The cochlea of the inner ear is the most critical structure in the auditory pathway, for it is there that the energy from sonically generated pressure waves is transformed into neural impulses. The cochlea not only amplifies sound waves and converts them into neural signals, but it also acts as a mechanical frequency analyzer, decomposing complex acoustical waveforms into simpler elements. Many features of auditory perception derive directly from the physical properties of the cochlea; hence, it is important to consider this structure in some detail.

The cochlea (from the Latin for “snail”) is a small (about 10 mm wide) coiled structure, which, were it uncoiled, would form a tube about 35 mm long (Figures 13.4 and 13.5). Both the oval window and the round window are at the basal end of this tube. The cochlea is bisected by the cochlear partition, which is a flexible structure that supports the basilar membrane and the tectorial membrane. There are fluid-filled spaces on each side of the cochlear partition, named the scala vestibuli and the scala tympani; a distinct channel, the scala media, runs within the cochlear partition. The cochlear partition does not extend all the way to the apical end of the cochlea; instead there is an opening, known as the helicotrema, that joins the scala vestibuli to the scala tympani. As a result of this structural arrangement, inward movement of the oval window displaces the fluid of the inner ear, which causes the round window to bulge out slightly and deforms the basilar membrane.

The manner in which the basilar membrane vibrates in response to sound is the key to understanding cochlear function. Measurements of the vibration of different parts of the basilar membrane, as well as the discharge rates of individual auditory nerve fibers, show that both these features are highly tuned; that is, they respond most intensely to a sound of a specific frequency. Frequency tuning within the inner ear is attributable in part to the geometry of the basilar membrane, which is wider and more flexible at the apical end and narrower and stiffer at the basal end. One feature of such a system is that regardless of where energy is supplied to it, movement always begins at the stiff end (i.e., the base), and then propagates to the more flexible end (i.e., the apex). Georg von Békésy, working at Harvard University, showed that a membrane that varies systematically in its width and flexibility vibrates maximally at different positions as a function of the stimulus frequency (Figure 13.5). Using tubular models and human cochleas taken from cadavers, he found that an acoustical stimulus initiates a traveling wave of the same frequency in the cochlea, which propagates from the base toward the apex of the basilar membrane, growing in amplitude and slowing in velocity until a point of maximum displacement is reached. This point of maximal displacement is determined by the sound frequency. The points responding to high frequencies are at the base of the basilar membrane, and the points responding to low frequencies are at the apex, giving rise to a topographical mapping of frequency (that is, to tonotopy). An important and striking feature of the tonotopically organized basilar membrane is that complex sounds cause a pattern of vibration equivalent to the superposition of the vibrations generated by the individual tones making up that complex sound, thus accounting for the decompositional aspects of cochlear function mentioned earlier.

Von Békésy's model of cochlear mechanics was a passive one, resting on the premise that the basilar membrane acts like a series of linked resonators, much as a concatenated set of tuning forks. Each point on the basilar membrane was postulated to have a characteristic frequency at which it vibrated most efficiently; because it was physically linked to adjacent areas of the membrane, each point also vibrated (if somewhat less readily) at other frequencies, thus permitting propagation of the traveling wave. It is now clear, however, that the tuning of the auditory periphery, whether measured at the basilar membrane or recorded as the electrical activity of auditory nerve fibers, is too sharp to be explained by passive mechanics alone. At very low sound intensities, the basilar membrane vibrates much more than would be predicted by linear extrapolation from the motion measured at high intensities. Therefore, the ear's sensitivity arises from an active biomechanical process, as well as from its passive resonant properties (Box B). The outer hair cells, which together with the inner hair cells comprise the sensory cells of the inner ear, are the most likely candidates for driving this active process. The details of this process are poorly understood.

The motion of the traveling wave initiates sensory transduction by displacing the hair cells that sit atop the basilar membrane. Because these structures are anchored at different positions, the vertical component of the traveling wave is translated into a shearing motion between the basilar membrane and the overlying tectorial membrane (Figure 13.6). This motion bends the tiny processes, called stereocilia, that protrude from the apical ends of the hair cells, leading to voltage changes across the hair cell membrane. How the bending of stereocilia leads to receptor potentials in hair cells is considered in the following section.