Anatomy of Inner ear
Introduction:
Inner ear anatomy in mammals is rather unique. It consists of the bony and membranous labyrinth. It is composed of two main functional parts:
1. Cochlea - Endorgan dedicated to hearing
2. Vestibular system - Dedicated to balance
This type of inner ear is found in all vertebrates and is innervated by the 8th cranial nerve.
The inner ear is embedded within the petrous portion of the temporal bone. It lies anterolateral to the posterior cranial fossa, wtih the medial wall of the middle ear (promotory) forming its lateral wall.
Bony labyrinth:
This is actually bony covering over the membranous labyrinth. The walls of the bony labyrinth are continuous with that of the surrounding temporal bone. The inner contours of the bony labyrinth more or less follows the contours of the membranous labyrinth. It can be subdivided into:
Vestibule
Semicircular canals (3 in number)
Cochlea
The bony walls of the labyrinth consists of dense bone everywhere except for 2 small areas near the base of the cochlea. The first is the round window which consists of a thin membranous partition that separates the perilymph of cochlear chambers from the air filled middle ear cavity. The other area is known as the oval window which is closed in life by the foot plate of stapes. Collagen fibers arising from the rim of the oval window connects to the foot plate of stapes.
Perilymph:
This fluid closely resembles CSF in its composition and flows between the bony and membranous labyrinths. Membranous labyrinth contains fluid which is known as the endolymph which is rich in potassium. Ionic concentration of endolymph differs from that of other body fluids.
Semicircular canals:
These bony canals are three in number (superior, lateral and posterior) enclose semicircular canal ducts. Receptors located at the dilated ampullated ends of semicircular canal ducts contains receptors that are stimulated by head rotation. The fluid filled chambers within the vestibule are continuous with that of semicircular canals.
These bony structures project from the vestibule in three different directions:
1. Superior / anterior semicircular canal: Projects vertically and in a perpendicular direction relative to the petrous bone.
2. Posterior semicircular canal: also projects vertically, along the vertical axis of the petrous bone
3. Lateral semicircular canal: projects horizontally and laterally with respect to the petrous bone.
At one end of each canal is an enlargement known as the amupulla. The ampullae of all three scc's open into the vestibule independently. Non ampullated ends ends of anterior and posterior canals fuse to form the common bony crus. The non ampullated end of the lateral canal is known as the simple bony crus.
The membranous portion i.e semicircular ducts occupy the semicircular canals are smaller than the bony canals. But, at the ampulla the membranous portion enlarges to occupy the entire space of the bony ampullae. The three ampullated ends and two non ampullated ends of the membranous semicircular ducts open into the utricle. The medial aspect of each ampullated end of the SCC ducts has a transverse crest which houses a specialized sensory neuroepithelium which is sensitive to angular acceleration of the head.
Cochlea:
This is a bony, and spiral shaped chamber containing the cochlear duct of the membranous labyrinth. The sense of hearing is perceived by receptors within the cochlear duct. A pair of perilymph filled chambers are found on each side of cochlear duct (scala tympani and scala vestibuli). The entire bony cochlea has about two and half turns around a central hub known as the modiolus resembling a snail shell. A thin bony ledge known as the spiral lamina projects from the modiolus into the cochlear canal. The membranous portion of the cochlea (cochlear duct) follows the spirals of the bony cochlea and terminates deep to the cochlear cupula. The cochlear duct is thinner than the bony cochlear canal, and it divides the cochlear canal into two parts i.e. stacla tympani which is found on the basal part and the scala vestibuli which is located on the apical region. The scala tympani communicates with scala vestibuli at the apex of the modiolus, and this region is known as the helicotrema. The distal end of the cochlear canal drains its perilymph via the cochlear aqueduct and the cochlear canaliculus into the subarachnoid space just inferior to the internal acoustic meatus in the anterior segment of the jugular foramen.
Cross section of the cochlea:
Cross sectional studies invovling the cochlea will reveal three spaces. The osseus spiral lamina projects horizontally into the cochlear aqueduct. A membranous band i.e. Reissner's membrane travels obliquely from the apical surface of the spiral lamina to the bony cochlea. Another membrane i.e. Basilar membrane extends horizontally from the apex of the spiral lamina to the osseus cochlea. Both the Reissner's membrane and basilar membrane are connected along the osseous cochlea by the spiral ligament.
The cochlear canal is covered by endosteum. It communictes with the middle ear cavity via fenestra cochlea (round window) which is covered by secondary tympanic membrane in life. The cochlear canal houses the membranous cochlear duct. Endolymph enters the cochlear duct from the saccule via the ductus reuniens.
Vestibule:
This is a hollow caviy located between the cochlea and the semicircular canals. It is located medial to the tympanic cavity, posterior to the cochlea and anterior to semicircular canals. Vestibule contains three recesses:
1. Elliptical recess: This recess lies closer to the ampullae of the superior and lateral semicircular canals
2. Cochlear recess: This recess is adjacent to the cochlea
3. Spherical recess: This recess lie adjacent to the opening of scala vestibuli
Six orifices open into the vestibule. Five of them belong to the semicircular canals and one from the scala vestibuli of the cochlea. There are smaller openings on the side adjacent to the internal acoustic meatus which serve as conduits for the 8th cranial nerve.
The vestibule communicates with the middle ear via the fenestra vestibuli (oval window). It is covered in life by the foot plate of stapes.
Membranous contents of vestibule include the saccule and utricle. The saccule is a small fibrous pouch that communicates inferiorly with the cochlear duct via the ductus reuniens. The saccule resides in the antero inferior compartment of the vestibule. The utricle is also a fibrous sac which communicates freely with the semicircular ducts. It occupies the posterior superior portion of the vestibule.
Close to the point at which the walls of both sacs abut, each structure gives off a small duct that fuses to form a Y shaped structure. The long stem of the Y is the endolymphatic duct that traverses the vestibular aqueduct and terminates as the endolymphatic sac deep to the dura mater in the posterior cranial fossa.
Membranous labyrinth is a closed system, and endolymph is removed from the ducts by reabsorption via the epithelia of the endolymphatic sac.
Arterial supply:
Supplied by labyrinthine artery which is a branch of basilar artery.
Stylomastoid artery a branch of the posterior auricular or occipital branches of external carotid artery provides supplementary blood supply.
Venous supply:
Occurs via the labyrinthine veins which travel through the internal acoustic meatus to drain their contents in to the inferior and superior petrosal sinuses.
Inner ear is devoid of lymphatics.
