Frequency-specific position shift in the guinea pig organ of Corti

Hidden hearing loss is associated with loss of ribbon synapses of cochlea inner hair cells

The present study aimed to observe the changes in the cochlea ribbon synapses after repeated exposure to moderate-to-high intensity noise. Guinea pigs received 95 dB SPL white noise exposure 4 h a day for consecutive 7 days (we regarded it a medium-term and moderate-intensity noise, or MTMI noise). Animals were divided into four groups: Control, 1DPN (1-day post noise), 1WPN (1-week post noise), and 1MPN (1-month post noise). Auditory function analysis by auditory brainstem response (ABR) and compound action potential (CAP) recordings, as well as ribbon synapse morphological analyses by immunohistochemistry (Ctbp2 and PSD95 staining) were performed 1 day, 1 week, and 1 month after noise exposure. After MTMI noise exposure, the amplitudes of ABR I and III waves were suppressed. The CAP threshold was elevated, and CAP amplitude was reduced in the 1DPN group. No apparent changes in hair cell shape, arrangement, or number were observed, but the number of ribbon synapse was reduced. The 1WPN and 1MPN groups showed that part of ABR and CAP changes recovered, as well as the synapse number. The defects in cochlea auditory function and synapse changes were observed mainly in the high-frequency region. Together, repeated exposure in MTMI noise can cause hidden hearing loss (HHL), which is partially reversible after leaving the noise environment; and MTMI noise-induced HHL is associated with inner hair cell ribbon synapses.

Characterisation of the static offset in the travelling wave in the cochlear basal turn

In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.

Auditory Distortions: Origins and Functions

To enhance weak sounds while compressing the dynamic intensity range, auditory sensory cells amplify sound-induced vibrations in a nonlinear, intensity-dependent manner. In the course of this process, instantaneous waveform distortion is produced, with two conspicuous kinds of interwoven consequences, the introduction of new sound frequencies absent from the original stimuli, which are audible and detectable in the ear canal as otoacoustic emissions, and the possibility for an interfering sound to suppress the response to a probe tone, thereby enhancing contrast among frequency components. We review how the diverse manifestations of auditory nonlinearity originate in the gating principle of their mechanoelectrical transduction channels; how they depend on the coordinated opening of these ion channels ensured by connecting elements; and their links to the dynamic behavior of auditory sensory cells. This paper also reviews how the complex properties of waves traveling through the cochlea shape the manifestations of auditory nonlinearity. Examination methods based on the detection of distortions open noninvasive windows on the modes of activity of mechanosensitive structures in auditory sensory cells and on the distribution of sites of nonlinearity along the cochlear tonotopic axis, helpful for deciphering cochlear molecular physiology in hearing-impaired animal models. Otoacoustic emissions enable fast tests of peripheral sound processing in patients. The study of auditory distortions also contributes to the understanding of the perception of complex sounds.

Non-linear response to amplitude-modulated waves in the apical turn of the guinea pig cochlea

Mechanical vibrations of the Hensen's cells were measured in the apical turn of the cochlea in living guinea pigs, in response to amplitude-modulated (AM) sound. The FFT of the input wave consisted of spectral components at the carrier frequency C and two sidebands (C+/-M) separated from the carrier by the modulation frequency M. The FFT of the velocity response consisted of components at: (i) the modulation frequency M, and harmonics n M; (ii) Carrier frequency C and sidebands (C+/-n M); (iii) harmonics of the carrier frequency and their side bands (2C+/-n M); (3C+/-n M); (4C+/-n M); em leader n=1,2,3, em leader,10. The carrier and the first pair of side bands were broadly tuned and nearly linear. Other components were sharply tuned and highly non-linear, suggesting a different origin. Evidence is presented that these components are generated in the non-linear stereocilia dynamics. An important function of this non-linearity is to demodulate the AM wave to extract information contained in the modulation.

Mechanics of the mammalian cochlea

In mammals, environmental sounds stimulate the auditory receptor, the cochlea, via vibrations of the stapes, the innermost of the middle ear ossicles. These vibrations produce displacement waves that travel on the elongated and spirally wound basilar membrane (BM). As they travel, waves grow in amplitude, reaching a maximum and then dying out. The location of maximum BM motion is a function of stimulus frequency, with high-frequency waves being localized to the "base" of the cochlea (near the stapes) and low-frequency waves approaching the "apex" of the cochlea. Thus each cochlear site has a characteristic frequency (CF), to which it responds maximally. BM vibrations produce motion of hair cell stereocilia, which gates stereociliar transduction channels leading to the generation of hair cell receptor potentials and the excitation of afferent auditory nerve fibers. At the base of the cochlea, BM motion exhibits a CF-specific and level-dependent compressive nonlinearity such that responses to low-level, near-CF stimuli are sensitive and sharply frequency-tuned and responses to intense stimuli are insensitive and poorly tuned. The high sensitivity and sharp-frequency tuning, as well as compression and other nonlinearities (two-tone suppression and intermodulation distortion), are highly labile, indicating the presence in normal cochleae of a positive feedback from the organ of Corti, the "cochlear amplifier." This mechanism involves forces generated by the outer hair cells and controlled, directly or indirectly, by their transduction currents. At the apex of the cochlea, nonlinearities appear to be less prominent than at the base, perhaps implying that the cochlear amplifier plays a lesser role in determining apical mechanical responses to sound. Whether at the base or the apex, the properties of BM vibration adequately account for most frequency-specific properties of the responses to sound of auditory nerve fibers.

Characteristics of the travelling wave in the low-frequency region of a temporal-bone preparation of the guinea-pig cochlea

This study provides a detailed quantitative description of the acoustically evoked vibration responses in the low-frequency region of the in vitro guinea-pig cochlea. Responses of the basilar membrane, the reticular lamina and Hensen cells were measured with a laser Doppler vibrometer, without the need for introducing artificial light reflectors. The apex of the cochlea was opened, leaving the helicotrema intact. Two response components were detected: a 'fast' component, which was probably caused by the hole in the cochlea, and a 'slow' component, which shared the features of a classical travelling wave. The velocity response of the 'slow' component exhibited a relatively flat low-frequency slope (15 dB/oct) and a much steeper high-frequency roll-off (third turn: -47 dB/oct; fourth turn: -35 dB/oct). The group delay was dependent on the characteristic frequency. In the fourth turn, the sharpness of the velocity tuning curves (Q(10 dB): 1.0) was similar to those of in vivo mechanical and neural recordings, whereas in the third turn the tuning (Q(10 dB): 1.1) was much less than for in vivo recordings. The results indicate that cochlear amplification, which is responsible for the high sensitivity and sharp tuning in the basal part of the cochlea, is much less pronounced in the apical turn of the cochlea.

Nonlinearity in the apical turn of living guinea pig cochlea

Mechanical vibrations were measured at the apical turn in living guinea pig cochlea, in response to sinusoidal acoustic stimuli, using heterodyne interferometry. The cochlea was sealed and the vibrations were measured at different cellular locations along a radial track at the level of reticular lamina and one point on the osseous spiral lamina. Averaged time waveforms were recorded at each test frequency. The nonlinearity in the apical turn is demonstrated by the distortion in the time waveforms and the richness of the harmonic components in their Fourier transforms. Tuning curves and input/output curves for the fundamental and harmonics components are shown. The fundamental component is essentially linear below about 90 dB SPL while the harmonics display strong nonlinearity and saturation. Negative feedback in the apical turn of the cochlea linearizes the response at the fundamental frequency.

Reticular lamina vibrations in the apical turn of a living guinea pig cochlea

The reticular lamina of the apical turn of a living guinea pig cochlea was viewed through the intact Reissner's membrane using a slit confocal microscope. Vibrations were measured at selected identified locations with a confocal heterodyne interferometer, in response to tones applied with an acoustic transducer coupled to the ear canal. The position coordinates of each location were recorded. Mechanical tuning curves were measured along a radial track at Hensen's cells, outer hair cells, inner hair cells and at the osseous spiral lamina, over a frequency range of 3 kHz, using five sound pressure levels (100, 90, 80, 70 and 60 dB SPL). The carrier to noise ratio obtained throughout the experiments was high. The response shape at any measuring location was not found to change appreciably with signal level. The response shape also did not change significantly with the radial position on the reticular lamina. However, the response magnitude increased progressively from the inner hair cell to the Hensen's cell. The observed linearity of response at the fundamental frequency is explained by the presence of negative feed back in the apical turn of the cochlea.

Mechanically evoked shortening of outer hair cells isolated from the guinea pig organ of Corti

Outer hair cells isolated from the mammalian hearing organ have been shown to respond to mechanical stimuli at acoustic frequencies by expressing a change in cell length (e.g. Canlon et al., 1988). The acoustically evoked response is characterised by both a tonic length change following the envelope of the stimulus, and a frequency-dependent phasic component. We show here that mechanical stimulation at much lower frequencies directed at the cell body also elicits length changes of the outer hair cells. When the apical pole of isolated outer hair cells was compressed with a quartz fibre, a shortening or contraction at the basal pole was observed. Transverse indentation at the lateral membrane elicited shortenings at both ends of the cells. The sensitivity to the mechanical manipulation was changed by an altered tonicity of the external solution, or exposure to salicylate. As the response occurs at very low stimulus frequencies, it may account for the mechanism by which the hearing organ responds to the low frequency modulation component in complex signals like speech.

Development of transient evoked otoacoustic emissions in the neonatal rat

Otoacoustic emissions (OAEs) represent acoustic energy generated by the cochlear amplifier which contributes to auditory sensitivity and frequency discrimination. Therefore the OAEs can serve as a noninvasive tool to study the cochlear amplifier. While transient evoked OAEs (TEOAEs) are generally recorded clinically in man, it has been difficult to record them in animals and instead cubic distortion product OAEs (DPOAEs) have been experimentally studied in animals. In a previous study, we perfected a method of recording TEOAEs routinely in rats and this technique was used here to study the development of OAEs in neonatal rats. TEOAEs were recorded and compared to the DPOAEs on several postnatal days. With increasing postnatal age, TEOAE peak-to-peak amplitude and spectral energy in the 2- to 4-kHz band increased, their threshold decreased and their input-output functions became less monotonic with a change in slope (notch and/or plateau) in the mid-intensity region. The DPOAEs to higher frequencies appeared first, then the TEOAEs, followed by the DPOAEs to lower frequencies. With age, their amplitude also increased, thresholds decreased and a notch appeared in their input-output functions. The TEOAEs were measurable during the continuum of the appearance of the DPOAEs and the developmental sequences of both types of OAEs were similar. This may be evidence that similar mechanisms account for their maturation which probably initially involves a reduction in the air-bone gap with maturation of the outer and middle ears, and then elevation of the endocochlear potential and additional micromechanical maturations.

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.

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.

Investigating routes to chaos in the guinea-pig cochlea using the continuous wavelet transform and the short-time Fourier transform

The continuous wavelet transform (CWT) and the short-time Fourier transform (STFT) were used to analyze the time course of cellular motion in the guinea pig inner ear. The velocity responses of individual outer hair cells and Hensen's cells to amplitude modulated (AM) acoustical signals applied to the ear canal displayed characteristics typical of nonlinear systems, such as the generation of spectral components at harmonics of the carrier frequency. Nonlinear effects were particularly pronounced at the highest stimulus levels, where half-harmonic (and sometimes quarter-harmonic) components were also seen. The generation of these components was consistent with the behavior of a dynamical system entering chaos via a period-doubling route. A negative-stiffness Duffing oscillator model yielded period-doubling behavior similar to that of the experimental data. We compared the effectiveness of the CWT and the STFT for analyzing the responses to AM stimuli. The CWT (calculated using a high-Q Morlet-wavelet basis) and the STFT were both useful for identifying the various spectral components present in the AM velocity response of the cell. The high-Q Morlet wavelet CWT was particularly effective in distinguishing the lowest frequency components present in the response, since its frequency resolution is appreciably better than the STFT at low frequencies. Octave-band-based CWTs (using low-Q Morlet, Meyer, and Daubechies 4-tap wavelets) were largely ineffective in analyzing these signals, inasmuch as the frequency spacing between neighboring spectral components was far less than one octave.

Sound-induced displacement responses in the plane of the organ of Corti in the isolated guinea-pig cochlea

Sound-induced displacement responses in the plane of the organ of Corti were studied in the apical turn in the isolated temporal-bone preparation of the guinea-pig cochlea. Swept sinusoidal sound stimuli (100-500 Hz) were delivered closed-field to the external auditory meatus. The surface of the organ of Corti was continuously monitored using a CCD video camera. Displacement responses in the plane of the organ of Corti were determined by analyzing the change of the location of the cells (pixel-by-pixel) within the visual field of the microscope. Displacement responses followed the stimulus amplitude and were observable at Hensen's cells, three rows of outer hair cells and inner hair cells. The most prominent displacement responses were over the outer hair cells; the maximum amplitude was 0.6-1.7 microns at 100 dB SPL. Tuned displacement responses were found; the Q10 dB was 1.3 +/- 0.6. The best frequency was tonotopically organized, decreasing toward the apex with a space constant of 0.4-0.9 mm/oct. The motion was directed either strial-apically or strial-basally in a frequency dependent manner. With the aid of laser interferometric measurements of the transverse displacement, it was concluded that sound stimulation does not induce slow DC motion in the organ of Corti for the isolated temporal-bone preparation.

Nonlinear mechanics at the apex of the guinea-pig cochlea

A heterodyne laser interferometer was used to observe the sound-evoked displacement patterns of Reissner's membrane and various other structures in the apical turn of the guinea-pig cochlea. Most structures (including the basilar membrane) were similarly tuned, and had best frequencies in the 200-350Hz range. A distinct notch was usually observed approximately 0.7 octaves above the best frequency, and amplitude- and phase-plateaus were observed at higher frequencies. In most other respects, however, the mechanical tuning resembled the frequency-threshold curves of low frequency cochlear nerve fibers. In five reasonably intact, in vivo preparations, the frequency of the mechanical sensitivity notch was intensity-dependent: Compressive nonlinearities were observed above approximately 80 dB SPL on the low-frequency side of the notch, with antagonistically expansive nonlinearities on the high-frequency side. Two-tone suppression was observed in one of these preparations. Stimulus-related baseline position shifts were observed in another in vivo preparation. No such nonlinearities were observed in structurally damaged and/or > 1 hour post-mortem preparations. However, more robust nonlinearities were observed in all preparations at higher levels of stimulation (e.g. > 100-110 dB SPL). These high-level nonlinearities diminished only slowly after death, and gave rise to various effects, including time-dependent (i.e. adapting) and severely distorted (e.g. peak-split and/or dc-shifted) responses.

Tuned phasic and tonic motile responses of isolated outer hair cells to direct mechanical stimulation of the cell body

Guinea pig outer hair cells (OHCs) isolated from the two apical turns of the cochlea and firmly attached to a suction pipette, were subjected to the stimulus of the near-field (particle) displacements of a calibrated oscillating fluid jet aimed at the lateral cell walls. The longitudinal length changes of the OHCs in response to stimulation, in a direction orthogonal to that of the fluid jet, were recorded by a photodiode array. The response had two components; a phasic length change which followed the frequency of the particle displacement of the jet cycle by cycle, and a tonic length change which took several milliseconds to develop depending on the magnitude of the mechanical stimulus. When the hair cell changed length the lateral walls of the OHC moved in antiphase, moving apart during shortening and together during lengthening. With increased stimulus level the phasic response grew in proportion to the stimulus magnitude and began to saturate at the highest stimulus levels, while the tonic response grew in proportion to the square of the stimulus magnitude. Both the direction of the tonic length change and the phase of the phasic component could alter with the level of stimulation. Isolevel and isoresponse-frequency functions of both the tonic and phasic length changes revealed that both response types were tuned to similar resonant frequencies (RF) between 150 and 2500 Hz. The phase of the phasic length change began to lag at frequencies just below the RF, lagged by about 90 degrees at the RF and lagged by a further 90 degrees at frequencies above RF. The frequency response properties of the OHCs closely corresponded to those of a damped, forced, mechanical resonance. The tonic response disappeared and the phasic response was reduced at low-levels as a consequence of intense mechanical stimulation and with time.

Evidence for calcium-binding proteins and calcium-dependent regulatory proteins in sensory cells of the organ of Corti

Calcium is thought to play a major signaling role in outer hair cells to control metabolism, cytoskeletal integrity, cell shape and cell excitability. For this to happen, in resting cells the concentration of free calcium ions must be maintained at low levels so that focal increases can trigger specific events. In this paper, the localization of calcium, calcium-binding and calcium-dependent regulatory proteins in sensory cells from the guinea pig inner ear was demonstrated using immunocytochemical and histochemical techniques. We found the calcium buffer and/or calcium sensor proteins calmodulin, calbindin and calsequestrin predominantly in sensory cells and that when present, these proteins can be enriched in the outer hair cells. Calmodulin is found in the stereocilia, in the cuticular plate and in the cytoplasm and calbindin is found only in the cuticular plate and cytoplasm of both the inner and outer hair cells. The staining for these proteins in the outer hair cells is homogeneous, with no apparent compartmentalization along the lateral wall. Calsequestrin, thought to store and release calcium from membrane bound intracellular storage sites is found only in the cytoplasm of outer hair cells. There, it has a more punctuate staining pattern than does calmodulin or calbindin suggesting that it may be present in calciosomes rather than soluble in the cytoplasm. We did not detect caldesmon and S-100. Using the potassium pyroantimonate technique, we found precipitates containing calcium ions distributed throughout the cytoplasm of outer hair cells, with no evidence that the subsurface cisterns along the lateral wall act as calcium storage sites. Thus, calcium in resting cells is found in the cytoplasm along with calbindin and calmodulin and appears to have a punctate distribution consistent with a co-localization with calsequestrin. The implications of this distribution with respect to the slow shortening and elongation seen in outer hair cells are discussed.

Spontaneous cellular vibrations in the guinea-pig cochlea

Mechanical vibrations of Hensen cells were measured with a laser-heterodyne interferometer in the third turn of the guinea-pig temporal-bone preparation without the application of an external stimulus. Smoothed periodograms (spectral-density estimates vs frequency) were constructed from the velocity vs time waveforms recorded from individual cells. For some cells, several peaks appear in the periodograms at levels as high as 10 dB above the noise floor, indicating the presence of spontaneous vibrations. The frequencies at which the peaks are located differ in different preparations, indicating that the observed peaks are not caused by the presence of ambient noise or ambient vibrations. It is demonstrated that smoothed-periodogram analysis is superior to fast-Fourier-transform analysis for discerning these spontaneous spectral components. The frequency tuning curves of cells from which spontaneous vibrations were measured (determined by applying an external stimulus to the ear) have single principal peaks. When the spontaneous spectral features are present, their frequencies lie, for the most part, within the principal-peak region of the tuning curve. We propose that these spontaneous vibrations originate at the outer hair cells and are the source of spontaneous otoacoustic emissions in the ear.

Changes in the mechanical tuning characteristics of the hearing organ following acoustic overstimulation

An in vitro preparation of the guinea-pig temporal bone was used to study the effects of acoustic overstimulation on the mechanical tuning characteristics of the inner ear. Using laser heterodyne interferometry, the vibratory responses of selected sensory and supporting cells within the hearing organ were measured in response to acoustic signals applied to the ear to obtain mechanical tuning curves before and after applying acoustic overstimulation. Following overstimulation the frequency at which the maximal vibration response occurred moved towards lower frequencies, the vibration amplitude generally increased and the shape of the mechanical tuning curves became considerably flatter. These effects were seen within minutes of overstimulation. The micromechanical changes were accompanied by distinct morphological changes mainly affecting the first row of outer hair cells, which were swollen and shortened. Hensen bodies and swelling of the subsurface cisternae were observed in the affected cells. Apart from this, most of the shortened cells looked structurally intact, had undamaged sensory hair bundles and made synaptic contacts to both afferent and efferent nerve fibres. The results demonstrate that the outer hair cells play a key role in determining the tuning of the hearing organ.

Spontaneous cellular vibrations in the guinea-pig temporal-bone preparation

Mechanical vibrations of Hensen cells were measured with a laser-heterodyne interferometer in the guinea-pig temporal-bone preparation without the application of an external acoustic stimulus. Smoothed periodograms (spectral-density estimates v. frequency) were constructed from the velocity v. time waveforms recorded from individual cells. Several peaks were seen in the periodograms at levels as high as 10 dB above the noise floor, indicating the presence of spontaneous vibrations. The frequencies at which the peaks were located differed in different preparations, indicating that the observed peaks were not caused by the presence of ambient noise or ambient vibrations. Furthermore, vibrations were seen only in fresh preparations. The tuning curves of cells from which spontaneous vibrations were measured (determined by applying an external stimulus to the ear) had single principal peaks. Several peaks in the periodogram were found to be located within the principal-peak region of the tuning curve. The spontaneous response does not arise from noise filtered through the tuning curve which would have a single peak. We propose that these spontaneous vibrations originate at the outer hair cells and are the source of spontaneous otoacoustic emissions in the ear.

Mechanical demodulation of hydrodynamic stimuli performed by the lateral line organ

Tonic displacements of the fish lateral line cupula were observed during stimulation of the organ with amplitude-modulated water motion. The modulation frequency was fixed at 2.4 Hz and the carrier frequency was varied from 25 to 500 Hz. The time waveforms of the cupular displacement at carrier frequencies below 280 Hz and above 470 Hz were essentially amplitude-modulated waves. Between 350 Hz and 410 Hz the magnitude at the modulation frequency increased sharply and the predominant shape of the displacement waveform changed to that of the modulating frequency. The mechanism for extraction of the modulation component may play a key role in the decoding of sensory information.

Two-tone suppression by a saturating feedback model of the cochlear partition

A model of a small strip of cochlear partition was computer simulated. The model is composed of two elements, approximations to the transfer functions of an inner hair cell (IHC) and an outer hair cell (OHC), respectively. The IHC element was insensitive to DC stimulation. Input was one or two sinusoids. One sinusoid, at the characteristic frequency (CF), was multiplied by the gain of the 'cochlear amplifier'. A second sinusoid, representing a tone with much lower frequency, was not affected by the amplifier gain. This gain was determined by the OHC transfer function. In one form of the model ('fixed-gain'), this gain was set at a fixed number determined from the furthest point reached on the OHC transfer function. This form of the model produced very realistic single-tone responses as well as showing 'two-tone suppression': that is, the IHC DC response produced by CF stimulation was reduced when the lower-frequency sinusoid, at suitable intensities, was added to the stimulus. When a DC component was added to the two-tone stimulus, the magnitude of this two-tone suppression was enhanced. In the second form of the model ('variable-gain'), the cochlear-amplifier gain varied throughout the stimulus cycle. Its value was re-calculated at each instant, determined by the point on the OHC transfer function current at that particular instant. This form of the model showed two-tone suppression only when a DC component was added to the two-tone stimulus.

The tuned displacement response of the hearing organ is generated by the outer hair cells

The motile responses of the guinea-pig hearing organ in response to a tone applied to the ear were measured by laser interferometry. Two types of responses can be recorded: (i) a vibration at the frequency of the applied tone; and (ii) a displacement response consisting of a shift in the position of the organ surface. The purpose of this study is to characterize the displacement response. The results are as follows. There is a relationship between the frequency of highest sensitivity (best-frequency) of the displacement response and the site from which it is recorded. High best-frequencies are noticed at more basal locations, low best-frequencies towards the apex. The displacement response is more frequency-selective than the vibration response. The displacement response is observed within physiological sound pressure levels. Its sharpness is dependent on the stimulus intensity, it shows biological variability and can be manipulated by drugs that are known to modify the receptor potential of the sensory cells, or to interfere with outer hair cell motility. These results suggest that the displacement response is an important step in the transduction process in the mammalian hearing organ and that it is generated by the motile action of the outer hair cells.

Responses to sound of the basilar membrane of the mammalian cochlea

Recent evidence shows that the frequency-specific non-linear properties of auditory nerve and inner hair cell responses to sound, including their sharp frequency tuning, are fully established in the vibration of the basilar membrane. In turn, the sensitivity, frequency selectivity and non-linear properties of basilar membrane responses probably result from an influence of the outer hair cells.

Sound induced displacement response of the guinea pig hearing organ and its relation to the cochlear potentials

The sound induced motion of the cells within the fourth turn of the guinea pig organ of Corti was studied in an in vitro preparation (Ulfendahl et al. 1989). The cells were visualised by relief microscopy, achieved by an oblique illumination technique. The motion of the sensory cells was observed during the recording of the extracellular receptor potentials; the cochlear microphonics (CM) and the summating potential (SP). Our results show that the temporal bone preparation sustains an endocochlear potential and maintains the receptor potentials for 3-4 h. During the tone stimulus the outer hair cells were seen to elongate and the surface of the organ of Corti was displaced in the direction of scala vestibuli. The displacement response showed two frequency maxima, one at 150 and one at 300 Hz. The mechanical tuning of the sensory organ coincided with the tuning of the receptor potentials. Both the mechanical and the electrical responses at the 300 Hz peak were vulnerable to the administration of methylene blue suggesting cyclic GMP dependence, whereas the 150 Hz peak was unaffected. We conclude that the outer hair cells provide active tuning in the organ of Corti.

Functional and morphological comparisons between cochlear outer hair cells and muscle tissues in the guinea-pig

The effect of polylysine on the motility of outer hair cells and various muscle types was compared. Poly-L-lysine and its stereoisomer, poly-D-lysine, inhibited acoustically induced length changes of isolated outer hair cells from the guinea-pig hearing organ. The frequency specific displacements of the hearing organ in response to a tone stimulus are also inhibited to polylysine (Brundin et al. 1991). Poly-L-lysine, and its stereoisomer, irreversibly attenuated motile responses to transmural stimulation of guinea-pig ileum, vas deferens and taenia coli in a dose dependent manner, but were without significant effect on motile responses in skeletal and heart muscle. L-lysine, D-lysine, and the negatively charged polyaminoacid poly-L-aspartate, were without significant effect on outer hair cell and smooth muscle motility. The inhibitory effect of polylysine in smooth muscle is a direct effect on the muscle cell since polylysine attenuated acetylcholine- and adenosine triphosphate-induced contractions in the ileum, and ATP- or noradrenaline-induced contractions in the vas deferens. Pillar structures, believed to be of importance to excitation contraction coupling, were compared. In heart and skeletal muscle the pillars span the gap between sarcoplasmic reticulum and T-tubuli, deeply recessed into the muscle cell. In smooth muscle and outer hair cell the pillars are in closer relation to the cell exterior. The length of the pillars of the outer hair cells exceeds by two times that of smooth and skeletal muscle. The susceptibility of outer hair cells and smooth muscle tissue to the positively charged polylysine may indicate similarities in membrane or channel composition.