The inner ear (auris interna) is the innermost part of the human ear, which also comprises the outer ear (auris externa) and the middle ear (auris media). It comprises the snail shaped cochlea (from Greek: kochlias = snail), which is our organ of hearing, and the vestibular system, which is our our organ of balance. The inner ear is located deep in the skull, and surrounded almost entirely by bone.
The cochlea is a tiny (about the size of a fingernail), yet very powerful sensory organ. Its function is to convert sound energy into neural signals which are carried to the brain. Sound pressure that enters the cochlea creates waves in the perilymph fluid inside. This motion is sensed by the stereocilia of hair cells and transduced by inner hair cells via neurotransmitters, primarily glutamate, into excitation of the auditory nerve.
At the time of birth, every human being with full auditory capabilities disposes of approximately 3,500 inner hair cells, 13,000 outer hair cells and 30,000 neurons per cochlea. Damage or loss of hair cells, neurons, nerve fibers or other structures in the cochlea from injury or other sources (e.g. ageing) can lead to impairment of the cochlea's function. Such cochlear dysfunction may manifest itself through symptoms such as hearing loss, sound intolerance, or tinnitus.