Whole cell currents and mechanical responses of isolated outer hair cells

4-aminopyridine in scala media reversibly alters the cochlear potentials and suppresses electrically evoked oto-acoustic emissions

Iontophoresis of 4-aminopyridine into scala media of the guinea pig cochlea caused elevation of the thresholds of the compound action potential of the auditory nerve, loss of amplitude of the extracellular cochlear microphonic response (CM), increase in the endocochlear potential (EP) and reduction in the amplitude of electrically evoked oto-acoustic emissions (EEOAEs). These changes were reversible over 10-20 min. The reciprocity of the changes in the CM and the EP was consistent with an interruption of both DC and AC currents through outer hair cells (OHCs), probably by blockade of mechano-electrical transduction (MET) channels in OHCs. Reductions in EEOAEs were consistent with the extrinsically applied generating current entering the OHC via the MET channels. Implications for the activation of OHC electromotility in vivo are discussed.

Identification of the nonlinearity governing even-order distortion products in cochlear potentials

In order to characterize the cochlear transducer nonlinearities which are involved in the generation of the summating potential (SP), we investigated the effect of a change in the electrical operating point of the cochlear transducer on the SP. The electrical operating point of the cochlear transducer was affected by suppressing reversibly the endocochlear potential (EP). This was realized by intravenous injection of furosemide in guinea pig. A differential recording technique was used in the basal turn of the cochlea to measure locally generated even-order distortion products: the SP and the second harmonic component (2F0) of the cochlear microphonics (CM). These potentials were evoked by 2 and 8 kHz stimuli presented at 60 dB SPL. Following furosemide injection, the SP changed polarity twice over time. The zero crossings of the SP coincided with a minimum in the amplitude of 2F0. Concomitantly, the phase of 2F0 shifted about 120 degrees. The changes in the electrical even-order products were comparable to the changes that occurred in a mechanical even-order intermodulation distortion product (the difference tone F2-F1 otoacoustic emission) after furosemide application (Mills et al., J. Acoust. Soc. Am. 94 (1993) 2108-2122). The combined results suggest that only one sigmoidal transfer function may account for the SP, 2F0, and the emission of the difference tone F2-F1, and that shifts in the operating point of the transfer function would be the major cause behind the furosemide-induced changes in the even-order distortion products. The sigmoidal transfer function is likely associated with the mechano-electrical transducer channel at the apical pole of the outer hair cell.

Modulation of outer hair cell compliance and force by agents that affect hearing

Force generated by outer hair cells is thought to be an essential source of mechanical input to the normal cochlea. Many disease processes in the inner ear act via outer hair cells. It is therefore plausible that such disease processes modulate outer hair cell force generation. The force generated by an isolated, electrically stimulated outer hair cell against a load may be represented by an intrinsic motor and a passive axial stiffness in series. Thus modulation of outer hair cell force generation may occur either by action on the motor or indirectly by an action on cell stiffness. In this study, the effects of agents that affect hearing on outer hair cell stiffness and force generation have been examined. Overstimulation and hypoosmotic challenge caused cells to decrease in length and increase in stiffness. The force generated by a constant voltage stimulus increased consequent to the stiffness increase. Hyperosmotic challenge elicited a stiffness decrease and a force decrease. In contrast, salicylate caused a decrease in force without stiffness change. The results suggest that outer hair cell force generation in vivo may be modulated in at least two ways.

The voltage dependence of the mechanoelectrical transducer modifies low frequency outer hair cell electromotility in vitro

The fast outer hair cell (OHC) electromotility is voltage dependent and is driven by changes in the OHC transmembrane potential. Those changes include the receptor potential generated by the variable conductance of the mechanoelectrical transducer (Evans and Dallos, 1993). In the experiments described here, we show that the voltage dependence of the mechanoelectrical transducer influences the low frequency motile responses of OHCs to an external electrical field. OHCs were fully inserted into a glass suction pipette, the microchamber, so that only the cuticular plate and hair bundle were exposed to the bath solution. With this technique, a rectification of the mechanical response, equivalent to an excitatory displacement of the hair bundle, was observed when the command voltage inside the microchamber depolarized the apical membrane. The shape of the response persisted when the OHC voltage-gated conductances were blocked. Following treatment of the hair bundle with BAPTA or dihydrostreptomycin, which are known to impair transduction function (Assad et al., 1991; Kroese et al., 1989), rectification of the motile response disappeared.

Effect of diamide on force generation and axial stiffness of the cochlear outer hair cell

We found that diamide, which affects spectrin, reduces the axial stiffness of the cochlear outer hair cell, the cylindrically shaped mechanoreceptor cell with a unique voltage-sensitive motility. This effect thus provides a means of examining the relationship between the stiffness and the motility of the cell. For measuring axial stiffness and force production, we used an experimental configuration in which an elastic probe was attached to the cell near the cuticular plate and the other end of the cell was held with a patch pipette in the whole-cell recording mode. Diamide at concentrations of up to 5 mM reduced the axial stiffness in a dose-dependent manner to 165 nN per unit strain from 502 nN for untreated cells. The isometric force elicited by voltage pulses under whole-cell voltage clamp was also reduced to 35 pN/mV from 105 pN/mV for untreated cells. Thus the isometric force was approximately proportional to the axial stiffness. Our observations suggest a series connection between the motor and cytoskeletal elements and can be explained by the area motor model previously proposed for the outer hair cell.

Expression of potassium channels in gerbil outer hair cells during development does not require neural induction

Mammalian outer hair cells (OHCs) contain Ca and K channels in their synaptic pole. We questioned if the ontogeny of potassium currents of OHCs depends on the neural induction of early afferent contact. By recording whole-cell currents of OHCs grown in organotypic cultures deprived of afferent innervation, we show that a Ca-activated K channel is expressed in these cells, suggesting that the ontogeny of the K channel is an intrinsic process.

Correspondence between middle frequency auditory loss in vivo and outer hair cell shortening in vitro

The aromatic hydrocarbon, toluene, has been reported to disrupt auditory system function both in occupational epidemiological and in laboratory animal investigations. This agent, along with several other organic solvents, impairs hearing preferentially at middle frequencies - a finding that distinguishes these agents from the traditional high frequency impairment observed with ototoxic drugs such as aminoglycoside antibiotics and cisplatin. Prior investigations performed in vivo have identified the outer hair cell as a probable target for toluene exposure. The purpose of this investigation was to determine directly whether outer hair cells isolated from the guinea pig cochlea show morphological alterations consistent with the toxic response seen in physiological studies with toluene exposure. The effect of toluene superfusion on outer hair cell shortening was assessed for cells harvested from different locations within the cochlea. Control studies included assessment of cell shortening among outer hair cells exposed to trimethyltin and cells exposed to benzene. Trimethyltin disrupts high frequency hearing preferentially and benzene does not produce hearing loss in vivo. Toluene at a concentration of 100 microM produced a marked shortening of outer hair cells although the effect was significantly greater among cells isolated from the apical half of the cochlea than from the basal half of the cochlea. By contrast, trimethyltin at the same concentration produced a preferential shortening among outer hair cells from the base of the cochlea. Benzene (100 microM) did not disrupt outer hair cell length of cells harvested from the apex. The results indicate that intrinsic features of outer hair cells contribute significantly to the site of ototoxic impairment observed in vivo for toluene.

Mechanical responses of the mammalian cochlea

Recent findings in auditory research have significantly changed our views of the processes involved in hearing. Novel techniques and new approaches to investigate the mammalian cochlea have expanded our knowledge about the mechanical events occurring at physiologically relevant stimulus intensities. Experiments performed in the apical, low-frequency regions demonstrate that although there is a change in the mechanical responses along the cochlea, the fundamental characteristics are similar across the frequency range. The mechanical responses to sound stimulation exhibit tuning properties comparable to those measured intracellularly or from nerve fibres. Non-linearities in the mechanical responses have now clearly been observed at all cochlear locations. The mechanics of the cochlea are vulnerable, and dramatic changes are seen especially when the sensory hair cells are affected, for example, following acoustic overstimulation or exposure to ototoxic compounds such as furosemide. The results suggest that there is a sharply tuned and vulnerable response related to the hair cells, superimposed on a more robust, broadly tuned response. Studies of the micromechanical behaviour down to the cellular level have demonstrated significant differences radially across the hearing organ and have provided new information on the important mechanical interactions with the tectorial membrane. There is now ample evidence of reverse transduction in the auditory periphery, i.e. the cochlea does not only receive and detect mechanical stimuli but can itself produce mechanical motion. Hence, it has been shown that electrical stimulation elicits motion within the cochlea very similar to that evoked by sound. In addition, the presence of acoustically-evoked displacements of the hearing organ have now been demonstrated by several laboratories.

Medial efferent inhibition suppresses basilar membrane responses to near characteristic frequency tones of moderate to high intensities

Efferent inhibition of low and medium spontaneous-rate fiber activity in response to characteristic frequency (CF) tones is greater at levels between 50 and 75 dB SPL than at lower levels, and even occurs at levels of 100 dB SPL [Guinan and Stankovic, J. Acoust. Soc. Am. 100, 1680-1690 (1996)]. The self-mixing effect of a laser diode was used to measure tone-evoked, basilar membrane (BM) displacements in the basal turn of the guinea pig cochlea during electrical stimulation of the medial efferent system. Efferent stimulation suppresses BM displacement and, for frequencies close to the CF of the measurement site and above, attenuation of tone-evoked displacements appears to be greatest for levels between 50 and 75 dB SPL, and efferent attenuation could exceed 10 dB SPL for tones at 90 dB SPL. By comparison, the attenuation of the BM vibrations caused by tones at frequencies below CF was found to be greatest for low sound pressure levels. The findings of Guinan and Stankovic therefore have their origin in events present in the vibrations of the BM.

Cochlear outer hair cell bending in an external electric field

We have used a high-resolution motion analysis system to reinvestigate shape changes in isolated guinea pig cochlear outer hair cells (OHCs) evoked by low-frequency (2-3 Hz) external electric stimulation. This phenomenon of electromotility is presumed to result from voltage-dependent structural changes in the lateral plasma membrane of the OHC. In addition to well-known longitudinal movements, OHCs were found to display bending movements when the alternating external electric field gradients were oriented perpendicular to the cylindrical cell body. The peak-to-peak amplitude of the bending movement was found to be as large as 0.7 microm. The specific sulfhydryl reagents, p-chloromercuriphenylsulfonic acid and p-hydroxymercuriphenylsulfonic acid, that suppress electrically evoked longitudinal OHCs movements, also inhibit the bending movements, indicating that these two movements share the same underlying mechanism. The OHC bending is likely to result from an electrical charge separation that produces depolarization of the lateral plasma membrane on one side of the cell and hyperpolarization on the other side. In the cochlea, OHC bending could produce radial distortions in the sensory epithelium and influence the micromechanics of the organ of Corti.

Mapping the distribution of outer hair cell voltage-dependent conductances by electrical amputation

The mammalian outer hair cell (OHC) functions not only as sensory receptor, but also as mechanical effector; this unique union is believed to enhance our ability to discriminate among acoustic frequencies, especially in the kilohertz range. An electrical technique designed to isolate restricted portions of the plasma membrane was used to map the distribution of voltage-dependent conductances along the cylindrical extent of the cell. We show that three voltage-dependent currents, outward K, I(K,n), and I(Ca) are localized to the basal, synaptic pole of the OHC. Previously we showed that the lateral membrane of the OHC harbors a dense population of voltage sensor-motor elements responsible for OHC motility. This segregation of membrane molecules may have important implications for auditory function. The distribution of OHC conductances will influence the cable properties of the cell, thereby potentially controlling the voltage magnitudes experienced by the motility voltage sensors in the lateral membrane, and thus the output of the "cochlear amplifier."

Force generation in the outer hair cell of the cochlea

The outer hair cell of the mammalian cochlea has a unique motility directly dependent on the membrane potential. Examination of the force generated by the cell is an important step in clarifying the detailed mechanism as well as the biological importance of this motility. We performed a series of experiments to measure force in which an elastic probe was attached to the cell near the cuticular plate and the cell was driven with voltage pulses delivered from a patch pipette under whole-cell voltage clamp. The axial stiffness was also determined with the same cell by stretching it with the patch pipette. The isometric force generated by the cell is around 0.1 nN/mV, somewhat smaller than 0.15 nN/mV, predicted by an area motor model based on mechanical isotropy, but larger than in earlier reports in which the membrane potential was not controlled. The axial stiffness obtained, however, was, on average, 510 nN per unit strain, about half of the value expected from the mechanical isotropy of the membrane. We extended the area motor theory incorporating mechanical orthotropy to accommodate the axial stiffness determined. The force expected from the orthotropic model was within experimental uncertainties.

Low-frequency modulation of inner hair cell and organ of Corti responses in the guinea pig cochlea

Low-frequency tones are used to study changes in responsiveness as a function of phase in inner hair cell (IHC) and organ of Corti (OC) responses recorded from second turn of the guinea pig cochlea. In these experiments a 40 Hz stimulus is combined with a variable frequency probe to determine the degree to which tones at and below best frequency (BF) are modulated. Changes in responsiveness produced by the low-frequency input are quantified and related to position of the basilar membrane which is estimated using the phase of the cochlear microphonic measured in the OC fluid space. Results obtained when 40 Hz is presented at its lowest effective level demonstrate that ac responses to low-level BF probes are reduced for basilar membrane displacements to scala tympani while probe tones well below BF are modulated in the opposite direction. The transition between these two response patterns occurs when the overall DC produced in the OC by the two-tone input changes from positive to negative. Because of this association, the frequency dependence exhibited in the bias results may be linked to mechanisms responsible for generating the two polarities of the summating potential and the DC receptor potentials that it reflects. An attempt is also made to relate bias-induced changes in hair cell receptor potentials to modulations in single-unit rate responses. In other words, to address variations in the temporal relationships between excitation and suppression measured in the auditory nerve.

Acetylcholine, outer hair cell electromotility, and the cochlear amplifier

The dominant efferent innervation of the cochlea terminates on outer hair cells (OHCs), with acetylcholine (ACh) being its principal neurotransmitter. OHCs respond with a somatic shape change to alterations in their membrane potential, and this electromotile response is believed to provide mechanical feedback to the basilar membrane. We examine the effects of ACh on electromotile responses in isolated OHCs and attempt to deduce the mechanism of ACh action. Axial electromotile amplitude and cell compliance increase in the presence of the ligand. This response occurs with a significantly greater latency than membrane current and potential changes attributable to ACh and is contemporaneous with Ca2+ release from intracellular stores. It is likely that increased axial compliance largely accounts for the increase in motility. The mechanical responses are probably related to a recently demonstrated slow efferent effect. The implications of the present findings related to commonly assumed efferent behavior in vivo are considered.

A theoretical basis for the high-frequency performance of the outer hair cell's receptor potential

The frequency response of the outer hair cell (OHC) was studied theoretically. An electrical model of the OHC was analyzed mathematically, taking into account the effect of its inherent voltage-dependent capacitance. It was found that the variations of the capacitance dependent on the membrane potential could enhance the high-frequency response of the OHC, so that its cutoff frequency could be extended into the audio range. It was found further that the enhancement of the frequency response of the OHC was strongly dependent on its resting potential and on the ratio of the maximum voltage-dependent capacitance to the membrane linear capacitance.

Outwardly rectifying currents in guinea pig outer hair cells

Studies of K+ conductances in hair cells report that big-conductance Ca(2+)-dependent K+ (BK) channels carry parts of the outwardly rectifying currents. Lin et al. (1995) suggested that in guinea pig outer hair cells (OHCs) a portion of these currents is carried via a voltage-dependent and Ca(2+)-independent K+ channel. The present study tests the hypothesis that there are two separable current components of the outwardly rectifying currents by using patch clamp methods in OHCs to characterize the voltage dependence and sensitivity of the outwardly rectifying currents to channel blockers. Lowering of external Ca2+ caused no change in the currents while charybdotoxin (ChTx; 100 nM), a BK K+ channel blocker, and Cd2+ (200 microM), and L-type calcium channel blocker, abolished about 50% of the currents. Both ChTx and Cd2+ caused a depolarizing shift in the half-activation voltage paralleled by a decrease in the voltage sensitivity. 4-Aminopyridine (4-AP, 0.01 mM), an A-type and delayed rectifier type channel blocker, abolished about 50% of the currents and caused a hyperpolarizing shift in the half-activation voltage together with an increase in the voltage sensitivity. The outwardly rectifying currents were more sensitive to block by 4-AP at membrane voltages around 40 mV compared to voltages around -20 mV. The differences in the current characteristics may be due to two separate channel types, one of which is similar to the delayed rectifier type channels while the other may be similar to the BK Ca(2+)-dependent K+ channels. In addition, the largest outwardly rectifying currents were present in long OHCs with the smallest present in short OHCs.

Effects of salicylate and lanthanides on outer hair cell motility and associated gating charge

Salicylate, one of the most widely used drugs, is known to induce reversible tinnitus and hearing loss. Salicylate interferes with outer hair cells (OHCs), which are believed to underlie normal auditory frequency selectivity and sensitivity. In the present experiments, the effects of salicylate and lanthanides on OHC motility and nonlinear capacitance were investigated by using isolated guinea-pig OHCs while attempting to avoid inadvertent intracellular pressure change, which itself can affect OHC motility and capacitance. Either extracellularly or intracellularly applied salicylate reduced nonlinear peak capacitance (Cmpk) and shifted the voltage at peak capacitance to depolarized levels. Concentration-response curves for reduction in Cmpk by salicylate and GdCl3 revealed a half-maximal concentration and Hill coefficient of 1.6 mM and 1.0, and 0.6 mM and 1.2, respectively. In comparable groups of OHCs, the normal Cmpk values of which were near 40 pF, average Cmpk decreased to 28 and 36 pF for intracellularly and extracellularly applied salicylate, respectively. Salicylate reduced, but did not completely block, the voltage-induced length change. Extracellularly, but not intracellularly, applied lanthanide blocked voltage-induced movement and capacitance almost completely. After intracellular trypsin treatment, salicylate reduced voltage-dependent capacitance reversibly, suggesting that salicylate directly acts on the sensor/motor and not via effects on intracellular structures, such as the subsurface cisternae. The results are consistent with the hypothesis that the dissociated, charged form of salicylate directly interacts with the sensor/motor on the inner aspect of the OHC plasma, whereas lanthanides interact on the outer aspect.

Acetylcholine response in guinea pig outer hair cells. I. Properties of the response

The properties of the ACh (acetylcholine) response in guinea pig outer hair cells (OHCs) are not well understood. It has been shown that the response to ACh involves the activation of a Ca2+ dependent K+ selective conductance (referred to as Ksub where sub stands for suberyldicholine). In the present study, we examined the voltage dependence, the time dependence, and the desensitization of the ACh response. In addition, we examined the K+ selectivity of K(sub). These properties are important for aiding in the determination of the type of K+ channels activated by ACh. Patch-clamp technique in the whole-cell mode was used to record from single OHCs isolated from adult pigmented guinea pigs. ACh (100 microM) was applied to the voltage-clamped OHCs and the ACh induced currents (IACh) were measured. A voltage dependence of the ACh response was found with the ACh induced currents decaying monoexponentially at potentials positive to -30 mV. The decay of the ACh induced currents was faster soon after establishing the whole-cell mode of recording when compared to the decay of the currents some time later. This effect, referred to as the time dependence, was different from the desensitization of the response upon prolonged application of ACh. The desensitization of the ACh induced currents was about 50% after 2 min of continuous application of 100 microM ACh. The examined characteristics of the ACh response in guinea pig OHCs indicate a voltage and time dependence of the response and strong K+ selectivity of the Ksub.

No evidence for functional GABA receptors in outer hair cells isolated from the apical half of the guinea-pig cochlea

Outer hair cells, isolated from the apical cochlear turns, did not respond to GABA during whole-cell recording. A few cells did respond to acetylcholine. Thus we have no evidence to suggest that GABA acts as an efferent neurotransmitter in the cochlea.

Differential expression of outer hair cell potassium currents in the isolated cochlea of the guinea-pig

1. Whole-cell currents were recorded from outer hair cells (OHCs) in undissociated tissues from the organ of Corti. The experiments allowed ionic currents to be measured in cells with precise localization on the three most apical cochlear turns. 2. Two major potassium currents were expressed in the cells. One current, named IK, was half-activated at -24 mV and was most prominent in the most apical turn, turn 4. A second, named IK.n, was half-activated at -92 mV and was the major contributor to the current-voltage (I-V) curve of cells from the more basal turns, turns 3 and 2, of the cochlea. 3. IK was specifically blocked by 100 microM 4-aminopyridine (4-AP). In contrast, IK.n was reduced by 5 mM external barium. Superfusion with zero calcium produced no effect on currents in the range from -60 to 0 mV, but reduced currents by a maximum of 15% outside this range. 4. The cell input conductance increased systematically from 3.4 nS in turn 4 to 40 nS in turn 2 measured at a holding potential of -70 mV. 5. The mean leak conductance, measured from the slope of the I-V curve at -110 mV, decreased systematically from 5.2 nS in turn 2, to 2.9 nS in turn 3 and 2.2 nS in turn 4. 6. These data show that hair cell properties can be determined in undissociated cells and are likely to provide a good estimate of the properties of the cells in the intact cochlea. Differences with the properties of isolated OHCs are discussed.

Acetylcholine response in guinea pig outer hair cells. II. Activation of a small conductance Ca(2+)-activated K+ channel

The type of K+ channel involved in the acetylcholine (ACh) evoked response (Ksub; sub stands for suberyldicholine) in guinea pig outer hair cells (OHCs) is still uncertain. The present study tests the hypotheses that Ksub is one of the following: a big conductance Ca(2+)-dependent K+ channel (BK), a small conductance Ca(2+)-dependent K+ channel (SK), a KA type of K+ channel, or a Kn type of K+ channel. Patch-clamp technique in the whole-cell mode was used to record from single guinea pig OHCs. ACh (100 microM) was applied to voltage-clamped OHCs and the ACh-induced currents (IACh) were measured. Charybdotoxin (100 nM) had no effect on IACh, while apamin (1 microM) blocked more than 90% of IACh. Lowering the external Ca2+ concentration caused a hyperpolarizing shift of the IACh monitored as a function of the prepulse voltage. Increasing internal Mg2+ (Mgi2+) concentration caused a reduction in the outward IACh without affecting the inward IACh. The Ksub channel was found to be permeable to Cs+. In Cs+ solutions, IACh was 45% of the IACh in K+ solutions. The block of IACh by apamin, the dependence on extracellular Ca2+, the incomplete block of IACh by Cs+, and the ACh-induced Cs+ currents favor the hypothesis that Ksub belongs to the SK type of channels. An ionotropic/nicotinic nature of the ACh mechanism of action is favored. It is suggested that, in vivo, the amplitude of the ACh-induced hyperpolarization may depend on the Ca2+/Mg2+ ratio inside and outside the cell.

Viscoelastic relaxation in the membrane of the auditory outer hair cell

The outer hair cell (OHC) in the mammalian ear has a unique membrane potential-dependent motility, which is considered to be important for frequency discrimination (tuning). The OHC motile mechanism is located at the cell membrane and is strongly influenced by its passive mechanical properties. To study the viscoelastic properties of OHCs, we exposed cells to a hypoosmotic solution for varying durations and then punctured them, to immediately release the osmotic stress. Using video records of the cells, we determined both the imposed strain and the strain after puncturing, when stress was reset to zero. The strain data were described by a simple rheological model consisting of two springs and a dashpot, and the fit to this model gave a time constant of 40 +/- 19 s for the relaxation (reduction) of tension during prolonged strain. For time scales much shorter or longer than this, we would expect essentially elastic behavior. This relaxation process affects the membrane tension of the cell, and because it has been shown that membrane tension has a modulatory role in the OHC's motility, this relaxation process could be part of an adaptation mechanism, with which the motility system of the OHC can adjust to changing conditions and maintain optimum membrane tension.

Mechanical properties of the lateral cortex of mammalian auditory outer hair cells

Mammalian auditory outer hair cells generate high-frequency mechanical forces that enhance sound-induced displacements of the basilar membrane within the inner ear. It has been proposed that the resulting cell deformation is directed along the longitudinal axis of the cell by the cortical cytoskeleton. We have tested this proposal by making direct mechanical measurements on outer hair cells. The resultant stiffness modulus along the axis of whole dissociated cells was 3 x 10(-3) N/m, consistent with previously published values. The resultant axial and circumferential stiffness moduli for the cortical lattice were 5 x 10(-4) N/m and 3 x 10(-3) N/m, respectively. Thus the cortical lattice is a highly orthotropic structure. Its axial stiffness is small compared with that of the intact cell, but its circumferential stiffness is within the same order of magnitude. These measurements support the theory that the cortical cytoskeleton directs electrically driven length changes along the longitudinal axis of the cell. The Young's modulus of the circumferential filamentous components of the lattice were calculated to be 1 x 10(7) N/m2. The axial cross-links, believed to be a form of spectrin, were calculated to have a Young's modulus of 3 x 10(6) N/m2. Based on the measured values for the lattice and intact cell cortex, an estimate for the resultant stiffness modulus of the plasma membrane was estimated to be on the order of 10(-3) N/m. Thus, the plasma membrane appears to be relatively stiff and may be the dominant contributor to the axial stiffness of the intact cell.

Diltiazem does not protect the ear from noise-induced hearing loss in mongolian gerbils

Recent data suggest that diltiazem reduces noise-induced hearing loss. Our study was designed to replicate and extend the results of Maurer et al. by using the gerbil as a model. In experiment A, subjects received diltiazem (30 mg/kg/day intraperitoneally) or saline for 3 days. After peripheral thresholds were measured, each subject was exposed to a 4-kHz tone (90-dB sound pressure level) for 20 minutes. Similar amounts of temporary threshold shifts (ITS) were measured in the saline and diltiazem groups. In experiment B, subjects were given saline or diltiazem (30 mg/kg/day intraperitoneally) for 3 days and then exposed to an octave band of noise centered at 4 kHz for 5 days, during which time the subjects continued to receive the drug or saline. The TTS and permanent threshold shifts were similar in the two groups. Measures of cochlear nonlinearities also showed no effect of diltiazem, suggesting that diltiazem does not protect the ear from the effects of noise.

Effects of kainic acid on the cochlear potentials and distortion product otoacoustic emissions in chinchilla

In absence of acoustic stimulation, the auditory nerve generates electrical noise with a spectral peak between 300 and 3000 Hz (Dolan et al., 1990). This electrical noise is eliminated when the dendrites of auditory nerve fibers are damaged by kainic acid (KA). We hypothesized that the KA-induced damage to the afferent dendrites might alter cochlear micromechanics or modify outer hair cell (OHC) electromotility. The KA-induced decrease in spontaneous electrical noise from the auditory nerve could conceivably reduce the spontaneous sounds recorded in the ear canal and the postulated change in cochlear micromechanics might alter distortion product otoacoustic emissions (DPOAE). To evaluate these hypotheses, we applied KA to the round window of the cochlea. KA reduced the spontaneous electrical noise recorded from the round window and significantly reduced the amplitude of the compound action potential (CAP) to tone bursts at 2, 4 and 8 kHz. KA caused only a slight reduction in the amplitude of the cochlear microphonic (CM) recorded from the round window: however, it had no effect on the spontaneous acoustic noise in the car canal or on 2 f1-f2 DPOAEs. These results suggest that the KA-induced reduction of electrical noise from the auditory nerve has no measurable effect on OHC electromotility as reflected in spontaneous otoacoustic emissions and that damage to the afferent dendrites has no effect on cochlear micromechanics as reflected in DPOAEs.

Effect of acetylcholine and GABA on the transfer function of electromotility in isolated outer hair cells

Outer hair cells (OHC) from high- and low-frequency regions were separately isolated from guinea pig cochleas. The cells were inserted with their ciliary pole first into a partitioning microchamber so that only 20-50% of the cell length was excluded. Somatic length changes due to transcellular electrical stimulation were measured at the cuticular plate in the inserted portion of the cells. Transfer curves of electromotility of the OHCs were obtained by both a series of brief (2.5 ms) and longer (30 ms) square pulses with opposite polarity and linearly increasing size from 40 to 280 mV in both negative and positive directions. Alterations in the transient and steady-state electromotility transfer curves were examined by application of acetylcholine (ACh) and gamma-aminobutyric acid (GABA) to the synaptic pole. ACh, in the concentration range of 10-30 microM, evoked a significant magnitude and gain increase of electromotility in both transient and steady-state responses without a measurable shift in the operating point of the displacement-voltage transfer curve. A tonotopic response magnitude difference is found for ACh challenge. Basal turn OHCs responded with greater magnitude increase (+90% increase from control) than apical turn OHCs (+40%). GABA exerted an opposite effect, again in a location-dependent manner. Magnitude response decreased about 30% for long cells and 14% for short ones. Atropin, a muscarinic receptor antagonist, completely blocked the increase in electromotility response due to ACh. However, D-tubocurarine, a nicorinic receptor antagonist, while not blocking the ACh effect, altered the cell's apparent operating point. Bicuculline methiodide, a GABAA-receptor antagonist, completely arrested GABA influences on the electromotility response. These results suggest that both ACh and GABA can change the electromotile activity of OHCs, in a tonotopically biased manner. ACh challenge evokes greater magnitude responses in basal turn OHCs, whereas GABA induces greater motility response decrease in apical turn OHCs. The control of the gain and magnitude of electromotility by the transmitter substances appear to involve at least two mechanisms. One is probably related to conformational changes of the voltage-to-movement converter molecules and a change in their number in an effective operational pool, the other operates via changing the electrical resistance of the basolateral cell membrane.

Human otosclerotic bone-derived peptide decreases the gain of the electromotility in isolated outer hair cells

An otosclerotic peptide (OP) (Sziklai et al., 1985a.b) was purified from perilymph and stapes footplate of otosclerotic patients by Sephadex G-25 gel column chromatography and subsequent isotachophoretic (ITP) separation. The transfer function of the electromotility was measured by inserting the isolated outer hair cells (OHC) into a partitioning microchamber (Evans et al., 1991) and applying a series of pairs of brief square-pulse stimuli with opposite polarity and with increasing magnitude. Somatic length changes of the inserted part of the OHCs were measured by an optoelectronic system. The isotachophoretically homogeneous peptide exerted a gain and magnitude decreasing effect on the transfer function of electromotility of isolated OHCs of the guinea pig, in vitro. The operating point of the electromotility did not change due to the effect of the peptide. The peptide decreased the electromotile performance within a minute and bath exchange to normal saline did not completely restore the transfer curve to baseline. Application of caffeine to the cells already under the effect of the otosclerotic peptide produced an opposite effect: gain and magnitude increase. Simultaneous application of acetylcholine (ACh) did not antagonize the effect of OP. The underlying mechanism of the action of OP on the transfer function of electromotility of OHCs is postulated to involve the modulation of intracellular Ca2- concentration.

Electromotile hearing: evidence from basilar membrane motion and otoacoustic emissions

Electrical stimulation of the cochlea is known to cause auditory sensations in humans and other animals. It also has been shown to produce emissions of sound from the inner ear. In the current study we investigate the relationship between electrically induced motion of the basilar membrane (BM) and the production of otoacoustic emissions. We test the hypothesis that electrical current-induced movements of the outer hair cell (OHC electromotility) result in intracochlear acoustic pressure which causes traveling waves on the BM. Our results demonstrate that the dominant response of the guinea pig inner ear to electric stimulation, at the round window membrane (RW) or across the cochlear duct, is a mechanical response of the organ of Corti. We observed that electrical stimulation of the cochlea produced traveling wave activity on the BM, measured with a laser Doppler velocimeter. The BM motion was accompanied by sound emitted by the cochlea for frequencies up to at least 25 kHz. Furthermore, bipolar rectangular current stimulation produced steady, bipolar displacements of the BM (to 2 nm), indicating functional elongation or contraction of OHCs occurs depending on the polarity of the current pulse. All of the evoked responses were absent after drug treatments eliminated the OHCs. Our data indicate that OHCs undergo electrically evoked displacements capable of producing high-fidelity, high-frequency acoustic energy. The electrically evoked intra-cochlear energy results in conventional traveling waves within the cochlea, as well as emissions of sound from the cochlea. These data provide direct support for a mechanism of cochlear sensitivity and tuning involving high-frequency OHC electromotility. Moreover, the data also indicate that any intra- or extracochlear electric current which affects the electric polarization of OHCs could induce BM traveling waves and cause 'electromotile hearing'. This form of hearing would be one component under the more general definition of the electrophonic effect.

Patch clamped responses from outer hair cells in the intact adult organ of Corti

Outer hair cells (OHCs) from the mammalian cochlea act as both sensory cells and motor cells. We report here whole-cell tight seal recordings of OHC activity in their natural embedding tissue, the intact organ of Corti, using a temporal bone preparation. The mean cell resting potential, -76 +/- 4 mV (n = 19) and input conductance (10 +/- 3 nS at -70 mV) of third turn hair cells were significantly lower than have been found in isolated cells. Two main K+ currents in the cell were identified. One current, activated positive to -100 mV, was reduced by 5 mM BaCl2. The other current, activated above -40 mV, was reduced by 100 microM 4-aminopyridine (4-AP) and by 30 mM tetraethylammonium (TEA). Both of these currents have been also identified in recordings reported from isolated cells. On stepping to different membrane potentials, cells imaged in the organ of Corti changed length by an amount large enough to cause visible distortions in neighbouring cells. By quantifying such distortions we estimate that the forces generated by OHCs can account for the enhanced response to sound required by the cochlear amplifier.

Dihydropyridines and verapamil inhibit voltage-dependent K+ current in isolated outer hair cells of the guinea pig

Dihydropyridines and verapamil are widely used as blockers of voltage-dependent Ca++ channels. In this work we show that these compounds can have a direct blocking action on a class of voltage-activated potassium channels. Voltage-dependent whole-cell currents were recorded from isolated guinea-pig outer hair cells (OHCs) under conditions such that the free Ca++ concentration in both the internal and external solutions was minimized. A substantial Ca(++)-independent K+ current was revealed by this procedure. Both conventional K+ and Ca++ channel ligands inhibited this current. The order of potency (in terms of the half inhibitory concentrations (IC50) of channel inhibitors) was: nimodipine (6 microM) > Bay K 8644 (8 microM) > verapamil (11 microM) > 4-aminopyridine (22 microM) > nifedipine (32 microM) > quinine (49 microM) > TEA (10236 microM). Except for verapamil, these channel ligands reduced the size of the K+ currents without much alteration of the time course of the currents. In contrast, verapamil caused a more than 10-fold increase in the apparent inactivation rate of the K+ currents without significantly altering the activation of the currents. The observation that relatively low concentrations of calcium channel ligands can directly inhibit potassium currents in isolated OHCs indicates that caution should be taken when these pharmacological agents are used as tools for studying cochlear hair cell physiology.

Nitroprusside suppresses cochlear potentials and outer hair cell responses

Biochemical and pharmacological evidence supports a role for nitric oxide (NO) in the cochlea. In the present experiments, we tested sodium nitroprusside (SNP), an NO donor, applied by intracochlear perfusions on sound-evoked responses of the cochlea (CM, cochlear microphonic; SP, summating potential; EP, endocochlear potential; CAP, compound action potential) and in vitro on outer hair cell (OHC) voltage-induced length changes and current responses. In vivo application of SNP in increasing concentrations (10, 33, 100, 330 and 1000 microM) reduced all sound-evoked responses starting at about 300 microM. The responses continued to decline after a postdrug wash. At 1 mM SNP decreased EP slowly (approximately 80 min) whereas at 10 mM it reduced EP more rapidly (approximately 20 min). Ferricyanide (1 mM) and S-nitroso-N-acetylpenicillamine (SNAP; 1 mM) had no effect on sound-evoked cochlear potentials. Ferricyanide (1 mM and 10 mM) and ferrocyanide (10 mM) had no effect on EP. In vitro, SNP (10 mM) significantly reduced both OHC voltage-induced length changes and whole-cell outward currents. Results suggest that SNP, possibly acting by released NO, influences cochlear function through effects at the stria vascularis and at the OHCs.

Direct measurements of Ca(2+)-activated K+ currents in inner hair cells of the guinea-pig cochlea using photolabile Ca2+ chelators

Intracellular photorelease of Ca2+ from caged Ca2+ (DM-nitrophen or nitr5) and the patch-clamp technique in the whole-cell configuration were used to investigate Ca(2+)-activated currents in inner hair cells (IHCs) of the mammalian cochlea. Photoliberation of intracellular Ca2+ activated outward currents with a mean amplitude of 260 +/- 110 pA when IHCs were voltage-clamped, near the resting membrane potential, at -50 mV. The photoactivated currents were reversibly blocked by extracellular application of tetraethylammonium (TEA, 10 mM), neomycin (1 mM) and charybdotoxin (1 microM), but not by apamin. The voltage dependence of membrane currents activated by photolysis of DM-nitrophen demonstrated a reversal potential near the K+ equilibrium potential (Ek) and saturation near 0 mV. The presence of Ca(2+)-activated currents was further confirmed by the effects of extracellular adenosine 5'-triphosphate (ATP, 10 microM) and the Ca2+ ionophore ionomycin (10 microM). Both agents raised intracellular Ca2+ and simultaneously activated outward currents when IHCs were voltage-clamped near the resting membrane potential. In experiments where currents were activated by depolarizing voltage steps, nifedipine (50 microM) and Cd2+ (1 mM) reduced significantly (20-50%) the whole-cell outward currents, suggesting the presence of L-type Ca2+ currents activating K+ currents. These results are the first direct evidence for Ca(2+)-activated K+ currents in mammalian IHCs, these currents being potentially important for cell repolarization during sound-induced depolarization and synaptic transmission.

Caffeine-induced shortening of isolated outer hair cells: an osmotic mechanism of action

The application of caffeine to the bathing medium of isolated cochlear outer hair cells (OHCs) induces shortening of the cells (Slepecky et al., 1988). This study was designed to test the hypothesis that a 'smooth muscle-like' mechanism was responsible for the caffeine-induced shortening of OHCs as suggested by Slepecky et al. OHCs were isolated from guinea pig cochleae and length measurements were taken during various drug perfusions. Antagonists of the ryanodine receptor/Ca(2+)-induced Ca2+ release (CICR; tetracaine, ruthenium red, and ryanodine) failed to block the caffeine-induced shortening of the OHCs. Application of the Ca2+ ionophore A23187 caused cell length to increase. These results did not support this hypothesis and therefore, an osmotic mechanism was proposed.

Effect of lidocaine injection of EOAE in patients with tinnitus

In the present study, evoked otoacoustic emissions (EOAEs) were measured in 30 patients with tinnitus before and after intravenous lidocaine injection (1 mg/kg). For EOAE recordings, 1 kHz tone burst stimuli were used. Intravenous lidocaine injection resulted in suppression of tinnitus in 22 (73%) ears, and changes of EOAE amplitude (increase or decrease) in 18 (60%) ears. Of the 18 ears with EOAE amplitude changes, tinnitus disappeared or decreased in 17 (94%) ears. In 12 ears without changes of EOAE amplitude, tinnitus was suppressed in only 5 (42%). Changes in latency were not detected in any of the ears. These results indicate that there is a relationship between the effect of lidocaine in tinnitus suppression and changes in cochlear micromechanics caused by lidocaine.

ATP modulation of L-type calcium channel currents in guinea pig outer hair cells

Ca2+ channel currents and their modulation by adenosine 5'-triphosphate (ATP) in acutely isolated guinea pig outer hair cells (OHCs) were investigated using the whole-cell patch-clamp technique. The current-voltage (I-V) relation of OHCs indicated that the Ca2+ channel opened near -30 mV, and the current reached a maximum at +10 and 0 mV in 20 mM Ca2+ and Ba2+ external solutions, respectively. BayK 8644 (BayK, 2 microM) caused a 3.5-fold increase in peak Ca2+ currents and shifted the I-V curves toward more negative potentials. These results suggest that the majority of Ca2+ channels in OHCs have L-type characteristics. The effects of ATP on Ca2+ channels of OHCs were heterogenous. ATP (100 microM) decreased Ca2+ channel currents by 31.7 +/- 5.6% at 0 mV and shifted Ca2+ tail activation curves toward more depolarized potentials in some cells (N = 6). By contrast, in others, ATP enhanced the currents by 43.5 +/- 12.5% at +10 mV (N = 6). In the presence of BayK, however, ATP-induced inhibition or enhancement of Ca2+ channel currents was attenuated. In addition, 100 microM ATP produced little effect on Ca2+ channel currents in another subpopulation of cells (N = 12). This heterogenous neuromodulation of Ca2+ channel currents by ATP may reflect a functional diversity among OHCs.

Membrane tension directly shifts voltage dependence of outer hair cell motility and associated gating charge

The unique electromotility of the outer hair cell (OHC) is believed to promote sharpening of the passive mechanical vibration of the mammalian basilar membrane. The cell also presents a voltage-dependent capacitance, or equivalently, a nonlinear gating current, which correlates well with its mechanical activity, suggesting that membrane-bound voltage sensor-motor elements control OHC length. We report that the voltage dependence of the gating charge and motility are directly related to membrane stress induced by intracellular pressure. A tracking procedure was devised to continuously monitor the voltage at peak capacitance (VpkCm) after obtaining whole cell voltage clamp configuration. In addition, nonlinear capacitance was more fully evaluated with a stair step voltage protocol. Upon whole cell configuration, VpkCm was typically near -20 mV. Negative patch pipette pressure caused a negative shift in VpkCm, which obtained a limiting value near the normal resting potential of the OHC (approximately -70 mV) at the point of cell collapse. Positive pressure in the pipette caused a positive shift that could reach values greater than 0 mV. Measures of the mechanical activity of the OHC mirrored those of charge movement. Similar membrane-tension dependent peak shifts were observed after the cortical cytoskeletal network was disrupted by intracellular dialysis of trypsin from the patch pipette. We conclude that unlike stretch receptors, which may sense tension through elastic cytoskeletal elements, the OHC motor senses tension directly. Furthermore, since the voltage dependence of the OHC nonlinear capacitance and motility is directly regulated by intracellular turgor pressure, we speculate that modification of intracellular pressure in vivo provides a mechanism for controlling the gain of the mammalian "cochlear amplifier".

High-frequency motility of outer hair cells and the cochlear amplifier

Outer hair cells undergo somatic elongation-contraction cycles in vitro when electrically stimulated. This "electromotile" response is assumed to underlie the high sensitivity and frequency selectivity of amplification in the mammalian cochlea. This process, presumably operating on a cycle-by-cycle basis at the frequency of the stimulus, is believed to provide mechanical feedback in vivo. However, if driven by the receptor potential of the cell, the mechanical feedback is expected to be severely attenuated at high frequencies because of electrical low-pass filtering by the outer hair cell basolateral membrane. It is proposed that electromotility at high frequencies is driven instead by extracellular potential gradients across the hair cell, and it is shown that this driving voltage is not subject to low-pass filtering and is sufficiently large. It is further shown that if the filtering properties of the cell membrane are canceled, taking advantage of the electrical characteristics of isolated outer hair cells in a partitioning glass microchamber, then the lower bound of the motor's bandwidth is approximately 22 kilohertz, a number determined only by the limitations of our instrumentation.

Electromotile responses and frequency tuning of isolated outer hair cells of the guinea pig cochlea

Isolated outer hair cells (OHC) of the guinea pig cochlea were exposed to external alternating electric fields parallel to the longitudinal axis of the cells. This resulted in oscillations of the cells' length that were measured photoelectrically using a ratiometric light amplifier. At 5 Hz and elongations up to 300 nm, amplitude of the cell length during oscillation was a linear function of the amplitude of the sinusoidal electric field. When increasing the stimulus frequency up to 32 kHz, OHC length changes followed the stimulus cycle-by-cycle. Oscillations at frequencies above 32 kHz escaped the experimental approach by their small amplitudes and could not be excluded. The frequency dependence of the motile response measured at 5-12,000 Hz had low-pass filter characteristics in cells of the second, third and fourth turns of the cochlea. However, frequency tuning of the motile response was absent in each OHC and systematic differences between different turns were not observed.

Motility voltage sensor of the outer hair cell resides within the lateral plasma membrane

The outer hair cell (OHC) from the organ of Corti is believed to be responsible for the mammal's exquisite sense of hearing. A membrane-based motile response of this cell underlies the initial processing of acoustic energy. The voltage-dependent capacitance of the OHC, possibly reflecting charge movement of the motility voltage sensor, was measured in cells during intracellular dialysis of trypsin under whole cell voltage clamp. Within 10 min after dialysis, light and electron microscopic examination revealed that the subplasmalemmal structures, including the cytoskeletal framework and subsurface cisternae, were disrupted and/or detached from adjacent plasma membrane. Dialysis of heat-inactivated trypsin produced no changes in cell structure. Simultaneous measures of linear and nonlinear membrane capacitance revealed minimal changes, indicating that contributions by subsurface structures to the generation of the nonlinear capacitance are unlikely. This study strongly suggests that voltage-dependent charge movement in the OHC reflects properties of the force generator's voltage sensor and that the sensor/motor resides solely within the lateral plasma membrane.

Acetylcholine activates a K+ conductance permeable to Cs+ in guinea pig outer hair cells

Acetylcholine (ACh), the major neurotransmitter released by efferent nerve fibers in the cochlea, has been shown to activate a Ca(2+)-dependent K+ conductance in outer hair cells (OHCs). Previously we reported that this ACh operated conductance is permeable to Cs+. The purpose of the present study was to characterize further this Cs(+)-permeable channel and its dependency on Ca2+ using isolated OHCs and the patch clamp technique in the whole cell configuration. The changes in the ACh response were examined when Cs+, Ba2+, Cd2+, N-methyl-D-glucamine (NMG+) and tetraethylammonium (TEA+) were placed in the external or internal solutions. Cs+ substituted for K+ in carrying the ACh-evoked Ca(2+)-dependent K+ current. When NMG+/TEA+ was substituted for internal K+ ACh-evoked an inward and an outward current, and Cs+ substituted for external K+ blocked the outward but not the inward current evoked by ACh suggesting it was carried by K+. In the NMG+/TEA+ condition, when the cell was held at different Vh values for an extended period of time, the ACh-induced K+ current rectified. In Ba2+ (3 mM) with zero Ca2+ ACh failed to induce any detectable current and the ACh response slowly recovered from the Ba2+ block, suggesting a block at an intracellular site. Cd2+ (1 mM) readily and reversibly blocked ACh-induced currents even when carried by Cs+. This data suggests that ACh opens a channel selective for K+, conductive to Cs+ and dependent on Ca2+.

First appearance and development of electromotility in neonatal gerbil outer hair cells

With the purpose of pinpointing the time of onset of electromotility, outer hair cells (OHCs) from apical and basal turns of the cochleae of postnatal gerbils, ranging in age from 6 to 19 days, were isolated and drawn into a glass microchamber. Length changes evoked by transcellular electrical stimulation were detected and measured with a photodiode detector. Motile responses first appeared in 3 out of 14 basal turn OHCs at 7 days after birth (DAB). At 8 DAB, 3 out of 13 apical turn cells also responded to the electrical stimulation. By 12 DAB, all the OHCs from both turns showed motile responses. Input-output functions relating applied stimulus and change in cell length revealed that the motile response threshold improved from 7 DAB to 12 DAB and the response amplitude kept increasing from 7 DAB until 13-14 DAB, when mature amplitudes were reached. Measurements of OHC length revealed only minor changes in basal turn hair cell length while apical hair cells continued to elongate until approximately 16 DAB. Since the onset of auditory function in gerbils occurs around 12 DAB and fine tuning develops between 14 and 17 DAB, our results suggest that the onset of OHC motility occurs earlier than that of auditory function and the maturation of the motility amplitude occurred earlier than the development of fine tuning. The maturation of OHC motility and the development of otoacoustic emissions are also compared and discussed.

Frequency response of mature guinea-pig outer hair cells to stereociliary displacement

Outer hair cells (OHC) were isolated from the apical two turns of the guinea-pig cochlea and their hair-bundle stimulated mechanically by a glass probe. In accordance with in vivo data (Dallos, 1985), the resting membrane potential was typically -64 mV (N = 200). The maximum amplitudes of the receptor potentials were between 0.4 and 5.2 mV peak-to-peak, with mean of 1.5 mV +/- 0.9 mV (N = 81). The sensitivity was 0.015 mV/nm or 2 mV/deg. The frequency response of the receptor potential followed a first order low-pass filter characteristic with a corner frequency of about 63 Hz. For frequencies up to at least 1.6 kHz, the frequency response of mechanoelectrical transduction was dominated by the electrical input impedance of the cell. The presence of a single time constant in the voltage response to stereociliary deflection implies that the frequency response of mechanoelectrical transduction far exceeds that of the electrical input impedance of the cell; its time constant must be faster than 100 microseconds. Under in vivo conditions, OHC should be capable of providing a sufficiently large receptor potential to supply enough energy for electromechanical feedback.

Mechanical and electromechanical properties of the stereovillar bundles of isolated and cultured hair cells of the chicken

Isolated single chicken hair cells and pieces of epithelium without the tectorial membrane, either freshly isolated or in tissue culture, were studied using water-jet stimulation of their stereovillar bundles and current injection. Responses were measured under enhanced video-microscopic observation or while using a differential photodiode technique sensitive down into the nanometer range. When stimulated with a water jet at low displacement amplitudes up to about 200 nm, the stereovillar bundle displacement was asymmetrical, indicating a lower stiffness in the excitatory direction, but the reverse was true at higher displacement amplitudes. Undamaged bundles showed no mechanical resonances below 1 kHz. In damaged bundles, however, such resonances were prominent and accompanied by splaying of the stereovilli. Hair cells in the epithelium showed small bundle movements (0.6 nm/mV) whose polarity depended on the polarity of the injected current. These movements probably resulted from activation of the bundle's adaptation motors.