The structure of the membranous canal of the cochlea and the spiral organ (diagram). Anatomy of the inner ear Scala cochlea and their shape
Hearing and balance
The recording of two sensory modalities—hearing and balance—occurs in the ear (Fig. 11–1). Both organs (hearing and balance) form the vestibule ( vestibulum) and snail ( cochlea) - vestibulocochlear organ. The receptor (hair) cells (Fig. 11–2) of the organ of hearing are located in the membranous canal of the cochlea (organ of Corti), and the organ of balance (vestibular apparatus) in the structures of the vestibule - the semicircular canals, the uterus ( utriculus) and pouch ( sacculus).
Rice . 11 – 1. Organs of hearing and balance . The outer, middle and inner ear, as well as the auditory and vestibular branches of the vestibulocochlear nerve (VIII pair of cranial nerves) extending from the receptor elements of the organ of hearing (organ of Corti) and balance (crests and spots).
Rice . 11 – 2. vestibulocochlear organ and receptor areas (top right, blackened) organs of hearing and balance. The movement of perilymph from the oval to the round window is indicated by arrows.
Hearing
Organ hearing(Fig. 11–1, 11–2) anatomically consists of the outer, middle and inner ear.
· External ear represented by the auricle and external auditory canal.
Ear sink- elastic cartilage of complex shape, covered with skin, at the bottom of which is the external auditory opening. The shape of the auricle helps direct sound into the external auditory canal. Some people can move their ears using weak muscles attached to the skull. Outer auditory passage- a blind tube 2.5 cm long, ending at the eardrum. The outer third of the passage consists of cartilage and is covered with fine protective hair. The internal parts of the passage are located in the temporal bone and contain modified sweat glands - ceruminous glands, which produce a waxy secretion - earwax - to protect the skin of the passage and fix dust and bacteria.
· Average ear. Its cavity communicates with the nasopharynx using the Eustachian (auditory) tube and is separated from the external auditory canal by a tympanic membrane with a diameter of 9 mm, and from the vestibule and scala tympani of the cochlea by oval and round windows, respectively. Drum membrane transmits sound vibrations to three small interconnected auditory bones: The malleus is attached to the tympanic membrane, and the stapes is attached to the oval window. These bones vibrate in unison and amplify the sound twenty times. The auditory tube maintains air pressure in the middle ear cavity at atmospheric pressure.
· Internal ear. The cavity of the vestibule, tympanic and vestibular scala of the cochlea (Fig. 11–3) are filled with perilymph, and the semicircular canals, utricle, saccule and cochlear duct (membranous canal of the cochlea) located in the perilymph are filled with endolymph. There is an electrical potential between the endolymph and perilymph - about +80 mV (intracochlear, or endocochlear potential).
à Endolymph- viscous liquid, fills the membranous canal of the cochlea and is connected through a special channel ( ductus reuniens) with the endolymph of the vestibular apparatus. Concentration K + in the endolymph 100 times more than in the cerebrospinal fluid (CSF) and perilymph; Na concentration + in the endolymph is 10 times less than in the perilymph.
à Perilymph its chemical composition is close to blood plasma and cerebrospinal fluid and occupies an intermediate position between them in terms of protein content.
à Endocochlear potential. The membranous canal of the cochlea is positively charged (+60–+80 mV) relative to the other two scalae. The source of this (endocochlear) potential is the stria vascularis. Hair cells are polarized by the endocochlear potential to a critical level, which increases their sensitivity to mechanical stress.
Rice . 11–3. Membranous canal and spiral (corti) organ [11]. The cochlear canal is divided into the scala tympani and vestibular canal and the membranous canal (middle scala), in which the organ of Corti is located. The membranous canal is separated from the scala tympani by a basilar membrane. It contains peripheral processes of neurons of the spiral ganglion, forming synaptic contacts with outer and inner hair cells.
Cochlea and organ of Corti
Conduction of sound to the cochlea
The chain of transmission of sound pressure is as follows: tympanic membrane ® malleus ® incus ® stapes ® oval window membrane ® perilymph ® basilar and tectorial membranes ® round window membrane (see Fig. 11-2). When the stapes is displaced, the perilymph moves along the scala vestibularis and then through the helicotrema along the scala tympani to the round window. Fluid displaced by the displacement of the oval window membrane creates excess pressure in the vestibular canal. Under the influence of this pressure, the basilar membrane moves towards the scala tympani. The oscillatory reaction in the form of a wave propagates from the basilar membrane to the helicotrema. The displacement of the tectorial membrane relative to the hair cells under the influence of sound causes their excitation. The resulting electrical reaction ( microphone effect) repeats the shape of the sound signal.
· Auditory bones. The sound vibrates the eardrum and transmits the energy of vibrations through the system of auditory ossicles to the perilymph of the scala vestibule. If the eardrum and ossicles did not exist, sound could reach the inner ear, but much of the sound energy would be reflected back due to differences in acoustic impedance ( impedances) air and liquid environments. That's why most important role drum membranes And chains auditory seeds is V creation compliance between impedances external air environment And liquid environment internal ear. The amplitude of movement of the sole of the stirrup during each sound vibration is only three-quarters of the amplitude of vibration of the hammer handle. Consequently, the oscillatory lever system of the ossicles does not increase the range of movement of the stapes. Instead, the lever system reduces the amplitude of vibrations, but increases their force by approximately 1.3 times. To this it should be added that the area of the eardrum is 55 mm 2 , while the area of the stapes sole is 3.2 mm 2 . The difference in the lever system by 17 times leads to the fact that the pressure on the fluid in the cochlea is 22 times higher than the air pressure on the eardrum. Equalizing the impedances between sound waves and sound vibrations of the liquid improves the clarity of perception of sound frequencies in the range from 300 to 3000 Hz.
· Muscles average ear. The functional role of the middle ear muscles is to reduce the impact of loud sounds on the auditory system. When loud sounds act on the transmitting system and signals enter the central nervous system, after 40–80 ms a sound-reducing reflex occurs, causing contraction of the muscles attached to the stapes and malleus. The malleus muscle pulls the handle of the malleus forward and downward, and the stapes muscle pulls the stapes outward and upward. These two opposing forces increase the rigidity of the ossicular lever system, reducing the conduction of low-frequency sounds, especially sounds below 1000 Hz.
· Sound reducing reflex can reduce the intensity of transmission of low-frequency sounds by 30-40 dB, at the same time without affecting the perception of loud voices and whispered speech. The significance of this reflex mechanism is twofold: protection snails from the damaging vibration effect of low sound and disguise low sounds in the environment. In addition, the muscles of the auditory ossicles reduce the sensitivity of a person's hearing to his own speech at the moment when the brain activates the vocal mechanism.
· Bone conductivity. The cochlea, enclosed in the bony cavity of the temporal bone, is capable of perceiving the vibrations of a hand-held tuning fork or the sound of an electronic vibrator applied to the protrusion of the upper jaw or the mastoid process. Bone conduction of sound under normal conditions is not activated even by loud sound transmitted through the air.
Movement of sound waves in the cochlea
For material in this section, see the book.
Hair cell activation
For material in this section, see the book.
Sound Characteristics Detection
For material in this section, see the book.
auditory pathways and centers
In Fig. Figure 11–6A shows a simplified diagram of the major auditory pathways. Afferent nerve fibers from the cochlea enter the spiral ganglion and from it enter the dorsal (posterior) and ventral (anterior) cochlear nuclei, located in the upper part of the medulla oblongata. Here, ascending nerve fibers form synapses with second-order neurons, the axons of which partly move to the opposite side to the nuclei of the superior olive, and partly end on the nuclei of the superior olive of the same side. From the superior olive nuclei, the auditory tract ascends through the lateral lemniscal tract; some of the fibers end in the lateral lemniscal nuclei, and most of the axons bypass these nuclei and follow to the inferior colliculus, where all or almost all auditory fibers form synapses. From here, the auditory pathway passes to the medial geniculate body, where all fibers end at synapses. The auditory pathway finally ends in the auditory cortex, located mainly in the superior gyrus of the temporal lobe (Fig. 11-6B). The basilar membrane of the cochlea at all levels of the auditory pathway is presented in the form of certain projection maps of various frequencies. Already at the level of the midbrain, neurons appear that detect several signs of sound on the principles of lateral and recurrent inhibition.
Rice . 11–6. A . Main auditory pathways (posterior view of the brainstem, cerebellum and cerebral cortex removed). B. Auditory cortex.
Auditory cortex
The projection areas of the auditory cortex (Fig. 11-6B) are located not only in the upper part of the superior temporal gyrus, but also extend to the outer side of the temporal lobe, capturing part of the insular cortex and parietal operculum.
Primary auditory bark directly receives signals from the internal (medial) geniculate body, while auditory associative region secondarily excited by impulses from the primary auditory cortex and thalamic areas bordering the medial geniculate body.
· Tonotopic cards. In each of the 6 tonotopic maps, high-frequency sounds excite neurons in the back of the map, while low-frequency sounds excite neurons in the front of the map. It is assumed that each individual area perceives its own specific features of sound. For example, one large map in the primary auditory cortex almost entirely discriminates against sounds that appear high-pitched to the subject. Another map is used to determine the direction of sound arrival. Some areas of the auditory cortex detect special qualities of sound signals (for example, sudden onset of sounds or modulations of sounds).
· Range sound frequencies, to which neurons of the auditory cortex respond narrower than for neurons of the spiral ganglion and brain stem. This is explained, on the one hand, by the high degree of specialization of cortical neurons, and on the other hand, by the phenomenon of lateral and recurrent inhibition, which enhances the resolving ability of neurons to perceive the required sound frequency.
· Many neurons in the auditory cortex, especially in the auditory association cortex, respond to more than just specific sound frequencies. These neurons “associate” sound frequencies with other types of sensory information. Indeed, the parietal part of the auditory association cortex overlaps somatosensory area II, which creates the possibility of associating auditory information with somatosensory information.
Determining the direction of sound
· Direction source sound. Two ears working in unison can detect the source of a sound by the difference in volume and the time it takes to reach both sides of the head. A person determines the sound coming to him in two ways.
à Time delays between receipt sound V one ear And V opposite ear. Sound travels first to the ear closest to the sound source. Low frequency sounds bend around the head due to their considerable length. If the sound source is located on the midline in front or behind, then even a minimal shift from the midline is perceived by a person. This subtle comparison of the minimum difference in the time of sound arrival is carried out by the central nervous system at the points where auditory signals converge. These convergence points are the superior olive, inferior colliculus, and primary auditory cortex.
à The difference between intensity sounds V two ears. At high sound frequencies, the size of the head noticeably exceeds the length of the sound wave, and the wave is reflected by the head. This results in a difference in the intensity of sounds coming to the right and left ears.
Auditory sensations
· Range frequencies, which is perceived by a person, includes about 10 octaves of the musical scale (from 16 Hz to 20 kHz). This range gradually decreases with age due to a decrease in the perception of high frequencies. Discrimination frequencies sound characterized by a minimal difference in frequency between two close sounds, which can still be detected by a person.
· Absolute threshold auditory sensitivity- the minimum sound intensity that a person hears in 50% of cases when it is presented. The hearing threshold depends on the frequency of sound waves. Maximum sensitivity hearing person located V region from 5 00 to 4000 Hz. Within these boundaries, sound is perceived as having extremely low energy. The region of sound perception of human speech is located in the range of these frequencies.
· Sensitivity To sound frequencies below 500 Hz progressively is decreasing. This protects a person from the possible constant sensation of low-frequency vibrations and noise produced by his own body.
Spatial orientation
The spatial orientation of the body at rest and in movement is largely ensured by reflex activity originating in the vestibular apparatus of the inner ear.
Vestibular apparatus
The vestibular (vestibulary) apparatus, or organ of balance (Fig. 11–2) is located in the petrous part of the temporal bone and consists of the bony and membranous labyrinths. The bony labyrinth is a system of semicircular ducts ( canales semicirculares) and the cavity communicating with them - the vestibule ( vestibulum). Membranous labyrinth- a system of thin-walled tubes and sacs located inside the bone labyrinth. In the bone ampullae, the membranous canals expand. In each ampullary extension of the semicircular canal there are scallops (crista ampullaris). In the vestibule of the membranous labyrinth, two interconnected cavities are formed: queen, into which the membranous semicircular canals open, and pouch. The sensitive areas of these cavities are spots. The membranous semicircular canals, the utricle and the sac are filled with endolymph and communicate with the cochlea, as well as with the endolymphatic sac located in the cranial cavity. The ridges and spots, the receptive areas of the vestibular organ, contain receptor hair cells. Rotational movements are recorded in the semicircular canals ( corner acceleration), in the uterus and pouch - linear acceleration.
· Sensitive spots And scallops(Fig. 11–7). The epithelium of the spots and scallops contains sensory hair and supporting cells. The epithelium of the spots is covered with a gelatinous otolithic membrane containing otoliths - crystals of calcium carbonate. The epithelium of the scallops is surrounded by a jelly-like transparent dome (Fig. 11-7A and 11-7B), which easily moves with the movements of the endolymph.
Rice . 11–7. Receptor area of the balance organ . Vertical sections through the comb (A) and spots (B, C). OM - otolith membrane, O - otoliths, PC - supporting cell, RK - receptor cell.
· Hairy cells(Fig. 11–7 and 11–7B) are located in the crests of each ampulla of the semicircular canals and in the spots of the vestibular sacs. Hair receptor cells in the apical part contain 40–110 immobile hairs ( stereocilia) and one mobile cilium ( kinocilium), located on the periphery of the bundle of stereocilia. The longest stereocilia are located near the kinocilium, and the length of the rest decreases with distance from the kinocilium. Hair cells are sensitive to the direction of the stimulus ( directional sensitivity, see fig. 11–8A). When the irritating effect is directed from the stereocilia to the kinocilium, the hair cell is excited (depolarization occurs). When the stimulus is directed in the opposite direction, the response is suppressed (hyperpolarization).
à There are two types of hair cells. Type I cells are usually located in the center of the ridges, while type II cells are located at their periphery.
Ú Cells type I They have the shape of an amphora with a rounded bottom and are located in the goblet-shaped cavity of the afferent nerve ending. Efferent fibers form synaptic terminals on afferent fibers associated with type I cells.
Ú Cells type II They look like cylinders with a round base. A characteristic feature of these cells is their innervation: the nerve endings here can be either afferent (the majority) or efferent.
à In the epithelium of the spots, kinocilia are distributed in a special way. Here the hair cells form groups of several hundred units. Within each group, the kinocilia are oriented in the same way, but the orientation of the kinocilia is different between different groups.
Stimulation of the semicircular canals
The receptors of the semicircular canals perceive rotational acceleration, i.e. angular acceleration (Fig. 11–8). At rest, there is a balance in the frequency of nerve impulses from the ampullae of both sides of the head. Angular acceleration of the order of 0.5° per second is sufficient to displace the dome and bend the cilia. Angular acceleration is recorded due to the inertia of the endolymph. When the head turns, the endolymph remains in the same position, and the free end of the dome deviates in the direction opposite to the turn. Movement of the dome bends the kinocilium and sterocilia embedded in the jelly-like structure of the dome. The tilt of the stereocilia toward the kinocilium causes depolarization and excitation; the opposite direction of tilt results in hyperpolarization and inhibition. When excited, a receptor potential is generated in the hair cells and a release occurs, which activates the afferent endings of the vestibular nerve.
Rice . 11–8. Physiology of recording angular acceleration. A - different reactions of hair cells in the scallops of the ampullae of the left and right horizontal semicircular canals when turning the head. B - Successively increasing images of the perceptive structures of the scallop.
The semicircular canals detect the rotation or rotation of the head. When the head suddenly begins to turn in any direction (this is called angular acceleration), the endolymph in the semicircular canals, due to its great inertia, remains in a stationary state for some time. The semicircular canals continue to move at this time, which causes endolymph flow in the direction opposite to the rotation of the head. This leads to activation of the endings of the vestibular nerve, and the frequency of nerve impulses exceeds the frequency of spontaneous impulses at rest. If the rotation continues, the pulse frequency gradually decreases and returns to its original level within a few seconds.
Reactions body, caused by stimulation semicircular channels. Stimulation of the semicircular canals causes subjective sensations in the form of dizziness, nausea and other reactions associated with excitation of the autonomic nervous system. To this are added objective manifestations in the form of changes in the tone of the eye muscles (nystagmus) and the tone of the anti-gravity muscles (falling reaction).
· Dizziness is a spinning sensation and can cause imbalance and falls. The direction of the rotation sensation depends on which semicircular canal was stimulated. In each case, the dizziness is oriented in the direction opposite to the displacement of the endolymph. During rotation, the feeling of dizziness is directed in the direction of rotation. The sensation experienced after the rotation stops is directed in the direction opposite to the actual rotation. As a result of dizziness, vegetative reactions occur - nausea, vomit, pallor, sweating, and with intense stimulation of the semicircular canals a sharp drop in blood pressure is possible ( collapse).
· Nystagmus And violations muscular tone. Stimulation of the semicircular canals causes changes in muscle tone, manifested in nystagmus, disruption of coordination tests and the fall reaction.
à Nystagmus- rhythmic twitching of the eye, consisting of slow and fast movements. Slow movement are always directed towards the movement of the endolymph and are a reflex reaction. The reflex occurs in the crests of the semicircular canals, impulses enter the vestibular nuclei of the brain stem and from there are switched to the muscles of the eye. Fast movement determined by the direction of nystagmus; they arise as a result of central nervous system activity (as part of the vestibular reflex from the reticular formation to the brainstem). Rotation in the horizontal plane causes horizontal nystagmus, rotation in the sagittal plane causes vertical nystagmus, rotation in the frontal plane causes rotational nystagmus.
à Rectifier reflex. Violation of the pointing test and the fall reaction are the result of changes in the tone of the anti-gravity muscles. The tone of the extensor muscles increases on the side of the body where the displacement of the endolymph is directed, and decreases on the opposite side. So, if the gravitational forces are directed towards the right foot, then the head and body of a person deviate to the right, displacing the endolymph to the left. The resulting reflex will immediately cause extension of the right leg and arm and flexion of the left arm and leg, accompanied by deviation of the eyes to the left. These movements are a protective righting reflex.
Stimulation of the uterus and sac
For material in this section, see the book.
projection pathways of the vestibular apparatus
The vestibular branch of the VIII cranial nerve is formed by the processes of approximately 19 thousand bipolar neurons, forming a sensory ganglion. The peripheral processes of these neurons approach the hair cells of each semicircular canal, utricle, and sac, and the central processes are sent to the vestibular nuclei of the medulla oblongata (Fig. 11-9A). Axons of second-order nerve cells are connected to the spinal cord (vestibular-spinal tract, olivo-spinal tract) and rise as part of the medial longitudinal fascicles to the motor nuclei of the cranial nerves, which control eye movements. There is also a pathway that carries impulses from the vestibular receptors through the thalamus to the cerebral cortex.
à Pre-door–spinal path (tractus vestibulospinalis). The lateral vestibular cord begins from the lateral vestibular nucleus (Deiters), passes through the anterior funiculus and reaches the anterior horns a - and g ‑motoneurons. Axons of neurons of the medial vestibular nucleus (Schwalbe) join the medial longitudinal fasciculus ( fasciculus longitudinalis medialis) and descend down in the form of the medial vestibule-spinal tract to the thoracic spinal cord.
à Olivo–spinal path (tractus olivospinalis). The nerve fibers of the bundle begin from the olivary nucleus, pass in the anterior cord of the cervical spinal cord and end in the anterior horns.
Rice . 11–9. A Ascending pathways of the vestibular apparatus (posterior view, cerebellum and cerebral cortex removed). B. Multimodal system spatial body orientation.
Vestibular apparatus is part multimodal systems(Fig. 11-9B), including visual and somatic receptors that send signals to the vestibular nuclei either directly or through the vestibular nuclei of the cerebellum or the reticular formation. Incoming signals are integrated in the vestibular nuclei, and output commands affect the oculomotor and spinal motor control systems. In Fig. Figure 11–9B shows the central and coordinating role of the vestibular nuclei, connected by direct and feedback connections to the main receptor and central systems of spatial coordination.
The inner ear contains the receptor apparatus of two analyzers: the vestibular (vestibule 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 sac. 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 of the endolymph, which is positively charged in relation to the 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 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 its 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 scala 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 just beyond 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 increasing 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. The 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.
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The mechanism of sound vibrations in the cochlea Sound waves are picked up by the auricle and sent through the ear canal 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 through the auditory nerve into impulses, where the final sensation of sound is created. |
The inner ear, or labyrinth, is located in the thickness of the pyramid of the temporal bone and consists of a bone capsule and a membranous formation included in it, the shape of which follows the structure of the bone labyrinth. There are three sections of the bony labyrinth:
middle - vestibule (vestibulum);
anterior - cochlea (cochlea);
posterior - a system of three semicircular canals (canalis semicircularis).
Laterally, the labyrinth is the medial wall of the tympanic cavity, into which the windows of the vestibule and cochlea face, medially it borders on the posterior cranial fossa, with which it is connected by the internal auditory canal (meatus acusticus internus), the vestibular aqueduct (aquaeductus vestibuli) and the cochlear aqueduct (aquaeductus cochleae).
Snail (cochlea) is a bony spiral canal, which in humans has approximately two and a half turns around a bone rod (modiolus), from which a bony spiral plate (lamina spiralis ossea) extends into the canal. The cochlea in section has the appearance of a flattened cone with a base width of 9 mm and a height of 5 mm, the length of the spiral bone canal is about 32 mm. The bony spiral plate, together with the membranous basilar plate, which is its continuation, and the vestibular (Reisner) membrane (membrana vestibuli) form an independent canal (ductus cochlearis) inside the cochlea, which divides the cochlear canal into two spiral corridors - upper and lower. The upper section of the canal is the scala vestibule (scala vestibuli), the lower section is the scala tympani (scala tympani). The staircases are isolated from each other along their entire length, only in the area of the apex of the cochlea they communicate with each other through an opening (helicotrema). The scala vestibule communicates with the vestibule, the scala tympani borders the tympanic cavity through the window of the cochlea and does not communicate with the vestibule. At the base of the spiral plate there is a canal in which the spiral ganglion of the cochlea (gangl. spirale cochleae) is located - here are the cells of the first bipolar neuron of the auditory tract. The bony labyrinth is filled with perilymph, and the membranous labyrinth located in it is filled with endolymph.
vestibule (vestibulum)- the central part of the labyrinth, phylogenetically the most ancient. This is a small cavity, inside of which there are two pockets: spherical (recessus sphericus) and elliptical (recessus ellipticus). In the first, closer to the cochlea, there is a spherical sac (sacculus), in the second, adjacent to the semicircular canals, there is a utricle (utriculus). The anterior part of the vestibule communicates with the cochlea through the scala vestibule, the posterior part communicates with the semicircular canals.
Semicircular canals (canalis semicircularis). The three semicircular canals are located in three mutually perpendicular planes: the lateral or horizontal (canalis semicircularis lateralis) is at an angle of 30° to the horizontal plane; anterior or frontal vertical canal (canalis semicircularis anterior) - in the frontal plane; The posterior or sagittal vertical semicircular canal (canalis semicircularis posterior) is located in the sagittal plane. In each canal, an expanded ampullary and a smooth genu are distinguished, facing the elliptical recess of the vestibule. The smooth bends of the vertical canals - frontal and sagittal - are merged into one common bend. Thus, the semicircular canals are connected to the elliptical recess of the vestibule by five openings. The ampulla of the lateral semicircular canal comes close to the aditus ad antrum, forming its medial wall.
Membranous labyrinth is a closed system of cavities and canals, basically repeating the shape of the bone labyrinth. The space between the membranous and bony labyrinth is filled with perilymph. This space is very small in the area of the semicircular canals and expands somewhat in the vestibule and cochlea. The membranous labyrinth is suspended within the perilymphatic space by connective tissue cords. The cavities of the membranous labyrinth are filled with endolymph. Perilymph and endolymph represent the humoral system of the ear labyrinth and are functionally closely related. Perilymph in its ionic composition resembles cerebrospinal fluid and blood plasma, endolymph - intracellular fluid. The biochemical difference concerns primarily the content of potassium and sodium ions: in the endolymph there is a lot of potassium and little sodium, in the perilymph the ratio is the opposite. The perilymphatic space communicates with the subarachnoid space through the cochlear aqueduct; the endolymph is located in the closed system of the membranous labyrinth and has no communication with brain fluids.
It is believed that endolymph is produced by the stria vascularis and is reabsorbed in the endolymphatic sac. Excessive production of endolymph by the stria vascularis and disruption of its absorption can lead to increased intralabyrinth pressure.
From anatomical and functional points of view, two receptor apparatuses are distinguished in the inner ear:
auditory, located in the membranous cochlea (ductus cochlearis);
vestibular, in the sacs of the vestibule (sacculus and utriculus) and in the three ampoules of the membranous semicircular canals.
webbed snail, or the cochlear duct (ductus cochlearis) is located in the cochlea between the scala vestibule and the scala tympani. In cross-section, the cochlear duct has a triangular shape: it is formed by the vestibule, tympanic and outer walls. The upper wall faces the stairs of the vestibule and is formed by a thin vestibular (Reisner) membrane (membrana vestibularis), consisting of two layers of flat epithelial cells.
The bottom of the cochlear duct is formed by a basilar membrane, separating it from the scala tympani. The edge of the bony spiral plate through the basilar membrane is connected to the opposite wall of the bony cochlea, where inside the cochlear duct there is a spiral ligament (lig. spirale), the upper part of which, rich in blood vessels, is called the vascular strip a vascularis). The basilar membrane has an extensive network of capillary blood vessels and is a formation consisting of transversely located elastic fibers, the length and thickness of which increases in the direction from the main curl to the apex. On the basilar membrane, located spirally along the entire cochlear duct, lies the spiral (Corti) organ - the peripheral receptor of the auditory analyzer. The spiral organ consists of neuroepithelial inner and outer hair cells, supporting and feeding cells (Deiters, Hensen, Claudius), outer and inner pillar cells forming the arches of Corti.
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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 the compact bone substance of the pyramid of the 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 auditory nerve fibers 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 fenestra vestibule; on the anterior wall there is an opening leading to the base of the cochlea; on the posterior wall there are 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 tympani (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 of the inner ear itself, 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 termination 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 ridge (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, 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 act as piezoelements.
Yu.M. Ovchinnikov, V.P. Gamow
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 vestibuli 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 - bone spiral 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. Nerve impulses are generated at the nerve endings 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, accordingly, the cells sitting on them are 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. Associated with the uterus are three mutually perpendicular semicircular canals with extensions - 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.
