Perception of sound requires that sound waves be transmitted through the outer ear canal, across the tympanic membrane, and through the ossicles to the oval window. Movement of the oval window initiates movement of perilymph, which causes movement of endolymph through the vestibular membrane. This fluid"s motion stimulates the neurosensory organs of hearing, the hair cells. Bending of the hair cells induces action potentials in the cochlear nerve, which projects to the brainstem. Neural projections to the auditory area in the temporal lobe result in sound perception. Balance is controlled by hair cells contained in the semicircular canals. Stimulation of these cells by head movement causes nerve impulses to be transmitted to the brain to keep individuals upright and control eye movement. Vertigo, the sensation of motion or aggravation of motion, is a cardinal symptom of disorders of the vestibular system. Vertigo is often associated with nystagmus and nausea.