Measurements and models of electrically-evoked motion in the gerbil organ of Corti

Abstract

A cell with extraordinary motile ability exists in our inner ear, the outer hair cell. Outer hair cell (OHC) motility can occur at acoustic frequencies and play a key role in mammalian cochlear frequency selectivity and hearing sensitivity. To date, the mechanism of cochlear amplification is not well understood and remains a matter of controversy. In order to understand the role of OHC motility in cochlear micromechanics we developed a technique to measure the mechanical responses within the organ of Corti (OC) due to OHC forces. We used an excised cochlea preparation because it provided a good view of the organ and allowed us to compare the resulting responses of hundreds of cells simultaneously. The tissue was stimulated electrically using sinusoidal current, and the resulting motion was captured at specific phases within the stimulus period using stroboscopic video microscopy. Animations of this motion were created, and the displacement magnitude and phase for each structure were calculated using two dimensional cross-correlation. With these techniques we were able to detect displacements as low as ten nanometers. The frequency responses of electrically-evoked vibrations from the apical and middle turn had low pass filtering characteristics with cutoff frequencies near or below the estimated characteristic frequency of the imaging location. Using a simple one dimensional electrical model of our excised cochlea preparation, we hypothesize that the electrical properties of the stria vascularis play an important role in shaping the frequency response of individual structures.

(cont.) The vibration pattern of the organ was complex and changed with frequency. These changes suggest that at least two OC vibration modes are excited by OHC motility. At all frequencies OHC motility induced oscillatory fluid flow in the tunnel of Corti. We modeled the tunnel of Corti as an elastic tube and showed that it can support a traveling wave. The tunnel of Corti wave could travel without significant attenuation for distances larger than the wavelength of the cochlear traveling wave at its peak. The classical view of cochlear partition vibration is that the structure simply bends in phase along the radial dimension, and that there is no coupling between adjacent sections other than that provided by the fluid above and below the OC. Our findings challenge the classical view of cochlear partition vibration, and support the existence of multiple vibration modes. In addition, the presence of fluid flow in the tunnel of Corti in response to OHC contractions suggests that a second traveling wave provides longitudinal coupling between adjacent sections. Such coupling may be critical for cochlear amplification.