Cochlea

The cochlea (Latin: cochlea) is a bony canal within the internal ear that forms a spiral shape, making 2.5 turns around its axis. In Greek, cochlea means snail, which suites this structure that resembles a snail. The cochlea is the part of the bony labyrinth that encloses the cochlear duct of the membranous labyrinth that is involved in the sense of hearing.

Structure of cochlea

The cochlea is a hollow chamber of bone. The cochlea lies in front and medially of the vestibule. The cochlea has an apex that is oriented anteroinferior and laterally and a base that is pointing approximately in the direction of the internal acoustic meatus. Within the cochlea, the sound waves go from the base, near the oval window, to the apex (the center of the spiral) of the cochlea.

The spiral canal of the cochlea, which is located after the base of the cochlea, is divided into three ducts or scalae by the bony spiral lamina and the basilar membrane on one side and the vestibular wall of the cochlear duct on the other side. The bony spiral lamina is found inside the spiral canal of the cochlea and is attached to the outer wall of the modiolus.

The modiolus is the conical axis of the cochlea. The modiolus forms the medial wall of the spiral canal. The spiral canal is hollowed out for housing the cochlear nerve (CN VIII). Since the spiral lamina is attached only to the modiolus, it incompletely divides the inner space of the spiral canal. These three ducts are:

• Scala vestibuli - also called the vestibular duct. It is located at the base of the cochlea and connects with the vestibule. The scala vestibuli lies superior to the cochlear duct and next to the oval window. The scala vestibuli contains perilymph;
• Scala tympani - also called the tympanic duct. It is placed in front of the round window on the labyrinthine wall of the tympanic cavity and is connected to the secondary tympanic membrane. The scala tympani is found below the cochlear duct. This duct also contains perilymph;
• Scala media - also called the cochlear duct. The scala media contains endolymph.

The confluence between the scala vestibuli and scala tympani at the apex of the cochlea is known as the helicotrema. The helicotrema forms because the bony spiral lamina and the cochlear duct end before reaching the apex of the cochlea.

The cochlea communicates with the posterior cranial fossa via its own cochlear aqueduct. The cochlear aqueduct starts near the round window, goes through the temporal bone, and ends with an opening on the posterior surface of the temporal bone in the cranium.

Reissner's membrane is a vestibular membrane that separates the vestibular duct from the cochlear duct. The Reissner's membrane and the basilar membrane create a compartment filled with endolymph which is important for the organ of Corti. The basilar membrane separates the cochlear duct from the tympanic duct.

The endolymph and perilymph contain proteins and electrolytes, but the perilymph is rich in sodium ions, while the endolymph is rich in potassium ions.