The fluid-filled inner ear (labyrinth) is a complex structure consisting of two major parts:
The cochlea and the vestibular system are connected to the brain by the 8th (vestibulocochlear) cranial nerve Overview of the Cranial Nerves Twelve pairs of nerves—the cranial nerves—lead directly from the brain to various parts of the head, neck, and trunk. Some of the cranial nerves are involved in the special senses (such as seeing... read more . One branch of this nerve, the auditory nerve, carries sound signals to the brain and another carries balance signals.
(See also Hearing Loss Hearing Loss Worldwide, about half a billion people (almost 8% of the world's population) have hearing loss. More than 10% of people in the United States have some degree of hearing loss that affects their... read more .)
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 come into your ear and are transmitted from the eardrum to a series of 3 small bones in the middle ear (the ossicles). The ossicles connect to the oval window in the inner ear. Movement of the oval window causes the fluid and cilia to vibrate. Vibrations of the cilia make the hair cells send signals through nerves to the brain. The brain interprets the nerve signals as sound.
Despite the protective effect of the acoustic reflex, in which tiny muscles in the middle ear contract to dampen the amount of movement of the ossicles, 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 Hearing Loss Worldwide, about half a billion people (almost 8% of the world's population) have hearing loss. More than 10% of people in the United States have some degree of hearing loss that affects their... read more and frequently noise or ringing in the ears (tinnitus Ear Ringing or Buzzing Ringing in the ears (tinnitus) is noise originating in the ear rather than in the environment. It is a symptom and not a specific disease. Tinnitus is very common—10 to 15% of people experience... read more ).
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 are located in the vestibule and contain cells that sense movement of the head in a straight line, that is, back and forth, sensing acceleration, or up and down, sensing gravity.
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) may develop.
Disorders of the inner ear can affect
Inner ear disorders include