Hearing research
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Comparative Study
Quantitative anatomical basis for a model of micromechanical frequency tuning in the Tokay gecko, Gekko gecko.
The basilar papilla of the Tokay gecko was studied with standard light- and scanning electron microscopy methods. Several parameters thought to be of particular importance for the mechanical response properties of the system were quantitatively measured, separately for the three different hair-cell areas that are typical for this lizard family. ⋯ Both of those areas showed prominent gradients in hair-cell bundle morphology, i.e., in the height of the stereovillar bundles and the number of stereovilli per bundle, as well as in hair cell density and the size of their respective tectorial covering. Based on the direction of the observed anatomical gradients, a 'reverse' tonotopic organization is suggested, with the highest frequencies represented at the apical end.
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This investigation examined the consequences of neonatal deafness and chronic electrical stimulation of the cochlea in the developing auditory system. Cats were bilaterally deafened by daily ototoxic drug administration for two weeks after birth. Electrical stimulation was initiated at 6-9 weeks of age and continued for up to 6 months, using monopolar round window electrodes that synchronously excited auditory neurons throughout the cochlea. ⋯ The results presented suggest that it is problematic to effect consistent 'whole' nerve stimulation using monopolar round window electrodes. Moreover, this mode of stimulation can induce profound functional alterations in the central nervous system and is substantially less effective in forestalling the degeneration of auditory neurons than intracochlear stimulation. Both these results contraindicate the implantation of such electrodes in young children for the purpose of maintaining the integrity of the auditory system for later application of a multichannel cochlear implant.
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This paper uses the quantitative details of the anatomy of the auditory papilla in the Tokay gecko Gekko gecko (as described in the companion paper) to make a quantitative model predicting the tonotopic organization of two of the three papillar areas. Assuming that hair-cell bundle stiffness is similar to that of other species, a model of resonance frequencies for the apical areas of the papilla was constructed, taking into account factors such as the number of hair cells per resonant unit, their bundle dimensions, the volume of the tectorial mass, etc. The model predicts that the apical pre- and postaxial areas, although anatomically adjacent, respond to different frequency ranges, a phenomenon not yet reported from any vertebrate. ⋯ Physiol. 142, 203-218] (0.8 to 5 kHz) for the high-frequency range for this species. Only physiological experiments tracing responses to specific papillar nerve fibres can confirm or refute these interesting predictions of the model. The model also indicates that, compared to free-standing hair-cell bundles, the semi-isolated tectorial structures called sallets not only lower the range of characteristic frequencies but also increase the frequency selectivity of the attached hair cells.