The inner ear, lying behind the eardrum and the bones of the middle ear, contains specialized structures that mediate both balance and hearing. In the inner ear, these structures convert sound waves and changes in the position of the head into electrical impulses that travel by nerves to brain centers that decode the information.
Anatomy
The entrance to the inner ear is called the oval window. In the inner ear the elastic basilar membrane of the cochlea divides this fluid-filled, snail-shaped organ into an upper and lower half. The basilar membrane supports the inner and outer hair cells that are critical for hearing. Moving toward the back of the head from the cochlea, the inner ear continues with the components of the labyrinth consisting of the fluid filled saccule, utricle and semi-circular canals. All of the parts of the labyrinth comprise the vestibular system.
Hearing
Sounds entering the ear cause the bones of the middle ear to vibrate, amplifying the sound and sending it along to the inner ear. The vibrations set off ripples in the fluid within the cochlea, rocking the basilar membrane and the hair cells lining it. Movement of the hairs triggers the opening of ion channels that generate an electrical impulse. The auditory nerve carries that signal to the brain for interpretation.
Hearing Disorders
Noise-induced hearing loss can occur with prolonged exposure to sounds of 85 decibels or more--a level of noise generated by heavy city traffic, according to the National Institute on Deafness and Other Communication Disorders, which can permanently impair hearing by damaging the sensitive outer hair cells of the inner ear. This level of noise triggers release of destructive free radical molecules that kill the hair cells, which cannot grow back.
Auditory neuropathy occurs when sound waves reach the inner ear normally, but transmission of the nervous impulse to the brain is garbled. In some cases, inner hair cells of the cochlea fail to generate the nerve signals to the brain. In other cases, the fault lies in the connection between the hair cells and the auditory nerve or in the auditory nerve itself.
Balance
The vestibular system mediates balance. Circular movement of the head tilts the endolymph fluid in the semi-circular canals of the labyrinth. The saccule and utricle are called otolithic organs because they contain small crystals or canaliths. They detect gravity and movement along a straight line. Similar to the hearing apparatus, movement of the fluid in the vestibular system triggers hair like sensors to send a signal along the vestibular nerve to the brain, relaying positional information.
Balance Disorders
Benign positional vertigo and Meniere’s disease both cause a false sensation of spinning or wheeling even when the body remains still due to an abnormality in the vestibular system. Benign positional vertigo occurs when canaliths move into the semi-circular canals, tickling the hair cells and sending false information to the brain. Meniere’s disease increases the volume or pressure of the endolymph with the same end result.
