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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.
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.
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 tune and protect the ear. The stapedius muscle is attached to the stirrup. 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 is a small tube that connects the middle ear to the airway in the back of the nose (nasopharynx). This tube allows outside air to enter the middle ear (behind the eardrum). The eustachian 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.
The inner ear (labyrinth) is a complex structure consisting of two major parts: the cochlea, which is the organ of hearing, and the vestibular system, which is the organ of 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 (see page Middle Ear), loud noise can damage and destroy hair cells. Once a hair cell is destroyed, it does not regrow. Continued exposure to loud noise causes progressive damage, eventually resulting in hearing loss and sometimes noise or ringing in the ears (tinnitus—see page Ear Ringing or Buzzing).
The vestibular system consists of two fluid-filled sacs called the saccule and the utricle and three fluid-filled tubes called the semicircular canals. These sacs and tubes gather information about the position and movement of the head. The brain uses this information to help maintain balance.
The saccule and utricle contain cells that sense movement of the head in a straight line, that is, back and forth or up and down.
The semicircular canals are three fluid-filled tubes at right angles to one another that sense rotation of the head. Rotation 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, which can occur in an upper respiratory infection or other temporary or permanent disorder, the person's sense of balance may be lost or a false sensation of moving or spinning (vertigo—see page Dizziness and Vertigo) may develop.
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