Abstract
The organ of Corti (OC) is the auditory epithelium of the mammalian cochlea comprising sensory hair cells and supporting cells riding on the basilar membrane. The outer hair cells (OHCs) are cellular actuators that amplify small sound-induced vibrations for transmission to the inner hair cells. We developed a finite element model of the OC that incorporates the complex OC geometry and force generation by OHCs originating from active hair bundle motion due to gating of the transducer channels and somatic contractility due to the membrane protein prestin. The model also incorporates realistic OHC electrical properties. It explains the complex vibration modes of the OC and reproduces recent measurements of the phase difference between the top and the bottom surface vibrations of the OC. Simulations of an individual OHC show that the OHC somatic motility lags the hair bundle displacement by ∼90 degrees. Prestin-driven contractions of the OHCs cause the top and bottom surfaces of the OC to move in opposite directions. Combined with the OC mechanics, this results in ∼90 degrees phase difference between the OC top and bottom surface vibration. An appropriate electrical time constant for the OHC membrane is necessary to achieve the phase relationship between OC vibrations and OHC actuations. When the OHC electrical frequency characteristics are too high or too low, the OHCs do not exert force with the correct phase to the OC mechanics so that they cannot amplify. We conclude that the components of OHC forward and reverse transduction are crucial for setting the phase relations needed for amplification.
Highlights
The organ of Corti (OC) is the sensory epithelium unique to the mammalian cochlea
We explored the dynamic relations between the OC mechanical variables such as the relative displacements of the tectorial membrane and reticular lamina determining hair bundle motion, and the electrical variables such as the Outer hair cells (OHCs) receptor potential and contractility dictating the feedback
OHC is a Phase Maker for the Cochlear Amplification The OHC somatic motility imposes a phase reversal between the basilar membrane and the reticular lamina [3,9] sometimes referred to as ‘negative feedback’ [18]. This opposing motion between the top and the bottom surface of the OC attributable to OHC somatic motility combined with the phase lag due to hair bundle motility (Fig. 3) results in,90 degree phase difference between the reticular lamina and the basilar membrane
Summary
The organ of Corti (OC) is the sensory epithelium unique to the mammalian cochlea. It is sandwiched between two tissues called the basilar membrane and the tectorial membrane, and these layers comprise the cochlear partition separating the two fluid compartments known as the scala media and scala tympani. Stroboscopic illumination and imaging demonstrated that the OC mechanics is highly complicated and dependent on the type of stimulation–acoustical or electrical [5,6]. Optical coherence tomography provided a clearer view of the relative motion within the OC [7,8] and showed that the vibration at the top surface of the OC leads the bottom by about 90 degrees at low stimulation levels, the phase difference diminishing as the stimulation level increases
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