The Anatomy of the Cochlea

While the cochlea is technically a fluid-filled structure within a bone, it plays a vital role in the function of hearing rather than simply being another component of the skeletal system. It is part of the inner ear and is often described as hollow and snail- or spiral-shaped.

Anatomy

The spiral shape of the cochlea is necessary for the transduction of different sound frequencies. The cochlea is approximately 10 millimeters (mm) wide. If it were uncoiled, the cochlea would be approximately 35 mm long.

Structure

The cochlea is filled with fluid (perilymph and endolymph) and is divided into three chambers called the scala vestibuli, scala media, and the scala tympani. Two of these fluid-filled chambers sense pressure changes (caused by sound) while the third chamber contains the organ of Corti, the cochlear duct and the basilar membrane.

The cochlear duct is another bony, hollow tube that sits between the scala vestibuli and the scala tympani. The cochlear duct contains endolymph. The scala tympani and cochlear duct are separated by the basilar membrane.

Also located within the cochlea are tiny hair cells. They are specifically found within the organ of Corti and are essential for proper hearing.

At birth we have about 12,000 hair cells. Hair cells can be damaged and lost throughout our lifetime from loud noises or other conditions, and once they are lost these cells do not regenerate. Given their essential role in hearing, the loss of hair cells results in permanent sensorineural hearing loss.

Location

The cochlea is one of two main structures that make up the inner ear. The inner ear is located behind the eardrum and deep within the middle ear. The other structures are called the semicircular canals, which are responsible for balance, while the cochlea is involved in hearing.

Behind the eardrum are the ossicles, tiny bones that play a vital role in hearing. At the bottom of the stapes sits the oval window.

The semicircular canals, which are filled with a fluid called endolymph, function to provide the body with a proper sense of balance. Directly adjacent to the semicircular canals, prior to the beginning of the snail-shaped tube that forms the cochlea, is the round window.

Anatomical Variations

Embryonically, the inner ear begins to form as early on as 4 weeks gestation. The cochlea itself is typically formed by 18 weeks gestation. The gene SOX2 is largely responsible for the formation of the cochlea, and mutations in SOX2 are associated with sensorineural hearing loss.

The cochlea can have large variations in cochlear lengths, angles between turns, and position in the skull base. This has implications for cochlear implant surgery, since the tiny electrode of the cochlear implant must be inserted into the cochlea.

Function

Sound waves are funneled into the ear and strike the eardrum (tympanic membrane), resulting in vibration. These vibrations travel to the ossicles—the tiny bones located within the middle ear called the malleus, incus. and stapes.

The stapes strikes the oval window, and vibrations are further conducted through the perilymph (fluid) located inside of the cochlea. Sound vibrations continue on through the scala vestibuli and scala tympani, eventually displacing the round window.

As the vibrations continue through the fluid, they activate the hair cells located on the basilar membrane and the organ of Corti. The hair cells then brush their stereocilia (the tiny hair-like projections that reside on top of the cell) against a structure called the tectorial membrane.

This movement of the hair cells results in the depolarization (a change in the balance of electrolytes in the fluid surrounding the cells) of the attached nerve fibers, and this is how sounds are sent to the brain for interpretation via the auditory nerve.

Associated Conditions

Several conditions may affect the cochlea.

Sensorineural Hearing Loss

Sensorineural hearing loss is technically defined as hearing loss that arises from any inner ear dysfunction. It includes sensory hearing loss that results from damaged hair cells within the cochlea.

Sensorineural hearing loss is extremely common, especially in the elderly population, but can also be congenital. It can be caused by exposure to loud noise, medications that are toxic to the ear, and a vast number of other conditions. It can also be associated with Meniere's disease.

Sensorineural hearing loss can be divided into central hearing loss or sensory hearing loss. As mentioned above, sensory hearing loss results from damaged hair cells, while central hearing loss can be the result of damage to the auditory nerve pathway.

Acoustic Neuroma (Vestibular Schwannoma)

Acoustic neuroma is a benign growth that arises from the nerves that supply the inner ear. It can cause problems with proper balance, resulting in dizziness, and can cause hearing loss or tinnitus (ringing in the ear).

Tinnitus

Tinnitus is ringing in the ear. It can also be an underlying buzzing, whistling, or chirping sound. Pulsatile tinnitus is when you can hear what sounds like your own heartbeat in your ears.

Tinnitus is strongly associated with exposure to loud noises and sensorineural hearing loss, and is also thought to be the result of damage to the hair cells in the cochlea.

Cochlear Implants

A cochlear implant is an electronic device that can improve hearing in individuals who experience deafness or profound hearing loss as a result of damage to the cochlea.

It has several parts, including a microphone, a speech processor, a transmitter and receiver, and an electrode array. Part of the cochlear implant is surgically placed under the skin, while an external portion is worn behind the ear.

Despite its name, a cochlear implant does not restore normal hearing. It stimulates the auditory nerve to give people who are deaf or have severe hearing loss a representation of different sounds and help them to understand speech. It takes proper training to interpret sound using a cochlear implant.

Tests

The health of the cochlea is assessed by several tests.

Rinne and Weber Tests

These types of hearing tests are sometimes referred to as tuning fork tests and are useful in detecting problems in the middle ear and inner ear. These tests are seldom used alone, but can be helpful in conjunction with other types of hearing tests when trying to determine if hearing loss is present or involves the cochlea.

Auditory Brainstem Response Testing

This test is often used for screening hearing loss in infants and is also called auditory evoked potential testing. It is useful in detecting problems with the nerve pathways involved in relaying sound impulses to the brain as well as problems with the cochlea.

Otoacoustic Emissions (OAE) Test

This test is easy to conduct: your healthcare provider will simply insert a probe in your ear and measure your response to certain noises. The OAE test specifically measures the function of the hair cells located in the cochlea.