Pathways of smell. Olfactory tract. II pair – optic nerve, nervus opticus. Visual and pupillary-reflex pathways
Peripheral section of the olfactory analyzer: d - diagram of the structure of the nasal cavity: 1 - lower nasal passage; 2 - lower, 3 - middle and 4 - upper nasal concha; 5 - upper nasal passage; B - diagram of the structure of the olfactory epithelium: 1 - body of the olfactory cell, 2 - supporting cell; 3 - mace; 4 - microvilli; 5 - olfactory filaments
The olfactory cell has two processes. One of them, through the holes of the perforated plate of the ethmoid bone, is directed into the cranial cavity to the olfactory bulbs, in which it is transmitted to those located there. Their fibers form olfactory pathways that connect to various sections. The cortical section of the olfactory analyzer is located in the hippocampal gyrus and the ammonian horn.
substances, their loosening and partial disappearance occur, which suggests that the function of olfactory cells is accompanied by changes in the distribution of RNA and in its quantity.
The olfactory cell has two processes. One of them, through the holes of the perforated plate of the ethmoid bone, is directed into the cranial cavity to the olfactory bulbs, in which excitation is transmitted to the neurons located there. Their fibers form olfactory pathways that connect to various sections. The cortical section of the olfactory analyzer is located in the hippocampal gyrus and the ammonian horn.
The second process of the olfactory cell has the shape of a rod 1 µm wide, 20-30 µm long and ends in an olfactory vesicle - a club, the diameter of which is 2 µm. There are 9-16 cilia on the olfactory vesicle.
Wiring department represented by conductive neural pathways in the form of the olfactory nerve, leading to the olfactory bulb (formation oval shape). Wiring department. The first neuron of the olfactory analyzer should be considered a neurosensory or neuroreceptor cell. The axon of this cell forms synapses, called glomeruli, with the main dendrite of the mitral cells of the olfactory bulb, which represent the second neuron. The axons of the mitral cells of the olfactory bulbs form the olfactory tract, which has a triangular extension (olfactory triangle) and consists of several bundles. The fibers of the olfactory tract go in separate bundles to the anterior nuclei of the visual thalamus.
Central department consists of the olfactory bulb, connected by branches of the olfactory tract with centers located in the paleocortex (ancient cortex of the cerebral hemispheres) and in the subcortical nuclei, as well as the cortical section, which is localized in the temporal lobes of the brain, the seahorse gyrus.
The central, or cortical, section of the olfactory analyzer is localized in the anterior part of the pyriform lobe of the cortex in the region of the seahorse gyrus.
Perception of smells. Molecules of the odorous substance interact with specialized proteins built into the membrane of the olfactory hair neurosensory receptor cells. In this case, adsorption of irritants occurs on the chemoreceptor membrane. According to stereochemical theory this contact is possible if the shape of the odorant molecule matches the shape of the receptor protein in the membrane (like a key and a lock). The mucus covering the surface of the chemoreceptor is a structured matrix. It controls the accessibility of the receptor surface to irritant molecules and is capable of changing the conditions of reception. Modern theory olfactory reception suggests that the initial link of this process can be two types of interaction: the first is contact charge transfer when molecules of an odorant substance collide with the receptive site and the second is the formation of molecular complexes and complexes with charge transfer. These complexes are necessarily formed with protein molecules of the receptor membrane, the active sites of which act as electron donors and acceptors. An essential point of this theory is the provision of multipoint interactions between molecules of odorant substances and receptive sites.
Features of adaptation of the olfactory analyzer. Adaptation to the action of an odorant in the olfactory analyzer depends on the speed of air flow over the olfactory epithelium and the concentration of the odorant. Typically, adaptation occurs in relation to one odor and may not affect other odors.
Olfactory receptors are very sensitive. To excite one human olfactory cell, 1 to 8 molecules of an odorous substance (butyl mercaptan) are sufficient. The mechanism of odor perception has not yet been established. It is assumed that olfactory hairs are like specialized antennas that are actively involved in the search and perception of odorous substances. There are different points regarding the mechanism of perception. Thus, Eimour (1962) believes that on the surface of the hairs of olfactory cells there are special receptive areas in the form of pits, slits of a certain size and charged in a certain way. The molecules of various odorant substances have a shape, size and charge that are complementary to different parts of the olfactory cell, and this determines the discrimination of odors.
Some researchers believe that the olfactory pigment present in the olfactory receptive zone is also involved in the perception of olfactory stimuli, like the retinal pigment in the perception of visual stimuli. According to these ideas, colored forms of pigment contain excited electrons. Odorous substances, acting on the olfactory pigment, cause the transition of electrons to a lower energy level, which is accompanied by discoloration of the pigment and the release of energy that is spent on the occurrence of impulses.
Biopotentials arise in the club and spread further along the olfactory pathways to the cerebral cortex.
Odor molecules bind to receptors. Signals from receptor cells enter the glomeruli (glomeruli) of the olfactory bulbs - small organs located in the lower part of the brain just above the nasal cavity. Each of the two bulbs contains approximately 2000 glomeruli - twice as many as there are types of receptors. Cells with receptors of the same type send a signal to the same glomeruli of the bulbs. From the glomeruli, signals are transmitted to mitral cells - large neurons, and then to special areas of the brain, where information from different receptors is combined to form an overall picture.
According to the theory of J. Eymour and R. Moncrieff (stereochemical theory), the smell of a substance is determined by the shape and size of the odorous molecule, which in configuration fits the receptor site of the membrane “like a key to a lock.” Concept of receptor sites different types, interacting with specific odorant molecules, suggests the presence of receptive sites of seven types (by type of odors: camphor, ethereal, floral, musky, pungent, minty, putrid). The receptive sites are in close contact with the odorant molecules, and the charge of the membrane region changes and a potential arises in the cell.
According to Eimur, the entire bouquet of odors is created by a combination of these seven components. In April 1991, employees of the Institute. Howard Hughes (Columbia University) Richard Axel and Linda Buck found that the structure of the receptor areas of the membrane of olfactory cells is genetically programmed, and there are more than 10 thousand species of such specific areas. Thus, a person is able to perceive more than 10 thousand odors.
Adaptation of the olfactory analyzer can be observed with prolonged exposure to an odor stimulus. Adaptation to the action of an odorous substance occurs rather slowly within 10 seconds or minutes and depends on the duration of action of the substance, its concentration and the speed of air flow (sniffing).
In relation to many odorous substances, complete adaptation occurs quite quickly, i.e. their smell ceases to be felt. A person ceases to notice such continuously acting stimuli as the smell of his body, clothes, room, etc. In relation to a number of substances, adaptation occurs slowly and only partially. With short-term exposure to a weak taste or olfactory stimulus: adaptation can manifest itself in an increase in the sensitivity of the corresponding analyzer. It has been established that changes in sensitivity and adaptation phenomena mainly occur not in the peripheral, but in the cortical part of the taste and olfactory analyzers. Sometimes, especially when frequent action the same taste or olfactory stimulus, a persistent focus of increased excitability appears in the cerebral cortex. In such cases, the sensation or smell to which increased excitability has arisen may also appear under the influence of various other substances. Moreover, the sensation of a corresponding smell or taste can become intrusive, appearing even in the absence of any taste or odor stimuli, in other words, illusions and hallucinations arise. If you say during lunch that a dish is rotten or sour, then some people develop corresponding olfactory and gustatory sensations, as a result of which they refuse to eat.
Adaptation to one odor does not reduce sensitivity to odorants of another type, because Different odorants act on different receptors.
The sense of smell is one of the first senses that a baby develops. Knowledge of the surrounding world and oneself begins with it. The taste that a person feels while eating is also a merit of smell, and not of the tongue, as it seemed before. Even the classics argued that our sense of smell can help in difficult situations. As J. R. R. Tolkien wrote: “If you are lost, always go where there is a better smell.”
Anatomy
The olfactory nerve belongs to the group of cranial nerves, as well as nerves of special sensitivity. It originates on the upper mucosa and the processes of neurosensory cells form the first neuron of the olfactory tract there.
Fifteen to twenty unmyelinated fibers enter the cranial cavity through the horizontal plate of the ethmoid bone. There they combine into the olfactory bulb, which is the second neuron of the pathway. Long nerve processes emerge from the bulb and go to the olfactory triangle. Then they are divided into two parts and immersed in the anterior perforated plate and transparent septum. There are the third neurons of the path.
After the third neuron, the tract goes to the cerebral cortex, namely to the area of the hook, to the olfactory nerve. This area ends. Its anatomy is quite simple, which allows doctors to identify disorders on different areas and eliminate them.
Functions
The name of the structure itself indicates what it is intended for. The functions of the olfactory nerve are to capture smell and decipher it. They cause appetite and salivation if the aroma is pleasant, or, on the contrary, they provoke nausea and vomiting when the amber leaves much to be desired.
In order to achieve this effect, the olfactory nerve passes through and is directed to the brain stem. There the fibers connect to the nuclei of the intermediate, glossopharyngeal and vagus nerves. This area also contains the nuclei of the olfactory nerve.
It is a known fact that certain smells evoke certain emotions in us. So, to ensure such a reaction, the fibers of the olfactory nerve communicate with the subcortical visual analyzer, the hypothalamus and the limbic system.
Anosmia
"Anosmia" is translated as "lack of sense of smell." If a similar condition is observed on both sides, then this indicates damage to the nasal mucosa (rhinitis, sinusitis, polyps) and, as a rule, does not threaten any serious consequences. But with unilateral loss of smell, you need to think about the fact that the olfactory nerve may be affected.
The causes of the disease may be an underdeveloped olfactory tract or fractures of the skull bones, for example, the cribriform plate. The course of the olfactory nerve is generally closely related to bone structures skulls Fibers can also be damaged by bone fragments after a broken nose, upper jaw, eye sockets. Damage to the olfactory bulbs is also possible due to a bruise of the brain substance when falling on the back of the head.
Inflammatory diseases, such as ethmoiditis, in advanced cases melt and damage the olfactory nerve.
Hyposmia and hyperosmia
Hyposmia is a decreased sense of smell. It can occur due to the same reasons as anosmia:
- thickening of the nasal mucosa;
- inflammatory diseases;
- neoplasms;
- injuries
Sometimes this is the only sign of a cerebral aneurysm or a tumor of the anterior cranial fossa.
Hyperosmia (increased or heightened sense of smell) is observed in emotionally labile people, as well as in some forms of hysteria. Increased sensitivity to odors is observed in people who inhale drugs, such as cocaine. Sometimes hyperosmia is caused by the fact that the innervation of the olfactory nerve extends to a large area of the nasal mucosa. Such people most often become workers in the perfume industry.
Parosmia: olfactory hallucinations
Parosmia is a perverted perception of smell that normally occurs during pregnancy. Pathological parosmia is sometimes observed in schizophrenia, damage to the subcortical centers of smell (parahippocampal gyrus and uncus), and in hysteria. In patients with iron deficiency anemia similar symptoms are observed: pleasure from the smell of gasoline, paint, wet asphalt, chalk.
Lesions of the olfactory nerve in the area temporal lobe causes a specific aura before epileptic seizures and causes hallucinations in psychosis.
Research methodology
In order to determine the state of a patient’s sense of smell, a neuropathologist conducts special tests to recognize various odors. Indicator aromas should not be too strong so as not to interfere with the purity of the experiment. The patient is asked to calm down, close his eyes and press his finger against his nostril. After this, a odorous substance is gradually brought to the second nostril. It is recommended to use odors that are familiar to humans, but avoid ammonia and vinegar, since when they are inhaled, in addition to the olfactory nerve, the trigeminal nerve is also irritated.
The doctor records the test results and interprets them relative to normal. Even if the patient cannot name the substance, the very fact of smelling it excludes nerve damage.
Brain tumors and sense of smell
For brain tumors various localizations, hematomas, impaired outflow of cerebrospinal fluid and other processes that compress the brain substance or press it against the bone formations of the skull. In this case, a unilateral or bilateral disturbance of smell may develop. The doctor should remember that they cross, so even if the lesion is localized on one side, the hyposmia will be bilateral.
Damage to the olfactory nerve is part of craniobasal syndrome. It is characterized not only by compression of the medulla, but also by its ischemia. Patients develop pathology of the first six pairs. Symptoms may be uneven, and various combinations occur.
Treatment
Pathologies of the olfactory nerve in its first section occur most often in the autumn-winter period, when there is a massive incidence of acute respiratory infections and influenza. Long-term progression of the disease can cause complete loss of smell. Restoring nerve function takes from ten months to a year. All this time it is necessary to carry out a course of treatment to stimulate regenerative processes.
IN acute period ENT prescribes physiotherapeutic treatment:
- nose and maxillary sinuses;
- ultraviolet irradiation of the nasal mucosa, 2-3 biodoses;
- magnetic therapy of the wings of the nose and sinuses of the upper jaw;
- infrared radiation with a frequency of 50-80 Hz.
You can combine the first two methods and the last two. This speeds up the restoration of lost functions. After clinical recovery, the following physiotherapeutic treatment is also carried out for rehabilitation:
- electrophoresis using the drugs “No-shpa”, “Prozerin”, as well as nicotinic acid or lidase;
- ultraphonophoresis of the nose and maxillary sinuses for ten minutes daily;
- irradiation with a red laser spectrum;
- endonasal electrical stimulation.
Each course of therapy is carried out for up to ten days with breaks of fifteen to twenty days until the function of the olfactory nerve is completely restored.
The olfactory analyzer provides the perception of olfactory stimuli, the conduction of nerve impulses to the olfactory centers, the analysis and integration of information received by them.
The receptors of the olfactory analyzer are located in olfactory region of the nasal mucosa and represent peripheral processes of olfactory cells (Fig. 1). The olfactory cells themselves are the bodies of the first neuron of the olfactory analyzer(Fig. 2, 3).
Rice. 1. (colored area of the mucous membrane of the lateral wall of the nasal cavity and nasal septum): 1 - olfactory bulb (bulbus olfactorius); 2 - olfactory nerves (nn. olfactorii; lateralis); 3 - olfactory tract (tractus olfactorius); 4 - superior nasal concha (concha nasalis superior); 5 - olfactory nerves (nn. olfactorii; medialis); 6 - nasal septum (septum nasi); 7 - inferior nasal concha (concha nasalis inferior); 8 - middle turbinate (concha nasalis media).
Rice. 2.: R - receptors - peripheral processes of sensitive cells of the mucous membrane of the olfactory region of the nasal cavity; I - first neuron - sensitive cells of the mucous membrane of the olfactory region of the nasal cavity; II - second neuron - mitral cells of the olfactory bulb (bulbus olfactorius); III - third neuron - cells of the olfactory triangle, anterior perforated substance and nuclei of the transparent septum (trigonum olfactorium, septum pellucidum, substantia perforata anterior); IV - cortical end of the olfactory analyzer - cells of the uncinate cortex and parahippocampal gyrus (uncus et gyrus parahippocampalis); 1 - olfactory region of the nasal cavity (pars olfactoria tunicae mucosae nasi); 2 - olfactory nerves (nn. olfactorii); 3 - olfactory bulb; 4 - olfactory tract and its three bundles: medial, intermediate and lateral (tractus olfactorius, stria olfactoria lateraris, intermedia et medialis); 5 - short path - to the cortical end of the analyzer; 6 - middle way- through the plate of the transparent septum, the fornix and the fimbria of the seahorse to the bark; 7 - long way - over corpus callosum as part of the waist bundle; 8 - mammillary bodies and the path from them to the thalamus (fasciculus mamillothalamicus); 9 - thalamic nuclei; 10 - superior colliculi of the midbrain and the path to them from the mastoid bodies (fasciculus mamillotegmentalis).
Rice. 3. .
The central processes of the olfactory cells constitute the olfactory nerves (nn. olfactorii), which penetrate into the cranial cavity through the openings of the cribriform plate (lamina cribrosa) of the ethmoid bone. The olfactory nerves travel to the olfactory bulb and come into contact with the mitral cells olfactory bulb (bodies of the second neuron).
The axons of the second neurons are part of olfactory tract, are divided into a medial bundle - to the olfactory bulb of the opposite side, a lateral bundle - to the cortical end of the analyzer, and an intermediate bundle, which approaches the bodies of the third neurons. Cell bodies of third neurons located in olfactory triangle, nuclei of the septum pellucida and the anterior perforated substance.
The axons of the third neurons are sent to the cortical end of the olfactory analyzer in three ways: from the cells in the olfactory triangle a long way above the corpus callosum, from the nuclei of the septum pellucidum there is a middle way through the fornix, and from the anterior perforated substance a short way leads directly to the hook.
The long path provides olfactory associations, the average search for the source of the odor, and the short path provides a motor defense reaction to a pungent odor. The cortical end of the olfactory analyzer is located in the crus and parahippocampal gyrus.
A feature of the olfactory analyzer is that nerve impulses initially enter the cortex, and then from the cortex to the subcortical centers: the papillary bodies and the anterior nuclei of the thalamus, interconnected by the papillary-thalamic fascicle.
The subcortical centers are in turn connected to the cortex frontal lobes, motor centers of the extrapyramidal system, limbic system and reticular formation, providing emotional reactions, protective motor reactions, changes in muscle tone, etc. in response to olfactory stimuli.
Development of the olfactory organ
The anlage of the olfactory organ occupies the very anterior edge of the neural plate. Then the anlage of the peripheral part of the olfactory analyzer is separated from the rudiment of the central nervous system and moves to the olfactory part of the developing nasal cavity. In the fourth month of intrauterine development, the cells in the olfactory part differentiate into supporting and olfactory cells. The processes of the olfactory cells grow through the still cartilaginous cribriform plate (lamina cribrosa) into the olfactory bulb. This is how the secondary connection of the olfactory organ with the central nervous system occurs.
Anomalies in the development of the olfactory organ
- Arinencephaly is the absence of the central and peripheral parts of the olfactory brain.
- Olfactory nerve defects.
- Weakening, lack of olfactory perception.
In diseases of the nasal mucosa, tumors of the base of the brain and frontal lobe, pathological decline sense of smell ( hyposmia) or its complete loss ( anosmia). In allergic conditions, an exacerbation of the sense of smell is possible ( hyperosmia).
Sources and literature
- Kondrashev A.V., O.A. Kaplunova. Anatomy of the nervous system. M., 2010.
The pathways of the olfactory analyzer (tractus olfactorius) have complex structure. The olfactory receptors of the nasal mucosa perceive changes in the chemistry of the air and are the most sensitive compared to the receptors of other senses. First neuron formed by bipolar cells located in the mucous membrane of the superior turbinate and nasal septum. The dendrites of the olfactory cells have club-shaped thickenings with numerous cilia that perceive chemical substances air; axons connect to olfactory filaments(fila olfactoria), penetrating through the openings of the cribriform plate into the cranial cavity, and are switched in the olfactory glomeruli olfactory bulb(bulbus olfactorius) to the second neuron . Axons of the second neuron(neutral cells) form olfactory tract and end at olfactory triangle(trigonum olfactorium) and in anterior perforated substance(substantia perforata anterior), where the cells of the third neuron are located. Axons of the third neuron grouped into three bundles - external, intermediate, medial, which are directed to various brain structures. External beam, going around the lateral sulcus of the cerebrum, reaches the cortical center of smell, located in hook(uncus) of the temporal lobe. Intermediate beam, passing in the hypothalamic region, ends in mastoid bodies and in the midbrain ( red core). Medial bundle is divided into two parts: one part of the fibers, passing through the gyrus paraterminalis, goes around the corpus callosum, enters the vaulted gyrus, reaches the hippocampus And hook; the other part of the medial fascicle forms olfactory-leash bundle nerve fibers passing through brain stripes(stria medullaris) of the thalamus on its own side. The olfactory-lead fascicle ends in the nuclei of the triangle of the frenulum of the suprathalamic region, where the descending pathway begins, connecting the motor neurons of the spinal cord. Nuclei of the triangular frenulum duplicated by a second system of fibers coming from the mastoid bodies.
The olfactory system has not undergone dramatic changes during evolution and has no representation in the neocortex.
Auditory sensory system
Auditory system , auditory analyzer - a set of mechanical, receptor and neural structures that perceive and analyze sound vibrations. The structure of the auditory system, especially its peripheral part, may vary in different animals. Thus, a typical sound receiver in insects is the tympanic organ; one of the sound receivers in bony fish is the swim bladder, the vibrations of which, under the influence of sound, are transmitted to the Weberian apparatus and further to the inner ear. In amphibians, reptiles and birds, additional receptor cells (basilar papilla) develop in the inner ear. In higher vertebrates, including most mammals, the auditory system consists of the outer, middle and inner ears, auditory nerve and successively connected nerve centers (the main ones are the cochlear and superior olive nuclei, the posterior tuberosities of the quadrigeminal, the auditory cortex).
The development of the central part of the auditory system depends on environmental factors and on the importance of the auditory system in animal behavior. The auditory nerve fibers travel from the cochlea to the cochlear nuclei. Fibers from the right and left cochlear nuclei go to both symmetrical sides of the auditory system. Afferent fibers from both ears converge in the superior olive. In the frequency analysis of sound, a significant role is played by the cochlear septum - a kind of mechanical spectral analyzer that functions as a series of mutually mismatched filters, spatially scattered along the cochlear septum, the vibration amplitude of which ranges from 0.1 to 10 nm (depending on the sound intensity).
For central departments The auditory system is characterized by a spatially ordered arrangement of neurons with maximum sensitivity to a specific sound frequency. The nervous elements of the auditory system, in addition to frequency, exhibit a certain selectivity to the intensity, duration of sound, etc. Neurons of the central, especially higher parts of the auditory system, selectively respond to complex signs of sounds (for example, to a certain frequency of amplitude modulation, to the direction of frequency modulation and sound movement ).
The auditory analyzer includes the hearing organ, the pathways of auditory information and the central representation in the cerebral cortex.
Hearing organ
Organa audites - labyrinth, which contains two types of receptors: one of them (organ of Corti) serve to perceive sound stimuli, others represent perceiving devices static-kinetic apparatus, necessary for the perception of the forces of gravity, to maintain balance and orientation of the body in space. At low stages of development, these two functions are not differentiated from each other, but the static function is primary. The prototype of a labyrinth in this sense can be a static bubble (oto- or statocyst), which is very common among invertebrate animals living in water, such as mollusks. In vertebrates, this initially simple form of the vesicle becomes significantly more complex as the functions of the labyrinth become more complex.
Genetically, the vesicle originates from the ectoderm by invagination followed by lacing, then the tube-like appendages of the static apparatus - the semicircular canals - begin to separate. Hagfish have one semicircular canal connected to a single vesicle, as a result of which they can move only in one direction; cyclostomes have two semicircular canals, thanks to which they are able to move their body in two directions. Starting with fish, all other vertebrates develop 3 semicircular canals, corresponding to the three dimensions of space existing in nature, allowing them to move in all directions.
As a result, vestibule of the labyrinth and semicircular canals having a special nerve - n. vestibularis. With access to land, with the advent of locomotion using limbs in terrestrial animals, and upright walking in humans, the importance of balance increases. While the vestibular apparatus is formed in aquatic animals, the acoustic apparatus, which is in its infancy in fish, develops only with access to land, when direct perception of air vibrations becomes possible. It gradually separates from the rest of the labyrinth, spiraling into a cochlea.
With the transition from the aqueous to the air environment, a sound-conducting apparatus is attached to the inner ear. Starting with amphibians, it appears middle ear- tympanic cavity with eardrum and auditory ossicles. The acoustic apparatus reaches its highest development in mammals, which have a spiral cochlea with a very complex sound-sensitive device. They have a separate nerve (n. cochlearis) and a number of auditory centers - subcortical (in the hindbrain and midbrain) and cortical. They also have outer ear with a recessed ear canal and auricle.
Auricle represents a later acquisition, playing the role of a speaker to amplify sound, and also serving to protect the external ear canal. In terrestrial mammals, the auricle is equipped with special muscles and easily moves in the direction of sound. It is absent in mammals leading an aquatic and underground lifestyle; in humans and higher primates it undergoes reduction and becomes immobile. At the same time, the emergence of oral speech in humans is associated with the maximum development of auditory centers, especially in the cerebral cortex, which form part of the second signaling system.
The embryogenesis of the organ of hearing and balance in humans proceeds similarly to phylogenesis. At the 3rd week of embryonic life, on both sides of the posterior medullary vesicle, an auditory vesicle appears from the ectoderm - the rudiment of the labyrinth. By the end of 4 weeks, a blind duct (ductus endolymphaticus) and 3 semicircular canals grow from it. The upper part of the auditory vesicle, into which the semicircular canals flow, represents the rudiment of the elliptical sac (utriculus), it is separated at the point where the endolymphatic duct departs from the lower part of the vesicle - the rudiment of the future spherical sac (sacculus). At the 5th week of embryonic life, from the anterior part of the auditory vesicle corresponding to the sacculus, a small protrusion (lagena) first occurs, growing into a spiral-twisted cochlea passage (ductus cochlearis). Initially, the walls of the vesicle cavity due to the ingrowth of peripheral processes nerve cells from the auditory ganglion lying on the front side of the labyrinth, turns into sensory cells (organ of Corti). The mesenchyme adjacent to the membranous labyrinth turns into connective tissue, creating perilymphatic spaces around the formed utriculus, sacculus and semicircular canals. At the 6th month of intrauterine life, around the membranous labyrinth with its perilymphatic spaces, a bone labyrinth arises from the perichondrium of the cartilaginous capsule of the skull through perichondral ossification, repeating general shape membranous.
Middle ear- tympanic cavity with auditory tube - develops from the first pharyngeal pouch and the lateral part of the upper wall of the pharynx, therefore, the epithelium of the mucous membrane of the middle ear cavities comes from the endoderm. Located in the tympanic cavity auditory ossicles formed from the cartilage of the first (malleus and incus) and second (stirrup) visceral arches. The outer ear develops from the first gill pouch.
In a newborn, the auricle is relatively smaller than in an adult and does not have pronounced convolutions and tubercles. Only by the age of 12 does it reach the shape and size of the auricle of an adult. After 50 - 60 years, the cartilage begins to dehydrate. The external auditory canal in a newborn is short and wide, and the bony part consists of a bony ring. The size of the eardrum in a newborn and an adult is almost the same. The eardrum is located at an angle of 180° to the upper wall, and in an adult - at an angle of 140°.
Tympanic cavity filled with fluid and connective tissue cells, its lumen is small due to the thick mucous membrane. In children under 2 - 3 years of age, the upper wall of the tympanic cavity is thin, has a wide stony-scaly gap filled with fibrous connective tissue with numerous blood vessels. The posterior wall of the tympanic cavity communicates with a wide opening with cells mastoid process. The auditory ossicles, although they contain cartilaginous points, correspond to the size of an adult. The auditory tube is short and wide (up to 2 mm). The shape and size of the inner ear do not change throughout life.
Sound waves, meeting the resistance of the eardrum, together with it vibrate the handle of the hammer, which displaces all the auditory ossicles. The base of the stapes presses on the perilymph of the vestibule of the inner ear. Since the fluid is practically incompressible, the perilymph of the vestibule displaces the fluid column of the scala vestibule, which moves through the opening at the apex of the cochlea (helicotrema) into the scala tympani. Its liquid stretches the secondary membrane covering the round window. Due to the deflection of the secondary membrane, the cavity of the perilymphatic space increases, which causes the formation of waves in the perilymph, the vibrations of which are transmitted to the endolymph. This leads to displacement of the spiral membrane, which stretches or bends the hairs of sensory cells. Sensory cells are in contact with the first sensory neuron.
Outer ear
The outer ear (auris externa) is a structural formation of the hearing organ, which includes Auricle, external auditory canal and eardrum, lying on the border of the outer and middle ear.
Auricle(auricula) - structural unit of the outer ear. The base of the auricle is represented by elastic cartilage covered with thin skin. The auricle has a funnel-shaped shape with indentations and protrusions on inner surface. Its free edge is curl(helix) - curved towards the center of the ear. Below and parallel to the helix is antihelix(anthelix), which ends at the bottom near the opening of the external auditory canal tragus(tragus). Posteriorly the tragus is located antitragus(antitragus). In the lower part of the auricle there is no cartilage and the skin forms a fold - lobe or ear lobule (lobulus auriculare). Above, behind and below, rudimentary striated muscles are attached to the cartilaginous part of the external auditory canal, which have actually lost their function, and displacement of the auricle does not occur.
External auditory canal(meatus acusticus externus) – structural formation of the outer ear. The outer third of the external auditory canal consists of cartilage (cartilago meatus acustici), related to the auricle; two-thirds of its length is formed by the bony part of the temporal bone. The external auditory canal has an irregular cylindrical shape. Opening on the side surface of the head, it is directed along the frontal axis into the depths of the skull and has two bends: one in the horizontal, the other in the vertical plane. This shape of the ear canal ensures that only sound waves reflected from its walls pass to the eardrum, which reduces its stretching. The entire ear canal is covered with thin skin, the outer third of which contains hair and sebaceous glands(gll. cereminosae). The epithelium of the skin of the external auditory canal continues to the eardrum.
Eardrum(membrana tympani) - a formation located on the border of the outer and middle ear. The eardrum develops along with the organs of the outer ear. It is an oval, 11x9 mm in size, thin translucent plate. The free edge of this plate is inserted into tympanic sulcus(sulcus tympanicus) in the bony part of the ear canal. It is strengthened in the furrow by a fibrous ring, not along the entire circumference. On the side of the ear canal, the membrane is covered flat epithelium, and from the side of the tympanic cavity by the epithelium of the mucous membrane.
The basis of the membrane consists of elastic and collagen fibers, which in its upper part are replaced by fibers of loose connective tissue. This part is poorly stretched and is called pars flaccida. In the central part of the membrane, the fibers are arranged circularly, and in the anterior, posterior and lower peripheral parts - radially. Where the fibers are oriented radially, the membrane is stretched and shines in reflected light. In newborns, the eardrum is located almost transversely to the diameter of the external auditory canal, and in adults - at an angle of 45°. In the central part it is concave and is called navel(umbo membranae tympani), where the handle of the hammer is attached to the side of the middle ear .
Middle ear
The middle ear (auris media) is a structural formation of the hearing organ. Comprises tympanic cavity with those imprisoned in it auditory ossicles and auditory tube reporting tympanic cavity with the nasopharynx.
Tympanic cavity
The tympanic cavity (cavum tympani) is a structural formation of the middle ear, located at the base of the pyramid of the temporal bone between the external auditory canal and the labyrinth (inner ear). It contains a chain of three small auditory ossicles that transmit sound vibrations from the eardrum to the labyrinth. The tympanic cavity has an irregular cuboid shape and a small size (volume about 1 cm3). The walls that limit the tympanic cavity border important anatomical structures: the inner ear, the internal jugular vein, the internal carotid artery, the cells of the mastoid process and the cranial cavity.
Anterior wall of the tympanic cavity(paries caroticus) - a wall close to the internal carotid artery. At the top of this wall is internal opening of the auditory tube(ostium tympanicum tubae anditivae), which gapes widely in newborns and young children, which explains the frequent penetration of infection from the nasopharynx into the middle ear cavity and further into the skull.
Membranous wall of the tympanic cavity(paries membranaceus) - lateral wall, formed by the eardrum and the bony plate of the external auditory canal. The upper, dome-shaped expanded part of the tympanic cavity forms supratympanic pocket(recessus epitympanicus), containing two bones: head of the malleus and incus. In case of illness pathological changes middle ear are most pronounced in the supratympanic recess.
Mastoid wall of the tympanic cavity(paries mastoideus) - posterior wall, delimits the tympanic cavity from the mastoid process. Contains a number of elevations and openings: pyramidal elevation(eminentia pyramidalis), which contains the stapes muscle (m. stapedius); projection of the lateral semicircular canal(prominentia canalis semicircularis lateralis); facial canal projection(prominentia canalis facialis); mastoid cave(antrum mastoideum), bordering on back wall external auditory canal.
The tegmental wall of the tympanic cavity(paries tegmentalis) - the upper wall, has a dome shape (pars cupularis) and separates the cavity of the middle ear from the cavity of the middle cranial fossa.
Jugular wall of the tympanic cavity(paries jugularis) - the lower wall, separates the tympanic cavity from the fossa of the internal jugular vein, where its bulb is located. In the posterior part of the jugular wall there is subulate protuberance(prominentia styloidea), a trace of pressure from the styloid process.
Auditory ossicles(ossicula auditus) - formations inside the tympanic cavity of the middle ear, connected by joints and muscles, providing air vibrations of varying intensity. The auditory ossicles include hammer, anvil and stirrup.
Hammer(malleus) – auditory ossicle. At the malleus they secrete neck(collum mallei) and handle(manubribm mallei). Hammer head(caput mallei) is connected by the incus-mallear joint (articulatio incudomallearis) to the body of the incus. The handle of the malleus fuses with the eardrum. And the muscle that stretches the eardrum (m. tensor tympani) is attached to the neck of the malleus.
Tensor tympani muscle(m. tensor tympani) is a striated muscle that originates from the walls of the muscular-tubal canal of the temporal bone and is attached to the neck of the malleus. By pulling the handle of the hammer inside the tympanic cavity, it strains the eardrum, so the eardrum is tense and concave into the cavity of the middle ear. Innervation of the muscle from the V pair of cranial nerves.
Anvil(incus) – auditory ossicle, has a length of 6-7 mm, consists of body(corpus incudis) and two legs: short (crus breve) and long (crus langum). The long leg bears a lenticular process (processus lenticularis) and is articulated by the incudostapedia joint with the head of the stapes (articulatio incudostapedia).
Stirrup(stapes) - auditory ossicle, has head ( caput stapedis), front and hind legs (crura anterius et posterius) and base(basis stapedis). The stapedius muscle is attached to the posterior leg. The base of the stapes is inserted into the oval window of the vestibule of the labyrinth. Ring ligament (lig. anulare stapedis) in the form of a membrane located between the base of the stapes and the edge oval window ensures mobility of the stirrup when exposed to air waves on the eardrum.
Stapes muscle(m. stapedius) - a striated muscle, begins in the thickness of the pyramidal eminence of the mastoid wall of the tympanic cavity and is attached to the posterior leg of the stapes. Contracting, it brings the base of the stirrup out of the hole. Innervation from the VII pair of cranial nerves. During strong vibrations of the auditory ossicles, together with the muscle that stretches the eardrum, it holds the auditory ossicles, reducing their displacement.
Eustachian tube
Eustachian tube (tuba auditiva), Eustachian tube, - the formation of the middle ear, which serves to allow air from the pharynx to enter the tympanic cavity, which maintains the same pressure with the external and inside eardrum. The auditory tube consists of bone and cartilage parts that are connected to each other. Bone part(pars ossea), 6 - 7 mm long and 1 - 2 mm in diameter, is located in the temporal bone. Cartilaginous part(pars cartilaginea), made of elastic cartilage, has a length of 2.3 - 3 mm and a diameter of 3 - 4 mm, located in the thickness of the lateral wall of the nasopharynx.
Originate from the cartilaginous part of the auditory tube tensor palatine muscle(m. tensor veli palatini), velopharyngeal muscle(m. palatopharyngeus), muscle lifting the velum(m. levator veli palatini). Thanks to these muscles, when swallowing, the auditory tube opens and the air pressure in the nasopharynx and middle ear is equalized. The inner surface of the tube is covered with ciliated epithelium; in the mucous membrane there are mucous glands(gll. tubariae) and accumulation of lymphatic tissue. It is well developed and forms the tubal tonsil at the mouth of the nasopharyngeal opening of the tube.
Inner ear
The inner ear (auris interna) is a structural formation related to both the organ of hearing and the vestibular apparatus. The inner ear consists of bony and membranous labyrinths. These labyrinths form vestibule, three semicircular canals(vestibular apparatus) and snail related to the organ of hearing.
Snail(cochlea) is an organ of the auditory system, part of the bony and membranous labyrinth. The bony part of the cochlea consists of spiral channel(canalis spiralis cochleae), limited by the bone substance of the pyramid. The channel has 2.5 circular strokes. Located in the center of the cochlea hollow bone rod(modiolus), located in the horizontal plane. It protrudes into the lumen of the cochlea from the side of the rod. bony spiral plate(lamina spiralis ossea). In its thickness there are openings through which blood vessels and auditory nerve fibers pass to the spiral organ.
Spiral plate The cochlea, together with the formations of the membranous labyrinth, divides the cochlear cavity into 2 parts: staircase vestibule(scala vestibuli), connecting to the cavity of the vestibule, and staircase drum(scala tympani). The place where the scala vestibule transitions into the scala tympani is called lucid opening of the cochlea(helicotrema). The window of the cochlea opens into the scala tympani. The cochlear aqueduct originates from the scala tympani and passes through the bony substance of the pyramid. On bottom surface the posterior edge of the pyramid of the temporal bone is located on the outer snail water pipe hole(apertura externa canaliculi cochleae).
Cochlear part the membranous labyrinth is represented cochlear duct(ductus cochlearis). The duct begins from the vestibule in the area cochlear recess(recessus cochlearis) of the bony labyrinth and ends blindly near the apex of the cochlea. In cross section, the cochlear duct has triangular shape, and most of it is located closer to the outer wall. Thanks to the cochlear duct, the cavity of the bony duct of the cochlea is divided into 2 parts: the upper one - the scala vestibule and the lower one - the scala tympani.
The outer (stria vascular) wall of the cochlear duct fuses with the outer wall of the bony duct of the cochlea. The upper (paries vestibularis) and lower (membrana spiralis) walls of the cochlear duct are a continuation of the bony spiral plate of the cochlea. They originate from its free edge and diverge towards the outer wall at an angle of 40 - 45°. On the bottom wall there is a sound-receiving apparatus - spiral organ(organ of Corti).
spiral organ(organum spirale) is located throughout the entire cochlear duct and is located on a spiral membrane, which consists of thin collagen fibers. Sensitive hair cells are located on this membrane. The hairs of these cells are immersed in a gelatinous mass called cover membrane(membrana tectoria). When a sound wave swells the basilar membrane, the hair cells standing on it sway from side to side and their hairs, immersed in the covering membrane, bend or stretch to the diameter of a hydrogen atom. These atom-sized changes in the position of the hair cells produce a stimulus that generates the generator potential of the hair cells.
One of the reasons high sensitivity hair cells is that the endolymph maintains a positive charge of about 80 mV relative to the perilymph. The potential difference ensures the movement of ions through the pores of the membrane and the transmission of sound stimuli. When removing electrical potentials from different parts snails discovered 5 different electrical phenomena. Two of them - the membrane potential of the auditory receptor cell and the endolymph potential - are not caused by the action of sound; they are also observed in the absence of sound. Three electrical phenomena - the microphonic potential of the cochlea, the summation potential and the potentials of the auditory nerve - arise under the influence of sound stimulation.
The membrane potential of an auditory receptor cell is recorded when a microelectrode is inserted into it. As with other nerve or receptor cells, the inner surface of the auditory receptor membranes is negatively charged (-80 mV). Since the hairs of auditory receptor cells are washed by positively charged endolymph (+ 80 mV), the potential difference between the inner and outer surfaces of their membrane reaches 160 mV. The significance of a large potential difference is that it greatly facilitates the perception of weak sound vibrations. The endolymph potential, recorded when one electrode is inserted into the membranous canal and the other into the area of the round window, is determined by the activity of the choroid plexus (stria vascularis) and depends on the intensity of oxidative processes. When breathing is impaired or tissue oxidative processes are suppressed by cyanide, the endolymph potential decreases or disappears. If you insert electrodes into the cochlea, connect them to an amplifier and a loudspeaker and apply sound, the loudspeaker accurately reproduces this sound.
The described phenomenon is called the cochlear microphone effect, and the recorded electrical potential is called the cochlear microphone potential. It has been proven that it is generated on the hair cell membrane as a result of hair deformation. The frequency of microphone potentials corresponds to the frequency of sound vibrations, and the amplitude, within certain limits, is proportional to the intensity of sounds acting on the ear. In response to strong sounds of high frequency, a persistent shift in the initial potential difference is noted. This phenomenon is called summation potential. As a result of the occurrence of microphonic and summation potentials in the hair cells under the influence of sound vibrations on them, pulsed excitation of the auditory nerve fibers occurs. The transfer of excitation from the hair cell to the nerve fiber occurs, apparently, both electrically and chemically.
The olfactory nerve (I pair) begins from the olfactory cells located in the mucous membrane of the upper part of the nasal cavity, the dendrites of which perceive aromatic substances. The axons of the olfactory cells in the form of 15-20 olfactory filaments form the olfactory nerve and pass through the openings in the ethmoid bone into the cranial cavity, where they end in the olfactory bulb. Here are the second neurons of the olfactory analyzer, the fibers of which are directed posteriorly, forming the right and left olfactory pathways (tractus olfactorius dexter et sinister), which are located in the olfactory grooves at the base of the frontal lobes of the brain. The fibers of the olfactory pathways follow to the subcortical olfactory centers: mainly to the olfactory triangle, as well as to the anterior perforated substance and the septum pellucidum, where they switch to third neurons. These neurons conduct olfactory stimuli from the primary olfactory centers to the cortical section of the olfactory analyzer on their own and the opposite side. The cortical center of smell is located on the inner surface of the temporal lobe in the anterior parts of the gyrus near the seahorse (parahippocampal), mainly in its hook (uncus). The fibers of the third neurons, having made a partial decussation, reach the cortical olfactory centers in three ways: some of them pass over the corpus callosum, another part under the corpus callosum, the third directly through the uncinate fasciculus (fasciculus uncinatu).
1 - olfactory threads; 2 - olfactory bulb; 3 - olfactory pathway; 4 - subcortical olfactory centers; 5 - olfactory fibers over the corpus callosum; 6 - olfactory fibers under the corpus callosum; 7 - cingulate gyrus; 8 - parahippocampal gyrus; 9 - cortical section of the olfactory analyzer.
Olfactory research. The patient is allowed to smell a weakly aromatic substance with each half of the nose separately. Strong irritating odors (vinegar, ammonia) should not be used, since the irritations they cause are perceived mainly by receptors trigeminal nerve. It is necessary to find out whether the patient senses and recognizes the smell, whether the sensation is the same on both sides, and whether he has olfactory hallucinations.
Impaired sense of smell can be in the form of decreased perception (hyposmia), total loss its (anosmia), exacerbation (hyperosmia), distortion of smell (parosmia), as well as olfactory hallucinations, when the patient perceives odors without a corresponding stimulus.
Bilateral impairment of smell is observed more often with inflammatory pathological processes in the nasal cavity that are not related to neurological pathology. Unilateral hypo- or anosmia occurs when the olfactory bulb, olfactory pathway and olfactory triangle reach the intersection of fibers heading to the cortical olfactory projection area. This pathology occurs when a tumor or abscess in the anterior cranial fossa damages the olfactory bulb or olfactory pathway. In this case, hypo- or anosmia occurs on the affected side. Unilateral damage to the fibers of the olfactory analyzer above the subcortical olfactory centers does not lead to loss of smell, since each of the subcortical centers and, accordingly, each half of the nose is connected to both cortical departments of smell. Irritation of the cortical areas of the olfactory analyzer in the temporal lobe leads to the appearance of olfactory hallucinations, often the aura of an epileptic seizure.