Cochlear duct

The cochlear duct of the membranous labyrinth (also termed the Reissner's canal or scala media, Latin: ductus cochlearis) is an endolymph-containing membranous tube within the cochlea situated between the scala vestibuli and scala tympani, bounded by the basilar membrane and the Reissner's membrane. The cochlear duct contains the organ of Corti, which is located on its lower wall and plays an essential role in the sense of hearing. Thus, the cochlear duct completely follows the anatomy of the bony labyrinth.

Structure of cochlear duct

Looking from aside, the cochlear duct has a triangular shape with three walls. Within the spiral membrane are hearing receptors or the organ of the Corti made of specific vibration catching cells. The cochlear duct has three walls:

• The floor or the basilar membrane - it separates the perilymph of the scala tympani from the endolymph in the bony cochlea. The spiral ligament of the cochlear duct is connected to the spiral lamina by the basilar membrane. This connection allows complete separation between the scala tympani and the scala vestibule;
• The outer wall of the spiral ligament - it is made of thick periosteum. Together with the other two walls, the spiral ligament encloses the cochlear duct;
• The roof or the vestibular membrane - it separates the endolymph from the perilymph in the scala vestibuli. The vestibular membrane starts from the modiolus and ends at the lateral wall of the cochlear duct.

Organ of Corti

Inside the cochlear duct's basilar membrane is the spiral organ or the organ of Corti. It contains mechanoreceptors called hair cells. The hair cells are positioned in three rows of outer hair cells (small and cylindrical) and one row of inner hair cells (pear-shaped). The hair cells are surrounded by Deiters cells, also known as the phalangeal cells (closely related to outer hair cells) and pillar cells (separate both types of hair cells).

The pillar cells make the most extensive support of this structure. The phalangeal cells are located on the thin outer portion of the spiral limbus. This thin outer portion is known as the tympanic lip. Three types of other cells are also found between the hair cells and the phalangeal cells - Hensen, Claudius, and Boettcher. Their function is not known.

The hair cells have projections. These cell tips project into the endolymph of the cochlear duct. The projections are called stereocilia. The stereocilia are arranged in a specific position that is believed to be the most important feature of the organ of Corti that allows the sensory cells tuning capability. The arrangement is that the shortest stereocilia are located on the outer rows, while the longest - on the inner row. The cochlea also has a unique characteristic - tonotopical organization. It means that different sound frequencies or waves affect different parts of the cochlea. The base of the cochlea is responsible for the high-frequency sounds, but the apex of the cochlea - for low-frequency sounds.

The tectorial membrane covers the hair cells. The tectorial membrane moves during oscillations of the endolymph. The oscillations happen every time a sound wave is transmitted. The movement of the tectorial membrane stimulates the hair cells that are followed by electrical impulses getting sent to the brain through the cochlear nerve. There are around 12 000 outer hair cells and 3 500 inner hair cells in the cochlea.