MEDICAL ANIMATION TRANSCRIPT: Hearing begins with the external ear funneling sound waves through the external auditory canal to the tympanic membrane. The tympanic membrane vibrates in response to the sound waves. Three connected bones called the malleus, incus, and stapes transmit the vibrations through the middle ear. Movement of the stapes causes vibrations of the membranous oval window of the inner ear. Oval window vibrations create fluid waves in the two perilymph-filled chambers in the cochlea ending at the round window. These waves of perilymph also push on a third fluid-filled chamber called the cochlear duct. Inside the cochlear duct, sensory receptor cells called hair cells line one of the walls of the duct called the basilar membrane. The tectorial membrane covers the hair cells. Collectively, this group of structures is called the organ of Corti. Fluid pressure in the perilymph pushes on the basilar membrane and its attached hair cells which bend when pushed against the stationary tectorial membrane. This event sets off a chain reaction within the hair cells converting the pressure wave to a nerve impulse in the nerve fibers. Nerve fibers throughout the cochlea combine to form the cochlear branch of the vestibulocochlear or auditory nerve which transmits the sound nerve impulse to the brain. Sensory neural hearing loss is caused by disease or damage to the nervous pathway from the organ of Corti to the vestibulocochlear nerve and central auditory system in the brainstem. Damage to the auditory nerve pathway can result from aging, chronic loud noise, viral infections, or physical trauma to the temporal bone. Another common cause of hearing loss is Meniere's disease in which a build up of perilymph fluid left untreated which result in the rupture of the membranous labyrinth. Diagnostic tests include tuning fork test and audiometer testing. Depending on the cause and severity of auditory nerve damage, sensory neural hearing loss is treated with devices such as hearing aides or cochlear implants.
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