Mechanical Motion in the Inner Ear in Response to Sound

The basilar membrane is a resonant structure varying systematically in width and stiffness. It is wider (0.420.65 mm) and more flaccid at the cochlear apex than at the base (0.08-0.16 mm). When a sound wave is transmitted to the fluid of the inner ear, the basilar membrane is set in motion. Basilar membrane motion is best described as a traveling wave of deformation, which begins at the cochlear base and moves apically toward a frequency-dependent place of maximal amplitude (Fig. 4). When very high-frequency sound waves reach the ear, only the region nearest the cochlear base vibrates. As the frequency of the sound is lowered, the place of maximal amplitude of vibration shifts toward the cochlear apex. Because of this resonance gradient, the basilar membrane is said to be "tonotopically" organized. Consequently, complex sound (e.g., speech) entering the inner ear is resolved into its component frequencies. This physical separation of sound energy into its spectral components, coupled with the focused innervation of the auditory nerve array, provides an orderly and spectrally segregated projection of the nerve into the auditory brain stem, thereby setting the stage for tonotopic organization along the entire central auditory pathway.

Normal cochlear vibration is not simply the result of passive mechanical resonance as once thought, but rather it involves active processes. Under certain conditions, OHCs are capable of changing shape,

Figure 4 Stylized mammalian cochlea, shown uncoiled to illustrate the flow of energy and pattern of vibration of the cochlear partition in response to a midfrequency tone of modest intensity. The scalae vestibuli and tympani are assigned the same shading as in cochlear cross sections shown in Fig. 2. [Adapted with permission from Geisler, C.D. (1998). From Sound to Synapse. Oxford Univ. Press, New York].

Figure 4 Stylized mammalian cochlea, shown uncoiled to illustrate the flow of energy and pattern of vibration of the cochlear partition in response to a midfrequency tone of modest intensity. The scalae vestibuli and tympani are assigned the same shading as in cochlear cross sections shown in Fig. 2. [Adapted with permission from Geisler, C.D. (1998). From Sound to Synapse. Oxford Univ. Press, New York].

which feeds energy back into the organ of Corti and alters the mechanical properties of the cochlear partition and possibly the transduction process. This active process may be controlled, in part, by feedback via olivocochlear axons originating in the auditory brain stem and profusely terminating at the base of OHCs. Apparently as the result of an active process, the organ of Corti acts not only to receive sound but also to generate it, as an otoacoustic emission (OAC) recorded by a microphone in the ear canal. There are several categories of OACs, reflecting perhaps more than one nonlinear active process in the cochlea. OACs are proving useful as an objective tool for diagnosing sensorineural hearing loss.

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