The cochlea is a small snail-shaped bone structure in the inner ear. There are several components of the structure and these, combined with the cochlea itself, make up the half of the inner ear structure that controls hearing. The other half of the inner ear labyrinth is the vestibular system, which controls balance.
There are many parts and structures within the cochlea. First there is the scala vestibuli and the scala tympani, which conduct and transmit the movement of air to the movement of the liquid inside the ear. This liquid then goes to the organ of Conti, which in turn transmits the sound by generating electrical impulses to the brain that travel up the cochlear nerve and cochlear nuclei to create the sense of sound.
The organ of Conti is encased in a cavity called the cochlear duct, or scala media, which surrounds the organ in fluid. A special membrane called the Reissner's membrane separates the scala media from the scala vestibuli. Another membrane, the basilar membrane, also serves as a barrier for the scala media, separating it from the scala tympani. Throughout the cochlea are tiny hair cells that move in response to the liquid passing through the various canals and ducts in the structure. These hair cells are what trigger the electrical impulses that go onto the cochlear nerve.
Most of the cochlea is either encased in, or transports, one of two special bodily fluids called perilymph and endolymph. Perilymph is the liquid inside the ear that travels to the organ of Conti, carrying sound with it. This liquid is similar in chemical composition to spinal fluid. Endolymph is found inside the cochlear duct and around the organ of Conti itself. Its unique chemical structure allows the electrical current from the organ of Conti to freely flow into the hair cells of the inner ear and onto the cochlear nerve that is connected to the brain. Although crucial to the hearing components of the inner ear, endolymph is also a vital component of the balance portion of the inner ear. Disruption of endolymph via sudden movements or spinning is what causes the sensation of dizziness.
If any of these components or structures are damaged either by disease, birth defect or accident then permanent hearing damage is often the result. Sometimes this can be treated via a cochlear implant, which functions like a bionic ear. When implanted in a person who has suffered hearing loss, these devices can sometimes bring back their hearing, or even give them hearing capabilities for the first time.