The cochlea is a hollow, spiral-shaped bone found in the inner ear that plays a key role in the sense of hearing and participates in the process of auditory transduction. Sound waves are transduced into electrical impulses that can be interpreted by the brain as individual frequencies of sound. The spiral shape of the cochlea allows for differing frequencies to stimulate specific areas along the spiral. This results in a tonotopic map that enables people to perceive various frequencies of sound. Specific areas along the cochlea are stimulated by vibrations carried within a fluid known as endolymph that is found in the cochlear duct. The vibrations are then converted to electrical impulses in the cochlear duct through mechanical stimulation of hair cells within a special structure known as the organ of Corti. These nerve impulses are carried by the vestibulocochlear nerve from the cochlea to the brain for interpretation.
Understanding cochlear anatomy is essential to understanding the physiology. The cochlear tube is formed by three membranous and fluid-filled canals which are the scala vestibuli (SV most superior and connected with the vestibule), scala media (SM) and scala tympani (ST most inferior and ends at the secondary tympanic membrane and the round window) forming a two-and-a-half spiral structure. SV and ST are filled with perilymphatic fluid while the endolymphatic fluid circulates in the SM that includes the organ of Corti supported by the basilar membrane. Auditory vibration transmitted from the tympanic membrane through the ossicles into the oval window then the perilymphatic fluid of the SV and ST, trigger the vibration of the basilar membrane that stimulates the organ of Corti which generates the afferent signals of the cochlear nerve.
Hair cells are specialized cells that play an important role in cochlear function. Hair cells are found within the organ of Corti and divided into inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are the true auditory receptor cells that synapse with bipolar spiral ganglion neurons to send afferent nerve impulses back to the brain via the cochlear nerve. Actin filaments connect the stereocilia at the tips of the IHCs, and mechanically gated potassium channels open in response to vibration leading to afferent cochlear nerve signaling. Ninety percent to 95% of spiral ganglion neurons synapse on IHCs. The remaining 5% to 10% of spiral ganglion neurons innervate the OHCs. The OHCs function to increase the maximum amplitude of the traveling wave of vibration. Efferent nerve fibers from the brain synapse on OHCs and decrease their ability to enhance the amplitude of the vibrational wave.
The inner ear develops embryologically from ectodermal cells by week 4 of gestation. Invagination of these cells forms the otic vesicle, which contains a dorsal and a ventral pouch. The dorsal pouch forms the endolymphatic duct along with vestibular structures, while the ventral pouch elongates to become the cochlea. The cochlea forms a two-and-a-half turn coil by 10 weeks gestation and reaches a maximum labyrinth length by 18 weeks. The organ of Corti develops from sensory neuroepithelium within the cochlear duct. SOX2 is an important transcription factor that plays a key role in cochlear development. Mutations in SOX2 have been shown to be associated with sensorineural hearing loss.
The cochlea is responsible for the phase of auditory transduction that takes place in the inner ear. The tonotopic map created by the spiral of the cochlea enables people to interpret a vast amount of different sounds simultaneously through vibrations carried from the perilymph to the endolymph in the cochlear duct. The anatomy of the cochlea allows it to efficiently carry vibrations that are ultimately converted to electrical impulses and interpreted by the auditory cortex of the brain.
The process of auditory transduction begins with sound waves entering the external acoustic meatus and striking the tympanic membrane, resulting in vibration. These vibrations are then transferred to the middle ear along the ossicular chain consisting of the malleus, incus, and stapes. The footplate of the stapes contacts the oval window in a piston-like motion that results in the transmission of vibrations to a fluid called perilymph within the cochlea. The vibrations travel up the cochlea to the apex through a hollow bony tube known as the scala vestibuli. The vibrations then travel from the apex down to the base through another hollow, bony tube called the scala tympani. At the end of the scala tympani, the vibrations within the perilymph displace the round window. The cochlear duct lies between the scala vestibuli and scala tympani. This is another hollow, bony tube that contains a fluid known as endolymph that has a higher positive potential than the surrounding perilymph. This positive potential is the result of high potassium ion concentration and low sodium ion concentration compared to the surrounding perilymph. The Reissner membrane maintains the differences in ion concentration between the endolymph and perilymph. The Reissner membrane separates the scala vestibuli from the cochlear duct and the stria vascularis, specialized cells lining the lateral wall of the cochlear duct. The structure separating the scala tympani from the cochlear duct is known as the basilar membrane. The basilar membrane contains a specialized structure known as the organ of Corti that plays a key role in auditory transduction. The difference in width and thickness of the basilar membrane between the base and the apex of the cochlea (BM narrow at the base and wide at the apex), allows for the perception of sounds with a wide frequency range (20-20,000 Hz). Vibrations traveling through perilymph in the scala vestibuli pass through the Reissner membrane and into the endolymph of the cochlear duct, eventually vibrating the organ of Corti. The organ of Corti contains hair cells that respond to vibration by brushing their stereocilia against a fixed structure called the tectorial membrane. The result of the hair cells bending against the tectorial membrane is depolarization of the attached nerve fibers. The frequency of the vibration traveling through the perilymph will correspond to an area along the cochlea that is maximally stimulated. This permits the interpretation of various frequencies of sound based on the tonotopic area along the cochlea (high frequencies at the base and low frequencies at closer to the apex) that resonates with the vibration the most.
Testing to determine if the cochlea or related structures are involved in hearing loss involves the Rinne and Weber tests. These tests are accomplished using a 512-Hz tuning fork to determine if the cause of hearing loss is conductive or sensorineural. Conductive hearing loss etiologies include more mechanical dysfunctions present in the middle ear, such as cerumen impaction, otitis media, and damage to the ossicles or tympanic membrane. Sensorineural hearing loss involves damage to the specialized nervous system that makes up the inner ear and involves the cochlea or nerves exiting the cochlea.
The Weber test is performed by striking the 512-Hz tuning fork and placing it on the center of the head. The patient is then asked if the sound is lateralized to one or both ears. The sound will lateralize to the dysfunctioning ear with conductive hearing loss and the properly functioning ear with sensorineural hearing loss.
The next step is to perform the Rinne test by striking the 512-Hz tuning fork and placing it on the mastoid process of each ear. The patient is instructed to indicate when the sound is no longer heard, at which point the tuning fork is moved to the auditory meatus, and the patient is instructed to repeat the process. Normal auditory function will have a 2:1 ratio of bone to air conduction times. If the hearing loss is conductive, bone conduction is heard longer than or equal to air conduction. If the hearing loss is sensorineural, air conduction is heard longer than bone conduction, but less than the normal 2:1 ratio.
There are many etiologies of hearing loss that occur through dysfunction of any part of the auditory transduction pathway. Etiologies specific to cochlear dysfunction include noise damage and Meniere disease.
Noise damage occurs during brief or sustained exposure to extremely loud noise. Exposure to sounds of 85 decibels or greater for an eight-hour period can lead to permanent cochlear damage. The damage associated with loud noise exposure destroys the hair cells in the cochlea. Loss of hair cells leads to an inability to stimulate afferent nerves and therefore an inability to hear the sound of various frequencies.
Meniere disease is caused by endolymphatic hydrops. This occurs when the volume of endolymphatic fluid within the cochlear duct is increased, causing distention. The etiology of Meniere disease involves dysregulation of aquaporin channels and osmotic disequilibrium involving the production of endolymph from perilymph through the Reissner membrane and stria vascularis.
From a clinical perspective, dysfunction of normal cochlear physiology will manifest as hearing loss. Cochlear dysfunction should be placed on the differential diagnosis for any patient presenting with hearing loss.
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