Cochlea: anatomy, function and treatment

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While the cochlea is technically a bone, it plays a vital role in auditory function and not just another component of the skeletal system. It is found in the inner ear and is often described as hollow, cochlear, or spiral-shaped.

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Anatomy

The spiral shape of the cochlea is necessary for the transmission of different sound frequencies. The snail is approximately 10 millimeters (mm) wide, and if the snail were unfolded it would be approximately 35 mm long .

Composition

The cochlea is filled with fluid (perilymph and endolymph) and is divided into three chambers called the vestibular staircase, the middle staircase, and the tympanic staircase. Two of these fluid-filled chambers perceive 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 hollow bony tube that lies between the vestibular staircase and the tympanic staircase. The cochlear duct contains endolymph. The tympanic staircase and the cochlear duct are separated by a basilar membrane .

There are also small hair cells within the cochlea. They are found in the organ of Corti and are essential for proper hearing .

At birth, we have around 12,000 hair cells. Hair cells can be damaged and lost throughout our lives due to loud noises or other conditions, and after loss, these cells do not regenerate. Given its important role in hearing, the loss of hair cells leads to irreversible sensorineural hearing loss .

Location

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

Behind the eardrum are bones, tiny bones that play a vital role in hearing. At the bottom of the stapes there is an oval window followed by semicircular canals (also called labyrinthine).

The semicircular canals are filled with a fluid called endolymph and work to provide the body with a sensation of proper balance. Directly to the semicircular canals, in front of the beginning of the cochlear tube that forms the cochlea, there is a round window .

Anatomical variations

Embryonic, the inner ear begins to form at 4 weeks gestation. The snail itself generally forms at 18 weeks gestation. The SOX2 gene is largely responsible for cochlea formation, and SOX2 mutations are associated with sensorineural hearing loss .

The cochlea has large differences in cochlear length, turn angles, and position at the base of the skull. This has implications for cochlear implant surgery.

Function

Sound waves travel to the ear and hit the eardrum (ear drum), causing vibrations. These vibrations are transmitted to the bones , tiny bones located in the middle ear called the hammer or anvil. and stirrups.

The stapes strikes the oval window and the vibrations are transmitted through the perilymph (fluid) located within the cochlea. The sound vibrations continue through the vestibular staircase and the drum, eventually displacing the circular window .

As the vibrations continue in the fluid, they activate the hair cells located in the basilar membrane and the organ of Corti. The hair cells then brush their stereocilia (small hair-like projections located 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 that surrounds the cells) of the attached nerve fibers, and this is how sounds are sent to the brain for interpretation through the nerve. auditory .

Related conditions

Several conditions can affect the snail.

Sensorineural hearing loss

Sensory neural hearing loss is technically defined as hearing loss resulting from inner ear dysfunction. It includes sensory hearing loss that results from damage to the hair cells of the cochlea.

Sensorineural hearing loss is extremely common, especially in the elderly, but it can also be congenital. It can be caused by exposure to loud noises, medications that are toxic to the ears, or due to Meniere's disease .

Neural-sensory hearing loss can be divided into central hearing loss or sensory hearing loss. As mentioned earlier, sensory hearing loss is the result of damage to the hair cells, while central hearing loss can result from damage to the auditory nerve pathways .

Acoustic neuroma (vestibular schwannoma)

An acoustic neuroma is a benign tumor that arises from the nerves that supply the inner ear. This can cause problems with proper balance, leading to dizziness and can cause hearing loss or tinnitus (ringing in the ear) .

Tinnitus

Ringing in the ears . It can also be an underlying buzzing, hissing, or grinding noise. Pulsating tinnitus is when you hear what sounds like your own heartbeat in your ears.

Tinnitus is closely associated with exposure to loud sounds, sensorineural hearing loss, and is also believed to be the result of damage to the hair cells of the cochlea .

Cochlear implants

A cochlear implant is an electronic device that can improve hearing in people who are deaf or have profound hearing loss due to damage to the cochlea.

It consists of several parts, including a microphone, a speech processor, a transmitter and a receiver, and an electrode array. One part of the cochlear implant is surgically placed under the skin and the outer part is placed behind the ear.

Despite its name, a cochlear implant does not restore normal hearing. It stimulates the auditory nerve so that people who are deaf or hard of hearing can reproduce a variety of sounds and help them understand speech. Adequate preparation is required to interpret sound with a cochlear implant .

Tests

The condition of the snail is evaluated through various tests.

Rinne and Weber tests

These types of hearing tests are sometimes called tuning fork tests and are helpful in identifying problems in the middle and inner ear. These tests are rarely used alone, but in combination with other types of hearing tests in an attempt to determine if there is hearing loss or if the cochlea is affected .

Auditory brainstem response (ABR) test

This test is often used to detect hearing loss in babies and is also called an auditory evoked potentials (AEP) test . This is useful for detecting problems with the nerve pathways involved in transmitting sound impulses to the brain, as well as problems with the cochlea.

Otoacoustic emission test (OAE)

This test is easy to do by simply inserting a probe into your ear and measuring its response to certain noises. The OAE test specifically measures the function of hair cells located in the cochlea.

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