Cochlear Micromechanics and Macromechanics

The main task of the mammalian cochlea is to analyze sound in terms of its intensity, timing, and frequency content. To achieve this, sound in air is “matched” acoustically to the denser water medium within the cochlea by the middle ear, efficiently producing pressure fluctuations in the cochlear fluids and vibration of the ribbonlike basilar membrane (BM) and the sensory organ of Corti (OC) perched upon it. The vibration of the cochlear partition (BM and OC) normally takes the form of waves or ripples that travel away from the stapes and toward the cochlear apex (See Fig. 1.5, Dallos, Chapter 1). For a pure-tone stimulus, the wave grows as it travels, reaching a maximum at a position known as the wave’s characteristic place, and then collapses abruptly so that no vibration exists beyond a cochlear position known as the wave’s cutoff region. The characteristic place and cutoff region are positioned along the cochlear length according to a place-frequency map: high frequencies toward the cochlear base, closer to the stapes, and low frequencies closer to the cochlear apex. The vibration of the BM and OC relative to the surrounding bony structures is known as macromechanical vibration, while the complex vibration of the parts of the OC relative to one another is known as micromechanical vibration. It is worth stressing here that both these vibrations are normally of atomic or at most molecular dimensions, comparable with the width of a cell membrane (less than 10 nm), and they are not independent: not only does macromechanical vibration of the BM “drive” the micromechanical vibration of the OC, but the micromechanics greatly affects the macromechanics.