Hearing occurs when sound waves are changed to neural signals inside the cochlea of the inner ear.
When the eardrum vibrates from sound waves, a chain of three tiny middle ear bones moves. The malleus, incus, and stapes (or hammer, anvil, and stirrup), together called the auditory ossicles. The arrangement of these bones forms a lever system that amplifies the energy of the sound waves before it reaches the cochlea.
The stapes fits into the membrane-covered oval window at the base of the cochlea. Within the bony cochlea is the cochlear duct. Alongside the cochlear duct are the vestibular duct and the tympanic duct. All three ducts are filled with fluid; endolymph in the cochlear duct and perilymph in both the vestibular duct and tympanic duct.
When the stapes pushes against the oval window, pressure waves in the perilymph travel up the spiral through the vestibular duct and back down in the tympanic duct. Pressure is relieved when the wave reaches the round window.
When looking at a cross section of the cochlear duct, specialized hair cells are shown being struck by pressure waves in the perilymph. The endolymph-filled cochlear duct now appears as the central triangular section. The vestibular duct and tympanic ducts, both containing perilymph, are on either side of it.
Sitting on the basilar membrane of the cochlear duct are hair cells of the organ of Corti. Above the hair cells is the tectorial membrane. When the basilar membrane is moved by pressure waves in the perilymph, hair cells are pressed against the tectorial membrane, and their stereocilia are distorted. The distortion generates a signal that travels through the cochlear branch of the vestibulocochlear nerve to the brain, where the signal is ultimately interpreted as a sound.