Sensory transduction and frequency selectivity in the basal turn of the guinea-pig cochlea.

Receptor potentials recorded from outer hair cells (OHC) and inner hair cells (IHC) in the basal high-frequency turn were compared. The DC component of the IHC receptor potential is maximized to ensure that IHCS can signal a voltage response to high-frequency tones. The OHC DC component is minimized so that OHCS transduce in the most sensitive region of their operating range. The phase and magnitude of OHC receptor potentials were recorded as an indicator of the magnitude and phase of the energy which is fed back to the basilar membrane to provide the basis for the sharp tuning and fine sensitivity of the cochlea to tones. IHC receptor potentials were recorded to assess the net effect of the feedback on the mechanics of the cochlea. It was concluded that OHCS generate feedback which enhances the IHC responses only at the best frequency. At frequencies below CF, IHC DC responses are elicited only when the OHC AC responses begin to saturate.     

Mathematical models of cochlear nucleus onset neurons: II. model with dynamic spike-blocking state

Onset (On) neurons in the cochlear nucleus (CN), characterized by their prominent response to the onset followed by little or no response to the steady-state of sustained stimuli, have a remarkable ability to entrain (firing 1 spike per cycle of a periodic stimulus) to low-frequency tones up to 1000 Hz. In this article, we present a point-neuron model with independent, excitatory auditory-nerve (AN) inputs that accounts for the ability of On neurons to both produce onset responses for high-frequency tone bursts and entrain to a wide range of low-frequency tones. With a fixed-duration spike-blocking state after a spike (an absolute refractory period), the model produces entrainment to a broad range of low-frequency tones and an On response with short interspike intervals (chopping) for high-frequency tone bursts. To produce On response patterns with no chopping, we introduce a novel, more complex, active membrane model in which the spike-blocking state is maintained until the instantaneous membrane voltage falls below a transition voltage. During the sustained depolarization for a high-frequency tone burst, the new model does not chop because it enters a spike-blocking state after the first spike and fails to leave this state until the membrane voltage returns toward rest at the end of the stimulus. The model entrains to low-frequency tones because the membrane voltage falls below the transition voltage on every cycle when the AN inputs are phase-locked. With the complex membrane model, On response patterns having moderate steady-state activity for high-frequency tone bursts (On-L) are distinguished from those having no steady-state activity (On-I) by requiring fewer AN inputs. Voltage-gated ion channels found in On-responding neurons of the CN may underlie the hypothesized dynamic spike-blocking state. These results provide a mechanistic rationale for distinguishing between the different physiological classes of CN On neurons.     

Effect of temperature upon auditory thresholds in two anuran species,Bombina v. variegata andAlytes o. obstetricans (Amphibia,Discoglossidae)

Summary Because it seemed likely that temperature affects not only the calling mechanism of anurans, but their auditory systems as well, we have measured the thresholds ofBombina variegata variegata andAlytes obstetricans obstetricans at 5°, 12°, 20° and 28°C by recording multiple-unit activity from the torus semicircularis. An increase in temperature from 5° to 28°C shortened the latencies considerably. InBombina v. variegata latencies fell from an average of 32 ms to 13 ms (600 Hz), and inAlytes o. obstetricans from an average of 22 ms to 11 ms (500 Hz). At frequencies below 500 Hz the decrease was still greater. Latency was also dependent on frequency, being shorter with high-frequency tones. At 5°C the auditory neurons ofBombina are rather insensitive and respond irregularly. At 12°C and at 20°C sensitivity is markedly increased. The minimum threshold in males was at 400–500 Hz (49 dB SPL), and that of females was at 450 Hz (47 dB SPL). There was no further increase in sensitivity at 28°C. InAlytes the auditory neurons were fully functional even at 5°C. At this temperature the audiogram had sensitivity maxima at 300, 1,100–1,300 and 1,800 Hz. In both males and females an increase in temperature to 20°C caused an extraordinary increase in sensitivity, primarily in the low-frequency range; the minimum threshold, at 400 Hz, was 44 dB SPL in males and 41 dB SPL in famales. In the intermediate frequency range there was also a marked increase in sensitivity, but not in the high-frequency range, where the best frequency was 1,800 Hz. At 28°C the threshold to low-frequency tones was increased.     

Prestin's role in cochlear frequency tuning and transmission of mechanical responses to neural excitation

The remarkable power amplifier [1] of the cochlea boosts low-level and compresses high-level vibrations of the basilar membrane (BM) [2]. By contributing maximally at the characteristic frequency (CF) of each point along its length, the amplifier ensures the exquisite sensitivity, narrow frequency tuning, and enormous dynamic range of the mammalian cochlea. The motor protein prestin in the outer hair cell (OHC) lateral membrane is a prime candidate for the cochlear power amplifier [3]. The other contender for this role is the ubiquitous calcium-mediated motility of the hair cell stereocilia, which has been demonstrated in vitro and is based on fast adaptation of the mechanoelectrical transduction channels [4, 5]. Absence of prestin [6] from OHCs results in a 40-60 dB reduction in cochlear neural sensitivity [7]. Here we show that sound-evoked BM vibrations in the high-frequency region of prestin(-/-) mice cochleae are, surprisingly, as sensitive as those of their prestin(+/+) siblings. The BM vibrations of prestin(-/-) mice are, however, broadly tuned to a frequency approximately a half octave below the CF of prestin(+/+) mice at similar BM locations. The peak sensitivity of prestin(+/+) BM tuning curves matches the neural thresholds. In contrast, prestin(-/-) BM tuning curves at their best frequency are >50 dB more sensitive than the neural responses. We propose that the absence of prestin from OHCs, and consequent reduction in stiffness of the cochlea partition, changes the passive impedance of the BM at high frequencies, including the CF. We conclude that prestin influences the cochlear partition's dynamic properties that permit transmission of its vibrations into neural excitation. Prestin is crucial for defining sharp and sensitive cochlear frequency tuning by reducing the sensitivity of the low-frequency tail of the tuning curve, although this necessitates a cochlear amplifier to determine the narrowly tuned tip.     

Variations in the loudness of a brief sound compared to another brief sound as a function of the duration of both sounds]

Loudness equalizations between two short 2500 Hz tones (15 to 120 ms, about 50 dB SPL) were made. One tone, either the first or the second one, was twice as long as the other. The intensity level differences between tones of the same loudness were calculated. Results show that the relations between duration and loudness of the tones differ for different subjects. Nevertheless the calculated differences diminished with subject experience. Subject evaluations in accordance with the intensity levels of tones, i.e. independently of the duration, were quite often obtained even for the pairs 15 ms-30 ms.     

Frequency dependence of electrical coupling in Deiters' cells of the guinea pig cochlea

Immunolabeling with antibodies against connexins 26 and 30 showed that, in the guinea pig cochlea, supporting Deiters' cells are massively interconnected and form an orderly network within the organ of Corti. In paired patch-clamp recordings the coupling ratio (CR) of adjacent Deiters' cells at the apex of the cochlea (approximately 0.31) was 3-fold smaller than in isolated cell pairs due to shunting afforded by multicellular connectivity. With sinusoidal current stimuli the delay in signal propagation between adjacent cells increased with increasing frequency whereas the amplitude did not change significantly up to 200 Hz (corner frequency Fc approximately 220 Hz). Depolarizing voltage commands applied to an outer hair cell (OHC) elicited outward potassium currents in the OHC and inward currents in the abutting Deiters' cells, supplying direct evidence for potassium buffering in the organ of Corti. Computational analysis indicates that electrical signals injected into a Deiters' cell are transmitted across a network segment spanning 8 cell diameters. Thus electrical coupling in the organ of Corti is unlikely to influence the selectivity of frequency filtering performed mechanically by the mammalian cochlea.     

Model of cochlear microphonic explores the tuning and magnitude of hair cell transduction current

The mammalian cochlea relies on the active forcing of sensory outer hair cells (OHCs) to amplify traveling wave responses along the basilar membrane. These forces are the result of electromotility, wherein current through the OHCs leads to conformational changes in the cells that provide stresses on surrounding structures. OHC transducer current can be detected via the voltage in the scala tympani (the cochlear microphonic, CM), and the CM can be used as an indicator of healthy cochlear operation. The CM represents a summation of OHC currents (the inner hair cell contribution is known to be small) and to use CM to probe the properties of OHC transduction requires a model that simulates that summation. We developed a finite element model for that purpose. The pattern of current generators (the model input) was initially based on basilar membrane displacement, with the current size based on in vitro data. The model was able to reproduce the amplitude of experimental CM results reasonably well when the input tuning was enhanced slightly (peak increased by ∼6 dB), which can be regarded as additional hair bundle tuning, and with a current/input value of 200–260 pA/nm, which is ∼4 times greater than the largest in vitro measures.     

Frequency selectivity of hearing in the green treefrog,Hyla cinerea

Frequency selectivity of hearing was measured in the green treefrog, Hyla cinerea. A psychophysical technique based on reflex modification was used to obtain masked threshold estimates for pure tones (300-5,400 Hz) presented against two levels of broadband masking noise. A pure tone (S-1) presented 200 ms prior to a reflex-eliciting stimulus (S-2) inhibited the motor reflex response to S-2. The magnitude of this reflex modification effect varied systematically with the sound pressure level (SPL) of S-1, and threshold was defined as the SPL of S-1 at which the reflex modification effect disappeared. Masked thresholds were used to calculate critical ratios, an index of the auditory system's frequency selectivity. The frequency selectivity of the treefrog's hearing is greatest and critical ratios are lowest (22-24 dB) at about 900 and 3,000 Hz, the two spectral regions dominant in the male treefrog's species-specific advertisement call. These results suggest that the treefrog's auditory system may be specialized to reject noise at biologically-relevant frequencies. As in other vertebrates, critical ratios remain constant when background noise level is varied; however, the shape of the treefrog's critical ratio function across frequencies differs from the typical vertebrate function that increases with increasing frequency at a slope of about 3 dB/octave. Instead, the treefrog's critical ratio function resembles its pure tone audiogram. Although the shape of the treefrog's critical ratio function is atypical, the critical ratio values themselves are comparable to those of many other vertebrates in the same frequency range. Critical ratio values here measured behaviorally do not match critical ratio values previously measured physiologically in single eighth nerve fibers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between evaluation of the duration and loudness of short sounds]

The relation between loudness and duration judgments of short tones was investigated. Sounds (15 to 120 ms, 2500 Hz, about dB SPL), were exposed in pairs; one tone of the pair, the first or the second one, was twice as long as the other. Subjects had to compare the loudness of the tones, or the duration, or the loudness and the duration at the same time. Results show that the loudness judgments are independent of experiences with duration judgments and of immediate duration judgments, but duration judgments were less precise when loudness judged at the same time.     

Modified Protein Expression in the Tectorial Membrane of the Cochlea Reveals Roles for the Striated Sheet Matrix

The tectorial membrane (TM) of the mammalian cochlea is a complex extracellular matrix which, in response to acoustic stimulation, displaces the hair bundles of outer hair cells (OHCs), thereby initiating sensory transduction and amplification. Here, using TM segments from the basal, high-frequency region of the cochleae of genetically modified mice (including models of human hereditary deafness) with missing or modified TM proteins, we demonstrate that frequency-dependent stiffening is associated with the striated sheet matrix (SSM). Frequency-dependent stiffening largely disappeared in all three TM mutations studied where the SSM was absent either entirely or at least from the stiffest part of the TM overlying the OHCs. In all three TM mutations, dissipation of energy is decreased at low (<8 kHz) and increased at high (>8 kHz) stimulus frequencies. The SSM is composed of polypeptides carrying fixed charges, and electrostatic interaction between them may account for frequency-dependent stiffness changes in the material properties of the TM. Through comparison with previous in vivo measurements, it is proposed that implementation of frequency-dependent stiffening of the TM in the OHC attachment region facilitates interaction among tones, backward transmission of energy, and amplification in the cochlea.     

Structure of the stereocilia side links and morphology of auditory hair bundle in relation to noise exposure in the chinchilla

Stereocilia side links are directly involved in the maintenance of stereociliary bundle integrity in hair cells. The structure of the stereocilia side links and morphology of the auditory hair bundle in relation to noise exposure in the chinchilla was investigated by transmission electron microscopy. The outer hair cell (OHC) stereocilia side link was suggested to consist of extracellular, juxta-membrane and thin filamentous regions. Two beaded filaments were folded at their distal ends and fastened in one globule in the center between stereocilia. An intracellular, submembraneous layer appeared to form a bridge between the actin core and the extracellular, juxta-membrane region of the side link. In normal physiological conditions, most OHC stereocilia had a regular distribution of side links, forming a ‘zipper-like’ lattice between stereocilium shafts. Side links of the inner hair cell (IHC) stereocilia had a similar filamentous appearance, but were observed less commonly and had decreased structural organization compared to those of the OHC stereocilia. Ultrastructural analysis of OHC and IHC stereocilia showed that a large number of the side links could survive acoustic stimulation of 114 dB SPL for 2 hrs or 123 dB SPL for 15 min, that resulted in temporarily elevated hearing thresholds in all animals. Disarray, separation, close attachment and fusion of stereocilia were more frequently observed for IHC stereocilia and OHC stereocilia that were poorly connected or that lacked side links. Most disarrayed OHC and IHC stereocilia recovered to a normal erect state with restored orientation of the side links after 14–28 days, which correlated with near-complete recovery of auditory sensitivity. However, direct attachment of plasma membranes, ruptured links, fusion and blebs were seen on some stereocilia even after 28 days and appear to be permanent.     

Resistance to neomycin ototoxicity in the extreme basal (hook) region of the mouse cochlea

Aminoglycoside ototoxicity results in permanent loss of the sensory hair cells in the mammalian cochlea. It usually begins at the basal turn causing high-frequency hearing loss. Here we describe previously unreported resistance of hair cells to neomycin ototoxicity in the extreme basal (hook) region of the developing cochlea of the C57BL/6 mouse. Organ of Corti explants from mice at postnatal day 3 were incubated (37 °C, 5% CO₂) in normal culture medium for 19.5 h prior to and after exposure to neomycin (1 mM, 3 h). To study neomycin uptake in the hair cells, cochlear explants were incubated with Neomycin Texas-red (NTR) conjugate. As expected, exposure to neomycin significantly reduced the survival of inner (IHC) and outer hair cells (OHC). IHC survival rate was high in the apical segment and low in the basal segment. OHC were well preserved in the apical and hook regions, with substantial OHC loss in the basal segment. The NTR uptake study demonstrated that the high survival rate in the extreme basal turn OHC was associated with low NTR uptake. Treatment with a calcium chelator (BAPTA), which disrupts the opening of mechanoelectrical (MET) transduction channels, abolished or reduced NTR uptake in the hair cells throughout the cochlea. This confirmed the essential role of MET channels in neomycin uptake and implied that the transduction channels could be impaired in the hook region of the developing mouse cochlea, possibly as a result of the cadherin 23 mutation responsible for the progressive deafness in C57BL/6 mice.     

Conservation of endocochlear potential in mice with profound hearing loss induced by co-administration of kanamycin and furosemide

Research in mammalian hair cell regeneration is hampered by a lack of in vivo model of adult mouse inner ear injury. In the present study we investigated the effects of a combination of a single dose of aminoglycoside followed by a loop diuretic in adult mice. The auditory brainstem response threshold shift, extent and defining characteristics of the cochlear lesion were assessed and verified at different time points post-treatment. Our data indicated that this drug combination caused the rapid and extensive death of outer hair cells (OHCs). OHC death presented throughout the cochlea that commenced in the basal turn by 24 h and progressed apically. In contrast, inner hair cell (IHC) loss was delayed and mild. Terminal deoxynucleotidyl transferase dUTP nick end labelling-positive nuclei demonstrated that the majority of OHCs died via an apoptotic pathway. Auditory threshold shifts of up to 90 dB SPL indicated a profound hearing loss. In addition, the endocochlear potential (EP) in the drug-treated animals displayed a significant decline at 12 h post-treatment followed by recovery by 48 h post-treatment. Despite this recovery, there was a significant and progressive decrease in strial vascularis thickness, which was predominantly due to atrophy of marginal cells. The present study reproduced an adult mouse model of aminoglycoside-induced hearing loss. The mechanism underlying the recovered EP in the model with extensive hair cell death is discussed.     

Nitric oxide and mitochondrial status in noise-induced hearing loss

The study investigated the distribution of nitric oxide (NO) within isolated outer hair cells (OHCs) from the cochlea, its relationship to mitochondria and its modulation of mitochondrial function. Using two fluorescent dyes--4,5-diamino-fluorescein diacetate (DAF-2DA), which detects NO, and tetramethyl rhodamine methyl ester (TMRM+), a mitochondrial membrane potential dye--it was found that a relatively greater amount of the DAF fluorescence in OHCs co-localized with mitochondria in comparison to DAF fluorescence in the cytosole. This study also observed reduced mitochondrial membrane potential of OHCs and increased DAF fluorescence following exposure of the cells to noise (120 dB SPL for 4 h) and to an exogenous NO donor, NOC-7 (>350 mm). Antibody label for nitrotyrosine was also increased, indicating NO-related formation of peroxynitrite in both mitochondria and the cytosol. The results suggest that NO may play an important physiological role in regulating OHC energy status and act as a potential agent in OHC pathology.     

Electrophysiological responses in guinea pig cochlea to low frequency sound stimuli: distortion of cochlear microphonic (CM) wave form

The present experiment investigated whether or not auditory responses of the middle and/or inner ear in guinea pigs to low frequency sound stimuli [ 60 Hz-2 kHz at 90-120 dB(SPL) ] exhibited the harmonic distortion phenomenon resulting from cochlear microphonics (CM). Measurement of CM leading in turn I by the differential electrode recording method involved measurement of 50 microV isopotential responses, output voltages and CM wave form distortion at each constant sound pressure. The results obtained were as follows: (1) On the 50 microV isopotential response curve and the output voltage curves, the changes at 60-90 Hz were different from those at higher frequencies. (2) At stimuli of 90 or 100 dB(SPL), CM wave form distortion appeared frequently at frequencies below 120 Hz, but were less pronounced above approximately 200 Hz. (3) When raised to 110 and 120 dB(SPL), almost all CM wave forms were distorted at all test frequencies between 60 and 500 Hz. (4) The patterns of CM wave form distortion at frequencies below approximately 120 Hz showed peak clipping and triangular wave distortions, while those at frequencies above approximately 200 Hz showed little of these distortions.     

Frequency Dependence of Electrical Coupling in Deiters″ Cells of the Guinea Pig Cochlea

Immunolabeling with antibodies against connexins 26 and 30 showed that, in the guinea pig cochlea, supporting Deiters″ cells are massively interconnected and form an orderly network within the organ of Corti. In paired patch-clamp recordings the coupling ratio (CR) of adjacent Deiters″ cells at the apex of the cochlea (～0.31) was 3-fold smaller than in isolated cell pairs due to shunting afforded by multicellular connectivity. With sinusoidal current stimuli the delay in signal propagation between adjacent cells increased with increasing frequency whereas the amplitude did not change significantly up to 200 Hz (corner frequency F_c ～220 Hz). Depolarizing voltage commands applied to an outer hair cell (OHC) elicited outward potassium currents in the OHC and inward currents in the abutting Deiters″ cells, supplying direct evidence for potassium buffering in the organ of Corti. Computational analysis indicates that electrical signals injected into a Deiters″ cell are transmitted across a network segment spanning 8 cell diameters. Thus electrical coupling in the organ of Corti is unlikely to influence the selectivity of frequency filtering performed mechanically by the mammalian cochlea.     

Response to a pure tone in a nonlinear mechanical-electrical-acoustical model of the cochlea

In this article, a nonlinear mathematical model is developed based on the physiology of the cochlea of the guinea pig. The three-dimensional intracochlear fluid dynamics are coupled to a micromechanical model of the organ of Corti and to electrical potentials in the cochlear ducts and outer hair cells (OHC). OHC somatic electromotility is modeled by linearized piezoelectric relations whereas the OHC hair-bundle mechanoelectrical transduction current is modeled as a nonlinear function of the hair-bundle deflection. The steady-state response of the cochlea to a single tone is simulated in the frequency domain using an alternating frequency time scheme. Compressive nonlinearity, harmonic distortion, and DC shift on the basilar membrane (BM), tectorial membrane (TM), and OHC potentials are predicted using a single set of parameters. The predictions of the model are verified by comparing simulations to available in vivo experimental data for basal cochlear mechanics. In particular, the model predicts more amplification on the reticular lamina (RL) side of the cochlear partition than on the BM, which replicates recent measurements. Moreover, small harmonic distortion and DC shifts are predicted on the BM, whereas more significant harmonic distortion and DC shifts are predicted in the RL and TM displacements and in the OHC potentials.     

Nitric oxide and mitochondrial status in noise-induced hearing loss

This study investigated the distribution of nitric oxide (NO) within isolated outer hair cells (OHCs) from the cochlea, its relationship to mitochondria and its modulation of mitochondrial function. Using two fluorescent dyes—4,5-diaminofluorescein diacetate (DAF-2DA), which detects NO, and tetramethyl rhodamine methyl ester (TMRM⁺), a mitochondrial membrane potential dye—it was found that a relatively greater amount of the DAF fluorescence in OHCs co-localized with mitochondria in comparison to DAF fluorescence in the cytosole. This study also observed reduced mitochondrial membrane potential of OHCs and increased DAF fluorescence following exposure of the cells to noise (120 dB SPL for 4 h) and to an exogenous NO donor, NOC-7 (>350 nm). Antibody label for nitrotyrosine was also increased, indicating NO-related formation of peroxynitrite in both mitochrondria and the cytosol. The results suggest that NO may play an important physiological role in regulating OHC energy status and act as a potential agent in OHC pathology.     

Electromotility of outer hair cells from the cochlea of the echolocating bat,Carollia perspicillata

Isolated outer hair cells (OHCs) and explants ot the organ of Corti were obtained from the cochlea of the echolocating bat, Carollia perspicillata, whose hearing range extends up to about 100 kHz. The OHCs were about 10–30 m long and produced resting potentials between-30 to -69 mV. During stimulation with a sinusoidal extracellular voltage field (voltage gradient of 2 mV/m) cyclic length changes were observed in isolated OHCs. The displacements were most prominent at the level of the cell nucleus and the cuticular plate. In the organ of Corti explants, the extracellular electric field induced a radial movement of the cuticular plate which was observed using video subtraction and photodiode techniques. Maximum displacements of about 0.3–0.8 m were elicited by stimulus frequencies below 100 Hz. The displacement amplitude decreased towards the noise level of about 10–30 nm for stimulus frequencies between 100–500 Hz, both in apical and basal explants. This compares well with data from the guinea pig, where OHC motility induced by extracellular electrical stimulation exhibits a low pass characteristic with a corner frequency below 1 kHz. The data indicate that fast OHC movements presumably are quite small at ultrasonic frequencies and it remains to be solved how they participate in amplifying and sharpening cochlear responses in vivo.Abbreviations BM basilar membrane - FFT fast Fourier Transfer - IHC inner hair cell - OHC outer hair cell     

The startle response during whiplash: a protective or harmful response?

Whiplash injuries are common following rear-end collisions. During such collisions, initially relaxed occupants exhibit brisk, stereotypical muscle responses consisting of postural and startle responses that may contribute to the injury. Using prestimulus inhibition, we sought to determine if the startle response elicited during a rear-end collision contributes to head stabilization or represents a potentially harmful overreaction of the body. Three experiments were performed. In the first two experiments, two groups of 14 subjects were exposed to loud tones (124 dB) preceded by prestimulus tones at either four interstimulus intervals (100-1,000 ms) or five prestimulus intensities (80-124 dB). On the basis of the results of the first two experiments, 20 subjects were exposed to a simulated rear-end collision (peak sled acceleration = 2 g; speed change = 0.75 m/s) preceded by one of the following: no prestimulus tone, a weak tone (85 dB), or a loud tone (105 dB). The prestimulus tones were presented 250 ms before sled acceleration onset. The loud prestimulus tone decreased the amplitude of the sternocleidomastoid (16%) and cervical paraspinal (29%) muscles, and key peak kinematics: head retraction (17%), horizontal head acceleration (23%), and head angular acceleration in extension (23%). No changes in muscle amplitude or kinematics occurred for the weak prestimulus. The reduced muscle and kinematic responses observed with loud tones suggest that the startle response represents an overreaction that increases the kinematics in a way that potentially increases the forces and strains in the neck tissues. We propose that minimizing this overreaction during a car collision may decrease the risk of whiplash injuries.