>> Pictured here is the structure of the human ear. It consists of three parts: the outer, middle, and inner ear. The outer ear includes the pinna and the external auditory canal and functions as a receiver. The pinna gathers and funnels sound waves into the external auditory canal, which then transmits the sound to the ear drum. The middle ear is composed of the Eustachian tube, the ear drum, and three tiny bones: the malleus, incus, and stapes. The Eustachian tube, a passageway leading from the middle ear to behind the nasal cavity, maintains equal air pressure between the middle ear and the atmosphere. When sound waves reach the eardrum, also called the tympanic membrane, the membrane begins to vibrate. This in turn sets the bones of the middle ear in motion. Together the three bones of the middle ear amplify the vibrations of the eardrum and pass them on to the inner ear. The inner ear consists of the oval window, the fluid filled cochlea, the round window, the vestibular apparatus, and the auditory nerve. Sound waves are transferred from the bones in the middle ear to the oval window. Vibration of the oval window sets the fluid in the cochlea in motion. Sensory cells within the cochlea convert the pressure waves in the cochlea to neural messages that are then sent to the brain via the auditory nerve. The round window, located at the opposite end of the cochlea from the oval window, serves to relieve the pressure created by the movement of the oval window. The cochlea houses the organ of Corti. The organ of Corti contains the sensory receptors that aid in the actual process of hearing. The receptors, or hair cells, are supported on the basilar membrane and contain tiny stereocilia, hairs embedded in the gelatinous tectorial membrane. The basilar membrane, sensory hair cells, and the tectorial membrane form the functional unit of hearing in the organ of Corti. When fluid movement causes the basilar membrane to vibrate, a shearing force is created between the basilar membrane and the tectorial membrane. The stereocilia of the hair cells move and bend, because of the shearing force. This movement causes the sensory events that ultimately result in nerve impulses in the auditory nerve, which are then sent to the brain for interpretation. The greater the pressure changes in the fluid, the stronger the bending of the basilar membrane and the more hair cells that are stimulated. The brain interprets the increased number of impulses as louder sound.