The Endocochlear Potential Alters Cochlear Micromechanics

Abstract

Acoustic stimulation gates mechanically sensitive ion channels in cochlear sensory hair cells. Even in the absence of sound, a fraction of these channels remains open, forming a conductance between hair cells and the adjacent fluid space, scala media. Restoring the lost endogenous polarization of scala media in an in vitro preparation of the whole cochlea depolarizes the hair cell soma. Using both digital laser interferometry and time-resolved confocal imaging, we show that this causes a structural refinement within the organ of Corti that is dependent on the somatic electromotility of the outer hair cells (OHCs). Specifically, the inner part of the reticular lamina up to the second row of OHCs is pulled toward the basilar membrane, whereas the outer part (third row of OHCs and the Hensen's cells) unexpectedly moves in the opposite direction. A similar differentiated response pattern is observed for sound-evoked vibrations: restoration of the endogenous polarization decreases vibrations of the inner part of the reticular lamina and results in up to a 10-fold increase of vibrations of the outer part. We conclude that the endogenous polarization of scala media affects the function of the hearing organ by altering its geometry, mechanical and electrical properties.

Active Outer Hair Cells Affect the Sound-Evoked Vibration of the Reticular Lamina

“…The reported vibration increase of OHC3 with positive current is in agreement with the interferometric measurements obtained from Hensen's cells [11,13]. However, the vibration decrease observed at OHC1 defies any obvious explanation, but a somewhat similar behavior was reported for alternating current stimulation at acoustic frequencies [17].…”

“…We have previously shown spatial differences in the sustained current-evoked repositioning of the reticular lamina [11]. In this report OHC1 moved in the opposite direction from OHC3 in response to quasistatic current injections, indicating different motion constraints for the individual OHC rows.…”

“…Sodium salicylate, a drug used to block somatic electromotility, abolishes any effect of current on the sound-evoked vibration amplitudes [11]. Furthermore, blocking the transduction channel also abolishes alterations of the sound-evoked vibration amplitudes [11].…”

“…The first image of each image series was discarded to avoid onset artifacts. Pixels were sorted accordingly to the phase of the electrical stimulus [11]. Images at various (12 equally spaced) acoustic phases were reconstructed using Fourier rows for each current polarity.…”

Basilar Membrane and Tectorial Membrane Stiffness in the CBA/CaJ Mouse

“…Dallos 1992). Experimental results are available that studied the micromechanics of the organ of Corti in excised cochlear preparations, including guinea pig temporal bone preparations, sections of the guinea pig, gerbil cochlea, and the gerbil hemicochlea (Chan and Hudspeth 2005;Edge et al 1998;Fridberger et al 2002a, b;Fridberger and de Monvel 2003;Hu et al 1995Hu et al , 1999Jacob et al 2011;Jia et al 2007;Karavitaki and Mountain 2007a, b;Karavitaki et al 1996;Mammano and Ashmore 1993;Morioka et al 1995;Nowotny and Gummer 2006;Reuter et al 1992;Dallos 2001, 2003;Richter et al 2002Richter et al , 2000von Tiedemann et al 2010). Results have been published using confocal microscopy or optical coherence tomography (OCT) to study micromechanics of the cochlea in vivo (Chen et al 2007(Chen et al , 2011Choudhury et al 2006;Gao et al 2011Gao et al , 2013Masaki et al 2009;Zha et al 2012).…”