Anatomy of the Cochlea

Cochlea

• The cochlea is bounded by the basilar membrane (floor), vestibular membrane (roof), and stria vascularis (outer wall).
• The organ of Corti lies on the basilar membrane and is responsible for hearing.
• It bathes in endolymph.

Organ of Corti

• Consists of supporting cells (outer and inner pillar cells, outer and inner phalangeal cells, border cells, and Hansen's cells) and sensory (hair) cells (inner and outer hair cells).
• Inner hair cells are flask-shaped, while outer hair cells are cylindrical.
• Each cell has apical stereocilia that receive hearing impulses and basal parts that synapse with the dendrites of bipolar nerve cells.

Tectorial Membrane

• A jelly-like membrane overhanging the organ of Corti, in contact with the stereocilia of the hair cells.

Mechanism of Hearing

• Sound waves vibrate the eardrum, stapes bone, and oval window, transmitting vibrations to the basilar membrane.
• This movement causes stereocilia to bend, exciting nerve endings and generating a nerve impulse.

Vestibule

• Part of the inner ear.
• Contains the membranous labyrinth (saccule and utricle).

Membranous Labyrinth

• Contains endolymph and is lined with simple squamous epithelium.
• The epithelium is modified to form maculae sacculi and utriculi.

Maculae Sacculi and Utriculi

• Neuro-epithelial thickenings in the walls of the saccule and utricle.
• Consists of supporting columnar cells and sensory (hair) cells (type I and type II).
• The apex of hair cells has a kino-cilium and many stereocilia.

Otolithic Membrane

• A gelatinous membrane covering the macula, containing calcium carbonate crystals.
• The stereocilia and kino-cilia are embedded in it.

Function of Maculae Sacculi and Utriculi

• Detects linear movements of the head.

Semicircular Canals

• Part of the inner ear, consisting of three membranous semicircular ducts.
• Contains endolymph and is lined with simple squamous epithelium.

Crista Ampullaris

• A neuro-epithelial structure in the wall of the ampulla of the semicircular ducts.
• Detects angular movements of the head.