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The basilar membrane is a thin strip of tissue located in the shell-shaped cochlea of the inner ear, directly underneath the organ of corti. It follows the contours of the cochlea and acts as a separator between the two liquid-filled tubes known as the scala media and the scala tympani, in addition to providing a base for the sensory receptors known as hair cells or stereocilia. In land vertebrates, the basilar membrane is an essential part of the auditory system.
This piece of tissue is only part of the larger structure known as the cochlea. The cochlea is shaped like a coiled shell and is composed of fluid-filled canals that help to transform sound waves transmitted by the tiny bones of the middle ear into electrical impulses that can be interpreted by the brain. The unique properties of the basilar membrane give it a key role in this sensory process.
The membrane follows the curvature of the cochlea, becoming gradually wider and less stiff the further away from the base of the cochlea it gets. It is covered with thousands of outer and inner hair cells — normally 16,000-20,000 in humans — that amplify sound waves. The sound waves are then transformed into electrical impulses and sent to the auditory areas of the brain for processing.
The “place theory” of pitch perception suggests that the basilar membrane is essential in the ability to differentiate the pitch of tones. Place theory states that different sound frequencies will vibrate different places on the basilar membrane, and that each location along the membrane responds to a distinctive, small range of frequencies. Higher-frequency sounds vibrate the membrane near its base, where the membrane is most stiff and narrow, and excite the hair cells of that particular area only. Lower-frequency sounds vibrate the membrane further up, where it is less rigid and wider.
According to place theory, humans recognize pitch based on the area of the basilar membrane that is stimulated. Of course, human pitch perception is more complex than this. Some “sharpening” of the perception is thought to occur at some point in the sensory process.
Damage to the basilar membrane itself can lead to nerve deafness, which can be caused by illness or injury. Hearing loss also can be caused by damage to the hair cells of the inner ear because of prolonged exposure to loud noise. In this case, if an individual is exposed to a loud noise of a specific frequency, hair cell damage might occur on only one area of the basilar membrane.