Subtectorial Space Research Articles

The Competition between the Noise and Shear Motion Sensitivity of Cochlear Inner Hair Cell Stereocilia

Acoustical excitation of the organ of Corti induces radial fluid flow in the subtectorial space (STS) that excites the hair bundles (HBs) of the sensory inner hair cell of the mammalian cochlea. The inner hair cell HBs are bathed in endolymphatic fluid filling a thin gap in the STS between the tectorial membrane and the reticular lamina. According to the fluctuation dissipation theorem, the fluid viscosity gives rise to mechanical fluctuations that are transduced into current noise. Conversely, the stochastic fluctuations of the mechanically gated channels of the HBs also induce dissipation. We develop an analytic model of the STS complex in a cross section of the gerbil organ of Corti. We predict that the dominant noise at the apex is due to the channel stochasticity whereas viscous effects dominate at the base. The net root mean square fluctuation of the HB motion is estimated to be at least 1.18 nm at the base and 2.72 nm at the apex. By varying the HB height for a fixed STS gap, we find that taller HBs are better sensors with lower thresholds. An integrated active HB model is shown to reduce the hydrodynamic resistance through a cycle-by-cycle power addition through adaptation, reducing the thresholds of hearing, hinting at one potential role for HB activity in mammalian hearing. We determine that a Couette flow approximation in the STS underestimates the dissipation and that modeling the entire STS complex is necessary to correctly predict the low-frequency dissipation in the cochlea. Finally, the difference in the noise budget at the base and the apex of the cochlea indicate that a sensing modality other than the shear motion of the TM that may be used to achieve low-noise acoustic sensing at the apex.

Effect of sub-tectorial space fluid dynamics on cochlear energy dissipation and inner hair cell mechano-transduction

In the mammalian cochlea, the mechano-transduction of the inner hair cell (IHC) stereocilia occurs in a micrometer-thick fluid space between the tectorial membrane and the reticular lamina. Using a computational model of the cochlea, we analyzed how the sub-tectorial space (STS) fluid mechanics affect cochlear power dissipation and IHC mechano-transduction. Based on the simulations of a single IHC stereociliary bundle in the STS (Prodanovic et al., 2015), the fluid-induced forces of the STS were reduced to simple equations. The reduced STS response was combined with a whole cochlear model consisting of: organ of Corti structural mechanics, cochlear fluid dynamics, and outer hair cell electro-physiology. Energy dissipation in the cochlea was quantified for three categories: macro-fluidic dissipation along the cochlear scalae, micro-fluidic dissipation in the STS, and other dissipation in the organ of Corti. The phase of IHC mechano-transduction current with respect to the basilar membrane displacement was ...

Power Dissipation in the Subtectorial Space of the Mammalian Cochlea Is Modulated by Inner Hair Cell Stereocilia

The stereocilia bundle is the mechano-transduction apparatus of the inner ear. In the mammalian cochlea, the stereocilia bundles are situated in the subtectorial space (STS)—a micrometer-thick space between two flat surfaces vibrating relative to each other. Because microstructures vibrating in fluid are subject to high-viscous friction, previous studies considered the STS as the primary place of energy dissipation in the cochlea. Although there have been extensive studies on how metabolic energy is used to compensate the dissipation, much less attention has been paid to the mechanism of energy dissipation. Using a computational model, we investigated the power dissipation in the STS. The model simulates fluid flow around the inner hair cell (IHC) stereocilia bundle. The power dissipation in the STS because of the presence IHC stereocilia increased as the stimulating frequency decreased. Along the axis of the stimulating frequency, there were two asymptotic values of power dissipation. At high frequencies, the power dissipation was determined by the shear friction between the two flat surfaces of the STS. At low frequencies, the power dissipation was dominated by the viscous friction around the IHC stereocilia bundle—the IHC stereocilia increased the STS power dissipation by 50- to 100-fold. There exists a characteristic frequency for STS power dissipation, CFSTS, defined as the frequency where power dissipation drops to one-half of the low frequency value. The IHC stereocilia stiffness and the gap size between the IHC stereocilia and the tectorial membrane determine the characteristic frequency. In addition to the generally assumed shear flow, nonshear STS flow patterns were simulated. Different flow patterns have little effect on the CFSTS. When the mechano-transduction of the IHC was tuned near the vibrating frequency, the active motility of the IHC stereocilia bundle reduced the power dissipation in the STS.

A computational study on different flow patterns around the inner hair cell stereocilia

The mechano-transduction of the mammalian cochlea occurs in the micro-fluid domain between the tectorial membrane and the reticular lamina called the subtectorial space. The subtectorial fluid bathes the bundled stereocilia of the inner hair cells (IHCs). These cells are responsible for the onset of neural impulses in the auditory nerve fibers. Despite the generally accepted postulation that the IHC stereocilia are deflected by shear flow between the two layers, there have been suggestions that other flow modes exist besides the shear flow. We developed a computational model of fluid dynamics in the subtectorial space. The model simulates IHC mechano-transduction excited by different flow patterns. In order to compare different modes of fluid dynamical stimulation, the power efficiency of IHC mechano-transduction was introduced (dissipated power normalized by IHC mechano-transduction current). Besides different flow patterns, the effect of mechanical parameters (such as the gap size between the stereociliary tip and the tectorial membrane, stereociliar bundle stiffness) were investigated. The results demonstrate that the power efficiency for the IHC mechano-transduction depends on the flow pattern in the subtectorial space.

Longitudinally propagating traveling waves of the mammalian tectorial membrane

Sound-evoked vibrations transmitted into the mammalian cochlea produce traveling waves that provide the mechanical tuning necessary for spectral decomposition of sound. These traveling waves of motion that have been observed to propagate longitudinally along the basilar membrane (BM) ultimately stimulate the mechano-sensory receptors. The tectorial membrane (TM) plays a key role in this process, but its mechanical function remains unclear. Here we show that the TM supports traveling waves that are an intrinsic feature of its visco-elastic structure. Radial forces applied at audio frequencies (2-20 kHz) to isolated TM segments generate longitudinally propagating waves on the TM with velocities similar to those of the BM traveling wave near its best frequency place. We compute the dynamic shear storage modulus and shear viscosity of the TM from the propagation velocity of the waves and show that segments of the TM from the basal turn are stiffer than apical segments are. Analysis of loading effects of hair bundle stiffness, the limbal attachment of the TM, and viscous damping in the subtectorial space suggests that TM traveling waves can occur in vivo. Our results show the presence of a traveling wave mechanism through the TM that can functionally couple a significant longitudinal extent of the cochlea and may interact with the BM wave to greatly enhance cochlear sensitivity and tuning.

Elektromechanische Transduktion

The somatic electromotility of the outer hair cells can be induced by an extracellular electrical field. This enables us to investigate the electromechanically induced motion of the organ of Corti. The electrically induced motion of the guinea-pig organ of Corti was measured with a laser Doppler vibrometer in three cochlear turns at ten radial positions on the reticular lamina (RL) and six on each of the upper and lower surfaces of the tectorial membrane (TM). We found a complex vibration pattern of the RL and TM, leading to a stimulus synchronous modulation of the depth of the subtectorial space in the region of the inner hair cells (IHCs). This modulation causes radial fluid motion inside the space up to at least 3 kHz. This motion is capable of deflecting the IHC stereocilia and provides an amplification mechanism additional to that associated with basilar-membrane motion.

Nanomechanics of the subtectorial space caused by electromechanics of cochlear outer hair cells.

The stereocilia of the cochlear inner hair cells (IHCs) transduce vibrations into the sensory receptor current. Until now, mechanisms for deflecting these stereocilia have not been identified experimentally. Here, we identify a mechanism by using the electromechanical properties of the soma of the outer hair cell to produce an intracochlear, mechanical force stimulus. It is known that the soma of this cell generates mechanical force in response to a change of its transmembrane potential. In the present experiments, the force was induced by intracochlear electrical stimulation at frequencies that covered the entire functionally relevant range of 50 kHz. Vibration responses were measured in the transverse direction with a laser Doppler vibrometer. For frequencies up to approximately 3 kHz in the first three turns of the guinea-pig cochlea, the apical surface of the IHC and the opposing surface of the tectorial membrane were found to vibrate with similar amplitudes but opposite phases. At high frequencies, there was little relative motion between these surfaces in the transverse direction. The counterphasic motion up to approximately 3 kHz results in a pulsatile motion of the fluid surrounding the stereocilia of the IHCs. Based on physical principles of fluid flow between narrowly spaced elastic plates, we show that radial fluid motion is amplified relative to transverse membrane motion and that the radial motion is capable of bending the stereocilia. In conclusion, for frequencies up to at least 3 kHz, there appears to be direct fluid coupling between outer hair cells and IHCs.

The cochlear amplifier as a standing wave: "squirting" waves between rows of outer hair cells?

This paper draws attention to symmetric Lloyd-Redwood (SLR) waves-known in ultrasonics as "squirting" waves-and points out that their distinctive properties make them well-suited for carrying positive feedback between rows of outer hair cells. This could result in standing-wave resonance-in essence a narrow-band cochlear amplifier. Based on known physical properties of the cochlea, such an amplifier can be readily tuned to match the full 10-octave range of human hearing. SLR waves propagate in a thin liquid layer enclosed between two thin compliant plates or a single such plate and a rigid wall, conditions found in the subtectorial space of the cochlea, and rely on the mass of the inter-plate fluid interacting with the stiffness of the plates to provide low phase velocity and high dispersion. The first property means SLR wavelengths can be as short as the distance between rows of outer hair cells, allowing standing wave formation; the second permits wide-range tuning using only an order-of-magnitude variation in cochlear physical properties, most importantly the inter-row spacing. Viscous drag at the two surfaces potentially limits SLR wave propagation at low frequencies, but this can perhaps be overcome by invoking hydrophobic effects.

Hair cell regeneration in the chick basilar papilla after exposure to wide-band noise: evidence for ganglion cell involvement

It has been demonstrated that the auditory epithelium in the chick basilar papilla may regenerate after acoustic or ototoxic damage. Both types of damage may elicit the appearance of new cells that may develop in to the sensory cells. Factors inducing this process and the role of ganglion cells, the first neuron cells in the auditory pathway, are still unknown. The pattern of auditory damage and regeneration, after octave-band and pure-tone noise exposure, has been well established in research studies on chicks, but there are scarce data on wide-band noise effects. The aim of this study was to investigate the effect of wide-band noise, with different exposure levels applied, on the chick basilar papilla and supporting cells. Further, it was also aimed to determine whether the proliferation of ganglion cells, after wide-band noise exposure, occurs. The morphological changes were assessed with fluorescent, light, and transmission electron microscopy. Cell proliferation was studied based on immunoreactivity assays of proliferating cell nuclear antigen (PCNA). The exposure to wide-band noise at 120 dB SPL for 72 h produced stripe-like lesion of tall hair cells along the neural edge of the basilar papilla, mainly in the middle and, at the lesser extend, in its proximal part. There was no patch-like damage to the region of short hair cells, commonly observed after the exposure to the octave-band or pure-tone noise. The lesion extend depended on the level of exposure. The lower equivalent levels of noise (120 dB SPL for 40 h intermittent exposure) produced proportionally less damage. No morphological changes at light and fluorescent microscopy (apart from tectorial membrane exfoliation) were observed at 110 dB SPL in case of 20 h intermittent exposure. The elimination of dying hair cells took place either by pulling a damaged cell down to the basilar membrane or by extruding the cell to the subtectorial space. New hair cells reappeared at the sensory epithelium on the fifth day after the end of exposure. Cell proliferation started prior to hair cell loss. PCNA-like immunoreactivity was observed after the exposure at all levels in both the damaged and intact areas. PCNA appeared not only in the supporting cells, as indicated in previous studies, but also in the ganglion cells, suggesting ganglion cell involvement in the process of regeneration.

Glycerol effect on the guinea pig tectorial membrane.

The tectorial membrane (Tm) of guinea pigs has been found to have an altered organization of its matrix fibers in response to intravenously administered glycerol. Following treatment, the Tm middle zone shows an increase in waviness and clumping of fibers in non-hydropic and several hydropic ears in contrast to non-treated control ears. Residue of the internal sulcus cells occasionally fills the subtectorial space. In the present study, additional investigations were performed with scanning electron microscopy in order to study the relationship between the Tm and the organ of Corti, as well as the relationship between Hensen's stripe and the inner hair cell. Present findings provide evidence for a connection between the inner hair cell stereocilia and Hensen's stripe which may be the molecular basis for the modulation of hearing during the glycerol test in a patient with Meniere's disease.

Scanning electron microscopic study on inner ear barotrauma: in the guinea pig with one eustachian tube occluded

In order to investigate the mechanism of inner ear barotrauma, guinea pigs, with one eustachian tube occluded, were subjected to compression and decompression between 1 absolute pressure and 2 absolute pressures in a high pressure chamber. The authors divided the animals into three groups according to the speed of compression and decompression. Each group contained 10 guinea pigs. Morphological changes in the cochlear sensory hairs were examined by means of scanning electron microscope. On the occluded side, hair cells were damaged more frequently and more severely by rapid compression than by rapid decompression. There was no difference in the degree of damage produced by slow decompression and rapid decompression. On the control side, slight or moderate damage to hair cells was observed mainly as a result of rapid decompression. These findings led us to hypothesize that, in the setting of normal eustachian tube function, inner ear barotrauma would generally be caused by the relative positive pressure in the middle ear cavity that occurs during decompression. Furthermore, animals deprived of eustachian tube function would not have sufficient endurance for rapid compression and would sustain inner ear barotrauma far more readily, and of far greater severity, with rapid compression than would animals with normal eustachian tube function. The difference may be due to the presence of another pressure transmission pathway to the inner ear. The mechanism of cochlear sensory hair damage due to atmospheric pressure changes is presumed to be distortion of the reticular lamina and cuticular plate caused by the difference in pressure between cortilymph (perilymph) and the subtectorial space (endolymph) resulting in injury to stereocilia.

Flow of endolymph in the inner spiral sulcus and the subtectorial space

Flows of endolymph within the inner spiral sulcus and the subtectorial space are studied analytically. These flows are driven by boundary displacements and scala media pressure, all prescribed in the form of traveling sine waves with uniform amplitude, wave speed, and wavelength. Prescribed boundary displacements have no axial components and are assumed "small." In the subtectorial space, waves are further assumed "long," and inner and outer hair cells' stereocilia are represented by permeable barriers. Boundary motions of the subtectorial space are assumed to be associated with basilar membrane motion. The localized boundary motions that would presumably accompany independent outer hair cell motions are not admitted. The pressure drop across the barrier representing the inner hair cells' stereocilia is evaluated for four specific input conditions. The results are used to assess its relative sensitivity to three types of boundary displacement and to examine the contribution of endolymph flow to the "second filter." The model predicts the introduction of appreciable tuning (102.3 dB/octave) into the auditory signal between the stages of relative normal displacement of the boundaries of the subtectorial space and the generation of a pressure drop across the inner hair cells' stereocilia. The outer barrier is removed from the model, and the pressure drop across the inner barrier is reevaluated for the same four cases to study its sensitivity to destruction of the outer hair cells' stereocilia. These results are examined in light of data by Robertson and Johnstone [D. Robertson and B. M. Johnstone, J. Acoust. Soc. Am. 66, 466-469 (1979)].

Guinea pig tectorial membrane profile in an in vitro cochlear preparation

The guinea pig cochlea was examined under high-magnification light microscopy in an in vitro preparation. After extraction of the otic capsule, the bulla was opened widely and a small hole made into the fourth turn of the scala vestibuli. The organ of Corti was visualized under artificial endolymph at 600 X magnification. Added 1-micron titanium dioxide particles settled on the upper surface of the transparent tectorial membrane. Particle positions showed that much of this upper surface lay in a flat sheet that extended centrifugally almost to the Hensen's cells, giving the impression it was attached there. The sheet extended at least to the level of the inner hair cells, where a tectorial membrane thickness of about 40 micron was reached. Titanium dioxide particles were seen regularly in immediate proximity to the hair cell cilia, indicating that scala media is continuous with the subtectorial space. Upon mechanical manipulation, Hensen's cells proved to be extremely cohesive and elastic. It is suggested that hair cell stereocilia provide major mechanical connections for the tectorial membrane.

Ionic environment of cochlear hair cells.

The scala media of the adult cochlea in mammals comprises a morphologically closed compartment sealed with tight junctions of the intermediate to tight types. The unique ionic composition of endolymph is maintained by the stria vascularis through active reabsorption of sodium and active secretion of potassium against ionic gradients. The subtectorial space is only a partially closed compartment which communicates with the endolymph via holes in the tectorial membrane at its outer insertion to the organ of Corti. Hardesty's membrane divides the subtectorial space into two compartments: one facing the surfaces of inner hair cells and one facing the surfaces of outer hair cells. In the study of comparative anatomy, hair cells, e.g. in the lizard, basilar papilla are of two types: those covered with a tectorial membrane and those being free-standing lacking the tectorial membrane. The ionic environment of the hair cell surface seems to be the same, independent of whether covered with a tectorial membrane or not. The tectorial membrane itself is semipermeable to ions in the endolymphatic space. Only the surface structures of the hair cell with the sensory hairs facing the subtectorial space are exposed to the high concentration of potassium, whereas the remaining parts of the hair cell are surrounded by a fluid having a more normal extracellular type of ionic composition (cortilymph/perilymph). During embryonic development the ionic composition of endolymph develops in parallel with the morphologic maturation of the stria vascularis. A completely mature composition of endolymph is reached before any electrophysiological potentials in the cochlea can be elicited. The sensory hair surface of hair cells has reached a mature morphology prior to the maturation of endolymph. In several species the tectorial membrane is morphologically only partially mature when the increase of the potassium concentration of endolymph starts. Drugs primarily affecting the stria vascularis causing a transient change of the ionic composition of endolymph result in a transient dysfunction of inner ear potentials. If the ionic changes persist for longer time, morphological changes can occur in both the stria vascularis and the hair cells of the organ of Corti. Whether such changes are primarily caused by the ototoxic drug itself or by changes in the ionic composition of endolymph has to be explored further.

Velocity and Displacement Coupling of Mammalian Inner Hair Cells and the Mechanical Resonance of the Free-Standing Stereocilia

Some controversy still exists as to whether the inner hair cells of the mammalian cochlea respond to the velocity or displacement of the basilar membrane or to a combination of these over their operating frequency range. A comparison between the nonlinear properties of the receptor potentials within inner hair cells of the basal turn of the guinea pig cochlea and the potentials recorded within the scala media of the same animals for stimulus frequencies between 200 and 3,200 Hz provides evidence that these inner hair cells change from velocity sensitivity to displacement sensitivity at about 1,000 Hz. We infer from this that viscosity within the subtectorial space is high enough to preclude mechanical resonance of the freestanding stereocilia of these cells as a frequency-selective mechanism within the mammalian cochlea.

Outer hair cell loss and supporting cell expansion following chronic gentamicin treatment.

A sequence of changes in the organ of Corti associated with the destruction of outer hair cells (OHCs) and their replacement by supporting cells following chronic gentamicin treatment has been examined using thin-sections and SEM. The progression of change of OHCs was matched by concomitant expansion of adjacent supporting cells. Hair cells ruptured in the lateral membrane. The apical fragment was retained in the reticular lamina and became surrounded basally by the expanded supporting cells. No large breaches at the surface of the organ of Corti were formed. Rather, it appeared that the tight junctions around the hair cell were maintained until junctions were established between newly adjacent supporting cells in the space once occupied by the hair cell body. Only then was the OHC apex disrupted and the debris released into the sub-tectorial space. Some features of the OHC degeneration process were reminiscent of the controlled, cellular self-destruction phenomenon of apoptosis. The results suggest the possibility that the processes of hair cell loss and replacement may be controlled enabling maintenance of permeability barriers during structural reorganisation.

Elemental composition of individual cells and tissues in the cochlea.

Localization of elements at the cellular and sub-cellular levels was performed with the energy dispersive X-ray microanalysis technique, using shock-frozen, freeze-dried and araldite-embedded mouse (CBA/CBA) cochleae sectioned dry. Anatomical identification occurred in the STEM (scanning transmission electron microscopy) mode. In inner hair cell stereocilia the K/Na ratio was 70:1 but only 20:1 in the cytoplasm. In outer hair cell cytoplasm the K/Na ratio was 11:1 while the ratio in stereocilia was similar to that in inner hair cells. Ca was identified in stereocilia and the upper part of the cytoplasm of both outer and inner hair cells. The elemental composition in the subtectorial space is endolymph-like and that in the inner tunnel of the organ of Corti is similar to extracellular fluid. Considerable regional differences in elemental composition occur in the tectorial membrane with regard to P, K and Ca. The highest concentration of Ca occurs in the basal part of the tectorial membrane towards the sensory hairs. The highest concentration of K occurs in the basal and outer parts whereas the middle part of the tectorial membrane contains low levels of both K and Ca. The elemental composition changes in two main directions: 1) from the limbal (growing) region to the tip of the tectorial membrane, and, 2) from upper to lower surfaces. The three cell types of the stria vascularis differ considerably in elemental composition. The highest concentration of K occurs in marginal cells. The basal cells contain more K than do the intermediate cells. A significantly higher concentration of Ca, Cl and Na occurs in marginal cell cytoplasm than in any other cell type in the stria vascularis.

Suppression of auditory nerve responses I: temporal analysis, intensity effects and suppression contours.

Two-tone suppression was studied in response patterns of single auditory nerve fibers in anesthetized cats. Utilizing suppression of discharge synchronization in response to low- and moderate-frequency tones as an index, it was found that (a) suppression behaves in the same manner when the suppressor tone presented alone is strongly excitatory as when it is ineffective in altering discharge rate; (b) suppression exists throughout an auditory nerve fiber's response area; (c) for fixed-intensity suppressors, suppression is maximal at a fiber's characteristic frequency; and (d) suppression magnitude over a wide intensity range depends only upon the parameters of the suppressor tone and not of the tone being suppressed. The data are in general agreement with previously published reports of suppression behavior, and they support the concept that suppression is generated primarily as a result of interactions occurring within hair cells or in the subtectorial space.

Experiments on the origin of two‐tone suppression

Two‐tone suppression has been shown to exist in virtually every response of cochlear origin. For comparable stimulus conditions, however, suppression magnitude measured in responses of hair cells and auditory nerve fibers is considerably greater than suppression magnitude measured in basilar membrane displacement patterns, and tho mechanical suppression threshold appears to be higher than the auditory nerve‐fiber suppression threshold. We recorded responses of single auditory nerve fibers in anesthetized cats and used discharge synchronization as an index of suppression magnitude. It was found that (a) when “notches” in input‐output rate functions exist, suppressed responses cannot be made to occur to the right of the upper leg of the unsuppressed function; (b) unlike results obtained for cochlear microphonics, the discharge phase angle of a suppressed response in auditory nerve fibers is the same as the angle of the pure‐tone response at the same intensity; and (c) for low‐frequency suppressors, the rate of growth of suppression with increasing suppressor‐tone intensity can easily exceed the presumed rate of growth of basilar membrane displacement. A possible explanation is that there are two generators for suppression. One generator operates at high intensities and originates in basilar membrane mechanics. The other generator operates at all suprathreshold intensities and originates either in the micromechanics of the subtectorial space or in the mechanical‐to‐neural transduction process occurring inside hair cells. [Work supported by NIH.]

Hereditary deafness in the cat: an electron-microscopic study of the tectorial membrane.

The tectorial membrane is affected at an early stage of the cochlear degeneration in the hereditarily deaf white cat. The membrane first descends towards the organ of Corti with obliteration of the intervening sub-tectorial space in the basal coil during the second post-natal week. Both the microvilli of supporting and Hensen's cells, and the hair cell stereocilia make deep indentations on the under-surface of the membrane. Cells are found insinuated between the tectorial membrane and Corti's organ, and numerous cellular processes occur within the former. A phagocytic function would appear probable for these cells, which seem to originate from the internal sulcus region. The membrane is retracted into the latter around the 2-month stage. At all ages, small spherical structures, which may represent altered interdental cell secretions, are found within the membrane, these becoming calcified in older animals.