Internal Ear structure and function


Internal ear or labyrinth is a membranous structure, enclosed by a bony labyrinth in petrous part of temporal bone. It consists the sense organs of hearing and equilibrium. Sense organ for hearing is the cochlea and the sense organ for equilibrium is the vestibular apparatus.

Internal Ear structure and function


Cochlea is a coiled structure like a snail’s shell (cochlea = snail’s shell). It consists of two structures:

1. Central conical axis formed by spongy bone called modiolus

2. Bony canal or tube, which winds around the modiolus.

In man, the bony canal makes two and a half turns, starting from the base of the cochlea and ends

at the top (apex) of cochlea. End of the canal is called cupula. Base of modiolus forms the bottom of internal auditory meatus, through which cochlear nerve fibers pass and enter the modiolus. Thus, a section through the axis of cochlea reveals the central bony pillar, modiolus and periotic or osseous canal, which coils around the modiolus. From modiolus, a bony ridge called osseous spiral

lamina projects into the canal, winding around modiolus like the thread of a screw. Spiral lamina follows the spiral turns of cochlea and ends at the cupula in a hookshaped process called hamulus.


Two membranous partitions extend between the osseous spiral lamina and outer wall of the spiral

canal. Both the membranes divide the spiral canal of cochlea into three compartments.

Membranes of cochlea:

1. Basilar membrane

2. Vestibular membrane.

1. Basilar Membrane

Basilar membrane is a connective tissue membrane. It stretches from the tip of the osseous spiral lamina

to tough dense fibrous band called spiral ligament, which lines the outer wall of the canal. Basilar membrane membrane is also called membranous spiral lamina. Along the basilar membrane are twenty thousand to thirty thousand tiny fibers that are called basilar fibers. Each fiber has different size and shape. Fibers near the oval window are short and stiff. While approaching towards helicotrema the basilar fibers gradually become longer and soft.

2. Vestibular Membrane

Vestibular membrane is also known as Reissner membrane and it is a thin membrane. It is placed

obliquely between the upper surface of osseous spiral lamina and upper part of spiral ligament.

Basilar membrane and vestibular membrane divide the spiral canal of cochlea into three compartments

called scalae .

Compartments of spiral canal of cochlea:

i. Scala vestibuli

ii. Scala tympani

iii. Scala media.

All the three compartments are filled with fluid. Scala vestibuli and scala tympani contain perilymph.

Scala media is filled with endolymph.

i. Scala vestibuli

Scala vestibuli lies above scala media. It arises from oval window (fenestra vestibuli), which is closed by the footplate of stapes. It follows the bony canal up to its apex. At the apex, it communicates with the scala tympani through a small canal called helicotrema.

ii. Scala tympani

Scala tympani lies below scala media. It is parallel to scala vestibuli and ends at the round window. Round window is closed by a strong thin membrane known as secondary tympanic membrane.

iii. Scala media

Scala media is otherwise called cochlear duct, membranous cochlea or otic cochlea. It is a triangular

compartment enclosed by basilar and vestibular membranes. It ends blindly at the apex and at the base

of cochlea. A slender ductus reuniens arises from the basal end and connects scala media with the saccule of otolith organ. Scala media is formed by upper, outer and lower walls. Upper wall or vestibular wall is formed by vestibular membrane. Outer wall is formed by spiral ligament, which is the thickening of periosteum. Lower wall is called tympanic wall. It is formed by basilar membrane (membranous spiral lamina) and a part of osseous spiral lamina. Scala media stretches between the tip of osseous spiral lamina and spiral ligament. Basilar membrane consists of straight unbranched connective tissue fibers, which are called basilar fibers or the auditory fibers. On the upper surface of the basilar membrane, epithelial cells are arranged in the form a special structure called the organ of Corti. It is the sensory part of cochlea.

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