Shirin Farrahi

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.

Lowered pH alters decay but not speed of tectorial membrane waves

Tectorial membrane (TM) traveling waves and mechanical shear impedances were measured in artificial endolymph baths at neutral and acidic pHs. Lowering pH from 7 to 4 significantly decreases the spatial extent of TM waves but has a relatively minor effect on wave speed. At pH 4, the imaginary component of TM shear impedance, which relates to the shear modulus, drops significantly; whereas, the real component, which relates to viscosity, is reduced less. These results suggest that shear modulus, and not viscosity, controls the extent of TM waves at lower pH.

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.

Tectorial Membrane Traveling Waves Underlie Impaired Hearing in Tectb Mutant Mice

We show that the Tectb mutation reduces the spatial extent and propagation velocity of tectorial membrane (TM) traveling waves. These results can account for all of the hearing abnormalities associated with the Tectb mutation, as follows. By reducing the spatial extent of TM waves, the Tectb mutation decreases spread of excitation and thereby increases frequency selectivity at mid‐ to high frequencies. Furthermore, the decrease in Tectb TM wave velocity at low frequencies reduces the number of hair cells that effectively couple energy to the basilar membrane, which thereby reduces sensitivity.

Cochlear mechanisms underlying the sharp frequency selectivity of hearing

Sharp frequency selectivity, which is a hallmark of mammalian hearing, originates in the cochlea. However, the underlying mechanisms remain unclear. The pioneering work of von Bekesy showed that sounds launch waves of motion along the spiraling basilar membrane, and subsequent hydrodynamic analysis has shown how mechanical properties of the cochlear partition can interact with fluid forces to support sharp frequency tuning. These analyses have generally presumed (or even purported to prove) that longitudinal mechanical coupling through cochlear structures is negligible. Here, we demonstrate that the visco-elastic structure of the tectorial membrane (TM), a gelatinous structure that overlies the sensory receptor cells and plays a key role in stimulating them, also supports traveling waves. The distance over which TM waves propagate provides a measure of mechanical coupling and, through the cochlear map, determines a range of frequencies that correlates strikingly well with direct measurements of cochlear t...

Tectorial membrane porosity controls spread of excitation and tuning in the cochlea

Modifications of genes that encode proteins found exclusively in the tectorial membrane (TM) alter mechanical properties and produce a wide range of hearing deficits. However, the changes in TM physical properties responsible for these deficits remain unclear. In particular, the cochlear tuning of Tectb−/− mice is significantly sharper than that of TectaY1870C/+ mice, even though the stiffnesses of TectaY1870C/+ and Tectb−/− TMs are similarly reduced relative to wild-type TMs. Here we show that differences in TM wave properties that are governed by shear viscosity account for these differences in tuning. The shear viscosity of the TM results from the interaction of interstitial fluid with the porous structure of the TM’s macromolecular matrix. In basal regions of the cochlea, nanoscale pores of TectaY1870C/+ TMs are significantly larger than those of Tectb−/− TMs. The larger pores in TectaY1870C/+ TMs gives rise to lower shear viscosity (by ∼70%), which in turn, reduces wave speed and increases wave decay...

The role of tectorial membrane stiffness and viscosity on traveling waves and resonance

Classical cochlear models have long suggested that the tectorial membrane (TM) is a resonant system, which contributes to both sensitivity and frequency selectivity. However, these models do not consider longitudinal coupling through the TM and assume damping in the subtectorial space controls sharpness of tuning. Recent TM wave motion results show that the TM can couple activity over several hundred rows of hair cells, suggesting that TM inertial and viscoelastic properties may overcome the dissipative effects of subtectorial damping. Here we manipulate the dynamic shear modulus (G′) and shear viscosity (η) of the TM by altering pH and adding polyethylene glycol (PEG) to the bath surrounding the TM. Analysis of a distributed impedance model shows that increasing TM shear viscosity (via addition of PEG) and decreasing shear modulus (via altering bath pH) both decrease wave decay constants by 35 − 42% over a broad range of frequencies. This reduction in spatial extent of TM waves is consistent with sharpen...