The membranous canal of the cochlea is filled. Inner ear. The structure of the snail. Microstructure of the organ of Corti. Conduction of sound vibrations in the cochlea. · Bone conduction. The cochlea, enclosed in the bony cavity of the temporal bone, is capable of perceiving vibrations from the hands

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The inner ear (auris interna) is divided into three parts: the vestibule, the cochlea, and the semicircular canal system. Phylogenetically, the organ of balance is a more ancient formation.

The inner ear is represented by the outer bone and inner membranous (formerly called leathery) sections - labyrinths. The cochlea belongs to the auditory analyzer, the vestibule and semicircular canals belong to the vestibular analyzer.

Bone labyrinth

Its walls are formed by compact bone substance pyramids temporal bone.

Snail (cochlea)

Fully lives up to its name and is a curled canal of 2.5 turns, twisting around a bone cone-shaped rod (modiolus), or spindle. From this spindle into the lumen of the helix, a bone plate extends in the form of a spiral, which has an unequal width as it moves from the base of the cochlea to the dome of the cochlea: at the base it is much wider and almost touches the inner wall of the helix, and at the apex it is very narrow and disappears.

In this regard, at the base of the cochlea the distance between the edge of the bony spiral plate and the inner surface of the cochlea is very small, and in the area of the apex it is noticeably wider. In the center of the spindle there is a canal for the fibers of the auditory nerve, from the trunk of which numerous tubules extend to the periphery towards the edge of the bone plate. Through these tubules, the fibers of the auditory nerve approach the spiral (corti) organ.

vestibule (vestibulum)

The bony vestibule is a small, almost spherical cavity. Its outer wall is almost entirely occupied by the opening of the window of the vestibule; on the front wall there is an opening leading to the base of the cochlea, on back wall- five openings leading to the semicircular canals. On the inner wall, small holes are visible through which the fibers of the vestibular nerve approach the receptor sections of the vestibule in the area of small depressions on this wall of a spherical and elliptical shape.

1 - elliptical sac (uterus); 2 — ampoule of the external canal; 3 - endolymphatic sac; 4 - cochlear duct; 5 - spherical bag; 6 - perilymphatic duct; 7 — cochlear window; 8 - window of the vestibule

Bone semicircular canals (canales semicircularesossei) are three arched thin tubes. They are located in three mutually perpendicular planes: horizontal, frontal and sagittal and are called lateral, anterior and posterior. The semicircular canals are not located strictly in the indicated planes, but deviate from them by 300, i.e. the lateral one is deflected from the horizontal plane by 300, the anterior one is turned to the middle by 300, the rear one is deflected posteriorly by 300. This should be taken into account when conducting a study of the function of the semicircular canals.

Each bony semicircular canal has two bony pedicles, one of which is expanded in the form of an ampulla (ampullary bone pedicle).

Membranous labyrinth

It is located inside the bone and completely follows its contours: cochlea, vestibule, semicircular ducts. All sections of the membranous labyrinth are connected to each other.

Cochlear duct

From the free edge of the bony spiral plate along its entire length towards the inner surface of the cochlear curls, the fibers of the “string” of the basilar plate (membrane) extend, and thus the cochlear curl is divided into two floors.

The upper floor - the staircase of the vestibule (scala vestibuli) begins in the vestibule, rises spirally to the dome, where through the opening of the cochlea (helicotrema) it passes into another, lower floor - the scala tympanum (scala tympani), and also spirals down to the base of the cochlea. Here the lower floor ends with a cochlear window covered by a secondary tympanic membrane.

On a cross section, the membranous labyrinth of the cochlea (cochlear duct) has the shape of a triangle.

From the place of attachment of the basilar plate (membrana basillaris), also towards the inner surface of the helix, but at an angle, another pliable membrane extends - the vestibular wall of the cochlear duct (vestibular, or vestibular, membrane; Reissner's membrane).

Thus, in the upper staircase, the staircase vestibule (scala vestibuli), an independent canal is formed, spiraling upward from the base to the dome of the cochlea. This is the cochlear duct. Outside of this membranous labyrinth in the scala tympani and in the scala vestibule there is a fluid - perilymph. It is generated by a specific system itself inner ear, represented by the vascular network in the perilymphatic space. Through the cochlear aqueduct, the perilymph communicates with the cerebral fluid of the subarachnoid space.

Inside the membranous labyrinth there is endolymph. It differs from perilymph in the content of K+ and Na+ ions, as well as in electrical potential.

Endolymph is produced by the vascular strip, which occupies the inner surface of the outer wall of the cochlear passage.

a - section of the cochlea of the rod axis; b - membranous labyrinth of the cochlea and spiral organ.

1 - cochlear opening; 2 - staircase vestibule; 3 - membranous labyrinth of the cochlea (cochlear duct); 4 - scala tympani; 5 - bone spiral plate; 6 - bone rod; 7 - vestibular wall of the cochlear duct (Reisner’s membrane); 8 - vascular strip; 9 — spiral (main) membrane; 10 - covering membrane; 11 - spiral organ
The spiral, or organ of Corti, is located on the surface of the spiral membrane in the lumen of the cochlear duct. The width of the spiral membrane is not the same: at the base of the cochlea, its fibers are shorter, more stretched, and more elastic than in areas approaching the dome of the cochlea. There are two groups of cells—sensory and supporting—that provide the mechanism for perceiving sounds. There are two rows (inner and outer) of supporting, or pillar, cells, as well as outer and inner sensory (hair) cells, with 3 times more outer hair cells than inner ones.

The hair cells resemble an elongated thimble, and their lower edges rest on the bodies of the Deuterian cells. Each hair cell has 20-25 hairs at its upper end. A covering membrane (membrana tectoria) extends over the hair cells. It consists of thin fibers fused to each other. The hair cells are approached by fibers that originate in the cochlear ganglion (spiral ganglion of the cochlea), located at the base of the bony spiral plate. Inner hair cells carry out “fine” localization and discrimination of individual sounds.

Outer hair cells “connect” sounds and contribute to a “complex” sound experience. Weak, quiet sounds are perceived by the outer hair cells, strong sounds by the inner ones. The outer hair cells are the most vulnerable and are damaged more quickly, and therefore, when the sound analyzer is damaged, the perception of weak sounds first suffers. Hair cells are very sensitive to the lack of oxygen in the blood and endolymph.

Membranous vestibule

It is represented by two cavities occupying a spherical and elliptical recess on the medial wall of the bony vestibule: a spherical sac (sacculus) and an elliptical sac, or utricle (utriculus). These cavities contain endolymph. The spherical sac communicates with the cochlear duct, the elliptical sac communicates with the semicircular ducts. Both sacs are also connected to each other by a narrow duct, which turns into an endolymphatic duct - the aqueduct of the vestibule (agueductus vestibuli) and ends blindly in the form of an endolymphatic sac (sacculus endolymphaticus). This small sac is located on the posterior wall of the pyramid of the temporal bone, in the posterior cranial fossa and can be a collector of endolymph and stretch when there is excess.

The elliptical and spherical sacs contain the otolithic apparatus in the form of spots (maculae). A. Scarpa was the first to draw attention to these details in 1789. He also pointed out the presence of “pebbles” (otoliths) in the vestibule, and also described the course and ending of the auditory nerve fibers in the “whitish tubercles” of the vestibule. Each sac of the “otolithic apparatus” contains terminal nerve endings of the vestibulocochlear nerve. The long fibers of the supporting cells form a dense network in which the otoliths are located. They are surrounded by a gelatinous mass that forms the otolithic membrane. Sometimes it is compared to wet felt. Between this membrane and the elevation, which is formed by the cells of the sensitive epithelium of the otolithic apparatus, a narrow space is defined. The otolithic membrane slides along it and deflects sensory hair cells.

The semicircular ducts lie in the semicircular canals of the same name. The lateral (horizontal or external) duct has an ampulla and an independent leg, with which it opens into an elliptical sac.

The frontal (anterior, upper) and sagittal (posterior, lower) ducts have only independent membranous ampoules, and their simple stalk is combined, and therefore only 5 openings open into the vestibule. At the border of the ampulla and the simple stalk of each canal there is an ampullar comb (crista ampularis), which is a receptor for each canal. The space between the expanded, ampullary part in the scallop area is delimited from the lumen of the semi-canal by a transparent dome (cupula gelotinosa). It is a delicate diaphragm and is revealed only with special staining of the endolymph. The dome is located above the scallop.

1 - endolymph; 2 — transparent dome; 3 - ampullary comb

The impulse occurs when the movable gelatin dome moves along the comb. It is assumed that these displacements of the dome can be compared with fan-shaped or pendulum-like movements, as well as with oscillations of a sail when the direction of air movement changes. One way or another, but under the influence of the endolymph current, the transparent dome, moving, deflects the hairs of sensitive cells and causes them to be excited and triggered.

The frequency of impulses in the ampullary nerve changes depending on the direction of deviation of the hair bundle, the transparent dome: when deviated towards the elliptical sac - an increase in impulses, towards the canal - a decrease. The transparent dome contains mucopolysaccharides that play the role of piezoelements.

Yu.M. Ovchinnikov, V.P. Gamow

HEARING ORGAN

Comprises outer, middle and inner ear.

Outer ear

Outer ear includes auricle, external auditory canal and eardrum.

Auricle consists of a thin plate of elastic cartilage covered with skin with a few thin hair and sebaceous glands. There are few sweat glands in its composition.

External auditory canal formed by cartilage, which is a continuation of the elastic cartilage of the shell, and a bone part. The surface of the passage is covered with thin skin containing hair and associated sebaceous glands. Deeper sebaceous glands There are tubular ceruminous glands that secrete earwax. Their ducts open independently on the surface ear canal or in excretory ducts sebaceous glands. The ceruminous glands are located unevenly along the auditory tube: in the inner two-thirds they are present only in the skin of the upper part of the tube.

Eardrum oval, slightly concave shape. One of the auditory ossicles of the middle ear - the malleus - is fused with the help of its handle to the inner surface of the eardrum. Pass from the malleus to the eardrum blood vessels and nerves. The middle part of the eardrum consists of two layers formed by bundles of collagen and elastic fibers and fibroblasts lying between them. The fibers of the outer layer are arranged radially, and the fibers of the inner layer are arranged circularly. In the upper part of the eardrum, the number of collagen fibers decreases. On its outer surface there is a very thin layer (E0-60 µm) of the epidermis, on the inner surface facing the middle ear there is a mucous membrane about 20-40 µm thick, covered with single-layer squamous epithelium.

Middle ear

The middle ear consists of tympanic cavity, auditory ossicles and auditory tube.

Tympanic cavity- a flattened space covered with single-layer squamous epithelium, sometimes turning into cubic or columnar epithelium. There are two openings, or “windows,” on the medial wall of the tympanic cavity. The first is the oval window. It contains the base of the stirrup, which is held in place by a thin ligament around the circumference of the window. The oval window separates the tympanic cavity from the scala vestibularis of the cochlea. The second window is round, located slightly behind the oval one. It is covered with a fibrous membrane. A round window separates the tympanic cavity from the scala tympani of the cochlea.

Auditory ossicles- the hammer, incus, and stirrup, as a system of levers, transmit vibrations of the eardrum of the outer ear to the oval window, from which the vestibular staircase of the inner ear begins.

Eustachian tube, connecting the tympanic cavity with the nasal part of the pharynx, has a well-defined lumen with a diameter of 1-2 mm. In the area adjacent to the tympanic cavity, the auditory tube is surrounded by a bone wall, and closer to the pharynx it contains islands of hyaline cartilage. The lumen of the tube is lined with multirow prismatic ciliated epithelium. It contains goblet glandular cells. On the surface of the epithelium, ducts of the mucous glands open. The auditory tube regulates the air pressure in the tympanic cavity of the middle ear.

Inner ear

The inner ear consists of bony labyrinth and located in it membranous labyrinth, which contains receptor cells - hair sensory epithelial cells of the organ of hearing and balance. They are located in certain areas of the membranous labyrinth: auditory receptor cells are in the spiral organ of the cochlea, and receptor cells of the balance organ are in the elliptical and spherical sacs and ampullary crests of the semicircular canals.

Development. In the human embryo, the organs of hearing and balance are formed together from the ectoderm. A thickening is formed from the ectoderm - auditory placode, which soon turns into auditory fossa, and then in otic vesicle and breaks away from the ectoderm and sinks into the underlying mesenchyme. The auditory vesicle is lined from the inside with multi-row epithelium and is soon divided into 2 parts by a constriction - from one part a spherical sac is formed - the sacculus and the cochlear membranous labyrinth is formed (i.e. hearing aid), and from the other part - an elliptical sac - the utriculus with semicircular canals and their ampoules (i.e., the organ of balance). In the multirow epithelium of the membranous labyrinth, cells differentiate into sensory sensory cells and supporting cells. Epithelium Eustachian tube connecting the middle ear with the pharynx and the epithelium of the middle ear develops from the epithelium of the 1st gill pouch. Somewhat later, the processes of ossification and formation of the bony labyrinth of the cochlea and semicircular canals occur.

Structure of the hearing organ (inner ear)

The structure of the membranous canal of the cochlea and spiral organ(scheme).

1 - membranous canal of the cochlea; 2 - vestibular staircase; 3 - scala tympani; 4 - spiral bone plate; 5 - spiral knot; 6 - spiral ridge; 7 - dendrites of nerve cells; 8 - vestibular membrane; 9 - basilar membrane; 10 - spiral ligament; 11 - epithelium lining 6 and another staircase; 12 - vascular strip; 13 - blood vessels; 14 - cover plate; 15 - outer sensoroepithelial cells; 16 - internal sensoroepithelial cells; 17 - internal supporting epithelialitis; 18 - external supporting epithelialitis; 19 - pillar cells; 20 - tunnel.

The structure of the hearing organ (inner ear). The receptor part of the hearing organ is located inside membranous labyrinth, located in turn in the bone labyrinth, having the shape of a snail - a bone tube spirally twisted into 2.5 turns. A membranous labyrinth runs along the entire length of the bony cochlea. On a cross section, the labyrinth of the bony cochlea has a rounded shape, and the transverse labyrinth has a triangular shape. The walls of the membranous labyrinth in cross section are formed by:

1. superomedial wall- educated vestibular membrane (8). It is a thin fibrillar connective tissue plate covered with single-layer squamous epithelium facing the endolymph and endothelium facing the perilymph.

2. outer wall- educated vascular strip (12), lying on spiral ligament (10). The stria vascularis is a multirow epithelium that, unlike all epithelia in the body, has its own blood vessels; this epithelium secretes endolymph, which fills the membranous labyrinth.

3. Bottom wall, base of the triangle - basilar membrane (lamina) (9), consists of individual stretched strings (fibrillar fibers). The length of the strings increases in the direction from the base of the cochlea to the top. Each string is capable of resonating at a strictly defined vibration frequency - strings closer to the base of the cochlea (shorter strings) resonate at higher vibration frequencies (higher sounds), strings closer to the top of the cochlea - at lower vibration frequencies (lower sounds) .

The space of the bony cochlea above the vestibular membrane is called vestibular staircase (2), below the basilar membrane - drum ladder (3). The scala vestibular and scala tympani are filled with perilymph and communicate with each other at the apex of the bony cochlea. At the base of the bony cochlea, the scala vestibular ends in an oval opening closed by the stapes, and the scala tympani ends in a round opening closed by an elastic membrane.

Spiral organ or organ of Corti - receptive part of the hearing organ , located on the basilar membrane. It consists of sensory cells, supporting cells and a covering membrane.

1. Sensory hair epithelial cells - slightly elongated cells with a rounded base, at the apical end they have microvilli - stereocilia. The dendrites of the first neurons of the auditory pathway approach the base of the sensory hair cells and form synapses, the bodies of which lie in the thickness of the bone rod - the spindle of the bony cochlea in the spiral ganglia. Sensory hair epithelial cells are divided into internal pear-shaped and external prismatic. The outer hair cells form 3-5 rows, while the inner hair cells form only 1 row. Inner hair cells receive about 90% of all innervation. The tunnel of Corti is formed between the inner and outer hair cells. Hangs over the microvilli of sensory hair cells. tectorial membrane.

2. SUPPORTING CELLS (SUPPORTING CELLS)

External pillar cells

Internal pillar cells

External phalangeal cells

Inner phalangeal cells

Supporting phalangeal epithelial cells- located on the basilar membrane and are a support for sensory hair cells, supporting them. Tonofibrils are found in their cytoplasm.

3. COVERING MEMBRANE (TECTORIAL MEMBRANE) - gelatinous formation, consisting of collagen fibers and amorphous connective tissue substance, extends from the upper part of the thickening of the periosteum of the spiral process, hangs over the organ of Corti, the tips of the stereocilia of hair cells are immersed in it

1, 2 - external and internal hair cells, 3, 4 - external and internal supporting (supporting) cells, 5 - nerve fibers, 6 - basilar membrane, 7 - openings of the reticular (reticular) membrane, 8 - spiral ligament, 9 - spiral bone plate, 10 - tectorial (cover) membrane

Histophysiology of the spiral organ. The sound, like air vibration, vibrates the eardrum, then the vibration is transmitted through the hammer and anvil to the stapes; the stapes through the oval window transmits vibrations to the perilymph of the scala vestibularis; along the vestibular scala, vibrations at the apex of the bony cochlea pass into the perilymph of the scala tympani and spiral downwards and rest against the elastic membrane of the round opening. Vibrations of the perilymph of the scala tympani cause vibrations of the strings of the basilar membrane; When the basilar membrane oscillates, the sensory hair cells oscillate in the vertical direction and their hairs touch the tectorial membrane. Bending of the microvilli of hair cells leads to the excitation of these cells, i.e. the potential difference between the outer and inner surfaces of the cytolemma changes, which is sensed by the nerve endings on the basal surface of the hair cells. IN nerve endings Nerve impulses are generated and transmitted along the auditory pathway to the cortical centers.

As determined, sounds are differentiated by frequency (high and low sounds). The length of the strings in the basilar membrane changes along the membranous labyrinth; the closer to the apex of the cochlea, the longer the strings. Each string is tuned to resonate at a specific vibration frequency. If the sounds are low, the long strings resonate and vibrate closer to the top of the cochlea and the cells sitting on them are accordingly excited. If high-pitched sounds resonate, short strings located closer to the base of the cochlea resonate, and the hair cells sitting on these strings are excited.

VESTIBULAR PART OF THE MEMBRANUS LABYRINTH - has 2 extensions:

1. Pouch - a spherical extension.

2. Uterus - an extension of an elliptical shape.

These two extensions are connected to each other by a thin tubule. Three mutually perpendicular semicircular canals with extensions are associated with the uterus - ampoules. Most of the inner surface of the sac, utricle and semicircular canals with ampoules is covered with single-layer squamous epithelium. At the same time, in the saccule, uterus and in the ampoules of the semicircular canals there are areas with thickened epithelium. These areas of thickened epithelium in the sac and utricle are called spots or macules, and in ampoules - scallops or cristae.

is a unique organ not only in its structure, but also in the functions it performs. Thus, it perceives sound vibrations, is responsible for maintaining balance and has the ability to hold the body in space in a certain position.

Each of these functions is performed by one of three parts of the ear: external and internal. Next, we will talk specifically about the internal section, and more specifically about one of its components - the cochlea.

The structure of the cochlea of the inner ear

Structure presented labyrinth, consisting of a bone capsule and a membranous formation that repeats the shape of the same capsule.

Location of the cochlea in the bony labyrinth of the inner ear

The bony labyrinth consists of the following sections:

semicircular canals;
vestibule;
snail.

Snail in the ear- this is a bone formation that has the appearance of a volumetric spiral in 2.5 turns around the bone shaft. The width of the base of the cochlea cone is 9 mm, and in height – 5 mm. The length of the bone spiral is 32 mm.

Reference. cochlea It also consists of a relatively durable material; according to some scientists, this material is one of the most durable in the entire human body.

Starting its path in the bone core, spiral plate goes inside the labyrinth. This formation at the beginning of the cochlea is wide, and towards its completion it gradually begins to narrow. The plate is all dotted with channels in which dendrites of bipolar neurons.

Section of the cochlea of the inner ear

Thanks to main (basilar) membrane, located between the unused edge of this plate and the wall of the cavity, occurs division of the cochlear canal into 2 passages or stairs:

Superior canal or scala vestibule- originates at the oval window and extends all the way to the apical point of the cochlea.
Inferior canal or scala tympani- extends from the apical point of the cochlea up to the round window.

Both canals at the apex of the cochlea are connected by a narrow opening - helicotrem. Also both cavities are filled perilymph, which has characteristics similar to cerebrospinal fluid.

The vestibular (Reissner's) membrane divides the upper canal into 2 cavities:

stairs;
membranous canal, called the cochlear duct.

IN cochlear duct located on the basilar membrane organ of corti– sound analyzer. It consists of supporting and auditory receptor hair cells, above which is located cover membrane, resembling a jelly-like mass in its appearance.

The structure of the organ of Corti, responsible for the beginning of sound processing

Functions of the cochlea of the inner ear

The main function of the cochlea in the ear- this is the transmission of nerve impulses coming from the middle ear to the brain, while the organ of Corti is a very important link in the chain, since it is where the primary formation analysis of sound signals. What is the sequence of performing such a function?

So, when sound vibrations reach the ear, they hit the membrane of the eardrum, thereby causing vibration in it. Then the vibration reaches 3 auditory ossicles(maleus, incus, stapes).

Connected with snail stapes affects fluid in the areas: scala vestibule and scala tympani. In this case, the liquid affects the basilar membrane, which includes the auditory nerves, and creates vibration waves on it.

From the generated vibration waves cilia of hair cells in the sound analyzer (organ of Corti) come into motion, irritating the plate located above them like a canopy (covering membrane).

Then this process comes to the final stage, where hair cells transmit impulses about the characteristics of sounds to the brain. Moreover, the latter is like a complex logic processor begins to separate useful audio signals from background noise, distributing them into groups according to various characteristics and looking for similar images in memory.

Spiral organ or organ of Corti - receptive part of the hearing organ , located on the basilar membrane. It consists of sensory cells, supporting cells and a covering membrane.

2. SUPPORTING CELLS (SUPPORTING CELLS)

External pillar cells

Internal pillar cells

External phalangeal cells

Inner phalangeal cells

Supporting phalangeal epithelial cells- located on the basilar membrane and are a support for sensory hair cells, supporting them. Tonofibrils are found in their cytoplasm.

Histophysiology of the spiral organ. The sound, like air vibration, vibrates the eardrum, then the vibration is transmitted through the hammer and anvil to the stapes; the stapes through the oval window transmits vibrations to the perilymph of the scala vestibularis; along the vestibular scala, vibrations at the apex of the bony cochlea pass into the perilymph of the scala tympani and spiral downwards and rest against the elastic membrane of the round opening. Vibrations of the perilymph of the scala tympani cause vibrations of the strings of the basilar membrane; When the basilar membrane oscillates, the sensory hair cells oscillate in the vertical direction and their hairs touch the tectorial membrane. Bending of the microvilli of hair cells leads to the excitation of these cells, i.e. the potential difference between the outer and inner surfaces of the cytolemma changes, which is sensed by the nerve endings on the basal surface of the hair cells. Nerve impulses are generated at the nerve endings and transmitted along the auditory pathway to the cortical centers.

VESTIBULAR PART OF THE MEMBRANUS LABYRINTH - has 2 extensions:

1. Pouch - a spherical extension.

2. Uterus - an extension of an elliptical shape.

The inner ear contains the receptor apparatus of two analyzers: the vestibular (vestibular and semicircular canals) and the auditory, which includes the cochlea with the organ of Corti.

The bony cavity of the inner ear, containing a large number of chambers and passages between them, is called labyrinth . It consists of two parts: the bony labyrinth and the membranous labyrinth. Bone labyrinth- a series of cavities located in the dense part of the bone; three components are distinguished in it: the semicircular canals are one of the sources of nerve impulses that reflect the position of the body in space; vestibule; and the snail - an organ.

Membranous labyrinth enclosed within the bony labyrinth. It is filled with a fluid, endolymph, and is surrounded by another fluid, perilymph, which separates it from the bony labyrinth. The membranous labyrinth, like the bony labyrinth, consists of three main parts. The first corresponds in configuration to the three semicircular canals. The second divides the bony vestibule into two sections: the utricle and the saccule. The elongated third part forms the middle (cochlear) scala (spiral canal), repeating the bends of the cochlea.

Semicircular canals. There are only six of them - three in each ear. They have an arched shape and begin and end in the uterus. The three semicircular canals of each ear are located at right angles to each other, one horizontally and two vertically. Each channel has an extension at one end - an ampoule. The six channels are arranged in such a way that for each there is an opposite channel in the same plane, but in a different ear, but their ampoules are located at mutually opposite ends.

Cochlea and organ of Corti. The name of the snail is determined by its spirally convoluted shape. This is a bone canal that forms two and a half turns of a spiral and is filled with fluid. The curls go around a horizontally lying rod - a spindle, around which a bone spiral plate is twisted like a screw, pierced by thin canaliculi, where the fibers of the cochlear part of the vestibulocochlear nerve - the VIII pair of cranial nerves - pass. Inside, on one wall of the spiral canal along its entire length there is a bony protrusion. Two flat membranes extend from this protrusion to the opposite wall so that the cochlea is divided along its entire length into three parallel channels. The two external ones are called the scala vestibuli and the scala tympani; they communicate with each other at the apex of the cochlea. Central, so-called the spiral canal of the cochlea ends blindly, and its beginning communicates with the sac. The spiral canal is filled with endolymph, the scala vestibule and scala tympani are filled with perilymph. Perilymph has a high concentration of sodium ions, while endolymph has a high concentration of potassium ions. The most important function endolymph, which is positively charged in relation to perilymph, is the creation of an electrical potential on the membrane separating them, which provides energy for the process of amplifying incoming sound signals.

The scala vestibule begins in a spherical cavity, the vestibule, which lies at the base of the cochlea. One end of the scala through the oval window (the window of the vestibule) comes into contact with the inner wall of the air-filled cavity of the middle ear. The scala tympani communicates with the middle ear through the round window (window of the cochlea). Liquid

cannot pass through these windows, since the oval window is closed by the base of the stapes, and the round window by a thin membrane separating it from the middle ear. The spiral canal of the cochlea is separated from the scala tympani so-called. the main (basilar) membrane, which resembles a miniature string instrument. It contains a number of parallel fibers of varying lengths and thicknesses stretched across a helical channel, with the fibers at the base of the helical channel being short and thin. They gradually lengthen and thicken towards the end of the cochlea, like the strings of a harp. The membrane is covered with rows of sensitive, hair-equipped cells that make up the so-called. the organ of Corti, which performs a highly specialized function - converts vibrations of the main membrane into nerve impulses. Hair cells are connected to the endings of nerve fibers that, upon exiting the organ of Corti, form the auditory nerve (cochlear branch of the vestibulocochlear nerve).

Membranous cochlear labyrinth, or duct, has the appearance of a blind vestibular protrusion located in the bony cochlea and blindly ending at its apex. It is filled with endolymph and is a connective tissue sac about 35 mm long. The cochlear duct divides the bony spiral canal into three parts, occupying the middle of them - the middle staircase (scala media), or cochlear duct, or cochlear canal. The upper part is the vestibular staircase (scala vestibuli), or the vestibular staircase, the lower part is the tympanic or tympanic staircase (scala tympani). They contain peri-lymph. In the area of the dome of the cochlea, both staircases communicate with each other through the opening of the cochlea (helicotrema). The scala tympani extends to the base of the cochlea, where it ends at the round window of the cochlea, closed by the secondary tympanic membrane. The scala vestibule communicates with the perilymphatic space of the vestibule. It should be noted that perilymph in its composition resembles blood plasma and cerebrospinal fluid; it has a predominant sodium content. Endolymph differs from perilymph in its higher (100 times) concentration of potassium ions and lower (10 times) concentration of sodium ions; in my own way chemical composition it resembles intracellular fluid. In relation to the peri-lymph, it is positively charged.

The cochlear duct in cross section has a triangular shape. The upper - vestibular wall of the cochlear duct, facing the staircase of the vestibule, is formed by a thin vestibular (Reissner) membrane (membrana vestibularis), which is covered from the inside with single-layer squamous epithelium, and on the outside - by endothelium. Between them there is fine fibrillar connective tissue. The outer wall fuses with the periosteum of the outer wall of the bony cochlea and is represented by a spiral ligament, which is present in all curls of the cochlea. On the ligament there is a vascular strip (stria vascularis), rich in capillaries and covered with cubic cells that produce endolymph. The lower - the tympanic wall, facing the scala tympani - is most complexly structured. It is represented by the basilar membrane, or plate (lamina basilaris), on which the spiral, or organ of Corti, which produces sounds, is located. The dense and elastic basilar plate, or basilar membrane, is attached at one end to the spiral bone plate, and at the opposite end to the spiral ligament. The membrane is formed by thin, weakly stretched radial collagen fibers (about 24 thousand), the length of which increases from the base of the cochlea to its apex - near the oval window, the width of the basilar membrane is 0.04 mm, and then towards the apex of the cochlea, gradually expanding, it reaches end 0.5 mm (i.e. the basilar membrane expands where the cochlea narrows). The fibers consist of thin fibrils anastomosing among themselves. The weak tension of the fibers of the basilar membrane creates conditions for their oscillatory movements.

The organ of hearing itself, the organ of Corti, is located in the bony cochlea. The organ of Corti is a receptor part located inside the membranous labyrinth. In the process of evolution, it arises on the basis of the structures of the lateral organs. It perceives vibrations of fibers located in the canal of the inner ear and transmits them to the auditory cortex, where sound signals are formed. In the Organ of Corti, the primary formation of the analysis of sound signals begins.

Location. The organ of Corti is located in the spirally curled bone canal of the inner ear - the cochlear passage, filled with endolymph and perilymph. The upper wall of the passage is adjacent to the so-called. staircase vestibule and is called Reisner's membrane; the lower wall bordering the so-called. scala tympani, formed by the main membrane attached to the spiral bone plate. The organ of Corti is composed of supporting, or supporting, cells, and receptor cells, or phonoreceptors. There are two types of supporting cells and two types of receptor cells - external and internal.

External supporting cells lie further from the edge of the spiral bone plate, and internal- closer to him. Both types of supporting cells converge at an acute angle to each other and form a triangular-shaped canal - an internal (Corti) tunnel filled with endo-lymph, which runs spirally along the entire organ of Corti. The tunnel contains unmyelinated nerve fibers coming from the neurons of the spiral ganglion.

Phonoreceptors lie on supporting cells. They are secondary sensory (mechanoreceptors) that transform mechanical vibrations into electrical potentials. Phonoreceptors (based on their relationship to the tunnel of Corti) are divided into internal (flask-shaped) and external (cylindrical) which are separated from each other by the arcs of Corti. The inner hair cells are arranged in a single row; their total number along the entire length of the membranous canal reaches 3500. Outer hair cells are arranged in 3-4 rows; their total number reaches 12,000-20,000. Each hair cell has an elongated shape; one of its poles is close to the main membrane, the second is located in the cavity of the membranous canal of the cochlea. At the end of this pole there are hairs, or stereocilia (up to 100 per cell). The hairs of the receptor cells are washed by the endolymph and come into contact with the integumentary, or tectorial, membrane (membrana tectoria), which is located above the hair cells along the entire course of the membranous canal. This membrane has a jelly-like consistency, one edge of which is attached to the bony spiral plate, and the other ends freely in the cavity of the cochlear duct a little further than the external receptor cells.

All phonoreceptors, regardless of location, are synaptically connected to 32,000 dendrites of bipolar sensory cells located in the spiral nerve of the cochlea. These are the first auditory pathways, which form the cochlear (cochlear) part of the VIII pair of cranial nerves; they transmit signals to the cochlear nuclei. In this case, signals from each inner hair cell are transmitted to bipolar cells simultaneously along several fibers (probably this increases the reliability of information transmission), while signals from several outer hair cells converge on one fiber. Therefore, about 95% of the auditory nerve fibers carry information from the inner hair cells (although their number does not exceed 3500), and 5% of the fibers transmit information from the outer hair cells, the number of which reaches 12,000-20,000. These data highlight the enormous physiological importance of inner hair cells in sound reception.

To hair cells Efferent fibers - axons of neurons of the superior olive - are also suitable. The fibers coming to the inner hair cells do not end on these cells themselves, but on afferent fibers. They are hypothesized to have an inhibitory effect on auditory signal transmission, promoting increased frequency resolution. Fibers coming to the outer hair cells affect them directly and, by changing their length, change their phono sensitivity. Thus, with the help of efferent olivo-cochlear fibers (Rasmussen's bundle fibers), higher acoustic centers regulate the sensitivity of phonoreceptors and the flow of afferent impulses from them to the brain centers.

Conduction of sound vibrations in the cochlea . Sound perception is carried out with the participation of phonoreceptors. Under the influence of a sound wave, they lead to the generation of a receptor potential, which causes excitation of the dendrites of the bipolar spiral ganglion. But how is the frequency and intensity of sound encoded? This is one of the most complex issues in the physiology of the auditory analyzer.

The modern idea of coding the frequency and intensity of sound comes down to the following. A sound wave, acting on the system of auditory ossicles of the middle ear, sets into oscillatory motion the membrane of the oval window of the vestibule, which, bending, causes wave-like movements of the perilymph of the upper and lower canals, which gradually attenuate towards the apex of the cochlea. Since all fluids are incompressible, these oscillations would be impossible if it were not for the membrane of the round window, which bulges when the base of the stapes is pressed on the oval window and returns to its original position when the pressure is released. Vibrations of the perilymph are transmitted to the vestibular membrane, as well as to the cavity of the middle canal, setting the endolymph and basilar membrane in motion (the vestibular membrane is very thin, so the fluid in the upper and middle canals vibrates as if both canals are one). When the ear is exposed to low frequency sounds (up to 1000 Hz), the basilar membrane is displaced along its entire length from the base to the apex of the cochlea. As the frequency of the sound signal increases, the oscillating column of liquid, shortened in length, moves closer to the oval window, to the most rigid and elastic part of the basilar membrane. When deformed, the basilar membrane displaces the hairs of the hair cells relative to the tectorial membrane. As a result of this displacement, an electrical discharge occurs in the hair cells. There is a direct relationship between the amplitude of the displacement of the main membrane and the number of auditory cortex neurons involved in the excitation process.

The mechanism of sound vibrations in the cochlea

Sound waves are captured auricle and through the auditory canal are directed to the eardrum. Vibrations of the eardrum, through the system of auditory ossicles, are transmitted through the stapes to the membrane of the oval window, and through it are transmitted to the lymphatic fluid. Depending on the vibration frequency, only certain fibers of the main membrane respond to fluid vibrations (resonate). The hair cells of the organ of Corti are excited when the fibers of the main membrane touch them and are transmitted along the auditory nerve into impulses, where the final sensation of sound is created.