Auditory System Cochlea

The cochlea is a tiny organ in the inner ear. Cochlear structures are important components in the hearing mechanism. “Cochlea” comes from the Latin word for “snail shell,” as it resembles a curving snail shell. Tiny hair cells in the cochlea send nerve impulses along the auditory nerve, transmitting auditory information to the brain.

Cochlear Structure

The outer layer of the cochlea is a bony shell, much like a snail shell. Within the cochlea, a series of tiny and complex structures work together to transmit sound information.

Throughout the cochlea, small hairs called cilia grow in the cochlea’s canals, which are filled with fluid. The base plate of the stapes (the innermost of the three small bones in the ear, or “ossicles”) inserts into a hole in the cochlea, called the oval window. The stapes moves in concert with the rest of the ossicles, triggered by movement of the tympanic membrane (eardrum). When the stapes moves, so does the fluid in the cochlea. Sound information travels through the cochlea from the base to the top, and causes movement in the cilia based on the sound frequency. The base of the cilia is connected to the auditory nerve, which carries this information to the brain.

The cochlear structure is set up such that certain sounds are processed in particular parts of the cochlea. High-frequency, or “high-pitched,” sounds are processed in the base of the cochlea. Low-frequency, or “low-pitched” sounds are processed at the top, or apex, of the cochlea. This distribution of processing is known as “tonotopic organization,” because of the tone-specific grouping in the cochlea.

The Cochlea - Auditory System

Problems with Cochlear Function

Damage to the cochlea can lead to hearing loss. This can include damage to the cilia, in which case the sound information cannot be translated into nerve impulses to be sent to the brain.

For those whose cochlea is not functioning properly, a cochlear implant may be appropriate to restore some cochlear function. Cochlear implants work by implanting an electrode directly into the cochlea. This electrode is connected to a sensor, which takes in environmental sound. The implant then sends nerve impulses to the brain based on the input it receives. Though cochlear implants can allow individuals to receive some auditory input, they do not restore “natural” hearing; the information they send to the brain is not as nuanced as that sent by the cochlea via the ear.

Resources

Dictionary.com (n.d.). Cochlea. Retrieved January 27, 2010, from Dictionary.com Web site: http://dictionary.reference.com/browse/cochlea.  

Martin, F.M.