19.7:
The Cochlea
Sound waves are transmitted to the cochlea in the inner ear by the ossicles vibrating the oval window, which pushes fluid through the cochlea, causing the basilar membrane to vibrate.
The basilar membrane is narrower and stiffer at the basal end, the side nearest to the oval window; and wider and more flexible at the apical end. As a result, the basal end vibrates maximally in response to high frequencies; and the apical end vibrates maximally in response to low frequencies, creating tonotopy, a topographic map of pitch.
Vibration of the basilar membrane creates a shearing force on the hair cells that are sandwiched between it and the stiffer tectorial membrane, generating a neural signal onto the auditory nerve cells in that location.
Therefore, high frequencies activate auditory nerve cells at the basal end of the cochlea, while low frequencies activate those at the apical end. This tonotopy is maintained through the auditory pathway to the brain where it aids in pitch discrimination.
19.7:
The Cochlea
The cochlea is a coiled structure in the inner ear that contains hair cells—the sensory receptors of the auditory system. Sound waves are transmitted to the cochlea by small bones attached to the eardrum called the ossicles, which vibrate the oval window that leads to the inner ear. This causes fluid in the chambers of the cochlea to move, vibrating the basilar membrane.
The basilar membrane extends from the basal end of the cochlea near the oval window to the apical end at its tip. Although the cochlea itself narrows towards the apical end, the basilar membrane has the opposite geometry—becoming wider and more flexible towards the apical end.
Primarily because of these physical characteristics, the apical end of the basilar membrane maximally vibrates when exposed to low-frequency sounds, while the narrower, stiffer basal end maximally vibrates when exposed to high frequencies. This gradient of frequency response creates tonotopy—a topographic map of pitch—in the cochlea.
The hair cells are stimulated by the shearing force created by the vibration of the basilar membrane below them, relative to the stiffer tectorial membrane above them. Because of the tonotopy of the basilar membrane, hair cells are maximally stimulated by different frequencies depending on where they are in the cochlea. Those at the basal end respond best to high frequencies, and those at the apical end respond best to low frequencies. Consequently, their postsynaptic cells—the auditory nerve cells—have the same tonotopic pattern of responses.
This tonotopy is maintained throughout the auditory pathway, with information from different regions of the cochlea traveling in organized, parallel pathways through the brain. Ultimately, the primary auditory cortex contains a “map” of inputs from the basal to the apical end of the cochlea. The neurons that are stimulated within this map correlate with the frequencies that were heard, aiding in pitch discrimination.
Therefore, the cochlea plays a vital role both in the transduction of sound information into neural signals and the initial encoding of the pitch.
Suggested Reading
Lenarz, Thomas. “Cochlear Implant – State of the Art.” GMS Current Topics in Otorhinolaryngology, Head and Neck Surgery 16 (February 19, 2018). [Source]
Wong, Ann C. Y., and Allen F. Ryan. “Mechanisms of Sensorineural Cell Damage, Death and Survival in the Cochlea.” Frontiers in Aging Neuroscience 7 (April 21, 2015). [Source]
Elliott, Stephen J, and Christopher A Shera. “The Cochlea as a Smart Structure.” Smart Materials & Structures 21, no. 6 (June 2012): 064001. [Source]