Phonoreceptor

The ear does double duty as the organ of both hearing and equilibrium, and is divided anatomically into three regions: the external ear, the middle ear and the internal (inner) ear. The external ear is made of the pinna (a flap of skin-covered elastic cartilage that collects sound waves), the external auditory meatus (a curved tube leading to the eardrum), and the tympanic membrane (eardrum) itself — a connective-tissue membrane covered by skin outside and mucous membrane inside. Fine hairs and wax-producing ceruminous glands in the auditory meatus combine to keep dust and foreign particles from reaching further in.

The middle ear is a small air-filled cavity in the temporal bone, connected to the throat by the Eustachian tube (which equalises air pressure on either side of the eardrum) and housing three tiny bones, the ossicles — malleus, incus and stapes — which mechanically relay and amplify vibrations from the eardrum onto the oval window of the inner ear; because of how the ossicles lever this force, the pressure delivered to the oval window ends up roughly 20 times greater than the pressure that first hit the eardrum. This amplified pressure sets up pressure waves in the perilymph fluid of the inner ear's cochlea, which in turn make the round window bulge alternately outward and inward while the basilar membrane — carrying the sensory organ of Corti — moves up and down. That up-and-down motion bends the stereocilia of the hair cells in the organ of Corti, alternately opening and closing mechanically gated ion channels at their base; the resulting action potentials are carried by the cochlear nerve to the brain, where they are perceived as sound. …