The inner ear is in the petrous area of the temporal bone. Within the bone is the osseous labyrinth, which encases the membranous labyrinth. The osseous labyrinth includes the vestibular system (made up of the semicircular canals and the vestibule) and the cochlea.
The vestibular system, responsible for balance and posture, consists of the saccule, utricle, and semicircular canals. The saccule and utricle contain cells that sense movement of the head in a straight line (sensing acceleration) or up and down (sensing gravity). The 3 semicircular canals sense angular rotation of the head. Depending on the direction the head moves, the fluid movement will be greater in one on the 3 canals than in the others. Hair cells in the canals respond to the fluid movement, initiating nerve impulses so that the brain can take appropriate action to maintain balance.
The cochlea, responsible for hearing, is filled with fluid. Within the cochlea is the organ of Corti, containing about 30,000 hair cells. Cilia from the hair cells extend into the fluid and are embedded in a gelatinous membrane. Sound vibrations are transmitted from the ossicles, through the middle ear and the oval window, into the inner ear where these vibrations cause the fluid and cilia to vibrate. These vibrations are then transformed into an electric signal that is sent to the brain. There are many environmental factors that can damage the cells within the inner ear and cause hearing loss. One of the most important is loud noise exposure. Despite the protective effect of the acoustic reflex which tenses the middle ear bones to blunt loud sounds, loud noise can damage and permanently destroy hair cells. Continued exposure to loud noise causes progressive damage, eventually resulting in hearing loss and sometimes in tinnitus.
Inner ear disorders include