Deciphering of Sound Wave Frequency Begins in the Cochlea
The physical properties of the basilar membrane are not uniform along its length. It is narrower and relatively stiffer at its base (near the oval window), and it becomes progressively wider and more flexible toward its apex (see Figure 17-4, B).
Thus the resonant properties of the membrane are not uniform. A region near the base is significantly displaced by a high-frequency sound, and as the sound frequency decreases, the region of significant displacement is located progressively closer to the apex (see Figure 17-4, C). Because the organ of Corti sits atop the basilar membrane, high-frequency sounds are most likely to affect hair cells and their associated eighth nerve neurons near the base of the membrane. As frequency decreases, the hair cells and neurons that are activated are located progressively closer to the apex. Given this orderly relationship between the frequency of a sound wave and the region of the cochlea that is activated by that frequency, the cochlea is said to have a tonotopic organization. Therefore a principal means by which the nervous system begins to decipher the frequency of a sound is through the location of the hair cells and neurons that are most affected by that sound.
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