E4 Cochlea: Basilar Membrane Mechanics and Tonotopia

Topic

As the stapes pushes cochlear perilymph through the oval window, it creates a pressure difference across the basilar membrane—the partition running through the interior of the cochlea from the base to the opposite end. This pressure difference sets the basilar membrane in motion, but it does not move the entire membrane at once or uniformly; movement begins at the base of the cochlea, near the oval window, and propagates along the membrane as a "traveling wave." This mechanism, described by von Békésy, involves the wave traversing the membrane from one end to the other rather than causing the entire structure to vibrate simultaneously.

The traveling wave does not propagate with equal ease across the entire basilar membrane because the membrane's stiffness varies along its length: it is much stiffer and narrower near the cochlear base, becoming progressively less stiff and wider as the distance from the base increases. A stiffer section of the membrane responds best to higher frequencies, whereas a less stiff section responds best to lower frequencies; consequently, as the traveling wave moves from the base toward the opposite end, it passes through zones of decreasing stiffness, each tuned to a progressively lower frequency.

For any given frequency, the traveling wave moves along the basilar membrane, gradually gaining amplitude until it reaches the specific zone where the membrane's stiffness is optimally tuned to that frequency. At this point, the wave reaches its peak amplitude; beyond this location, it rapidly loses amplitude and virtually disappears. Since each frequency has its own position of maximum amplitude along the basilar membrane—determined by the membrane's stiffness at each point—there is a fixed correspondence between a sound's frequency and the specific location on the basilar membrane that responds to it with the greatest amplitude; this fixed correspondence is known as tonotopy, and the location where a specific frequency reaches its maximum amplitude is referred to as that location's characteristic frequency.

Outer hair cells are distributed along the entire length of the basilar membrane, but those situated precisely at the position of maximum amplitude for a given frequency experience the most intense movement of the membrane in response to that frequency, and they also contribute the most to its amplification. Outer hair cells located at the position of maximum amplitude for their specific characteristic frequency selectively amplify the basilar membrane's movement at that point by forty to sixty decibels—an active amplification that the passive traveling wave alone could not achieve without the contribution of these cells. This active amplification provided by the outer hair cells, concentrated right at the position of maximum amplitude, causes the peak of the traveling wave to be much higher and narrower than it would be if the basilar membrane moved solely due to the pressure difference delivered by the stapes.

Thanks to this combination of the position of maximum amplitude—determined by the basilar membrane's stiffness at each point—and the active amplification provided by the outer hair cells at that exact location, the cochlea is able to separate a complex sound (composed of multiple simultaneous frequencies) into a series of distinct positions along the basilar membrane, with each position responding primarily to its own characteristic frequency. This same tonotopy—initiated by the traveling wave from the stapes at the base of the cochlea—is what subsequently enables each outer and inner hair cell to transmit information specific to a particular frequency, rather than a single, undifferentiated signal representing the entire sound entering the cochlea.

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