What is the Scala Tympani?

The scala tympani is a channel in the cochlea, the part of the ear where sounds are converted into electrical signals and sent to the brain. It is essential for hearing. Sound waves are converted to mechanical vibrations in the middle ear, and then into waves within the fluid-filled cavity of the scala tympani. Hair cells suspended in fluid then detect these waves, and relay the sound information through the cochlear nerve to the brain. If the fluid-filled channels are damaged, impaired hearing or deafness can result.

The cochlear channel called the scala tympani receives sound vibrations from the the bones of the middle ear through a membrane and is filled with a fluid called perilymph. Along with the scala vestibuli, it is one of the two channels of the cochlear labyrinth where amplified sound waves are processed. Vibrations first come into contact with the tympanic membrane, then travel through the three ossicles of the middle ear, where they are amplified extensively before reaching the fluid compartments of the inner ear.

The scala tympani and scala vestibuli are separated by Reissner's membrane. They convert the vibrations of the middle ear bones — the malleus, incus, and stapes — into liquid waves within the cochlea. This motion reaches the hair cells of the organ of Corti. The hair cells are attached to receptor neurons and suspended in fluid, making them sensitive to the liquid movements caused by sound waves. When the hairs activate the neurons of the cochlear nerve, they cause an action potential that reaches the brain. This electrical signaling by the nerve enables the perception of sounds.

Adjacent to the scala tympani is the cochlear duct housing the organ of Corti. The two structures are divided from each other by the basilar membrane. This is essential because perilymph that fills both of the scala channels has a specific electrolyte composition necessary for proper hearing. Conversely, the cochlear duct is filled with endolymph, which has very different chemical and electrical properties and cannot be mixed with perilymph without interfering with the function of the hair cells.

Damage to the scala tympani or any other part of the cochlea can result in hearing loss. Trauma to the hair cells from excessively loud sound amplification is one common cause, but mixing of fluids in between the organ of Corti and the scala can also be a problem. Ménière's disease is believed to be caused by a spillover of endolymph into the surrounding channels. In certain kinds of bacterial infection, the scala tympani can become ossified, as bone forms within the canals of the cochlea that should instead be filled with fluid. This can cause deafness by impeding sound waves from reaching the hair cells.