Distortion product otoacoustic emissions (DPOAE) were recorded from wild-type mice and mutant Tecta
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mice with detached tectorial membranes (TM) under combined ketamine/xylaxine ...anesthesia. In Tecta
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mice, DPOAEs could be detected above the noise floor only when the levels of the primary tones exceeded 65 dB SPL. DPOAE amplitude decreased with increasing frequency of the primaries in Tecta
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mice. This was attributed to hair cell excitation via viscous coupling to the surrounding fluid and not by interaction with the TM as in the wild-type mice. Local minima and corresponding phase transitions in the DPOAE growth functions occurred at higher DPOAE levels in wild-type than in Tecta
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mice. In less-sensitive Tecta
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mice, the position of the local minima varied nonsystematically with frequency or no minima were observed. A bell-like dependence of the DPOAE amplitude on the ratio of the primaries was recorded in both wild-type and Tecta
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mice. However, the pattern of this dependence was different in the wild-type and Tecta
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mice, an indication that the bell-like shape of the DPOAE was produced by a combination of different mechanisms. A nonlinear low-frequency resonance, revealed by nonmonotonicity of the phase behavior, was seen in the wild-type but not in Tecta
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mice.
School of Life Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, United Kingdom
Submitted 15 July 2003;
accepted in final form 24 September 2003
Distortion product otoacoustic emissions ...(DPOAE) were recorded from wild-type mice and mutant Tecta ENT/ ENT mice with detached tectorial membranes (TM) under combined ketamine/xylaxine anesthesia. In Tecta ENT/ ENT mice, DPOAEs could be detected above the noise floor only when the levels of the primary tones exceeded 65 dB SPL. DPOAE amplitude decreased with increasing frequency of the primaries in Tecta ENT/ ENT mice. This was attributed to hair cell excitation via viscous coupling to the surrounding fluid and not by interaction with the TM as in the wild-type mice. Local minima and corresponding phase transitions in the DPOAE growth functions occurred at higher DPOAE levels in wild-type than in Tecta ENT/ ENT mice. In less-sensitive Tecta ENT/ ENT mice, the position of the local minima varied nonsystematically with frequency or no minima were observed. A bell-like dependence of the DPOAE amplitude on the ratio of the primaries was recorded in both wild-type and Tecta ENT/ ENT mice. However, the pattern of this dependence was different in the wild-type and Tecta ENT/ ENT mice, an indication that the bell-like shape of the DPOAE was produced by a combination of different mechanisms. A nonlinear low-frequency resonance, revealed by nonmonotonicity of the phase behavior, was seen in the wild-type but not in Tecta ENT/ ENT mice.
Address for reprint requests and other correspondence: A. N. Lukashkin, School of Biological Sciences, University of Sussex, Falmer, Brighton, BN1 9QG, UK (E-mail: a.lukashkin{at}sussex.ac.uk ).
This paper is based on our model Dolgobrodov et al., 2000. Hear. Res., submitted for publication in which we examine the significance of the polyanionic surface layers of stereocilia for ...electrostatic interaction between them. We analyse how electrostatic forces modify the mechanical properties of the sensory hair bundle. Different charge distribution profiles within the glycocalyx are considered. When modelling a typical experiment on bundle stiffness measurements, applying an external force to the tallest row of stereocilia shows that the asymptotic stiffness of the hair bundle for negative displacements is always larger than the asymptotic stiffness for positive displacements. This increase in stiffness is monotonic for even charge distribution and shows local minima when the negative charge is concentrated in a thinner layer within the cell coat. The minima can also originate from the co-operative effect of electrostatic repulsion and inter-ciliary links with non-linear mechanical properties. Existing experimental observations are compared with the predictions of the model. We conclude that the forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena, which have been recorded from the auditory periphery.
This paper provides theoretical estimates for the forces of electrostatic interaction between adjacent stereocilia in auditory and vestibular hair cells. Estimates are given for parameters within the ...measured physiological range using constraints appropriate for the known geometry of the hair bundle. Stereocilia are assumed to possess an extended, negatively charged surface coat, the glycocalyx. Different charge distribution profiles within the glycocalyx are analysed. It is shown that charged glycocalices on the apical surface of the hair cells can support spatial separation between adjacent stereocilia in the hair bundles through electrostatic repulsion between stereocilia. The charge density profile within the glycocalyx is a crucial parameter. In fact, attraction instead of repulsion between adjacent stereocilia will be observed if the charge of the glycocalyx is concentrated near the membrane of the stereocilia, thereby making this type of charge distribution unlikely. The forces of electrostatic interaction between stereocilia may influence the mechanical properties of the hair bundle and, being strongly non-linear, contribute to the non-linear phenomena that have been recorded from the periphery of the auditory and vestibular systems.
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.
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