and
vestibule, viewed from above. The labyrinth can be divided by layer or by region.
Bony and membranous labyrinths The
bony labyrinth, or osseous labyrinth, is the network of passages with bony walls lined with
periosteum. The three major parts of the bony labyrinth are the
vestibule of the ear, the
semicircular canals, and the
cochlea. The
membranous labyrinth runs inside of the bony labyrinth, and creates three parallel fluid filled spaces. The two outer are filled with
perilymph and the inner with endolymph.
Vestibular and cochlear systems In the
middle ear, the energy of
pressure waves is translated into mechanical vibrations by the three auditory ossicles. Pressure waves move the tympanic membrane which in turns moves the malleus, the first bone of the middle ear. The malleus articulates to incus which connects to the stapes. The footplate of the stapes connects to the oval window, the beginning of the inner ear. When the stapes presses on the oval window, it causes the perilymph, the liquid of the inner ear, to move. The middle ear thus serves to convert the energy from sound pressure waves to a force upon the perilymph of the inner ear. The oval window has only approximately 1/18 the area of the tympanic membrane and thus produces a higher
pressure. The cochlea propagates these mechanical signals as waves in the fluid and membranes and then converts them to nerve impulses which are transmitted to the brain. The vestibular system is the region of the inner ear where the semicircular canals converge, close to the cochlea. The vestibular system works with the visual system to keep objects in view when the head is moved. Joint and muscle receptors are also important in maintaining balance. The brain receives, interprets, and processes the information from all these systems to create the sensation of balance. The vestibular system of the inner ear is responsible for the sensations of balance and motion. It uses the same kinds of fluids and detection cells (
hair cells) as the cochlea uses, and sends information to the brain about the attitude, rotation, and linear motion of the head. The type of motion or attitude detected by a hair cell depends on its associated mechanical structures, such as the curved tube of a semicircular canal or the calcium carbonate crystals (
otolith) of the
saccule and
utricle.
Development The human inner ear develops during week 4 of
embryonic development from the
auditory placode, a thickening of the
ectoderm which gives rise to the
bipolar neurons of the
cochlear and
vestibular ganglions. As the auditory placode invaginates towards the embryonic
mesoderm, it forms the auditory vesicle or
otocyst. The
auditory vesicle will give rise to the utricular and saccular components of the
membranous labyrinth. They contain the sensory hair cells and
otoliths of the
macula of utricle and
of the saccule, respectively, which respond to
linear acceleration and the force of
gravity. The utricular division of the auditory vesicle also responds to
angular acceleration, as well as the
endolymphatic sac and
duct that connect the saccule and utricle. Beginning in the fifth week of development, the auditory vesicle also gives rise to the
cochlear duct, which contains the spiral
organ of Corti and the
endolymph that accumulates in the membranous labyrinth. The
vestibular wall will separate the cochlear duct from the perilymphatic
scala vestibuli, a cavity inside the cochlea. The
basilar membrane separates the cochlear duct from the
scala tympani, a cavity within the cochlear labyrinth. The lateral wall of the cochlear duct is formed by the
spiral ligament and the
stria vascularis, which produces the
endolymph. The
hair cells develop from the lateral and medial ridges of the cochlear duct, which together with the
tectorial membrane make up the organ of Corti.
Microanatomy showing the
organ of Corti. Rosenthal's canal or the spiral canal of the cochlea is a section of the bony labyrinth of the inner ear that is approximately 30 mm long and makes 2¾ turns about the
modiolus, the central axis of the cochlea that contains the
spiral ganglion. Specialized inner ear cell include: hair cells, pillar cells, Boettcher's cells, Claudius' cells, spiral ganglion neurons, and Deiters' cells (phalangeal cells). The hair cells are the primary auditory receptor cells and they are also known as auditory sensory cells, acoustic hair cells, auditory cells or cells of Corti. The
organ of Corti is lined with a single row of inner hair cells and three rows of outer hair cells. The hair cells have a hair bundle at the apical surface of the cell, which consists of an array of actin-based stereocilia. Each stereocilium inserts as a rootlet into a dense filamentous actin mesh known as the cuticular plate. Disruption of these bundles results in hearing impairments and balance defects. Inner and outer pillar cells in the organ of Corti support hair cells. Outer pillar cells are unique because they are free standing cells which only contact adjacent cells at the bases and apices. Both types of pillar cell have thousands of cross linked
microtubules and
actin filaments in parallel orientation. They provide mechanical coupling between the
basement membrane and the
mechanoreceptors on the hair cells.
Boettcher's cells are found in the organ of Corti where they are present only in the lower turn of the cochlea. They lie on the basilar membrane beneath Claudius' cells and are organized in rows, the number of which varies between species. The cells interdigitate with each other, and project
microvilli into the intercellular space. They are supporting cells for the auditory hair cells in the organ of Corti. They are named after German pathologist
Arthur Böttcher (1831–1889).
Claudius' cells are found in the organ of Corti located above rows of Boettcher's cells. Like Boettcher's cells, they are considered supporting cells for the auditory hair cells in the organ of Corti. They contain a variety of
aquaporin water channels and appear to be involved in ion transport. They also play a role in sealing off endolymphatic spaces. They are named after the German anatomist
Friedrich Matthias Claudius (1822–1869).
Deiters' cells (phalangeal cells) are a type of
neuroglial cell found in the organ of Corti and organised in one row of inner phalangeal cells and three rows of outer phalangeal cells. They are the supporting cells of the hair cell area within the cochlea. They are named after the German pathologist Otto Deiters (1834–1863) who described them.
Hensen's cells are high columnar cells that are directly adjacent to the third row of Deiters' cells.
Hensen's stripe is the section of the tectorial membrane above the inner hair cell.
Nuel's spaces refer to the fluid-filled spaces between the outer pillar cells and adjacent hair cells and also the spaces between the outer hair cells.
Hardesty's membrane is the layer of the tectoria closest to the reticular lamina and overlying the outer hair cell region.
Reissner's membrane is composed of two cell layers and separates the scala media from the scala vestibuli.
Huschke's teeth are the tooth-shaped ridges on the spiral limbus that are in contact with the tectoria and separated by interdental cells.
Blood supply The bony labyrinth receives its blood supply from three arteries: 1 – Anterior tympanic branch (from maxillary artery). 2 – Petrosal branch (from middle meningeal artery). 3 – Stylomastoid branch (from posterior auricular artery). The
membranous labyrinth is supplied by the
labyrinthine artery. Venous drainage of the inner ear is through the labyrinthine vein, which empties into the
sigmoid sinus or
inferior petrosal sinus. ==Function==