The Cochlea

Sound waves vibrate the ossicles, which push the oval window and create fluid movement through the cochlea. This fluid motion causes the basilar membrane to vibrate. The vibrating basilar membrane then stimulates hair cells sandwiched between it and the tectorial membrane, generating neural signals sent to the auditory nerve.

The basilar membrane varies in physical properties along its length: the basal end is narrow and stiff, while the apical end is wider and flexible. High frequencies maximally vibrate the stiff basal end, while low frequencies maximally vibrate the flexible apical end. This creates tonotopy, a topographic map of pitch across the cochlea.

Tonotopy is a topographic map of pitch created by the basilar membrane's frequency-dependent vibration pattern. This organization is maintained throughout the auditory pathway to the brain, where neurons in the primary auditory cortex form a frequency map. This spatial organization aids in pitch discrimination by correlating neural stimulation patterns with specific frequencies heard.

Hair cells experience a shearing force created by the relative motion between the vibrating basilar membrane below and the stationary tectorial membrane above. This mechanical shearing stimulates the hair cells, generating neural signals that activate auditory nerve cells at that location. The frequency of vibration determines which hair cells are maximally stimulated.

The basilar membrane's structural gradient determines its frequency response. The narrow, stiff basal end near the oval window responds maximally to high frequencies, while the wide, flexible apical end responds maximally to low frequencies. This mechanical property creates the cochlea's ability to separate and encode different sound frequencies spatially.

The tonotopic organization established in the cochlea is preserved throughout the auditory pathway. Hair cells at different cochlear locations activate corresponding auditory nerve cells, which maintain this frequency-based organization through parallel pathways in the brain. The primary auditory cortex contains a complete frequency map reflecting inputs from the basal to apical cochlear regions.

The tectorial membrane is a stiffer structure positioned above the hair cells. As the basilar membrane vibrates below, the relative motion between these two membranes creates a shearing force on the hair cells sandwiched between them. This mechanical interaction is essential for converting basilar membrane vibrations into the neural signals that encode sound information.