Jonathan B. Sellon

Porosity Controls Spread of Excitation in Tectorial Membrane Traveling Waves

Cochlear frequency selectivity plays a key role in our ability to understand speech, and is widely believed to be associated with cochlear amplification. However, genetic studies targeting the tectorial membrane (TM) have demonstrated both sharper and broader tuning with no obvious changes in hair bundle or somatic motility mechanisms. For example, cochlear tuning of Tectb(-/-) mice is significantly sharper than that of Tecta(Y1870C/+) mice, even though TM stiffnesses are similarly reduced relative to wild-type TMs. Here we show that differences in TM viscosity can account for these differences in tuning. In the basal cochlear turn, nanoscale pores of Tecta(Y1870C/+) TMs are significantly larger than those of Tectb(-/-) TMs. The larger pore size reduces shear viscosity (by ∼70%), thereby reducing traveling wave speed and increasing spread of excitation. These results demonstrate the previously unrecognized importance of TM porosity in cochlear and neural tuning.

Longitudinal spread of mechanical excitation through tectorial membrane traveling waves

Significance The sharp frequency selectivity of auditory neurons, which is a hallmark of mammalian hearing, originates mechanically in the cochlea. Local resonance of the tectorial membrane (TM) is thought to play a key role. However, the presence of TM traveling waves suggests an entirely different mechanism. In this paper, we present experiments to measure longitudinal spread of mechanical excitation via TM traveling waves and discuss implications for the resulting tuning. We show that increasing viscosity or decreasing stiffness of the TM reduces the longitudinal spread of mechanical excitation, which would sharpen frequency selectivity. These trends are opposite those trends for a resonant TM, where increasing viscous loss or decreasing stiffness would broaden tuning. The mammalian inner ear separates sounds by their frequency content, and this separation underlies important properties of human hearing, including our ability to understand speech in noisy environments. Studies of genetic disorders of hearing have demonstrated a link between frequency selectivity and wave properties of the tectorial membrane (TM). To understand these wave properties better, we developed chemical manipulations that systematically and reversibly alter TM stiffness and viscosity. Using microfabricated shear probes, we show that (i) reducing pH reduces TM stiffness with little change in TM viscosity and (ii) adding PEG increases TM viscosity with little change in TM stiffness. By applying these manipulations in measurements of TM waves, we show that TM wave speed is determined primarily by stiffness at low frequencies and by viscosity at high frequencies. Both TM viscosity and stiffness affect the longitudinal spread of mechanical excitation through the TM over a broad range of frequencies. Increasing TM viscosity or decreasing stiffness reduces longitudinal spread of mechanical excitation, thereby coupling a smaller range of best frequencies and sharpening tuning. In contrast, increasing viscous loss or decreasing stiffness would tend to broaden tuning in resonance-based TM models. Thus, TM wave and resonance mechanisms are fundamentally different in the way they control frequency selectivity.

Electrokinetic properties of the mammalian tectorial membrane

The tectorial membrane (TM) clearly plays a mechanical role in stimulating cochlear sensory receptors, but the presence of fixed charge in TM constituents suggests that electromechanical properties also may be important. Here, we measure the fixed charge density of the TM and show that this density of fixed charge is sufficient to affect mechanical properties and to generate electrokinetic motions. In particular, alternating currents applied to the middle and marginal zones of isolated TM segments evoke motions at audio frequencies (1–1,000 Hz). Electrically evoked motions are nanometer scaled (∼5–900 nm), decrease with increasing stimulus frequency, and scale linearly over a broad range of electric field amplitudes (0.05–20 kV/m). These findings show that the mammalian TM is highly charged and suggest the importance of a unique TM electrokinetic mechanism.

Motion microscopy for visualizing and quantifying small motions

Significance Humans have difficulty seeing small motions with amplitudes below a threshold. Although there are optical techniques to visualize small static physical features (e.g., microscopes), visualization of small dynamic motions is extremely difficult. Here, we introduce a visualization tool, the motion microscope, that makes it possible to see and understand important biological and physical modes of motion. The motion microscope amplifies motions in a captured video sequence by rerendering small motions to make them large enough to see and quantifies those motions for analysis. Amplification of these tiny motions involves careful noise analysis to avoid the amplification of spurious signals. In the representative examples presented in this study, the visualizations reveal important motions that are invisible to the naked eye. Although the human visual system is remarkable at perceiving and interpreting motions, it has limited sensitivity, and we cannot see motions that are smaller than some threshold. Although difficult to visualize, tiny motions below this threshold are important and can reveal physical mechanisms, or be precursors to large motions in the case of mechanical failure. Here, we present a “motion microscope,” a computational tool that quantifies tiny motions in videos and then visualizes them by producing a new video in which the motions are made large enough to see. Three scientific visualizations are shown, spanning macroscopic to nanoscopic length scales. They are the resonant vibrations of a bridge demonstrating simultaneous spatial and temporal modal analysis, micrometer vibrations of a metamaterial demonstrating wave propagation through an elastic matrix with embedded resonating units, and nanometer motions of an extracellular tissue found in the inner ear demonstrating a mechanism of frequency separation in hearing. In these instances, the motion microscope uncovers hidden dynamics over a variety of length scales, leading to the discovery of previously unknown phenomena.

Geometric Requirements for Tectorial Membrane Traveling Waves in the Presence of Cochlear Loads

Recent studies suggest that wave motions of the tectorial membrane (TM) play a critical role in determining the frequency selectivity of hearing. However, frequency tuning is also thought to be limited by viscous loss in subtectorial fluid. Here, we analyze effects of this loss and other cochlear loads on TM traveling waves. Using a viscoelastic model, we demonstrate that hair bundle stiffness has little effect on TM traveling waves calculated with physiological parameters, that the limbal attachment can cause small (<20%) increases in TM wavelength, and that viscous loss in the subtectorial fluid can cause small (<20%) decreases in TM wave decay constants. However, effects of viscous loss in the subtectorial fluid are significantly increased if TM thickness is decreased. In contrast, increasing TM thickness above its physiological range has little effect on the wave, suggesting that the TM is just thick enough to maximize the spatial extent of the TM traveling wave.

Electromechanical role of fixed charge in the mammalian tectorial membrane

The mammalian tectorial membrane (TM) is thought to play a purely mechanical role in stimulating cochlear sensory receptors, but the presence of glycosaminoglycans and associated fixed charge groups suggests that electromechanical properties also may be important. Here, we measure the fixed charge concentration of the TM (−7.1 mmol/L at physiological pH), and show that this concentration of fixed charge is sufficient to generate electrokinetic motions of the TM. Electrically-evoked TM motions were nanometer-scaled (5-200 nm), increased linearly with electric field amplitude (0.05-20 kV/m) and decreased with frequency (1–1000 Hz). This frequency dependence can be understood in terms of the interplay between electrophoresis and electro-osmosis. Although the electric fields applied in this study were large, they are comparable in amplitude to the electric fields generated near hair cell transduction channels. TM electrokinetics could thus play a role in the deflection of cochlear hair bundles in vivo.

Characterizing anisotropic, viscoelastic material properties of the tectorial membranes of wild-type and mutant mice using longitudinally propagating waves

The tectorial membrane (TM) is an important structural component of the mammalian inner ear. Examination of cochlear physiology in genetically modified mice has demonstrated that mutation of genes affecting TM proteins causes changes in key characteristics of cochlear function. Characterizing the differences in material properties between wild-type and mutant mice could provide insight into the source of reported differences in cochlear physiology. In this study, optical images of isolated TM segments of wild-type and genetically modified mice in response to harmonic radial excitation at acoustic frequencies are used to determine the amplitude and phase of the motion. Wave propagation on the TM segments is modeled using finite element models that take into account the anisotropy, viscoelasticity, and finite dimensions of the TM and the presence of a viscous boundary layer. An automated least-square fitting algorithm is used to find anisotropic, viscoelastic material parameters of wild-type and mutant mice TMs at acoustic frequencies. The resulting material properties are compared to previous estimates of the TM properties.The tectorial membrane (TM) is an important structural component of the mammalian inner ear. Examination of cochlear physiology in genetically modified mice has demonstrated that mutation of genes affecting TM proteins causes changes in key characteristics of cochlear function. Characterizing the differences in material properties between wild-type and mutant mice could provide insight into the source of reported differences in cochlear physiology. In this study, optical images of isolated TM segments of wild-type and genetically modified mice in response to harmonic radial excitation at acoustic frequencies are used to determine the amplitude and phase of the motion. Wave propagation on the TM segments is modeled using finite element models that take into account the anisotropy, viscoelasticity, and finite dimensions of the TM and the presence of a viscous boundary layer. An automated least-square fitting algorithm is used to find anisotropic, viscoelastic material parameters of wild-type and mutant mice...

The Tectorial Membrane: Mechanical Properties and Functions

The tectorial membrane (TM) is widely believed to play a critical role in determining the remarkable sensitivity and frequency selectivity that are hallmarks of mammalian hearing. Recently developed mouse models of human hearing disorders have provided new insights into the molecular, nanomechanical mechanisms that underlie resonance and traveling wave properties of the TM. Herein we review recent experimental and theoretical results detailing TM morphology, local poroelastic and electromechanical interactions, and global spread of excitation via TM traveling waves, with direct implications for cochlear mechanisms.

Examining the effects of anisotropy on longitudinally propagating waves on isolated tectorial membranes

Knowledge of the mechanical properties of the tectorial membrane (TM) would improve the understanding of TM function in hearing mechanics. In recent years, several groups have used measurements of wave propagation on isolated TMs to estimate the mechanical properties of the TM at acoustic frequencies. However, these studies have ignored the curvature, finite dimensions and anisotropy of the TM. In this study, optical images of an isolated wild-type basal TM segment in response to harmonic radial excitation are used to determine the amplitude and phase of the motion at acoustic frequencies. Wave propagation on the TM segment is modeled using a finite element model that takes into account the geometry, anisotropy and viscoelasticity of the TM. An automated least-square fitting algorithm is used to find material parameters of the TM. The resulting material properties are compared to previous estimates.Knowledge of the mechanical properties of the tectorial membrane (TM) would improve the understanding of TM function in hearing mechanics. In recent years, several groups have used measurements of wave propagation on isolated TMs to estimate the mechanical properties of the TM at acoustic frequencies. However, these studies have ignored the curvature, finite dimensions and anisotropy of the TM. In this study, optical images of an isolated wild-type basal TM segment in response to harmonic radial excitation are used to determine the amplitude and phase of the motion at acoustic frequencies. Wave propagation on the TM segment is modeled using a finite element model that takes into account the geometry, anisotropy and viscoelasticity of the TM. An automated least-square fitting algorithm is used to find material parameters of the TM. The resulting material properties are compared to previous estimates.

Effects of geometry and cochlear loads on tectorial membrane traveling waves

It has been suggested that the decay constants of tectorial membrane (TM) traveling waves contribute to differences in the tuning of Tectb −/– mutant mice, wild-type mice, and humans. However, the experiments underlying these results were obtained in vitro, in the absence of cochlear loads. In this work, we analyze effects of cochlear loads on TM traveling waves using a viscoelastic model. Results demonstrate that hair bundle stiffness has little effect on TM waves, the limbal attachment tends to increase the wavelength of TM waves, and viscous loss to subtectorial fluid tends to decrease the wave decay constant. To understand how the TM is able to support traveling waves despite these loads we examined how TM wave decay constants depend on thickness of the TM. We observe that increasing TM thickness tends to moderate all effects of cochlear loads and that as TM thickness increases, the wave decay constant approaches an asymptotic limit that is independent of cochlear loads. For material properties corres...