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INNER EAR

The inner ear, auris interna, or labyrinth, labirinthus, is located in the depths of the pyramid of the temporal bone and consists of several intricately constructed cavities and channels that communicate with each other. A bone labyrinth or labyrinth capsule and the connective tissue or membranous labyrinth contained in it are isolated.

The bony labyrinth, labyrinthus osseus, (Fig. 1.1.7) has walls consisting of a compact bone substance, 2-3 mm thick. This substance is particularly hard compared to the surrounding bone tissue of the pyramid, so it can be extracted from the temporal bone as a separate structure. Macroscopically, the system of cavities of the bone labyrinth is divided into three sections: the central — the vestibule, the anterior — the cochlea, and the posterior — semicircular canals.

Fig. 1.1.7

The vestibule, vestibulum, and the main scroll of the cochlea comprise the medial wall of the tympanum. The vestibule communicates with the latter using oval and round windows.

The semicircular canals, canales semicirculares ossei, are located in approximately three mutually perpendicular planes: lateral, canalis semicircularis lateralis - in the horizontal; front, canalis semicircularis anterior - in the frontal and posterior, canalis semicircularis posterior - in the sagittal. The lateral canal is raised relative to the horizontal plane by 300. The anterior and posterior canals form an angle of 450 with the frontal and sagittal planes. The anterior canal of one side and the posterior one of the opposite side lie in the same plane. Each channel has a simple smooth leg, crus osseum simplex, and an expanded ampulla end, ampulla ossea. The simple ends of the anterior and posterior semicircular canals merge into a common leg with a single canal. The diameter of the bony semicircular canals is 1.5 mm, the length is 12-18 mm.

The cochlea cochlea consists of a spirally curled around the rod, modiolus, a channel 28-30 mm long, which in humans represents 21 / 2-23 / 4 turns. In the section, the cochlea has the appearance of a truncated cone with a base width of about 9 mm and a height of 5 mm. A transverse bone plate, wrapped around it in the form of a spiral - lamina spiralis ossea, leaves the core deep into the bone canal. At the base of the spiral plate there is a channel in which the spiral ganglion is placed - the first bipolar neuron of the auditory tract.

The membranous labyrinth, labyrinthus membranaceus, (Fig. 1.1.8) is a closed system of cavities and channels, fortified in a bone labyrinth capsule by connective tissue cords. It consists of two vestibule sacs, three membranous semicircular canals, a membranous cochlea, and an endolymphatic duct with an endolymphatic sac. The cavities of the membranous labyrinth are filled with a fluid - endolymph, endolympha, and the spaces between the bony and membranous labyrinths contain perilymph, perilympha. In the vestibule sacs and semicircular canals is the organ of balance, and in the cochlea is the hearing organ.

Figure 1.1.8

In the vestibule there are two webbed formations - the uterus, utriculus, and sac, sacculus. Both bags are interconnected by a thin channel - ductus utriculo-saccularis, which continues into the endolymphatic duct, ductus endolymphaticus. The latter is placed in the osseous canal - the vestibule of the vestibule, aquaeductus vestibuli, and ends with the expansion - the endolymphatic sac, saccus endolymphaticus, lying in the duplicate of the dura mater on the posterior surface of the temporal bone pyramid.

There are static spots in the utriculus and sacculus, macula statica. Macula utriculi is located on the inner spherical surface of the uterine fundus, and macula sacculi is located on the inner spherical surface of the medial wall of the sac. Static spots contain supporting and receptor neuroepithelial hair cells. The receptor cell on the apical part has many short hairs - stereocilia and one long hair - kinocilia. Kinocilia is enclosed in an elastic substance, on which lies the otolith membrane (from the Greek otos ear, litos stone) with calcium carbonate crystals included in it. From the name of this membrane comes the term - otolithic apparatus. Due to the mass of otoliths, larger than the mass of the endolymph, the otolithic apparatus is irritated when the position of the head in space and linear accelerations change (Fig. 1.1.9).

Fig. 1.1.9

The membranous semicircular canals are located inside the bony and in the legs are adjacent to one of their walls. The ampullar ends of the membranous labyrinth almost completely fill the bone extensions. They contain ampullar scallops, cristae ampullares, on which supporting and receptor neuroepithelial hair cells are located, which have stereocilia and kinocilia, as in the otolith apparatus. Kinocilia are glued together by an elastic substance and form a flap - cupula, cupula. The cupula almost completely covers the lumen of the ampoule, so any movement of the endolymph in the channel causes its deflection and irritation of the receptor cells (Fig. 1.1.10).

Figure 1.1.10

Ampoule and otolith receptor formations belong to the organ of equilibrium - the vestibular apparatus.

The receptor cells of the vestibular apparatus are specialized secondary-sensory hair cells that contact the synaptic ends of the peripheral processes of the bipolar neurons of the vestibular ganglion, ganglion vestibularae (I neuron), located at the bottom of the internal auditory canal (Fig. 1.1.11). The central processes of the vestibular ganglion cells form the vestibular root, radix vestibularis, and the vestibulocochlear nerve. In the internal auditory meatus, it connects to the cochlear root of the same nerve and, in the area of ​​the cerebellopontine angle, penetrates into the substance of the brain (bridge).

Fig. 1.1.11

In the bridge, the fibers of the vestibular root are divided into ascending and descending paths and sent to the vestibular nuclei located in the very lateral corner of the rhomboid fossa, in the vestibular field, area vestibularis (II neuron). The ascending fibers end with synapses on the cells of the upper vestibular nucleus, nucl.vestibularis superior (Ankylosing spondylitis), the descending ones - on the cells of the medial vestibular nucleus, nucl.vestibularis medialis (Schwalbe), the lateral vestibular nucleus, nucl.vestibularis lateralis (Deuteris and vestibular nuclei) nucl. vestibularis inferior (Rollerball).

Axons of the cells of the vestibular nuclei form several bundles (tracts), which are sent to the cerebral cortex, spinal cord, cerebellum, to the medial and posterior longitudinal bundles (Fig. 1.1.12).

Fig. 1.1.12

The vestibulo-cortical path, tr. vestibulo-corticalis (vestibular sensory tract), switches in the thalamic nuclei (III neuron) and ends on the cells of the temporoparietal region of the cerebral cortex (IV neuron). This sensory pathway is called the vestibular analyzer (according to I.P. Pavlov) or the vestibular sensory system.

The vestibule-spinal path, tr.vestibulospinalis, ends segmentally on the cells of the motor nuclei of the anterior horns of the spinal cord, carrying out motor impulses to the muscles of the neck, trunk and extremities. It provides unconditionally reflexive maintenance of body balance.

The pre-cerebellar afferent pathway, tr.vestibulocerebellaris, passes through the lower legs of the cerebellum and ends on the cells of the cerebellar worm cortex. Through the cerebellar-vestibular efferent tract, tr.cerebellovestibularis, through the Deuterium nucleus, the cerebellum exerts a corrective effect on the spinal cord.

The connections of the vestibular nuclei through the medial longitudinal bundle, tractus vestibulolongitudinalis, with the nuclei of the oculomotor, block, and abducent nerves (III, IV, VI pairs) provide a compensatory rotation of the eyes with a change in the position of the head and a slow component of the vestibular nystagmus.

Part of the axons of the cells of the Deuterium nucleus enters into the posterior longitudinal beam. Through this associative tract connecting the posterior hypothalamic nuclei with the autonomic nuclei of the oculomotor, facial, glossopharyngeal and vagus nerves (III, VII, IX, X couples), as well as with the sympathetic and parasympathetic centers of the spinal cord, autonomic reactions are realized with irritation of the vestibular apparatus.
The humoral vestibular reactions of the hypothalamic-pituitary-adrenal system are triggered, probably due to the vestibulo-thalamo-hypothalamic connections.

The auditory part of the membranous labyrinth is represented by the cochlear duct, ductus cochlearis, located in the bone cochlea. The webbed snail is a tube communicating with the sacculus and ending in the apical curl with a blind protrusion (Fig. 1.1.8).

The cochlear duct does not completely fill the lumen of the bone cochlea. On the cross section of the bone cochlea (Fig. 1.1.13), three spaces are distinguished. As they climb the scrolls of the snail, they are called stairs. The cochlear duct corresponds to the triangular median staircase, scala media, filled with endolymph. The bottom of the duct is represented by a main or basilar membrane connecting the edge of the bony spiral plate of the modiolus with the opposite wall of the bone cochlea through a spiral ligament, lig.spirale. The upper part of this ligament, adjacent to the wall of the bone cochlea, is called the vascular strip, stria vascularis, as it is rich in blood vessels. The roof of the cochlear duct is the vestibular (Reisner) membrane, membrana vestibularis, consisting of two layers of squamous epithelial cells. The triangle of the cochlear duct divides two perilymphatic spaces or stairs. Above the cochlear duct above the Reisner membrane is the vestibule, scala vestibuli, and below the basilar membrane is the tympanic ladder, scala tympani. They communicate with each other through a hole on the top of the cochlea called helicotrema, helicotrema.

Fig. 1.1.13

On the basilar membrane located spirally along the entire course of the cochlear duct, there is a hearing organ — a spiral organ or a Corti organ, organum spirale seu organum Corti (Fig. 1.1.14). At the inner side of the Corti's organ, the periosteum of the upper surface of the bone spiral plate is thickened and forms an elevation - a spiral limb, limbus spiralis, which protrudes into the lumen of the cochlear duct. From the upper lip of the limb stretches a thin jelly-like integumentary membrane, membrana tectoria, lying above the hair cells of the Corti's organ and in contact with them. The organ of Corti consists of one row of internal hair cells, three rows of external hair cells, supporting cells, and pillar cells. Between the outer hair cells are the supporting cells of Deiters, and outside of them are the supporting cells of Hensen and Claudius. Pillar cells form a tunnel of the Corti's organ.

Fig. 1.1.14

The basilar membrane consists of 2400 transverse fibers - auditory strings. They are the longest and thickest at the top of the cochlea, and short and thinest at its base. Cochlear nerve fibers come into contact with internal (4000) and external (20,000) hair cells, which, as in the vestibular apparatus, are secondary-sensory mechanoreceptor cells with about 50 short hairs - stereocilia and one long - kinocilia. The cochlear duct hair cells are washed by a special fluid called cortilympha.

Hair cells are synaptically connected to the peripheral processes of the bipolar cells of the spiral ganglion, ganglion spirale, located in the spiral channel of the bone cochlea (I neuron). The central processes of bipolar neurons make up the cochlear root, radix cochlearis, the vestibulo-cochlear nerve (VIII), passing in the internal auditory canal of the temporal bone. In the bridge-cerebellar corner, the fibers of the cochlear root enter the substance of the brain (bridge) and end in the lateral corner of the rhomboid fossa on the cells of the ventral cochlear nucleus, nucl.cochlearis ventralis and the dorsal cochlear nucleus, nucl.cochlearis dorsalis (II neuron) (Fig. 1.1.15) .

Fig. 1.1.15

Axons from neurons of the ventral nucleus are divided into two bundles: most of them (70%) go to the opposite side of the bridge and end in the medial accessory olive kernel and trapezius, corpus trapezoideum (III neuron). A smaller part of the fibers goes to the same formations on their side. The axons of the upper olive and the nuclei of the trapezoidal body form a lateral loop, lemniscus lateralis, which rises up and ends in the lower mounds of the roof of the midbrain and in the medial cranked body, corpus geniculatum mediale (IV neuron). A part of the neurons of the ventral and dorsal cochlear nuclei enters the lateral loop, passing through the trapezoid nuclei in transit. The axons of the superior olive and the nuclei of the trapezius are subcortical auditory centers. Lower mounds play an important role in spatial hearing and the organization of orientational behavior.

The axons of the cells of the posterior trapezoidal body go to the bottom of the rhomboid fossa and at the level of the median groove plunge into the depth of the bridge and partially go to the opposite side, making up striae acusticae, and then join the lateral loop, contacting the neurons of the lower mounds of the roof of the midbrain and the medial cranked body. Thus, in the lateral loop there are auditory tracts from both ear labyrinths.

From the cells of the medial cranked body, axons pass as part of the back leg of the inner capsule, then they are sent in the form of radiatio acustica to the middle part of the superior temporal gyrus (Geschl gyrus) of the cerebral cortex (V neuron). The cortical center of hearing receives information mainly from the opposite ear. The auditory cortex has associative connections with other areas of the cerebral cortex: the posterior speech field, the visual and sensorimotor zones. The fibers connecting the right and left auditory fields of the hemispheres pass through the corpus callosum, corpus collozum, and the anterior commissure.

In addition to the sensory tract, the auditory system is represented by efferent fibers at its various levels. From the cerebral cortex and the descending bundle of nerve fibers. Shorter conductors end in the medial cranked body and lower mounds, and longer conductors end in the upper olive kernel. The olive-cochlear path, tr. olivocochlearis containing straight and crossed fibers. They reach the hair cells of the spiral organ. The axons of the cells of the nucleus of the lower mound through the integration center of the midbrain located in the upper mound, through the tympanospinal and nuclear tibial tracts go to the motor nuclei of the anterior horns of the spinal cord, cranial nerves (facial, abducent) and reticular formation. Thanks to these conductors, unconditionally reflex motor reactions of the muscles of the body, limbs, head and eyes to sudden sound irritations are carried out. The cells of the lower mound nucleus have connections with the cells of the motor nuclei of the trigeminal and facial nerve, which provide an accommodation-protective mechanism for the auditory muscles.

The auditory system provides the perception of sound vibrations, the conduct of nerve impulses to the auditory nerve centers, the analysis of information and the organization of effector adaptive and protective reactions.

Maze blood supply. The arteries of the inner ear come from the artery of the labyrinth, a.labyrinthi (r.meatus acustici interni), which is a branch of the main artery, a.basilaris (Fig. 1.1.16). The artery of the labyrinth, entering together with the vestibulo-cochlear nerve into the internal auditory canal, is divided into three branches: the vestibular, cochlear, and the vestibular cochlear artery.

Fig. 1.1.16

The vestibular artery, a.vestibularis, feeds the posterior part of the sac and the posterior part of the uterus, together with ampoules and legs of the semicircular canals. The cochlear artery, a.cochlearis, is designed to feed the cochlea. The vestibular-cochlear artery, a.vestibulocochlearis, anastomoses with the cochlear artery in the region of the main curl, and also supplies blood to the lower part of the cochlea and the posterior-lower part of the vestibule with the corresponding legs of the semicircular canals.

The veins of the vestibule and semicircular canals collect in the vein of the vestibule, v.aqueductus vestibuli, flowing into the transverse sinus, sinus transversus or sigmoid sinus, sinus sigmoideus. The veins of the cochlea flow into the vein of the cochlear canal, v. Canaliculi cochleae, flowing into the lower stony sinus, sinus petrosus inferior.
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INNER EAR

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