Inner ear

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

The labyrinthus osseus, whose walls are formed by the compact bone substance of the temporal bone pyramid, lies between the drum cavity on the lateral side and the inner auditory meatus medially. The size of the bone labyrinth along its long axis is about 20 mm. In the bone labyrinth, the vestibule is distinguished; The front of it is a snail, behind it are semicircular canals.

The vestibule (vestibulum) is a cavity of large dimensions, irregular in shape. On the lateral wall of the bone labyrinth there are two windows. One of them is oval and opens on the threshold. From the side of the drum cavity it is covered by the base of the stirrup. The second window of the cochlea is round, it opens at the beginning of the spiral channel of the cochlea and is closed by the secondary eardrum. On the back wall of the vestibule you can see five small holes, which open semicircular canals in the vestibule, and on the front wall there is a rather large hole leading to the channel of the cochlea. On the medial wall of the vestibule there is a crest of the vestibula (crista vestibuli), which separates two pits from each other. The anterior fossa is rounded, called the spherical depression (recessus sphericus). The posterior fossa is elongated, it lies closer to the semicircular canals - this is an elliptical depression (recessus ellipticus). In the elliptical depression there is an internal aperture of the aqueduct of the vestibule (apertura interna aqueductus vestibuli - BNA).

The cochlea is the front part of the bone labyrinth. It is a convoluted coiled channel of the cochlea (canalis spiralis cochleae), which forms around the axis of the cochlea two and a half turns. The base of the cochleae is drawn medially, toward the internal auditory canal. The top is the dome cupula (cupula cochleae) directed towards the drum cavity. The axis of the cochlea, which lies horizontally, is the bone marrow (modiolus). A bone spiral plate (lamina spiralis ossea), which does not completely block the coiled channel of the cochlea, is wound around the core. In the area of the dome with the help of a hook of a spiral plate (hamulus laminae spiralis), the bone plate confines the oval hole of the cochlea (helicotria). The stem is penetrated by thin longitudinal channels of the rod (canaies longitudinals modioli), in which the fibers of the cochlear portion of the pre-collateral nerve cochlea are located. At the base of the bone spiral plate, the spiral canal of the rod (canalis spiralis modioli) passes, where the nervous cochlear node (spiral node of the cochlea) lies. At the base of the cochlea, at the beginning of the tympanic staircase, is the inner opening of the cochlear duct (apertura interna canaliculi cochleae - BNA).

Bone semicircular canals (canales semicirculares ossei) are three arcuate curved tubes lying in three mutually perpendicular planes. The width of the lumen of each bone semicircular canal in a transverse 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 other semicircular canals, and its upper point on the front wall of the pyramid of the temporal bone forms an arcuate elevation.

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

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

Three semicircular canals open in the vestibule with five holes. Neighboring bone legs (crura ossea) of the anterior and posterior semicircular canals merge into the common bone leg (crus osseum commune), and the remaining 4 legs of the semicircular canals open on the vestibule independently. One of the legs of each semicircular canal before its entry into the vestibule is expanded in the form of a bone ampulla (ampulla ossea). Therefore, this leg was called the ampullar bone leg (crus osseum ampullarae). One of the legs of the lateral semicircular canal, which does not have an ampoule, is a simple bone leg (cnis osseum simplex) also opens on the threshold itself.

The membranous labyrinthus mibranaceus is located inside the bone, basically repeating its outlines. The walls of the membranous labyrinth consist of a thin connective tissue plate covered with a flat epithelium. Between the inner surface of the bone labyrinth and the membranous labyrinth is a narrow slit - a perilymphatic space (spatium perilymphaticum) filled with a liquid - perilymph (perilympha). From this space along the perilymphatic duct (ductus perilymphaticus) passing through the cochlea of the cochlea, 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 endolympha (endolympha), which through the endolymphatic duct (ductus endolymphaticus), passing in the aqueduct of the vestibule to the back surface of the pyramid, can flow into the endolymphatic sac (saccus endolymphaticus), which lies in the thickness of the hard shell of the brain on the back surface of the pyramid.

In the membranous labyrinth, elliptical and spherical sacs are distinguished, three semicircular ducts and a cochlear duct. The oblong elliptical sac, or the mantle (utriculus), is located in the eponymous pit of the vestibule, and the pear-shaped spherical sac (sacculus) occupies a spherical depression. Elliptical and spherical sacs communicate with each other by means of a thin tubule, a duct of elliptical and spherical sacs (ductus utriculosaccularis), from which the endolymphatic duct extends. In its lower part, the spherical sac passes into the connecting duct (ductus reuniens), which flows into the cochlear duct. In the elliptical sac, five openings of anterior, posterior and lateral semicircular ducts open in the same semicircular canal bones. The semicircular ducts (ductus semicirculares) are thinner than the bone channels. In the places of enlargement of the bone semicircular canals - bone ampullae, each membranous semicircular duct has a membranous ampulla. Accordingly, the ducts distinguish the anterior membranous ampulla (ampulla membranacea anterior), the posterior membranous ampulla (ampulla membranacea posterior) and the lateral membranous ampulla (ampulla membranacea lateralis).

In the elliptical and spherical sacs, as well as on the inner surface of the walls of membranous ampullae of semicircular ducts, there are gel-like substance-containing formations containing hair sensory (sensitive) cells. In the pouches, this is an off-white spot (maculae): a spot of the elliptical sac (macula utriculi) and a spot of a spherical sac (macula sacculi). With the participation of oscillations of the endolymph in these spots, the static positions of the head and rectilinear movements are perceived. In the membranous ampullas of the semicircular ducts there are in the form of transverse folds the ampullar scallops (cnstae ampullares), catching the turns of the head in various directions. The hair sensory cells, which are in spots and ampullar scallops, with their apices facing the cavity of the labyrinth. These cells are divided into two types. Type I cells (pear-shaped cells) have a wide base, to which a nerve ending adjoins in the form of a bowl. 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) with a length of approximately 40 μm leave . Another variety of cells is the 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 a lot of fine cytoplasmic microvilli on the tops. The surface of the macular epithelium is covered with a statolithic membrane - a special gelatinous substance containing crystals of calcium carbonate (otoliths, or statoconia). The apical part of the epithelium of the ampullar scallops is surrounded by a gelatinous transparent dome shaped like a bell without a cavity (about 1 mm long).

Irritations of the hair sensory cells present in spots and scallops are transmitted to the sensitive endings of the vestibule of the pre-collateral nerve cochlear. The bodies of the neurons of this nerve are in the vestibule node, which lies at the bottom of the internal auditory canal. The central processes of these neurons in the pre-collar nerve conduction are guided through the internal auditory canal into the cranial cavity, and then into the brain to the vestibular nuclei lying in the area of the vestibular field of the rhomboid fossa. The processes of the cells of the vestibular nuclei (the next neuron) are directed to the nuclei of the marrow and the spinal cord, forming the pre-spinal cord, and also enter the dorsal longitudinal fascicle (Bechterew's bundle) of the brain stem. Part of the fibers of the vestibule of the pre-collar nerve is directed directly to the cerebellum - to the nodule (podulus), bypassing the vestibular nuclei.

The membranous snail labyrinth - the cochlear duct (ductus cochlearis) begins blindly on the eve, behind the confluence of the connecting duct, and continues forward inside the spiral channel of the cochlea. In the area of the tip of the cochlea, the cochlear duct also ends blindly. On the cross-section it has the form of a triangle. The outer wall of the cochlear duct (paries externus ductus cochlearis), which is a vascular stria (stria vascularis), is fused to the periosteum of the outer wall of the cochlear spiral canal. The vascular band is rich in blood capillaries involved in the formation of the endolymph, which also nourishes the structures of the spiral organ.

The lower drum wall of the cochlear duct (spiral membrane, paries tympanicus ductus cochlearis, s. Membrana spiralis) is, as it were, an extension of the bone spiral plate. On it is located the sound-receiving spiral organ of the inner ear. The third is 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 osseous spiral plate obliquely upward to the outer wall of the cochlear duct.

The cochlear duct occupies the middle part of the cochlear spiral canal of the cochlea and separates the lower part of it - the scala tympani, bordering the spiral membrane, from the upper scala vestibuli adjacent to the vestibule. In the area of the dome of the cochlea, both ladders communicate with each other with the help of a snail (helicotria) opening. At the base of the snail, the drum staircase ends at the window closed by the secondary eardrum. 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 stirrup.

Inside the cochlear duct, on the spiral membrane, is the auditory spiral organ (organum spirale; corti organ). At the base of the spiral organ lies a basilar lamina basilaris or membrane that contains up to 2,400 thin collagen fibers stretched from the free edge of the bone spiral plate to the opposite wall of the cochlear spiral canal. Longer (up to 500 μm) fibers are in the area of the tip of the cochlea, short (about 105 μm) - at its base. These collagen fibers are arranged in a homogeneous main substance and act as string-resonators. From the side of the tympanic staircase, the basilar plate is covered with flat cells of mesenchymal origin.

On the basilar plate, along the entire length of the cochlear duct, there is a sound-receiving spiral organ. The spiral organ (organium spirale) consists of two groups of cells: supporting (supporting) and hair (sensory) cells that capture the mechanical vibrations of the perilymph, located in the staircase of the vestibule and in the tympanic staircase.

Supporting cells , internal and external, are located directly on the basement membrane. Between the inner and outer supporting cells there is a narrow channel filled with the endolymph - the inner (corti) tunnel. Through the tunnel, along its entire length (along the entire spiral organ), there pass uncarried nerve fibers, which are dendrites of the neurons of the spiral node. The nerve endings of these dendrites terminate on the bodies of the hair sensory cells.

Hair sensor cells are also divided into internal and external. Internal hair (sensory) epithelial cells in an amount up to 3500 are located in a row on the supporting cells. They have a jug-shaped shape, an enlarged base, 30-60 short microvilli (stereocilia) on the apical surface covered with 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 peaks of the sparial sensory cells of the spiral organ, over the entire length of the cochlear duct, a cover membrane (membrape tectoria) is located. This membrane is a thin jelly-like consistency of the plate, floating freely in the endolymph. The cover membrane consists of thin, radially oriented collagen fibers located in a transparent glueing amorphous substance.

Sound sensations in the hair sensory cells arise as a result of the oscillation of the perilymph and along with it the spiral organ and the contact of the microvilli (stereocilia) of these cells in the cover membrane. The perilymph oscillations are caused by motions of the base of the stirrup in the window of the vestibule and are transmitted to the basilar plate. In the staircase of the vestibule these oscillations extend to the side of the dome of the cochlea, and then through the holes of the cochlea to the perilymph in the tympanic staircase closed at the base of the cochlea by the secondary tympanic membrane. Due to the elasticity of this membrane, practically incompressible fluid - perilymph - comes into motion.

The sound vibrations of the perilymph in the tympanic staircase are transmitted to the basilar plate (membrane) on which the spiral (auditory) organ is located, and the endolymph in the cochlear duct. The vibrations of the endolymph and basilar plate activate the sound-receiving apparatus, the hair (sensory, receptor) cells of which transform mechanical movements into a nerve impulse. The impulse is perceived by the endings of bipolar cells, the bodies of which lie in the cochlear node (spiral node of the cochlea). The central processes of these cells form the cochlear part of the pre-collateral nerve, which is directed through the internal auditory canal to the brain, to the anterior (ventral) and posterior (dorsal) cochlear nuclei located in the bridge in the region of the vestibular field of the rhomboid fossa. Here, the impulse is transferred 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). Axons of the posterior (dorsal) nucleus come to the surface of the rhomboid fossa and in the form of the cerebral strips of the IV ventricle they go to the midline of the rhomboid fossa, then they sink into the substance of the brain and continue in the fibers of the trapezoidal body. On the opposite side of the bridge, the fibers of the trapezoidal body make a bend facing laterally, giving rise to the lateral loop (lemniscus lateralis). Further, these fibers follow the subcortical centers of hearing: the medial geniculate corpus (corpus geniculatum mediale) and the lower hillock (hillock) of the midbrain roof plate. Part of the fibers of the auditory path (axons of cochlear cores) ends in the medial geniculate body, where it transmits an impulse to the next neuron, the processes of which, passing through the subclavian part of the inner capsule, are directed to the auditory center (cortical end of the auditory analyzer). The cortical center of hearing is in the cortex of the upper temporal gyrus (in the transverse temporal convolutions, or in the convolutions of Geshel). Here there is a higher analysis of the nerve impulses coming from the sound-receiving apparatus. Another part of the nerve fibers passes through the medial geniculate body, and then through the handle of the lower mound enters its nucleus, where it ends. From here begins one of the extrapyramidal tracts (tractus tectospinalis), which transmits impulses from the lower hillocks of the midbrain roof (the lower tubercles of the quadruple) to the cells of the nuclei (motor) of the anterior horns of the spinal cord.