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The ear, which is the organ of hearing and balance, consists of the outer, middle, and inner ear. The outer, middle, and inner ear function together to convert sound waves into nerve impulses that travel to the brain, where they are perceived as sound. The inner ear also helps to maintain balance.
Outer Ear
The outer ear consists of the external part of the ear (pinna or auricle) and the ear canal (external auditory meatus). The pinna consists of cartilage covered by skin and is shaped to capture sound waves and funnel them through the ear canal to the eardrum (tympanic membrane), a thin membrane that separates the outer ear from the middle ear.
Middle Ear
The middle ear consists of the eardrum and a small air-filled chamber containing a chain of three tiny bones (ossicles) that connect the eardrum to the inner ear. The ossicles are named for their shapes. The hammer (malleus) is attached to the eardrum. The anvil (incus) is the middle bone between the hammer and the stirrup (stapes), which is attached to the oval window, a thin membrane at the entrance to the inner ear. Vibrations of the eardrum are amplified mechanically by the ossicles and transmitted to the oval window.
The middle ear also contains two tiny muscles. The tensor tympani muscle is attached to the hammer and helps helps to tune and protect the ear. The stapedius muscle is attached to the stirrup and oval window. This muscle contracts in response to a loud noise, making the chain of ossicles more rigid so that less sound is transmitted. This response, called the acoustic reflex, helps protect the delicate inner ear from sound damage.
The eustachian tube, a small tube that connects the middle ear with the back of the nose, allows outside air to enter the middle ear. This tube, which opens when a person swallows, helps maintain equal air pressure on both sides of the eardrum and prevents fluid from accumulating in the middle ear. If air pressure is not equal, the eardrum may bulge or retract, which can be uncomfortable and distort hearing. Swallowing or voluntary "popping" of the ears can relieve pressure on the eardrum caused by sudden changes in air pressure, as often occurs when flying in an airplane. The eustachian tube's connection with the middle ear explains why upper respiratory infections (such as the common cold), which inflame and block the eustachian tube, can lead to middle ear infections or changes in middle ear pressure, resulting in pain.
Inner Ear
The inner ear (labyrinth) is a complex structure consisting of two major parts: the cochlea, the organ of hearing; and the vestibular system, the organ of balance. The vestibular system consists of the saccule and the utricle, which determine position sense, and the semicircular canals, which help maintain balance.
The cochlea, a hollow tube coiled in the shape of a snail's shell, is filled with fluid. Within the cochlea is the organ of Corti, which consists, in part, of about 20,000 specialized cells, called hair cells. These cells have small hairlike projections (cilia) that extend into the fluid. Sound vibrations transmitted from the ossicles in the middle ear to the oval window in the inner ear cause the fluid and cilia to vibrate. Hair cells in different parts of the cochlea vibrate in response to different sound frequencies and convert the vibrations into nerve impulses. The nerve impulses are transmitted along fibers of the cochlear nerve to the brain. The round window is a small, membrane-covered opening between the fluid-filled cochlea and the middle ear. This window helps dampen the pressure caused by sound waves in the cochlea.
Despite the protective effect of the acoustic reflex, loud noise can damage and destroy hair cells. Once a hair cell is destroyed, it does not seem to regrow. Continued exposure to loud noise causes progressive damage, eventually resulting in hearing loss and sometimes noise or ringing in the ears (tinnitus).
The semicircular canals are three fluid-filled tubes at right angles to one another. Movement of the head causes the fluid in the canals to move. Depending on the direction the head moves, the fluid movement will be greater in one of the canals than in the others. The canals contain hair cells that respond to this movement of fluid. The hair cells initiate nerve impulses that tell the brain which way the head is moving, so that appropriate action can be taken to maintain balance.
If the semicircular canals malfunction, as may occur in an upper respiratory infection and other conditions both temporary and permanent, the person's sense of balance may be lost or a whirling sensation (vertigo) may develop.
Last full review/revision March 2006 by Harold C. Pillsbury, III, MD; Austin S. Rose, MD
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