Inner ear

Inner ear(auris interna) is located in the thickness of the pyramid of the temporal bone and is separated from the tympanic cavity by its labyrinthine wall. The inner ear consists of a bony labyrinth and a membranous labyrinth inserted into it.

The bony labyrinth (labyrinthus osseus), the walls of which are formed by the compact bone substance of the pyramid of the temporal bone, lies between the tympanic cavity on the lateral side and the internal auditory canal medially. The size of the bony labyrinth along its long axis is about 20 mm. In the bony labyrinth, a vestibule is distinguished; in front of it lies the cochlea, behind - the semicircular canals.

The vestibule is a large, irregularly shaped cavity. There are two windows on the lateral wall of the bony labyrinth. One of them is oval and opens into the vestibule. From the side of the tympanic cavity, it is closed by the base of the stapes. The second window of the cochlea is round, it opens into the beginning of the spiral canal of the cochlea and is closed by the secondary tympanic membrane. On the posterior wall of the vestibule, five small openings are visible, through which the semicircular canals open into the vestibule, and on the anterior wall there is a rather large opening leading into the cochlear canal. On the medial wall of the vestibule there is a vestibular ridge (crista vestibuli), separating two fossae from each other. The anterior fossa is rounded, called the spherical recess (recessus sphericus). The posterior fossa is elongated, lies closer to the semicircular canals - this is the elliptical recess (recessus ellipticus). The internal opening of the vestibular aqueduct (apertura interna aqueductus vestibuli - BNA) is located in the elliptical depression.

The cochlea is the anterior part of the bony labyrinth. It is a twisted spiral canal of the cochlea (canalis spiralis cochleae), forming two and a half turns around the axis of the cochlea. The base of the cochlea (basis cochleae) faces medially, towards the internal auditory canal. The apex - the dome of the cochlea (cupula cochleae) is directed towards the tympanic cavity. The axis of the cochlea, which lies horizontally, is the bony rod (modiolus). Around the rod, the bony spiral plate (lamina spiralis ossea) is wound, which does not completely block the spiral canal of the cochlea. In the area of the dome, with the help of the hook of the spiral plate (hamulus laminae spiralis), the bony plate limits the oval opening of the cochlea (helicotria). The rod is penetrated by thin longitudinal rod canals (canaies longitudinals modioli), in which the fibers of the cochlear part of the vestibulocochlear nerve are located. At the base of the bony spiral plate passes the spiral rod canal (canalis spiralis modioli), where the nervous cochlear ganglion (spiral ganglion of the cochlea) lies. At the base of the cochlea, at the beginning of the tympanic ladder, is the internal opening of the cochlear canal (apertura interna canaliculi cochleae - BNA).

The bony semicircular canals (canales semicirculares ossei) are three arcuately curved tubes lying in three mutually perpendicular planes. The width of the lumen of each bony semicircular canal in cross-section is about 2 mm.

The anterior (sagittal, superior) semicircular canal (canalis semicircularis anterior) is oriented perpendicular to the longitudinal axis of the pyramid. It lies above the other semicircular canals, and its upper point on the anterior wall of the pyramid of the temporal bone forms an arcuate elevation.

The posterior (frontal) semicircular canal (canalis semicircularis posterior) is the longest of the canals and lies almost parallel to the posterior surface of the pyramid.

The lateral (horizontal) semicircular canal (canalis semicircularis lateralis) forms a protrusion on the labyrinthine wall of the tympanic cavity - the protrusion of the lateral semicircular canal (prominentia canalis semicircularis lateralis). This canal is shorter than the other semicircular canals.

Three semicircular canals open into the vestibule through five openings. The adjacent bone crura (crura ossea) of the anterior and posterior semicircular canals merge into a common bone crus (crus osseum commune), and the remaining four crura of the semicircular canals open into the vestibule independently. One of the crura of each semicircular canal is expanded in the form of a bone ampulla (ampulla ossea) before it enters the vestibule. Therefore, such a crus is called the ampullar bone crus (crus osseum ampullarae). One of the crura of the lateral semicircular canal, which does not have an ampulla, is a simple bone crus (cnis osseum simplex) and also opens into the vestibule independently.

The membranous labyrinth (labyrinthus mibranaceus) is located inside the bony labyrinth, basically repeating its outlines. The walls of the membranous labyrinth consist of a thin connective tissue plate covered with flat epithelium. Between the inner surface of the bony labyrinth and the membranous labyrinth there is a narrow gap - the perilymphatic space (spatium perilymphaticum), filled with fluid - perilymph (perilympha). From this space, along the perilymphatic duct (ductus perilymphaticus), passing in the cochlear canal, the perilymph can flow into the subarachnoid space on the lower surface of the pyramid of the temporal bone. The membranous labyrinth is filled with endolymph, which through the endolymphatic duct (ductus endolymphaticus), passing in the aqueduct of the vestibule to the posterior surface of the pyramid, can flow into the endolymphatic sac (saccus endolymphaticus), lying in the thickness of the dura mater of the brain on the posterior surface of the pyramid.

The membranous labyrinth comprises the elliptical and spherical saccules, three semicircular ducts and the cochlear duct. The elongated elliptical saccule, or utriculus, is located in the recess of the vestibule of the same name, and the pear-shaped spherical saccule (sacculus) occupies the spherical recess. The elliptical and spherical saccules communicate with each other by means of a thin canal, the duct of the elliptical and spherical saccules (ductus utriculosaccularis), from which the endolymphatic duct departs. In its lower part, the spherical saccule passes into the connecting duct (ductus reuniens), which flows into the cochlear duct. Five openings of the anterior, posterior and lateral semicircular ducts, located in the bony semicircular canals of the same name, open into the elliptical saccule. The semicircular ducts (ductus semicirculares) are thinner than the bone canals. In places where the bone semicircular canals expand - the bone ampullae - each membranous semicircular duct has a membranous ampulla. According to the ducts, the anterior membranous ampulla (ampulla membranacea anterior), the posterior membranous ampulla (ampulla membranacea posterior) and the lateral membranous ampulla (ampulla membranacea lateralis) are distinguished.

In the elliptical and spherical saccules, as well as on the inner surface of the walls of the membranous ampullae of the semicircular ducts, there are formations covered with a jelly-like substance containing hair sensory (sensitive) cells. In the saccules, these are whitish spots (maculae): the spot of the elliptical saccule (macula utriculi) and the spot of the spherical saccule (macula sacculi). With the participation of endolymph fluctuations, static positions of the head and rectilinear movements are perceived in these spots. In the membranous ampullae of the semicircular ducts, there are ampullar ridges (cnstae ampullares) in the form of transverse folds that detect turns of the head in different directions. The hair sensory cells located in the spots and ampullar ridges have their apices facing the cavity of the labyrinth. These cells are divided into two types. Type I cells (pear-shaped cells) have a broad base to which the nerve ending is attached in the form of a cup. Type II cells (columnar cells) have a prismatic shape. On the outer surface of both types of hair cells there is a cuticle from which 60-80 hairs (stereocilia) approximately 40 μm long extend. Another type of cells are supporting cells. They are located between the sensory cells. They are characterized by the presence of a dark oval nucleus, a significant number of mitochondria and many thin cytoplasmic microvilli at the apices. The surface of the macula epithelium is covered with a statolith membrane - a special gelatinous substance containing calcium carbonate crystals (otoliths, or statoconia). The apical part of the epithelium of the ampullar ridges is surrounded by a gelatinous transparent dome shaped like a bell without a cavity (about 1 mm long).

Stimulation of the hair cells located in the maculae and combs is transmitted to the sensory endings of the vestibular part of the vestibulocochlear nerve. The bodies of the neurons of this nerve are located in the vestibular ganglion, which lies at the bottom of the internal auditory canal. The central processes of these neurons as part of the vestibulocochlear nerve are directed through the internal auditory canal into the cranial cavity, and then into the brain to the vestibular nuclei located in the area of the vestibular field (area vestibularis) of the rhomboid fossa. The processes of the cells of the vestibular nuclei (the next neuron) are directed to the nuclei of the cerebellar tent and the spinal cord, forming the vestibulospinal tract, and also enter the dorsal longitudinal fasciculus (Bechterew's fasciculus) of the brainstem. Some of the fibers of the vestibular part of the vestibulocochlear nerve go directly to the cerebellum - to the nodule (podulus), bypassing the vestibular nuclei.

The membranous labyrinth of the cochlea - the cochlear duct (ductus cochlearis) begins blindly in the vestibule, behind the confluence of the connecting duct, and continues forward inside the spiral canal of the cochlea. In the region of the apex of the cochlea, the cochlear duct also ends blindly. In cross section, it has the shape of a triangle. The outer wall of the cochlear duct (paries externus ductus cochlearis), which is a vascular strip (stria vascularis), is fused with the periosteum of the outer wall of the spiral canal of the cochlea. The vascular strip is rich in blood capillaries that participate in the formation of endolymph, which also nourishes the structures of the spiral organ.

The lower tympanic wall of the cochlear duct (spiral membrane; paries tympanicus ductus cochlearis, s. membrana spiralis) is a kind of continuation of the bony spiral plate. The sound-perceiving spiral organ of the inner ear is located on it. The third - the upper vestibular wall of the cochlear duct (vestibule membrane, Reissner's membrane) paries vestibularis cochlearis. s. membrana vestibularis) extends from the free edge of the bony spiral plate obliquely upward to the outer wall of the cochlear duct.

The cochlear duct occupies the middle part of the bony spiral canal of the cochlea and separates its lower part, the tympanic staircase (scala tympani), bordering the spiral membrane, from the upper staircase of the vestibule (scala vestibuli), adjacent to the vestibular membrane. In the area of the cochlear dome, both staircases communicate with each other by means of the cochlear opening (helicotria). At the base of the cochlea, the tympanic staircase ends at the window, closed by the secondary tympanic membrane. The staircase of the vestibule communicates with the perilymphatic space of the vestibule, the oval window of which is closed by the base of the stapes.

Inside the cochlear duct, on the spiral membrane, is located the auditory spiral organ (organum spirale; organ of Corti). At the base of the spiral organ is the basilar (main) plate (lamina basilaris) or membrane, which contains up to 2400 thin collagen fibers stretched from the free edge of the bony spiral plate to the opposite wall of the spiral canal of the cochlea. Longer (up to 500 μm) fibers are located in the area of the apex of the cochlea, short (about 105 μm) - at its base. These collagen fibers are located in a homogeneous ground substance and act as resonator strings. From the side of the scala tympani, the basilar plate is covered with flat cells of mesenchymal origin.

On the basilar plate, along the entire length of the cochlear duct, is located the sound-perceiving spiral organ. The spiral (organum spirale) of Corti consists of two groups of cells: supporting (supporting) and hair (sensory) cells that detect mechanical vibrations of the perilymph located in the scala vestibuli and in the scala tympani.

The supporting cells, internal and external, are located directly on the basement membrane. Between the internal and external supporting cells there is a narrow channel filled with endolymph - the internal (Corti's) tunnel. Through the tunnel along its entire length (along the entire spiral organ) pass non-medullar nerve fibers, which are the dendrites of the neurons of the spiral ganglion. The nerve endings of these dendrites end on the bodies of the hair sensory cells.

Sensory hair cellsare also divided into internal and external. Internal hair (sensory) epithelial cells in the amount of up to 3500 are located in one row on the supporting cells. They have a pitcher-shaped form, an expanded base, 30-60 short microvilli (stereocilia) on the apical surface covered with a cuticle. The nucleus of these cells occupies a basal position in the cytoplasm. External hair sensory cells in the amount of 12,000-20,000 also lie on the supporting cells.

Above the tops of the spikelet sensory cells of the spiral organ, along the entire length of the cochlear duct, is the tectorial membrane (membraна tectoria). This membrane is a thin, gelatinous plate that floats freely in the endolymph. The tectorial membrane consists of thin, radially oriented collagen fibers that are in a transparent, adhesive, amorphous substance.

Sound sensations in the hair sensory cells arise as a result of vibrations of the perilymph and, together with it, the spiral organ and the contact of the microvilli (stereocilia) of these cells in the tectorial membrane. Vibrations of the perilymph are caused by movements of the base of the stapes in the vestibular window and are transmitted to the basilar plate. In the vestibular scala, these vibrations spread toward the dome of the cochlea, and then through the openings of the cochlea - to the perilymph in the tympanic scala, closed at the base of the cochlea by the secondary tympanic membrane. Due to the elasticity of this membrane, a virtually incompressible fluid - the perilymph - begins to move.

Sound vibrations of the perilymph in the scala tympani are transmitted to the basilar plate (membrane), on which the spiral (auditory) organ is located, and to the endolymph in the cochlear duct. Vibrations of the endolymph and basilar plate activate the sound-perceiving apparatus, the hair (sensory, receptor) cells of which transform mechanical movements into a nerve impulse. The impulse is received by the endings of the bipolar cells, the bodies of which are located in the cochlear ganglion (spiral ganglion of the cochlea). The central processes of these cells form the cochlear part of the vestibulocochlear nerve, as part of which they are directed through the internal auditory canal to the brain, to the anterior (ventral) and posterior (dorsal) cochlear nuclei, located in the bridge in the area of the vestibular field of the rhomboid fossa. Here the impulse is transmitted to the next neuron, the cells of the auditory nuclei. The processes of the cells of the anterior (ventral) nucleus are directed to the opposite side, forming a bundle of nerve fibers called the trapezoid body (corpus trapezoideum). The axons of the posterior (dorsal) nucleus emerge on the surface of the rhomboid fossa and, in the form of cerebral stripes of the fourth ventricle, are directed to the median groove of the rhomboid fossa, then plunge into the brain matter and continue in the fibers of the trapezoid body. On the opposite side of the bridge, the fibers of the trapezoid body make a bend facing the lateral side, giving rise to the lateral loop (lemniscus lateralis). Then these fibers go to the subcortical auditory centers: the medial geniculate body (corpus geniculatum mediale) and the inferior colliculus (tubercle) of the midbrain roof plate. Some of the fibers of the auditory tract (axons of the cochlear nuclei) end in the medial geniculate body, where they transmit an impulse to the next neuron, the processes of which, having passed through the sublenticular part of the internal capsule, are directed to the auditory center (the cortical end of the auditory analyzer). The cortical center of hearing is located in the cortex of the superior temporal gyrus (in the transverse temporal gyri, or in Heschl's gyri). Here, the highest analysis of nerve impulses coming from the sound-perceiving apparatus occurs. Another part of the nerve fibers passes in transit through the medial geniculate body, and then through the handle of the inferior colliculus enters its nucleus, where it ends. From here, one of the extrapyramidal tracts (tractus tectospinalis) begins, which transmits impulses from the inferior colliculi-plate of the midbrain roof (inferior colliculi quadrigemina) to the cells of the nuclei (motor) of the anterior horns of the spinal cord.