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Inner Ear Anatomy- Development and Relationship

Development

It starts in the 3rd week of the intrauterine life and is completed by the 16th week of the intrauterine life.
Membranous labyrinth develops from the otic capsule. This differentiates into various structures, like sensory end organ of hearing and equilibrium.
Bony labyrinth develops from the otic capsule.

This is a mesenchymal condensation surrounding the membranous labyrinth. Soon this is converted into cartilage. Between the cartilage and the labyrinth is loose periotic tissue. This tissue disappears around the utricle and saccule to form the vestibule. It also disappears around the semi­ circular ducts to form the semicircular canals.
In the cochlea two spaces are formed on either side of the cochlear duct known as scala vestibuli and scala tympani.

Anatomy

The inner ear is well protected and lies inside the petrous temporal bone, between the medial wall ‘ of the middle ear and the internal auditory canal. It is composed of:
1. The bony labyrinth has a central part called bony vestibule which is connected anteriorly to the bony cochlea and posteriorly to the three bony semicircular canals.
2. The membranous labyrinth has the same named structures as the bony labyrinth which floats on the perilymph and itself has endolymph .

Anatomy of the Bony Labyrinth
Divided into 3 parts:
(a) Bony vestibule
(b) Semicircular canals.
(c) Cochlea
(a) Bony Vestibule

(a)Vestibule is the central part of the bony labyrinth and is compared to a standard aspirin tablet (5 mm). It lies between the medial wall of the middle ear and lateral to the internal acoustic meatus, anterior to the semicircular canal and posterior to the cochlea.
On the lateral wall of the vestibule there is a bean-shaped opening called fenestra vestibulae (oval window) occupied by the footplate of the stapes and surrounded by annular ligament. On the front half of the medial wall there is a marked depression called spherical recess. This is a space for saccule. This wall is perforated by minute holes called maculae cribrosa media for passage of the inferior vestibular nerve filaments. Behind is another depression called the elliptical recess containing utricle. The two are separated by a crest, the anterior end being the estibu1ar pyramid.

Vestibular crest splits posteriorly to enclose the cochlear recess for cochlear nerve filaments. The pyramid and elliptical recess are perforated by small holes called macula cribrosa superior also called Mike’s dot, (It is an important landmark in translabyrinthine approach) for nerves to utricle and ampulla of superior and lateral semicircular canals respectively. Below the elliptical recess, there is a diverticulum called aqueduct of vestibule plugged in life by the endo- lymphatic duct and one or two small veins.

(b) Semicircular Canals:

They are three in number:

1. Superior
2. Posterior or vertical
3. Lateral semicircular canals.

Each occupies two-thirds of a circle and is unequal in length. The diameter is 0.8 mm. All three canals show dilatation at one end called ampulla containing vestibular sensory epithelium.
Superior semicircular canals length 15 to 20 mm lies transverse to the bony axis of the petrous portion of temporal bone. Anterolateral end is ampullated and opens in the upper lateral part of the vestibule. The other end fuses with the superior limb of the posterior vertical canal to form crus commune, 4 mm length, which opens in the medial part of the vestibule.
The lateral semicircular canal projects as a rounded bulge into aditus and antrum of the middle ear cleft It is 12 to 15 mm long, lies at an angle of 30° to the horizontal plane. The ampullary end opens into the upper part of the vestibule, posterior end into the lower part below the orifice of the crus commune.

Anatomy of the Ear I 23 Posterior semicircular canal 18 to 22 mm long lies parallel and very close to the posterior surface of the petrous portion of the temporal bone. Lower ampullated limb opens into the lower part of the vestibule and the upper limb joins the crus commune. The angle formed by three semicircular canal is solid angle, whereas the triangle bounded by the bony labyrinth anteriorly, sigmoid sinus posteriorly and dura superiorly is known as Trautman’s triangle, which is a weakest part.

(c) Cochlea

Cochlea resembles a common snail. It forms the anterior portion of the bony labyrinth.
It is 5 mm from base to apex and 9 mm around its base, length of the tube is 30 mm. It is a hollow tube having 2 and three-fourth turns around a conical central axis called ‘modiolus’. The base of modiolus is directive toward internal auditory meatus and is perforated for the passage of cochlear nerve. The apex lies medial to tensor tympani muscle (internal carotid artery).

The osseous spiral lamina winds around modiolus and along the basilar membrane, it separates the scala media (cochlear duct) from scala tympani. Within this bony canal lies the membranous cochlear duct. There are three longitudinal channels within the cochlea: scala vestibuli above, scala tympani below and scala media in between. Scala vestibuli communicates with scala tympani at the apex of the cochlea called helicotrema.

Scala vestibuli is in continuity with the vestibule at the oval window closed by the stapes footplate. Scala tympani is separated from the tympanic cavity by the secondary tympani membrane at the fenestra cochlea. Central perforation leads to a foramen central in the body of modiolus, where nerves for the apex are accommodated. The nerve for the first turn and 3/4th of the cochlear tube pass through the peripheral tractus spiralis foraminosa. At once the nerves pair off towards the margin of ganglion and from here nerves communicate via the osseous spiral lamina with the organ of corti.

Anatomy of the Membranous Labyrinth
The membranous labyrinth can be broadly divided into three parts based on physiology:
1. Membranous vestibular labyrinth
2. Membranous semicircular canal
3. Membranous cochlear labyrinth
The membranous labyrinth contains endolymph and the specialized vestibular and cochlear receptors. It lies within the bony labyrinth, floating on the perilymph.

1. Membranous Vestibular Labyrinth It consists of:
• Saccule
• Utricle
• The endolymphatic duct and sac.
Saccule is connected to the cochlear labyrinth by the membranous cochlear reuniens. Saccule and utricle are connected to each other indirectly by the endolymphatic duct. Saccule occupies the spherical recess in the bony vestibule and it contains specialized vestibular epithelium. Utricle is bigger in size than the saccule and occupies the elliptical recess of the bony vestibule. The three semicircular canals open into the posterior wall of the utricle by five openings. Anteriorly, it connects to the saccule indirectly via the endolymphatic duct. It also contains specialized vestibular receptor organs.
Vestibular Receptor Organs
• Macula
• Cristae Macula
Vestibular receptor organs of the saccule and utricle are called macula. Macula of the saccule lies vertically in the medial surface of the saccule, whereas in the utricle it lies horizontally. These specialized vestibular receptor organs are composed of hair cells, supporting cells and gelatinous mass.
This gelatinous mass is composed of muco­ polysaccharides thought to be secreted by the supporting cells. Macular gelatinous mass contains additional materials made up of calcium carbonate crystals known as otolith or statoconia. Hence, the gelatinous mass is sometimes known as statoconial membrane.

2. Membranous Semicircular Canal
Cristae ampullaris: The membranous semicircular canal occupies the bony semicircular canals. It opens through the five openings into the posterior wall of the utricle. One end of each semicircular canal gets dilated before entering the utricle. The dilated part is filled in the ampullary end of the bony semicircular canal. It accommodates the specialized vestibular epithelium known as cristae. This cristae also has hair cells, supporting cells and gelatinous mass. The gelatinous mass in the cristae is dome shaped hence called cupula.
The hair cells are of two types:
Type 1 are flask shaped with nerve chalice; Type I cells are predominantly seen in the summit of the cristae.
Type 2 are cylindrical with no nerve chalice. Type 2 cells are found more towards the periphery in the cristae.
The hair cells consist of one kinocilium which is tall and prominent; many small cilia (60-110) known as steriocilia which are smaller than the kinocilium. The kinocilium in the macula are not uniformly arranged. A curve line called striola divides each macula into medial and lateral halves.

3. Membranous Cochlea
Cochlear duct (scala media): It occupies mid pertion of the cochlear canal and is triangular in cross section. Floor is formed by basilar membrane, roof by Reissner’s membrane and lateral wall by stria vascularis and bony wall of cochlea. Basilar membrane supports organ of corti, containing the sound receptors.
The thin area of basilar membrane in its inner part is called Zona Areuata, thicker outer part is Zona Peetinata.

Organ of corti is spread like a ribbon along the entire basilar membrane. It consists of the tunnel of corti which is composed of two rows of rods of inner and outer hair cells. It forms a triangle with the basilar membrane and contains cortilymph. There is one row of hair cells on the inner whereas outer row has 3 or 4 rows of hair cells. Inner rods are 3500 and outer rods are 12000. Rods are expanded like a cap on top. Sensory cells arranged in two groups as inner and outer hair cells. In the fetus and newborn, there are 3500 inner hair cells and 13000 outer hair cells. As age advances there is generalized reduction in the number of hair cells. Hair cells are supported by pillar cells, Deiter’s cells and Hensen’s cells. The tips of the outer hair cells are attached to the under surface of tectorial membrane.

Tectorial membrane: It consists of a gelatinous matrix with delicate fibers, it e organ of corti. The shearing force between the hair cells and tectorial produce stimulus to hair cells.
Lateral wall ‘stria vascularis’: It is thought to play an active role in the maintance of the ionic composition and electrical potential of the endolymph.
Roof formed by Reissner’s membrane.

Blood Supply of Inner Ear
Internal auditory artery arises from the anterior inferior cerebellar artery (AICA). It accompanies the facial and vestibulocochlear nerves in the internal acoustic meatus and usually divides into three branches to supply the inner ear:
• Anterior vestibular artery to supply the macula of utricle and crista of superior and lateral semicircular canals
• Vestibulocochlear branch to supply the posterior semicircular canal
• Cochlear branch to supply the cochlea.

AUDITORY PATHWAY
First order neurons are located in the spiral ganglion and are bipolar. Peripheral processes innervate the organ of Corti and central processes terminate in the dorsal and ventral cochlear nuclei.
Second order neurons lie in the dorsal and ventral cochlear nuclei. Most of the axons cross over in the trapezoid body and terminate in the superior olivary nucleus. Many end in the trapezoid body or lateral lemniscus and some remain uncrossed.
Third order neurons lie in the superior olivary nucleus. The axons cross from lateral lemniscus and reach the inferior colliculus.
Fourth order neurons lie in the inferior colliculus. Their axons pass through the inferior brachium to reach the medial geniculate body (some fibers directly reach the medial geniculate body).
Fifth order neuron lie in medial geniculate body. Their axons form auditory radiation which passed through the part of the internal capsule to reach auditory area in the temporal lobe.

Vestibular Pathway
Vestibular receptors are the macula of the saccule and utricle and the cristae of the ampullae. They are innervated by the peripheral processes of bipolar neurons of the vestibular ganglion which is situated in the internal acoustic meatus. The central processes form the vestibular nerve which ends in the vestibular nuclei.

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