Abstract
The mechanical and electrical responses of the mammalian cochlea to acoustic stimuli are nonlinear and highly tuned in frequency. This is due to the electromechanical properties of cochlear outer hair cells (OHCs). At each location along the cochlear spiral, the OHCs mediate an active process in which the sensory tissue motion is enhanced at frequencies close to the most sensitive frequency (called the characteristic frequency, CF). Previous experimental results showed an approximate 0.3 cycle phase shift in the OHC-generated extracellular voltage relative the basilar membrane displacement, which was initiated at a frequency approximately one-half octave lower than the CF. Findings in the present paper reinforce that result. This shift is significant because it brings the phase of the OHC-derived electromotile force near to that of the basilar membrane velocity at frequencies above the shift, thereby enabling the transfer of electrical to mechanical power at the basilar membrane. In order to seek a candidate physical mechanism for this phenomenon, we used a comprehensive electromechanical mathematical model of the cochlear response to sound. The model predicts the phase shift in the extracellular voltage referenced to the basilar membrane at a frequency approximately one-half octave below CF, in accordance with the experimental data. In the model, this feature arises from a minimum in the radial impedance of the tectorial membrane and its limbal attachment. These experimental and theoretical results are consistent with the hypothesis that a tectorial membrane resonance introduces the correct phasing between mechanical and electrical responses for power generation, effectively turning on the cochlear amplifier.
Highlights
MethodsExperimental measurements of local cochlear microphonic and basilar membrane (BM) motion.This paper is primarily a modeling paper, with experimental data included to bolster previous experimental findings
The same basilar membrane (BM) displacement phase is shown in each phase panel, and this enables a common reference for the comparison local cochlear microphonic (LCM) phase to the BM displacement phase similar to that of Fig. 2
We provide experimental and theoretical evidence supporting the role of the TM as the controlling factor for activation of the cochlear amplifier
Summary
Experimental measurements of local cochlear microphonic and BM motion.This paper is primarily a modeling paper, with experimental data included to bolster previous experimental findings. Experimental measurements of local cochlear microphonic and BM motion. The wg[165] data of Fig. 2 and the BM motion data of Fig. 429 were previously published. Other data from these figures are unpublished, similar LCM and OCT-based displacement data have been presented and methods fully described in recent work[12,17]. To keep the focus on the modeling results, the description of experimental methods for the unpublished data is kept short. The stimulus generation and acquisition were performed using MATLAB-based programs and a Tucker Davis Technologies (TDT) System. Pure tone stimuli were used for LCM measurements and multi-tone stimuli were used for the BM motion measurement of Fig. 2. The multi-tone stimulus was a Zwuis tone complex composed of 60
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