What is the Cochlea?

The Cochlea and Our Hearing

Cochlea. Photo © A.D.A.M.

The ear, which is responsible for our sense of hearing, is made up of three parts: the outer ear, middle ear, and inner ear. The outer ear is responsible for funneling sounds from the world and directing them towards the middle ear. The middle ear is responsible for transmitting and amplifying the sound through the vibrations of the ear drum (which marks the beginning of the middle ear) which then causes the movement of the ossicles (middle ear bones).

As the ossicles move, the last bone, the stapes, continues to transmit the sound vibrations by vibrating against the the oval window. Of note, the eustachian (auditory) tube connects to the middle ear. The oval window, which is part of the cochlea, marks the beginning of the inner ear.The inner ear is responsible for converting sound into electrical signals that our brain can use to create the sensation of hearing.

The Cochlea

Derived from the Greek word for snail, kochlías, the cochlea looks remarkably like a shell of a snail. The cochlea is divided into three different chambers (scala vestibuli, scala tympani and the scala media or cochlear duct). The scala vestibuli and scala tympani and contains fluids called perilymph, while the cochlear duct contains a fluid called endolymph. as well as tiny, sensitive hairs called cilia on the inner surface (Organ of Corti). When the stapes vibrates at the oval window, the perilymph is put into motion.

This motion travels down the scala vestibuli to the end called the helicotrema where it interacts with the fluid of the scala tympani. The fluid continues to vibrate until it ends at the round window.

The round window works opposite of the oval window. For example, when the oval window is pushed in, the round window is pushed out and vice versa.

This allows the fluid to vibrate and travel. Certain congenital disorders cause the round window to not function properly causing hearing loss.

Converting Vibrations to Electrical Signals in the Cochlea

As the fluid (perilymph) in the cochlea vibrates along the chambers, the Organ of Corti's hairs (cilia) are moved. The cilia then convert these vibrations into nerve impulses which are conducted to the brain through the cochlear portion of the auditory nerve. The brain then can interpret the electrical signals as sound.

The Cochlea is Similar to a Microphone

During the process of hearing, sound waves vibrate against the ear drum. Vibrations in ear require far less energy than vibrating fluid. Since the cochlea is fluid filled, there is a need to increase the amount of energy in order to adequately vibrate the fluid. In order to achieve this, the ear drum is about 13 times the size of the oval window. The ossicles, or middle ear bones act as the amplifier to narrow the vibrations of the ear drum down to the size of the oval window.

This allows a more intense signal to reach the brain. This is similar to the way a microphone amplifies our own voices.

Sources:

American Academy of Otolaryngology: Head and Neck Surgery. (2015). Hearing Loss. Access on November 6, 2015 from http://www.entnet.org/?q=node/1319.

American Speech-Language-Hearing Association. (n.d.) The Inner Ear. Access on November 6, 2015 from http://www.asha.org/public/hearing/Inner-Ear/.

Swensen, R.S. (2006).Review of Clinical and Functional Neuroscience. Access on November 6, 2015 from https://www.dartmouth.edu/~rswenson/NeuroSci/.

Jarvis, C. (2004). Ears. In Physical Examination & Health Assessment (341-370). St. Louis, Missouri:Saunders.

Continue Reading