odors: floral, ethereal, musky, camphorous, iota, putrefactive, acrid, etc. Chemically similar substances may be in different odor classes, and conversely, substances with similar odors may have completely different chemical natures. Odors that occur in nature are usually varied mixtures on a conventional scale of odors, in which certain components predominate.

Peripheral division of the olfactory sensory system.

Olfactory receptors in humans are located in the nasal cavity (Fig. 5.16), which is divided into two halves by the nasal septum. Each of the halves, in turn, is divided into three nasal conchas, covered with mucous membrane: upper, middle and lower. Olfactory receptors are mainly located in the upper mucous membrane and, in the form of islands, in the middle nasal turbinates. The rest of the mucous membrane of the nasal cavity is called the respiratory lining. It is lined with multirow ciliated epithelium, which includes numerous secretory cells.

Rice. 5.16.

Olfactory epithelium formed by two types of cells - receptor and supporting. At the outer pole, facing the surface of the epithelium in the nasal cavity, the receptor cells have modified cilia, immersed in a layer of mucus covering the olfactory epithelium. Mucus is secreted by single-celled glands of the epithelium of the respiratory part of the nasal cavity, supporting cells and special glands, the ducts of which open onto the surface of the epithelium. The flow of mucus is regulated by the cilia of the respiratory epithelium. When inhaled, molecules of the odorous substance are deposited on the surface of the mucus, dissolve in it and reach the cilia of the receptor cells. Here the molecules interact with special receptor sites on the membrane. The presence of a large number of odorants suggests that the same receptor molecule on the cell membrane can bind to several chemical stimuli. It is known that receptor cells have selective sensitivity to various substances, while at the same time, under the influence of the same stimulus, neighboring receptor cells are excited differently. Usually, with an increase in the concentration of odorous substances, the frequency of impulses in the olfactory nerve increases, but some substances can inhibit the activity of receptor cells.

Odorous substances, in addition to stimulating receptor cells, can excite the endings of the afferent fibers of the trigeminal nerve (V pair). They are believed to be sensitive to acrid and burning odors.

Distinguish detection threshold And recognition threshold smell. Calculations have shown that to detect some substances, contacts of no more than eight molecules of the substance with one receptor cell are sufficient. Animals have much lower olfactory thresholds and higher sensitivity than humans, since the sense of smell plays a much larger role in their lives than in humans. At low concentrations of an odorous substance, barely sufficient to cause the sensation of “some” odor, a person, as a rule, cannot determine it. They can only identify substances in concentrations exceeding the threshold.

With prolonged exposure to the stimulus, the sense of smell weakens: adaptation occurs. With prolonged intense stimulation, adaptation can be complete, i.e. the sensation of smell disappears completely.

The receptors of the olfactory sensory system are located among the cells of the mucous membrane in the area of the upper nasal passages and look like separate islands in the middle passages.
The olfactory epithelium lies on the side of the main respiratory tract, so when odorous substances enter, a person takes deep breaths and sniffs.
The thickness of the epithelium is approximately 100-150 microns, the diameter of the receptor cells located between the supporting cells is 5-10 microns. Olfactory receptors are the primary bipolar sensory cells. Their total number in humans is about
100 million. On the surface of each olfactory cell there is a spherical thickening. This is an olfactory mace. 6-12 thin (0.3 microns) hairs 10 microns long protrude from it. Olfactory hairs are immersed in liquid, produced by the olfactory glands. Thanks to the olfactory hair, the receptor area that comes into contact with molecules
odorous substances increases tenfold. It is quite possible that olfactory hairs also have a motor function, which increases the reliability of capturing molecules of odorous substances and contact with them. The olfactory club is an important cytochemical center of the olfactory cell: it generates RP.
Olfactory receptors are classified as chemoreceptors. Molecules of the odorous substance come into contact with the mucous membrane of the nasal passages, which leads to interaction with specialized membrane receptor proteins. Due to a complex, not yet sufficiently studied chain of reactions, a RP is generated in the receptor, and then an impulse excitation is transmitted, which is transmitted by the fibers of the olfactory nerve to the olfactory bulb, the primary nerve center of the olfactory analyzer. Using electrodes, an electroolfactogram can be obtained. The electrodes are located directly on the surface of the olfactory organ. epithelium and record their total electrical activity. A monophasic negative wave with an amplitude of up to 10 mV and a duration of several seconds occurs even with short-term exposure to an odorous substance; mainly, a slight positivity can be noticed on the electroolfactograms, preceding the main negative wave, and with a sufficient duration of exposure, a large negative wave is recorded. in response to its termination (off-reaction). Sometimes fast oscillations are superimposed on the slow wave of the electroolfactogram, which reflect synchronous pulse discharges of a significant number of receptors.
As evidenced by the results of microelectrode studies, single receptors respond by increasing the frequency of impulses, which depends on the quality and intensity of the stimulus. Each receptor can respond to a large number of odorants, but it gives preference to some of them. It is believed that the encryption of olfactory stimuli and their recognition in the centers of the olfactory analyzer can be based on these properties of receptors, which differ in their mood for different groups of substances. Adaptation in the olfactory analyzer occurs relatively slowly (tens of seconds and minutes) and depends on the speed of air flow over the olfactory epithelium and the concentration of the odorous substance. There is cross-adaptation, which consists in the fact that with prolonged intake of any odorous substance, the threshold of sensitivity not only to it, but also to other substances increases.
Electrophysiological studies of the olfactory bulbs revealed that the parameters of the electrical response, which is recorded during the action of odors, depend on the type of odorous substance. With different odors, the spatial mosaic of excited and inhibited areas of the olfactory bulbs changes. Whether this serves as a means of encrypting olfactory information is difficult to judge.
The sensitivity of the human olfactory analyzer is extremely high: one olfactory receptor can be excited by one or several molecules of an odorous substance, and the stimulation of a small number of receptors leads to the emergence of a feeling. At the same time, the change in the intensity of the effect of a substance (the limit of the difference) is assessed by a person quite roughly (the smallest perceived difference in odor strength is 30-60% of its initial concentration). In many animals, especially dogs, these figures are 3-6 times less. One of the characteristic features of the olfactory analyzer is that its afferent fibers do not switch in the thalamus and do not move to the opposite side of the cerebral cortex.
In the olfactory bulb, when analyzing incoming information, the phenomena of convergence and inhibition are widely used. Afferent control from overlying centers or the contralateral olfactory bulb also occurs here. The olfactory tract consists of several bundles that are sent to various parts of the brain: the anterior olfactory nucleus, the olfactory tubercle, the preperiform cortex, the periamygdala cortex and part of the nuclei of the amygdala complex. The connection of the olfactory bulb with the hippocampus, periform cortex and other parts of the olfactory brain occurs through several switches. Electrophysiological studies and experiments on animals with conditioned reflexes indicate that smell recognition does not require a significant number of centers of the olfactory brain (rhinencephalon). In this regard, most of the projection areas of the olfactory tract can be considered as associative centers that provide communication between the olfactory system and other sensory systems and the formation on this basis of a number of complex forms of behavior - food, protective, sexual.
The connection between the ruch analyzer and the limbic system provides
the presence of an emotional component in olfactory perception. . Odor can evoke feelings of pleasure or disgust and probably plays a role in shaping sexual behavior (especially in animals). The sensitivity of olfactory neurons is controlled by sex hormones.
In clinical practice, there are patients with various olfactory disorders, ranging from decreased sensitivity (hypo-or anosmia) to a variety of olfactory hallucinations and parosmia (incorrect perception of odors).

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Peripheral department The olfactory sensory system is the receptor cells in the epithelial (olfactory) lining of the nasal cavity. It is located in the superior nasal concha and the corresponding part of the nasal septum, is yellowish in color (due to the presence of pigment in the cells) and occupies about 2.5–5 cm 2 in the nasal cavity.

The mucous membrane of the nasal cavity in the area of the olfactory lining is somewhat thickened compared to the rest of the mucous membrane. It is formed by receptor and supporting cells (see Atl.). Olfactory receptor cells are primary sensory cells. In their apical part there is a long thin dendrite ending in a club-shaped thickening. Numerous cilia extend from the thickening, having a normal structure and immersed in mucus. This mucus is secreted by supporting cells and glands lying under the epithelial layer (Bowman's glands).

A long axon is located in the basal part of the cell. The unmyelinated axons of many receptor cells form rather thick bundles under the epithelium, called olfactory fibers (fila olfactoria). These axons pass into the holes of the perforated plate of the ethmoid bone and are directed to olfactory bulb, lying on the lower surface of the brain (see).

Excitation of receptor cells occurs when a stimulus interacts with cilia, then it is transmitted along the axon to the brain. Although olfactory cells are neurons, they, unlike the latter, are capable of renewal. The lifespan of these cells is approximately 60 days, after which they degenerate and are phagocytosed. The replacement of receptor cells occurs due to the division of the basal cells of the olfactory lining.

Conductive and central sections of the olfactory sensory system

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IN olfactory bulb There are five layers located concentrically (Fig. 3.72):

Rice. 3.72. Olfactory bulb:
A – drawing from a histological specimen; B – diagram, 1 – grain cells; 2 – granular layer; 3 – mitral cells, 4 – inner and 5 – outer reticular layers; 6 – periglomerular cells, 7 – glomeruli; 8 – processes of olfactory receptor cells

1 layer form fibers of the olfactory nerve - processes of olfactory receptor cells;

2 layer formed by glomeruli with a diameter of 100–200 μm, here synaptic contact of olfactory fibers with the processes of neurons of the next order occurs,

3 layer – outer reticulate (plexiform), formed by periglomerular cells in contact with several glomeruli each,

4 layer – internal reticular (plexiform), contains the largest cells of the olfactory bulb - mitral cells(second neuron). These are large neurons, the apical dendrites of which form one glomerulus in layer 2, and the axons form the olfactory tract. Within the bulb, the axons of mitral cells form collaterals that contact other cells. During electrophysiological experiments, it was found that odor stimulation causes different activity of mitral cells. Cells located in different parts of the olfactory bulb respond to certain types of odors;

5 layer – granular, form granule cells, on which efferent fibers coming from the center end. These cells are able to control the activity of mitral cells.

Derived from the olfactory bulb olfactory tract, formed by the axons of mitral cells. It carries olfactory signals to other areas of the brain (see Atl.). The tract ends with the lateral and medial olfactory stripes. Through lateral olfactory stripe impulses enter mainly the ancient cortex olfactory triangle, where the third neuron lies, and then into the amygdala.

Fibers medial olfactory strip end in the old cortex of the subcallosal field, the transparent septum, in the cells of the gray matter in the depths of the groove of the corpus callosum. Having gone around the latter, they reach the hippocampus. This is where the fibers originate. vault – projection system of the old cortex, ending partly in the transparent septum and in mamillary body hypothalamus. It starts from him mamillo-thalamic tract, going to one of the nuclei (anterior) of the thalamus, and mamillo-tectal tract, ending in the interpeduncular nucleus of the tegmentum of the cerebral peduncles, from where impulses are conducted to other efferent nuclei of the central nervous system.

From the anterior nucleus of the thalamus, impulses are sent to the limbic cortex. In addition, from the primary olfactory cortex, nerve fibers reach the medioventral nucleus of the thalamus, where there are also inputs from the taste system. The axons of the neurons of this nucleus go to the frontal (frontal) region of the cortex, which is considered as the highest integrative center of the olfactory system.

The hypothalamus, hippocampus, amygdala and limbic cortex are interconnected; they are part of limbic system and take part in the formation of emotional reactions, as well as in regulating the activity of internal organs. The connection of the olfactory pathways with these structures explains the participation of the sense of smell in nutrition, emotional status, etc.

With the help of smell, a person is able to distinguish thousands of odors, but nevertheless he is classified as microsmatic, since in humans this system is much less developed than in animals, which use it to navigate the environment. Peripheral department The olfactory sensory system is the receptor cells in the epithelial (olfactory) lining of the nasal cavity. It is located in the superior nasal concha and the corresponding part of the nasal septum, is yellowish in color (due to the presence of pigment in the cells) and occupies about 2.5–5 cm 2 in the nasal cavity. The mucous membrane of the nasal cavity in the area of the olfactory lining is somewhat thickened compared to the rest of the mucous membrane. It is formed by receptor and supporting cells (see Atl.). Olfactory receptor cells are primary sensory cells. In their apical part there is a long thin dendrite ending in a club-shaped thickening. Numerous cilia extend from the thickening, having a normal structure and immersed in mucus. This mucus is secreted by supporting cells and glands lying under the epithelial layer (Bowman's glands). A long axon is located in the basal part of the cell. The unmyelinated axons of many receptor cells form rather thick bundles under the epithelium, called olfactory fibers (fila olfactoria). These axons pass into the holes of the perforated plate of the ethmoid bone and are directed to olfactory bulb, lying on the lower surface of the brain (see Fig. 3.15). Excitation of receptor cells occurs when a stimulus interacts with cilia, then it is transmitted along the axon to the brain. Although olfactory cells are neurons, they, unlike the latter, are capable of renewal. The lifespan of these cells is approximately 60 days, after which they degenerate and are phagocytosed. The replacement of receptor cells occurs due to the division of the basal cells of the olfactory lining.

Conductive and central sections of the olfactory sensory system. IN olfactory bulb There are five layers arranged concentrically: 1 layer form fibers of the olfactory nerve - processes of olfactory receptor cells; 2 layer formed by glomeruli with a diameter of 100–200 µm, here synaptic contact of olfactory fibers with the processes of neurons of the next order occurs, 3rd layer – outer reticulate (plexiform), formed by periglomerular cells in contact with several glomeruli each, 4 layer – internal reticular (plexiform), contains the largest cells of the olfactory bulb - mitral cells(second neuron). These are large neurons, the apical dendrites of which form one glomerulus in layer 2, and the axons form the olfactory tract. Within the bulb, the axons of mitral cells form collaterals that contact other cells. During electrophysiological experiments, it was found that odor stimulation causes different activity of mitral cells. Cells located in different parts of the olfactory bulb respond to certain types of odors; 5 layer – granular, form granule cells, on which efferent fibers coming from the center end. These cells are able to control the activity of mitral cells. Derived from the olfactory bulb olfactory tract, formed by the axons of mitral cells. It carries olfactory signals to other areas of the brain. The tract ends in the lateral and medial olfactory stripes. Through lateral olfactory stripe impulses enter mainly the ancient cortex olfactory triangle, where the third neuron lies, and then into the amygdala. Fibers medial olfactory strip end in the old cortex of the subcallosal field, the transparent septum, in the cells of the gray matter in the depths of the groove of the corpus callosum. Having gone around the latter, they reach the hippocampus. This is where the fibers originate. vault – projection system of the old cortex, ending partly in the transparent septum and in mamillary body hypothalamus. It starts from him mamillo-thalamic tract, going to one of the nuclei (anterior) of the thalamus, and mamillo-tectal tract, ending in the interpeduncular nucleus of the tegmentum of the cerebral peduncles, from where impulses are conducted to other efferent nuclei of the central nervous system. From the anterior nucleus of the thalamus, impulses are sent to the limbic cortex. In addition, from the primary olfactory cortex, nerve fibers reach the medioventral nucleus of the thalamus, where there are also inputs from the taste system. The axons of the neurons of this nucleus go to the frontal (frontal) region of the cortex, which is considered as the highest integrative center of the olfactory system. The hypothalamus, hippocampus, amygdala and limbic cortex are interconnected; they are part of limbic system and take part in the formation of emotional reactions, as well as in regulating the activity of internal organs. The connection of the olfactory pathways with these structures explains the participation of the sense of smell in nutrition, emotional status, etc.

Development of the olfactory organ in the prenatal period of ontogenesis. In the second month of intrauterine development, ectodermal outgrowths form on the surface of the embryo’s head, which then invaginate. Their thickened epithelium becomes the bottom olfactory fossa. At first they are quite far apart from each other, located almost on the sides of the facial area of the embryo. Elevations appear along the edges of the olfactory fossae, which turn into medial and lateral nasal processes. Simultaneously with the growth of the maxillary protrusions, the formation of the facial structures of the eye occurs and the nasal fossae shift from their original lateral position to the midline. By the end of the second month of intrauterine development, the formation of the upper jaw is completed. On the medial edges of the anlage of the maxillary bones, palatal processes appear, which grow towards the midline and divide the oral cavity into the oral and nasal chambers. The medial nasal processes fuse with each other to form the nasal septum. Thus, simultaneously with the separation of the oral cavity from the nasal cavity, the latter is divided into right and left halves. The roof of each nasal area is differentiated olfactory area. Olfactory receptor cells - bipolar neurons - differentiate in the epithelium itself among long columnar cells called supporting cells. The processes of receptor cells facing the surface of the epithelium form extensions - clubs, crowned with a bunch of modified cilia, which carry chemical receptors on their surface. The opposite processes of these cells elongate and establish connections with neurons in the olfactory bulb, which transmit nerve impulses to the corresponding centers of the brain.

Taste sensory system - The gustatory and olfactory sensory systems allow a person to evaluate the chemical composition of food and the surrounding air. For this reason, they are collectively called chemosensory systems. This also includes splanchnic chemoreceptors (carotid sinus, digestive tract, and others). Chemical reception is one of the most phylogenetically ancient forms of communication between an organism and its environment.

The receptor section of the taste sensory system is located in the oral cavity and is represented by taste receptor cells. They are collected in taste buds, which are located mainly in the papillae on the dorsal surface of the tongue - mushroom-shaped, leaf-shaped and groove-shaped. Single taste buds are scattered in the mucous membrane of the soft palate, tonsils, posterior wall of the pharynx and epiglottis. In children, their area of distribution is wider than in adults; with old age their number decreases.

The most typical structure in humans is the taste buds of the circumvallate papillae. Each kidney is an oval formation that occupies the entire thickness of the epithelium and opens onto its surface sometimes taste. The bud is about 70 µm in height, 40 µm in diameter and is formed by 40–60 elongated cells arranged like segments in an orange. Among the cells of taste buds, receptor, supporting and basal are distinguished. The first two types of cells occupy the entire length of the bud from its basal part to the taste pore. There is still controversy regarding the receptor function of these cells. It is assumed that supporting cells may also participate in the receptor process. Taste receptor cells are secondary sensory cells. Embedded in their apical membrane, facing the taste pore, are receptor molecules to which various chemicals bind. As a result, the cell membrane enters an excited state. Through synaptic contacts in the basolateral part of the cell, excitation is transmitted to the nerve fiber, and then to the brain. A person distinguishes four basic tastes (sweet, salty, bitter, sour) and several additional ones (metallic, alkaline, etc.). Reception of taste substances becomes possible when these substances enter the surface of the tongue, dissolve in saliva, pass through the taste pore and reach the apical membrane of the receptor cells. The lifespan of receptor and supporting cells is short - about 10 days. Their renewal occurs due to mitotic cell division in the basal part of the kidney.

Conductive and central sections of the taste sensory system. Taste afferent fibers from the anterior two-thirds of the tongue, from the taste buds of the fungiform papillae of the anterior part of the tongue and several foliate papillae, pass as part of the facial nerve (drum string)(branch of the VII pair), and from the posterior third, posterior leaf-shaped and groove-shaped - as part of the glossopharyngeal nerve (IX pair). The taste buds of the posterior wall of the oral cavity and pharynx are innervated by the vagus nerve (X pair). These fibers are peripheral processes of neurons lying in the ganglia of these nerves: VII pair - in the geniculate ganglion, IX pair - in the petrosal ganglion. The fibers of all nerves through which taste sensitivity is transmitted end in the nucleus of the solitary tract . From here, ascending fibers follow to the neurons of the dorsal part of the pons (parabrachial nucleus) and to the ventral nuclei of the thalamus. From the thalamus, part of the impulses goes to the new cortex - to the lower part postcentral gyrus(field 43). It is assumed that with the help of this projection the discrimination of taste occurs. Another part of the fibers from the thalamus is sent to the structures of the limbic system (parahippocampal gyrus, hippocampus, amygdala and hypothalamus). These structures provide the motivational coloring of taste sensations and the participation in it of memory processes, which underlie taste preferences acquired with age. The fibers of the trigeminal nerve (V pair) also end in the mucous membrane of the anterior part of the tongue. They get here as part of the lingual nerve. These fibers transmit tactile, temperature, pain and other sensitivity from the surface of the tongue, which complements information about the properties of the stimulus in the oral cavity.

Development of the taste organ in the prenatal period of ontogenesis. In a 4-week-old human embryo, the facial area is just beginning to form. The oral cavity at this time is represented by an ectodermal invagination adjacent to the foregut, but not connected to it. A thin plate consisting of ecto- and endoderm later breaks through and the oral cavity connects with other parts of the digestive tract. On the sides of the oral cavity there are types of anlage of the upper and lower jaws, which grow towards the midline of the mouth, forming the jaws. An increase in the relative size of the midface occurs throughout the entire intrauterine period and continues after birth. At the beginning of its formation, the tongue is a hollow outgrowth of the mucous membrane of the posterolateral parts of the oral cavity, filled with growing muscles. Most of the mucous membrane of the tongue is of ectodermal origin, however, in the region of the tongue root it develops from the endoderm. Muscles and connective tissue are derivatives of the mesodermal layer. Outgrowths form on the surface of the tongue - taste And tactile papillae. Taste buds containing receptor cells develop in the taste buds. In humans, they first appear at the 7th week of embryogenesis as a result of interaction between the fibers of the sensory cranial nerves (VII and IX) and the integumentary epithelium of the tongue. There is evidence that the fruit is able to sense taste. It is hypothesized that this function may be used by the fetus to control the amniotic fluid surrounding it.

Somatosensory system - The human body is covered with skin. The skin consists of a superficial epithelial layer and deep layers (dermis), formed by dense, unformed connective tissue and subcutaneous fatty tissue. In addition, there are derivatives of the skin - hair, nails, sebaceous and sweat glands. The structure of the skin is described in detail in Chapter 5. In addition to the integumentary (protective) skin, it performs a number of other functions. It is involved in thermoregulation and excretion, and also carries a large number of receptor formations. These receptors perceive information about tactile, pain, temperature and other irritations applied to various areas of the skin. In other words, the surface of our body (soma) has sensitivity, which is called somatic. To carry out this impulse, there are several pathways along which information is transmitted to various parts of the central nervous system, including the cerebral cortex. Each type of sensitivity has its own projections, the somatotopic organization of which allows us to determine to which part of our body the irritation is applied, what its strength and modality are (touch, pressure, vibration, temperature or pain, etc.). There are several types of receptor formations for the perception of these stimuli. All of them belong to the primary senses, i.e. They are the terminal branches of sensory nerve fibers. Depending on the presence or absence of additional structures around them in the form of connective tissue and other capsules, they can be respectively encapsulated or non-encapsulated (free).

Free nerve endings. These endings of nerve fibers represent their terminal branches, devoid of a myelin sheath. They are located in the dermis and in the deep layers of the epidermis, rising to the granular layer (Fig. 3.76). Such endings perceive mechanical stimuli and also respond to heating, cooling and pain (nociceptive) influences. The endings are formed by thin myelinated or unmyelinated fibers. So, for example, in case of a burn, the first fibers provide a quick reaction (withdrawing the hand), and the second fibers provide a rather prolonged burning sensation. Thin myelinated fibers are sensitive to cooling, while unmyelinated fibers are sensitive to heating. At the same time, very strong cooling or heating can cause pain and subsequent itching.

In addition, in the hairy skin, the hair shafts and follicles are surrounded by the endings of 5–10 sensory fibers (Fig. 3.76). These fibers lose their myelin sheath and become embedded in the basal lamina of the hair shaft. They react to the slightest deviation of the hair.

Encapsulated nerve endings They are specialized formations for the perception of a certain type of stimulus. They are the endings of thicker myelinated fibers than those that form free nerve endings. This is due to the higher speed of signal transmission to the central structures. Vater-Pacini corpuscles (Pacini corpuscles) – one of the largest receptor structures of this kind (Fig. 3.77, A). They are located in the deep layers of the dermis, as well as in the connective tissue membranes of the muscles, periosteum, mesenteries, etc. At one pole, a myelinated nerve fiber penetrates the body and immediately loses its myelin sheath. The fiber passes through the body in the inner flask and expands at the end, forming irregularly shaped outgrowths. Above the inner flask is an outer flask formed by numerous concentrically located plates - derivatives of Schwann cells, between which there are collagen fibers and tissue fluid. Outside, the body is covered with a connective tissue capsule, which continuously passes into the endoneurium of the afferent fiber. The deeper the Pacinian corpuscle is located, the greater the number of layers in the inner and outer flasks it contains. These endings are sensitive to touch, pressure and rapid vibration, which is important for the perception of the texture of an object. When stimulation is applied, for example in the form of pressure, the layers of the capsule are displaced and excitation occurs in the afferent fiber. Merkel discs lie more superficially under the epithelium, near its lower border. They are sensitive to static tactile stimuli (touch, pressure). Meissner's corpuscles lie at the base of the dermal papillae and are sensitive to light touch and vibration. They are especially numerous in the skin of the palms and soles, lips, eyelids, and nipples of the mammary glands. Meissner corpuscles are oval formations about 100 µm long, located perpendicular to the surface of the epithelium. The body is formed by flattened modified Schwann cells, layered on top of each other, lying mostly transversely. The myelinated afferent fiber approaches the Meissner corpuscle, loses its myelin, and branches repeatedly. Thus, the body contains up to 9 branches. They are arranged in a spiral in the spaces between cells. Externally, the body is covered with a connective tissue capsule, beyond which it passes into the endoneurium. With the help of bundles of collagen fibers, the capsule of the body is attached to the lower border of the epithelium. Taurus Ruffini lie in the deep layers of the dermis, they are especially numerous on the plantar surface of the foot and are oval bodies measuring 1x0.1 mm. A thick myelinated afferent fiber approaches the corpuscle, loses its sheath and branches. Numerous terminal fibers are intertwined with collagen fibers, which also form the core of the body. When collagen fibers are displaced, afferents are excited. The thin capsule of the body passes into the endoneurium. Krause end flasks located in the conjunctiva of the eye, tongue, external genitalia. The corpuscle is surrounded by a thin-walled capsule. Before entering the capsule, the afferent fiber loses its myelin and branches. These endings probably perform a mechanoreceptor function. In addition to the fact that the nervous system receives information about stimuli acting on the skin, it receives impulses from the musculoskeletal system, signaling the position of the body in space. Previously, this sensitivity system was called the motor analyzer, but now other terminology has become generally accepted.

As the table shows, these three terms overlap to some extent. Proprioception integrates sensory signals from the skeleton and muscles and therefore includes muscle feeling. Kinesthesia – it is a sense of body position and limb movement, as well as sensations of effort, strength and heaviness. All receptors of the musculoskeletal system and skin are involved in its provision. The receptor structures that provide these types of sensitivity have a rather complex structure.

Muscle receptors - muscle spindles - serve to determine the degree of muscle stretching. There are especially many of them in the muscles that control precise movements. These receptors are spindle-shaped formations enclosed in a thin extensible connective tissue capsule. The spindles are located longitudinally in the muscles and stretch when the muscle is stretched. Each spindle is formed by several fibers (from 2 to 12), called intrafusal(from lat. fusus - spindle) (Fig. 3.78). These fibers are washed by tissue fluid. Intrafusal fibers are of two types. In the central part of most fibers there is a chain of one row of cell nuclei. The second type of fibers carries a nuclear cluster in the center (fibers with a nuclear bag); these fibers are longer and thicker than the first. The peripheral ends of both types of fibers are capable of stretching. Intrafusal fibers are innervated by afferent myelinated nerve fibers. In this case, a thick nerve fiber, which has a high speed of impulse conduction, approaches the central part of the intrafusal fiber and spirals around the nuclear bag or area containing a chain of nuclei. This ending is called primary. On the sides of the primary endings, thinner afferent fibers form secondary endings, the shape of which may resemble a bunch. The primary ending responds to the degree and speed of muscle stretching, and the secondary ending only to the degree of stretching and changes in the position of the muscle. When a muscle is stretched, information from the nerve endings enters the spinal cord, where part of it is switched to the motor neurons of the anterior horns. Their reflex impulse response leads to muscle contraction. The other part of the impulses switches to interneurons and enters other parts of the nervous system (see below). Muscle spindles also have efferent innervation, which controls the degree of their stretch. Efferent fibers approach the muscle spindles from motor neurons of the spinal cord, but not from those that innervate the muscle itself, the fibers of which are called extrafusal. However, in some cases, muscle spindles receive motor innervation via collaterals from axons to the muscles. This is observed, for example, in the muscles of the eyeball.

In addition to the receptor endings that lie in the muscles themselves and respond to the degree of their stretching, there are receptors at the junction of the muscles and tendons. They are called Golgi tendon organs (receptors)(Fig. 3.79). They are covered by a capsule and innervated by thick myelin fibers. The sheath of the fibers is lost as it passes through the capsule, and the fiber forms terminal branches between the bundles of collagen fibers of the tendon. These endings are excited when they are compressed by tendon fibers during muscle contraction, while the muscle spindles are inactive, and vice versa, when the muscle is stretched, the activity of the spindles increases and the tendon receptors decrease.

A large number of receptor endings are located in the joints (Fig. 3.79). In the articular ligaments there are receptors similar to tendon ones, in the connective tissue joint capsules there are large numbers of free nerve endings, as well as structures similar to the Pacinian and Rufini corpuscles. They are sensitive to stretching and compression that occurs during movement, and thus signal the position of the body in space and the movement of its individual parts (kinesthesia). Free nerve endings can also sense pain.

Conductor and central sections of the somatosensory system. Nerve impulses from the receptors of the skin and the musculoskeletal system, except for the head, reach the spinal ganglia along the spinal nerves, and then enter the spinal cord through the dorsal roots. The afferent fibers of each dorsal root conduct impulses from a specific area of the body - the dermatome (see Atl.). The information received by the spinal cord is used for two purposes: it participates in local reflexes, the arcs of which close at the level of the spinal cord, and is also transmitted to the overlying parts of the central nervous system along the ascending pathways. At the same time, a somatotopic organization can be traced in the ascending tracts: axons connected at a higher level are located on the side of the gray matter. Accordingly, axons coming from the lower part of the body lie more superficially.

As mentioned above, the gray matter of the spinal cord can be represented in the form of plates. Thin unmyelinated fibers approaching the spinal cord from pain and mechanoreceptors terminate in the superficial plates, mainly in the substantia gelatinosa. Thin myelin fibers mainly reach only the marginal zone (Fig. 3.80). Thick myelin fibers bend around the dorsal horn, give off collaterals to neurons of layers III–IV and enter the posterior cord of the white matter. It has been established that most dorsal horn neurons receive only one type of afferentation, but there are neurons on which impulses from different receptors converge. This may be the basis for the interaction of different receptor systems. The axons of the dorsal horn neurons can go into the white matter - into the ascending tracts, or reach the motor neurons of the anterior horns and participate in a number of spinal reflexes. Thus, impulses from skin receptors trigger the flexion reflex. It appears when a limb is withdrawn from a painful stimulus (during a burn, etc.). Impulses from the receptors of the somatosensory system are carried through the thin and cuneate fasciculi, as well as through the spinothalamic and spinocerebellar tracts and the trigeminal lemniscus. Thin Bun carries impulses from the body below the V thoracic segment, and wedge-shaped bundle - from the upper body and arms. These pathways are formed by the axons of sensory neurons, the bodies of which lie in the spinal ganglia, and the dendrites form receptor endings in the skin, muscles and tendons. Having passed the entire spinal cord and the posterior part of the medulla oblongata, the fibers of the thin and cuneate fasciculi end on neurons thin And wedge-shaped nuclei. The axons of the neurons of these nuclei go in two directions. Some are called external arcuate fibers – move to the opposite side, where in the composition inferior cerebellar peduncle end on cells worm bark(see Atl.). The latter's neurites connect the worm's cortex with cerebellar nuclei. The axons of the neurons of these nuclei, as part of the inferior cerebellar peduncles, are directed to vestibular nuclei of the pons. Another, most of the fibers from the neurons of the thin and cuneate nuclei in front of the central canal of the medulla oblongata cross and form medial loop or lemniscus Therefore, both of these paths are called lemniscal system. The medial loop passes through the medulla oblongata, the tegmentum of the pons and the midbrain and ends in lateral And ventral nuclei of the thalamus. On their way through the brain stem, the fibers of the medial lemniscus give off collaterals to the reticular formation. Fibers of thalamic neurons pass as part of the thalamic radiation to the cortex central regions cerebral hemispheres. Both the nuclei of the medulla oblongata and the thalamic and cortical projections of the thin and cuneate tracts have a somatotopic organization. These pathways (especially the sphenoid fasciculus) transmit fine sensitivity from the upper extremities, making fine and precise movements of the fingers possible. This is also facilitated by the presence of a small number of switches from neuron to neuron - there is no “spreading” of excitation throughout the structures of the brain and spinal cord.

Spinothalamic tract conducts stimulation from receptors, the irritation of which causes pain and temperature sensations (see Atl.). There are also fibers from articular and tactile receptors. The bodies of sensory neurons in this pathway also lie in the spinal ganglia. The central processes of these neurons enter the spinal cord as part of the dorsal roots, where they end on the bodies of the interneurons of the dorsal horns at the level of plates IV–VI. The axons of the dorsal horn neurons partially move to the opposite side, the rest remain on their side and form the spinothalamic tract deep in the lateral cord. The latter passes through the spinal cord, tegmentum of the medulla oblongata, pons and cerebral peduncles and ends on the cells ventral nucleus of the thalamus. Along the way through the brain stem, collaterals depart from the fibers of this tract to the reticular formation. From the thalamus, the fibers go as part of the thalamic radiance to the cortex, where they end mainly in post-central region. Spinocerebellar posterior And front path carry out excitation from proprioceptors of the motor apparatus (see Atl.). Sensitive neurons of these pathways are located in the spinal ganglia, and intercalary neurons are located in hind horns spinal cord. The neurites of the interneurons that are part of the posterior spinocerebellar tract remain on the same side of the spinal cord in the lateral cord, and those forming the anterior tract move to the opposite side, where they are also located in the lateral cord. Both pathways enter the cerebellum: the posterior one through its lower peduncles, and the anterior one along its upper peduncles. They end on cells worm bark. From here, impulses follow the same paths as those passing along the external arcuate fibers from the medulla oblongata. Thanks to the spinocerebellar pathways, information from the muscle and joint receptors of the limbs and the cerebellar mechanisms necessary for coordinating movements, maintaining muscle tone and posture is integrated. This is especially important for the work of the lower extremities in a standing position and when moving

Trigeminal loop transmits impulses from mechano-, thermo- and pain receptors of the head (see Atl.) Cells serve as sensitive neurons trigeminal node. The peripheral fibers of these cells pass as part of the three branches of the trigeminal nerve, innervating the skin of the face (Fig. 3.28). The central fibers of sensory neurons leave the ganglion as part of the sensory root of the trigeminal nerve and penetrate the pons at the place where it passes into the middle cerebellar peduncles. In the pons, these fibers are divided T-shaped into ascending and long descending branches (spinal tract), which end on neurons that form the main structure in the pons tegmentum. sensory nucleus of the trigeminal nerve, and in the medulla oblongata and spinal cord - its spinal nucleus(see Atl.). The central fibers of the neurons of these nuclei cross in the upper part of the pons and, as a trigeminal loop, pass along the tegmentum of the midbrain to the thalamus, where they end independently or together with the fibers of the medial loop above its cells ventral nucleus. The processes of the neurons of this nucleus are sent as part of the thalamic radiation to the cortex of the lower part post-central region, where the sensitivity coming from the structures of the head is mainly localized

Somatosensory projections in the cerebral cortex are located in the postcentral gyrus. Fibers from the thalamus are suitable here, bringing impulses from all receptors of the skin and musculoskeletal system. Here, as well as in the thalamus, the somatotopic organization of projections is well expressed (Fig. 3.81). In addition to the primary projection zone, which receives afferentation only from the thalamus, there is also a secondary zone, on whose neurons, along with thalamic ones, fibers from the primary zone terminate. In this zone, sensory signals are processed, from here they are sent to others, including the motor areas of the cortex and subcortical structures.