Sound waves travel through the air and into the outer ear canal where they cause the eardrum (tympanum) to vibrate. This causes a series of small bones (ossicles) to vibrate, which vibrates a small membrane called the oval window. This causes fluid inside the cochlea to move back and forth.
A portion of the cochlea known as the organ of Corti contains hair cells that convert this fluid movement into what our brain perceives as sound. There are two types of hair cells. Outer hair cells amplify quiet sounds, while inner hair cells primarily detect and transmit sound signals to the brain via the auditory nerve. There are roughly 3,200 inner hair cells and 12,000 outer hair cells in the human organ of Corti.
Outer and inner hair cells are arranged linearly within the cochlea tonotopic array. Hair cells at the base of the cochlea detect the highest frequencies. Those located at the apex, or tip, detect the lowest frequencies.
For normal hearing function, a variety of additional cell types and cellular structures must work together in concert. Broadly speaking, these include (but are not limited to):
Stereocilia: structures that project from hair cells and respond to movement of fluid within the cochlea, enabling the sensation of sound.
The stria vascularis: a group of cells that produce endolymph, a fluid in the cochlea that moves in response to sound waves.
Synapses: specialized connections that are required for the efficient transfer of sound information from hair cells to spiral ganglion neurons.
Spiral ganglion neurons: cells that transmit information from the cochlea to the brain, where sound is perceived.
Damage or dysfunction within any of these cells and cell structures can lead to symptoms of hearing loss, tinnitus or hyperacusis.