Where Is the Cochlea?
It helps to picture taking a journey through the ear. First you pass through the outer ear, which is the part of the ear we can see. The outer ear has the eardrum. Next, you pass through the middle ear, which is between the eardrum and the inner ear. At last, inside the inner ear, we find the cochlea.
Cochlea's Appearance and Function
Have you seen a snail? That is what a cochlea looks like. In fact, the name "cochlea" is based on the Latin word for "snail." It's small, only about 1 cm wide. The cochlea is filled with fluid, and is coiled up tightly. When the auditory ossicles (kind of like little rocks) in the middle ear vibrate, they cause that fluid to move within the cochlea. Inside the cochlea, there is a special part called the Organ of Corti. The Organ of Corti has hair cells that respond to sound by generating nerve impulses that are ultimately transmitted to the brain to be interpreted.
When viewing a cross-section of the cochlea, these three scalae can be seen: the scala vestibuli at the top, the scala media in the middle, and the scala tympani at the bottom. Each scalae are filled with types of fluid -- scala vestibuli is filled with perilymph, the scala media with endolymph, and the scala tympani with perilymph. Running across the middle like a bridge is the basilar membrane, which has both inner hair cells and outer hair cells. On top of the basilar membrane is another membrane, the tectorial membrane.
Researchers have found that when there is a sound stimulus, it affects the basilar membrane. The sound stimulus causes a "traveling wave" that travels along the basilar membrane, displacing it and the hair cells on top of it. (How much the basilar membrane is displaced depends on whether the sound is high frequency or low frequency.) The basilar membrane is pushed either up or down, and there is a shearing that happens between the basilar membrane and the aforementioned tectorial membrane. During this shearing, the tectorial membrane is pushed across the top of the hair cells on top of the basilar membrane, bending the top portion of the hair cells. This top portion is referred to as the stereocilia.
Cochlea's Role in Deafness
If the hair cells in the cochlea are damaged, they cannot carry out their function in the hearing mechanism. This renders a person deaf. Often, this can be treated with a cochlear implant.
In a cochlear implant, the cochlea becomes the home to a set of electrodes that are surgically threaded into the cochlea. These electrodes transmit electrical impulses that stimulate the hearing nerve. The electrodes are an imperfect substitute for the damaged hair cells.
Damage to Cochlea Itself
Cochlear implants may not be possible if the cochlea itself has become scarred. This can happen as a result of a disease like meningitis. The meningitis may cause ossification of the cochlea, which is the formation of scar tissue and new bone inside the inner ear space. This can prevent surgical implantation of a cochlear implant, so if a child is deafened by meningitis, cochlear implant surgery needs to take place as soon as possible.
Anther cause of cochlear ossification is cochlear otosclerosis. Normally otosclerosis involves the growth of bone inside the middle ear. In cochlear otosclerosis, that bone growth can extend into the cochlea from the middle ear. Another cause is labyrinthitis ossificans, which is a complication of bacterial infection (e.g. chronic otitis media) in the inner ear. It too causes the formation of bone, in the basal turn (lower part) of the cochlea.
Cochlear anatomy. Cochlear Fluids Lab, Washington University. http://oto2.wustl.edu/cochlea/intro2.htm
Evans, Michael G and Corné J Kros. The cochlea – new insights into the conversion of sound into electrical signals. J Physiol. 2006 October 1; 576(Pt 1): 3-5. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1995630/
Green JD Jr, Marion MS, Hinojosa R. Labyrinthitis ossificans: histopathologic consideration for cochlear implantation. Otolaryngology Head Neck Surgery. 1991 Mar;104(3):320-6.
Otosclerosis. Deafness Research UK. http://www.deafnessresearch.org.uk/factsheets/otosclerosis.pdf
Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001. http://www.ncbi.nlm.nih.gov/books/NBK10946/
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