Biology subjects

Puria, S.

Publications and source records attributed to Puria, S..

3 recordsLinked to original sources

bioRxiv2022

A Cochlea-Slice Model using Floquet Boundary Conditions shows Global Tuning

A common assumption about the cochlea is that the local characteristic frequency (CF) is determined by a local resonance of basilar-membrane (BM) stiffness with the mass of the organ-of-Corti (OoC) and entrained fluid. We modeled the cochlea while avoiding such a priori assumptions by using a finite-element model of a 20-m-thick cross-sectional slice of the middle turn of a passive gerbil cochlea. The model had anatomically accurate structural details with physiologically appropriate material properties and interactions between the fluid spaces and solid OoC structures. The longitudinally-facing sides of the slice had a phase difference that mimicked the traveling-wave wavelength at the location of the slice by using Floquet boundary conditions. A paired volume-velocity drive was applied in the scalae at the top and bottom of the slice with the amplitudes adjusted to mimic experimental BM motion. The development of this computationally efficient model with detailed anatomical structures is a key innovation of this work. The resulting OoC motion was greatest in the transverse direction, stereocilia-tip deflections were greatest in the radial direction and longitudinal motion was small in OoC tissue but became large in the sulcus at high frequencies. If the source velocity and wavelength were held constant across frequency, the OoC motion was almost flat across frequency, i.e., the slice showed no local resonance. A model with the source velocity held constant and the wavelength varied realistically across frequency, produced a low-pass frequency response. These results indicate that tuning in the gerbil middle turn is not produced by a resonance due to local OoC mechanical properties, but rather is produced by the characteristics of the traveling wave, manifested in the driving pressure and wavelength. STATEMENT OF SIGNIFICANCEThe sensory epithelium of hearing, the organ of Corti, is encased in the bone of the fluid-filled cochlea and is difficult to study experimentally. We provide a new method to study the cochlea: making an anatomically-detailed finite-element model of a small transverse slice of the cochlea using Floquet boundary conditions and incorporating global cochlear properties in the slice drive and the wavelength-frequency relationship. The model shows that the slice properties do not show a mechanical resonance and therefore do not produce the frequency-response tuning of the cochlea. Instead, tuning emerges from global cochlear properties carried by the traveling wave.

biophysics↗

bioRxiv2022

Motion of the Cochlear Reticular Lamina Varies Radially Across Outer-Hair-Cell Rows

The basilar membrane (BM) is connected to the reticular lamina (RL) through three rows of Y-shaped structures consisting of an outer hair cell (OHC) and a Deiters cell (DC) with a phalangeal process (PhP) that forms part of the RL mosaic surface. Morphological differences in the anatomy of the Y-shaped structures across the three OHC rows suggest differences in motion across the rows. Here we report OoC transverse motions measured across several radial locations for the gerbil basal region corresponding to ~45 kHz. Cross-sectional imaging and vibrometry measurements were made using a high-resolution (2.23 um axially in water) spectral-domain optical-coherence-tomography (SD-OCT) system. The stimuli were pure tones (2-63 kHz) at ear-canal sound pressure levels (SPLs) of 30-95 dB SPL in anesthetized gerbils (N=9) with healthy cochleae. We report displacements at the RL regions of OHC rows 1-3 (RL1-3), at the OHC-DC junctions of OHC rows 1-3 (OHC-DC-junction1-3), and at the arcuate zone, arcuate-pectinate junction, and pectinate zone of the BM (BMAZ, BMAPJ, and BMPZ, respectively). The in vivo BM displacements showed classic compressive nonlinearity and traveling-wave delays. The RL gain was similar to the BM gain at low frequencies (<20 kHz), but increased with frequency. Near the best frequency (BF), the RL gain was greater than the high-level BM gain by 40 {+/-}5 dB (mean{+/-}std), and had greater compressive nonlinearity. RL motion varied radially, and the RL3 gain was significantly greater than that of RL1 by 10 {+/-}1 dB (p<0.001). In contrast, the OHC-DC-junction gain varied little radially across OHCs. At low frequencies the OHC-DC-junction gain was constant across SPLs, and 14 {+/-} 3 dB greater than the BM gain. As the frequency increased, the OHC-DC-junction gain decreased to a level similar to the BM gain at BF. The RL2, 3 phase was advanced by 0.25-0.375 cycles relative to the BM phase at low frequencies, but the RL2, 3 phase lead decreased as the frequency increased, became similar to the BM phase at BF, and lagged behind the BM phase by 0.25-0.5 cycles above BF. The OHC-DC-junction phases were mostly similar to the BM phase at low frequencies, but became delayed relative to the BM as the frequency increased, typically by 0.25-0.5 cycles near BF and by up to 1 cycle above BF. Our results show the most detailed picture of motion around the three OHC rows yet published, indicating that RL motion varied radially. Surprisingly, there was little motion difference across the three OHC rows in the OHC-DC-junction region, indicating that the tops of the DCs move in unison. Our data show a rich array of OoC amplitude and phase variations that are not explained by current theories.

physiology↗

bioRxiv2021

Watching Death in the Gerbil Cochlea Using Optical Coherence Tomography (OCT)

Because it is 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 of 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 in vivo and postmortem micron-scale volumetric images of the high-frequency hook region of the gerbil cochlea through the round-window membrane. The anatomical structures and fluid spaces of the organ of Corti were segmented and quantified in vivo and over a 90-minute postmortem period. The results show that some aspects of the organ of Corti are significantly altered over the course of death, such as the volumes of the fluid spaces, whereas the dimensions of other features change very little. We postulate that the fluid space of the outer tunnel and its surrounding tectal cells form a resonant structure that can affect the motion of the reticular lamina and thereby have a profound effect on outer-hair-cell transduction and thus cochlear amplification. In addition, the in vivo fluid pressure of the inner spiral sulcus is postulated to effectively inflate the connected sub-tectorial gap between the tectorial membrane and the reticular lamina. This gap height decreases after death, which is hypothesized to reduce and disrupt hair-cell transduction

neuroscience↗