The fluid-filled inner ear (labyrinth) is a complex structure consisting of two major parts:
The organ of hearing (cochlea)
The organ of balance (vestibular system)
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.
Sound waves come into the ear and cause the eardrum to vibrate. These vibrations are transmitted to three small bones in the middle ear (called ossicles), then through the oval window into the inner ear, where they enter the cochlea.
A Look Inside the Ear
 |
Cochlea
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 contains about 20,000 specialized cells called hair cells. These cells have small hairlike projections (cilia) that extend into the fluid. The sound vibrations cause the fluid and cilia in the cochlea 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.
To help prevent damage to hair cells, the muscles in the middle ear contract to decrease the movement of the ossicles caused by loud noises, This response to loud noises is called the acoustic reflex. However, despite this protective reflex, loud noise can still damage and destroy hair cells. Once a hair cell is destroyed, it does not regrow. Continued exposure to loud noise destroys more and more hair cells, 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 15% 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 ).
Vestibular system
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 from side to side (horizontally), sensing acceleration, or up and down (vertically), 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 is 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 disorders, the person's sense of balance may be lost or a false sensation of moving or spinning (vertigo Dizziness and Vertigo Dizziness is an inexact term people often use to describe various related sensations, including Faintness (feeling about to pass out) Light-headedness Dysequilibrium (feeling off balance or... read more ) may develop.
Disorders of the inner ear
Disorders of the inner ear can affect
Hearing
Balance
Both hearing and balance
Inner ear disorders include
Some middle ear disorders Middle Ear Disorders can affect the inner ear and vice versa.
