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Tichacek O, Mistrík P, Jungwirth P. From the outer ear to the nerve: A complete computer model of the peripheral auditory system. Hear Res 2023; 440:108900. [PMID: 37944408 DOI: 10.1016/j.heares.2023.108900] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/28/2023] [Revised: 10/03/2023] [Accepted: 10/23/2023] [Indexed: 11/12/2023]
Abstract
Computer models of the individual components of the peripheral auditory system - the outer, middle, and inner ears and the auditory nerve - have been developed in the past, with varying level of detail, breadth, and faithfulness of the underlying parameters. Building on previous work, we advance the modeling of the ear by presenting a complete, physiologically justified, bottom-up computer model based on up-to-date experimental data that integrates all of these parts together seamlessly. The detailed bottom-up design of the present model allows for the investigation of partial hearing mechanisms and their defects, including genetic, molecular, and microscopic factors. Also, thanks to the completeness of the model, one can study microscopic effects in the context of their implications on hearing as a whole, enabling the correlation with neural recordings and non-invasive psychoacoustic methods. Such a model is instrumental for advancing quantitative understanding of the mechanism of hearing, for investigating various forms of hearing impairment, as well as for devising next generation hearing aids and cochlear implants.
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Affiliation(s)
- Ondrej Tichacek
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo nam. 2, 160 00 Prague 6, Czech Republic.
| | | | - Pavel Jungwirth
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo nam. 2, 160 00 Prague 6, Czech Republic.
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2
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Altoè A, Charaziak KK. Intracochlear overdrive: Characterizing nonlinear wave amplification in the mouse apex. THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA 2023; 154:3414-3428. [PMID: 38015028 PMCID: PMC10686682 DOI: 10.1121/10.0022446] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/06/2023] [Revised: 10/02/2023] [Accepted: 11/01/2023] [Indexed: 11/29/2023]
Abstract
In this study, we explore nonlinear cochlear amplification by analyzing basilar membrane (BM) motion in the mouse apex. Through in vivo, postmortem, and mechanical suppression recordings, we estimate how the cochlear amplifier nonlinearly shapes the wavenumber of the BM traveling wave, specifically within a frequency range where the short-wave approximation holds. Our findings demonstrate that a straightforward mathematical model, depicting the cochlear amplifier as a wavenumber modifier with strength diminishing monotonically as BM displacement increases, effectively accounts for the various experimental observations. This empirically derived model is subsequently incorporated into a physics-based "overturned" framework of cochlear amplification [see Altoè, Dewey, Charaziak, Oghalai, and Shera (2022), J. Acoust. Soc. Am. 152, 2227-2239] and tested against additional experimental data. Our results demonstrate that the relationships established within the short-wave region remain valid over a much broader frequency range. Furthermore, the model, now exclusively calibrated to BM data, predicts the behavior of the opposing side of the cochlear partition, aligning well with recent experimental observations. The success in reproducing key features of the experimental data and the mathematical simplicity of the resulting model provide strong support for the "overturned" theory of cochlear amplification.
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Affiliation(s)
- Alessandro Altoè
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90007, USA
| | - Karolina K Charaziak
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90007, USA
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3
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Altoè A, Shera CA. The Long Outer-Hair-Cell RC Time Constant: A Feature, Not a Bug, of the Mammalian Cochlea. J Assoc Res Otolaryngol 2023; 24:129-145. [PMID: 36725778 PMCID: PMC10121995 DOI: 10.1007/s10162-022-00884-w] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/15/2022] [Accepted: 12/23/2022] [Indexed: 02/03/2023] Open
Abstract
The cochlea of the mammalian inner ear includes an active, hydromechanical amplifier thought to arise via the piezoelectric action of the outer hair cells (OHCs). A classic problem of cochlear biophysics is that the RC (resistance-capacitance) time constant of the hair-cell membrane appears inconveniently long, producing an effective cut-off frequency much lower than that of most audible sounds. The long RC time constant implies that the OHC receptor potential-and hence its electromotile response-decreases by roughly two orders of magnitude over the frequency range of mammalian hearing, casting doubt on the hypothesized role of cycle-by-cycle OHC-based amplification in mammalian hearing. Here, we review published data and basic physics to show that the "RC problem" has been magnified by viewing it through the wrong lens. Our analysis finds no appreciable mismatch between the expected magnitude of high-frequency electromotility and the sound-evoked displacements of the organ of Corti. Rather than precluding significant OHC-based boosts to auditory sensitivity, the long RC time constant appears beneficial for hearing, reducing the effects of internal noise and distortion while increasing the fidelity of cochlear amplification.
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Affiliation(s)
- Alessandro Altoè
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, CA, USA.
| | - Christopher A Shera
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, CA, USA.
- Department of Physics & Astronomy, University of Southern California, Los Angeles, CA, USA.
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4
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Guinan JJ. Cochlear amplification in the short-wave region by outer hair cells changing organ-of-Corti area to amplify the fluid traveling wave. Hear Res 2022. [DOI: 10.1016/j.heares.2022.108641] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/04/2022]
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5
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Cho NH, Puria S. Cochlear motion across the reticular lamina implies that it is not a stiff plate. Sci Rep 2022; 12:18715. [PMID: 36333415 PMCID: PMC9636238 DOI: 10.1038/s41598-022-23525-x] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2022] [Accepted: 11/01/2022] [Indexed: 11/06/2022] Open
Abstract
Within the cochlea, the basilar membrane (BM) is coupled to the reticular lamina (RL) through three rows of piezo-like outer hair cells (OHCs) and supporting cells that endow mammals with sensitive hearing. Anatomical differences across OHC rows suggest differences in their motion. Using optical coherence tomography, we measured in vivo and postmortem displacements through the gerbil round-window membrane from approximately the 40-47 kHz best-frequency (BF) regions. Our high spatial resolution allowed measurements across the RL surface at the tops of the three rows of individual OHCs and their bottoms, and across the BM. RL motion varied radially; the third-row gain was more than 3 times greater than that of the first row near BF, whereas the OHC-bottom motions remained similar. This implies that the RL mosaic, comprised of OHC and phalangeal-process tops joined together by adhesion molecules, is much more flexible than the Deiters' cells connected to the OHCs at their bottom surfaces. Postmortem, the measured points moved together approximately in phase. These imply that in vivo, the RL does not move as a stiff plate hinging around the pillar-cell heads near the first row as has been assumed, but that its mosaic-like structure may instead bend and/or stretch.
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Affiliation(s)
- Nam Hyun Cho
- Department of Otolaryngology-Head and Neck Surgery, Harvard Medical School, Boston, MA, 02114, USA
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA
| | - Sunil Puria
- Department of Otolaryngology-Head and Neck Surgery, Harvard Medical School, Boston, MA, 02114, USA.
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA.
- Speech and Hearing Bioscience and Technology Program, Harvard University, Cambridge, MA, 02138, USA.
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6
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Altoè A, Dewey JB, Charaziak KK, Oghalai JS, Shera CA. Overturning the mechanisms of cochlear amplification via area deformations of the organ of Corti. THE JOURNAL OF THE ACOUSTICAL SOCIETY OF AMERICA 2022; 152:2227. [PMID: 36319240 PMCID: PMC9578757 DOI: 10.1121/10.0014794] [Citation(s) in RCA: 9] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/28/2022] [Revised: 08/17/2022] [Accepted: 09/23/2022] [Indexed: 06/16/2023]
Abstract
The mammalian ear embeds a cellular amplifier that boosts sound-induced hydromechanical waves as they propagate along the cochlea. The operation of this amplifier is not fully understood and is difficult to disentangle experimentally. In the prevailing view, cochlear waves are amplified by the piezo-electric action of the outer hair cells (OHCs), whose cycle-by-cycle elongations and contractions inject power into the local motion of the basilar membrane (BM). Concomitant deformations of the opposing (or "top") side of the organ of Corti are assumed to play a minor role and are generally neglected. However, analysis of intracochlear motions obtained using optical coherence tomography calls this prevailing view into question. In particular, the analysis suggests that (i) the net local power transfer from the OHCs to the BM is either negative or highly inefficient; and (ii) vibration of the top side of the organ of Corti plays a primary role in traveling-wave amplification. A phenomenological model derived from these observations manifests realistic cochlear responses and suggests that amplification arises almost entirely from OHC-induced deformations of the top side of the organ of Corti. In effect, the model turns classic assumptions about spatial impedance relations and power-flow direction within the sensory epithelium upside down.
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Affiliation(s)
- Alessandro Altoè
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90033, USA
| | - James B Dewey
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90033, USA
| | - Karolina K Charaziak
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90033, USA
| | - John S Oghalai
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90033, USA
| | - Christopher A Shera
- Caruso Department of Otolaryngology, University of Southern California, Los Angeles, California 90033, USA
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Meenderink SWF, Lin X, Park BH, Dong W. Sound Induced Vibrations Deform the Organ of Corti Complex in the Low-Frequency Apical Region of the Gerbil Cochlea for Normal Hearing : Sound Induced Vibrations Deform the Organ of Corti Complex. J Assoc Res Otolaryngol 2022; 23:579-591. [PMID: 35798901 PMCID: PMC9613840 DOI: 10.1007/s10162-022-00856-0] [Citation(s) in RCA: 7] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/31/2022] [Accepted: 06/16/2022] [Indexed: 10/17/2022] Open
Abstract
Human speech primarily contains low frequencies. It is well established that such frequencies maximally excite the cochlea near its apex. But, the micromechanics that precede and are involved in this transduction are not well understood. We measured vibrations from the low-frequency, second turn in intact gerbil cochleae using optical coherence tomography (OCT). The data were used to create spatial maps that detail the sound-evoked motions across the sensory organ of Corti complex (OCC). These maps were remarkably similar across animals and showed little variation with frequency or level. We identify four, anatomically distinct, response regions within the OCC: the basilar membrane (BM), the outer hair cells (OHC), the lateral compartment (lc), and the tectorial membrane (TM). Results provide evidence that active processes in the OHC play an important role in the mechanical interplay between different OCC structures which increases the amplitude and tuning sharpness of the traveling wave. The angle between the OCT beam and the OCC makes that we captured radial motions thought to be the effective stimulus to the mechano-sensitive hair bundles. We found that TM responses were relatively weak, arguing against a role in enhancing mechanical hair bundle deflection. Rather, BM responses were found to closely resemble the frequency selectivity and sensitivity found in auditory nerve fibers (ANF) that innervate the low-frequency cochlea.
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Affiliation(s)
| | - Xiaohui Lin
- VA Loma Linda Healthcare System, Loma Linda, CA, 92374, USA
| | - B Hyle Park
- Department of Bioengineering, University of California, Riverside, Riverside, CA, 92521, USA
| | - Wei Dong
- VA Loma Linda Healthcare System, Loma Linda, CA, 92374, USA.
- Loma Linda University Health, Loma Linda, CA, 92350, USA.
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He W, Burwood G, Fridberger A, Nuttall AL, Ren T. An outer hair cell-powered global hydromechanical mechanism for cochlear amplification. Hear Res 2022; 423:108407. [PMID: 34922772 PMCID: PMC9156726 DOI: 10.1016/j.heares.2021.108407] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/06/2021] [Revised: 11/23/2021] [Accepted: 11/30/2021] [Indexed: 11/04/2022]
Abstract
It is a common belief that the mammalian cochlea achieves its exquisite sensitivity, frequency selectivity, and dynamic range through an outer hair cell-based active process, or cochlear amplification. As a sound-induced traveling wave propagates from the cochlear base toward the apex, outer hair cells at a narrow region amplify the low level sound-induced vibration through a local feedback mechanism. This widely accepted theory has been tested by measuring sound-induced sub-nanometer vibrations within the organ of Corti in the sensitive living cochleae using heterodyne low-coherence interferometry and optical coherence tomography. The aim of this short review is to summarize experimental findings on the cochlear active process by the authors' group. Our data show that outer hair cells are able to generate substantial forces for driving the cochlear partition at all audible frequencies in vivo. The acoustically induced reticular lamina vibration is larger and more broadly tuned than the basilar membrane vibration. The reticular lamina and basilar membrane vibrate approximately in opposite directions at low frequencies and in the same direction at the best frequency. The group delay of the reticular lamina is larger than that of the basilar membrane. The magnitude and phase differences between the reticular lamina and basilar membrane vibration are physiologically vulnerable. These results contradict predictions based on the local feedback mechanism but suggest a global hydromechanical mechanism for cochlear amplification. This article is part of the Special Issue Outer hair cell Edited by Joseph Santos-Sacchi and Kumar Navaratnam.
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Affiliation(s)
- Wenxuan He
- Department of Otolaryngology, Head and Neck Surgery, Oregon Health & Science University, Portland OR, United States
| | - George Burwood
- Department of Otolaryngology, Head and Neck Surgery, Oregon Health & Science University, Portland OR, United States
| | - Anders Fridberger
- Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden
| | - Alfred L Nuttall
- Department of Otolaryngology, Head and Neck Surgery, Oregon Health & Science University, Portland OR, United States
| | - Tianying Ren
- Department of Otolaryngology, Head and Neck Surgery, Oregon Health & Science University, Portland OR, United States.
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9
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Cho NH, Wang H, Puria S. Cochlear Fluid Spaces and Structures of the Gerbil High-Frequency Region Measured Using Optical Coherence Tomography (OCT). J Assoc Res Otolaryngol 2022; 23:195-211. [PMID: 35194695 PMCID: PMC8964889 DOI: 10.1007/s10162-022-00836-4] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2021] [Accepted: 01/04/2022] [Indexed: 10/19/2022] Open
Abstract
Since it has been difficult to directly observe the morphology of the living cochlea, our ability to infer the mechanical functioning of the living ear has been limited. Nearly all our knowledge about cochlear morphology comes from postmortem tissue that was fixed and processed using procedures that possibly distort the structures and fluid spaces of the organ of Corti. In this study, optical coherence tomography was employed to obtain volumetric images of the high-frequency hook region of the gerbil cochlea, as viewed through the round window, with far better resolution capability than had been possible before. The anatomical structures and fluid spaces of the organ of Corti were segmented and quantified in vivo and over a 90-min postmortem period. We find that the arcuate-zone and pectinate-zone widths change very little postmortem. The volume of the scala tympani between the round-window membrane and basilar membrane and the volume of the inner spiral sulcus decrease in the first 60-min postmortem. While textbook drawings of the mammalian organ of Corti and cortilymph prominently depict the tunnel of Corti, the outer tunnel is typically missing. This is likely because textbook drawings are typically made from images obtained by histological methods. Here, we show that the outer tunnel is nearly twice as big as the tunnel of Corti or the space of Nuel. This larger outer tunnel fluid space could have a substantial, little-appreciated effect on cochlear micromechanics. We speculate that the outer tunnel forms a resonant structure that may affect reticular-lamina motion.
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Affiliation(s)
- Nam Hyun Cho
- Department of Otolaryngology-Head and Neck Surgery, Harvard Medical School, Boston, MA, 02114, USA
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA
| | - Haobing Wang
- Department of Otolaryngology-Head and Neck Surgery, Harvard Medical School, Boston, MA, 02114, USA
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA
| | - Sunil Puria
- Department of Otolaryngology-Head and Neck Surgery, Harvard Medical School, Boston, MA, 02114, USA.
- Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA.
- Speech and Hearing Bioscience and Technology Program, Harvard University, Cambridge, MA, 02138, USA.
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10
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Kim W, Liu D, Kim S, Ratnayake K, Macias-Escriva F, Mattison S, Oghalai JS, Applegate BE. Vector of motion measurements in the living cochlea using a 3D OCT vibrometry system. BIOMEDICAL OPTICS EXPRESS 2022; 13:2542-2553. [PMID: 35519276 PMCID: PMC9045890 DOI: 10.1364/boe.451537] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/16/2021] [Revised: 03/11/2022] [Accepted: 03/11/2022] [Indexed: 05/22/2023]
Abstract
Optical coherence tomography (OCT) has become an important tool for measuring the vibratory response of the living cochlea. It stands alone in its capacity to measure the intricate motion of the hearing organ through the surrounding otic capsule bone. Nevertheless, as an extension of phase-sensitive OCT, it is only capable of measuring motion along the optical axis. Hence, measurements are 1-D. To overcome this limitation and provide a measure of the 3-D vector of motion in the cochlea, we developed an OCT system with three sample arms in a single interferometer. Taking advantage of the long coherence length of our swept laser, we depth (frequency) encode the three channels. An algorithm to depth decode and coregister the three channels is followed by a coordinate transformation that takes the vibrational data from the experimental coordinate system to Cartesian or spherical polar coordinates. The system was validated using a piezo as a known vibrating element that could be positioned at various angles. The angular measurement on the piezo was shown to have an RMSE of ≤ 0.30° (5.2 mrad) with a standard deviation of the amplitude of ≤ 120 pm. Finally, we demonstrate the system for in vivo imaging by measuring the vector of motion over a volume image in the apex of the mouse cochlea.
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Affiliation(s)
- Wihan Kim
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
- Contributed equally
| | - Derek Liu
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
- Contributed equally
| | - Sangmin Kim
- Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843, USA
| | - Kumara Ratnayake
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
| | - Frank Macias-Escriva
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
| | - Scott Mattison
- Department of Biomedical Engineering, Texas A&M University, College Station, TX 77843, USA
- Current address: Department of Engineering and Physics, University of Central Oklahoma, Edmond, OK 73034, USA
| | - John S. Oghalai
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
| | - Brian E. Applegate
- Caruso Department of Otolaryngology - Head and Neck Surgery, University of Southern California, Los Angeles, CA 90033, USA
- Department of Biomedical Engineering, University of Southern California, Los Angeles, CA. 90089, USA
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11
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Fallah E, Strimbu CE, Olson ES. Nonlinearity of intracochlear motion and local cochlear microphonic: Comparison between guinea pig and gerbil. Hear Res 2021; 405:108234. [PMID: 33930834 DOI: 10.1016/j.heares.2021.108234] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/20/2020] [Revised: 03/08/2021] [Accepted: 03/26/2021] [Indexed: 12/19/2022]
Abstract
Studying the in-vivo mechanical and electrophysiological cochlear responses in several species helps us to have a comprehensive view of the sensitivity and frequency selectivity of the cochlea. Different species might use different mechanisms to achieve the sharp frequency-place map. The outer hair cells (OHC) play an important role in mediating frequency tuning. In the present work, we measured the OHC-generated local cochlear microphonic (LCM) and the motion of different layers in the organ of Corti using optical coherence tomography (OCT) in the first turn of the cochlea in guinea pig. In the best frequency (BF) band, our observations were similar to our previous measurements in gerbil: a nonlinear peak in LCM responses and in the basilar membrane (BM) and OHC-region displacements, and higher motion in the OHC region than the BM. Sub-BF the responses in the two species were different. In both species the sub-BF displacement of the BM was linear and LCM was nonlinear. Sub-BF in the OHC-region, nonlinearity was only observed in a subset of healthy guinea pig cochleae while in gerbil, robust nonlinearity was observed in all healthy cochleae. The differences suggest that gerbils and guinea pigs employ different mechanisms for filtering sub-BF OHC activity from BM responses. However, it cannot be ruled out that the differences are due to technical measurement differences across the species.
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Affiliation(s)
- Elika Fallah
- Department of Biomedical Engineering, Columbia University, New York City, NY, United States
| | - C Elliott Strimbu
- Department of Otolaryngology-Head and Neck Surgery, Columbia University, New York City, NY, United States
| | - Elizabeth S Olson
- Department of Biomedical Engineering, Columbia University, New York City, NY, United States; Department of Otolaryngology-Head and Neck Surgery, Columbia University, New York City, NY, United States.
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12
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Nankali A, Wang Y, Strimbu CE, Olson ES, Grosh K. A role for tectorial membrane mechanics in activating the cochlear amplifier. Sci Rep 2020; 10:17620. [PMID: 33077807 PMCID: PMC7573614 DOI: 10.1038/s41598-020-73873-9] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/13/2020] [Accepted: 09/21/2020] [Indexed: 12/02/2022] Open
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.
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Affiliation(s)
- Amir Nankali
- Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA
| | - Yi Wang
- Biomedical Engineering, Columbia University, New York, NY, USA
| | | | - Elizabeth S Olson
- Otolaryngology, Head and Neck Surgery, Columbia University, New York, NY, USA.,Biomedical Engineering, Columbia University, New York, NY, USA
| | - Karl Grosh
- Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA. .,Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA.
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