Hair cell and organ of corti responses to normal and intense acoustic stimulation
Author: Fridberger, Anders
Date: 1997-04-25
Location: Föresläsningssalen, Fysiologiska instiutionen, Doktorsringen 6A
Time: 9.00
Department: Inst för fysiologi och farmakologi / Dept of Physiology and Pharmacology
Abstract
The principal aims of the studies described in this thesis were to develop an in vitro model for studying acoustic overstimulation at the cellular level, to define the electrical and mechanical response characteristics of the perfused temporal bone preparation, and to investigate the effects of intense sound stimulation on the calcium levels of the hair cells in the intact hearing organ. In the in vitro model for acoustic overstimulation, isolated cochlear outer hair cells were subjected to a pressure jet emanating from a glass micropipette aimed at the cell body. The pressure jet was generated by the vibrating shaft of a minishaker, hydraulically coupled to the micropipette. Such stimulation was found to cause increases of the cytoplasmic calcium concentration in most auditory sensory cells. The calcium changes were sustained, and no evidence of recovery of the elevated levels were seen after the termination of the stimulus.
A system was also developed to measure the pressures delivered to the cells. This system was based on a piezoresistive pressure transducer connected to a glass micropipette brought into the immediate vicinity of cells subjected to the pressure jet, allowing highly localized pressure changes to be measured. The peak pressure that could be generated by the stimulus system was found to be 144 dB SPL. Taking the middle ear transfer function into account, this level would correspond to approximately 120 dB SPL at the tympanic membrane during normal sound stimulation. Using laser heterodyne interferometry, the sound-induced vibrations of the low-frequency regions of the inner ear were investigated. The responses of the perfused isolated temporal bone preparation was found to be similar to that of living animals, both in terms of sharpness of tuning and the presence of nonlinearities. These characteristics were also reflected in the extracellularly recorded receptor potentials of the hair cells. The mechanical and electrical responses of the low-frequency regions of the cochlea were substantially different from the high-frequency regions, however.
Methods were developed to load the organ of Corti with fluorescent dyes and to measure the fluorescence after various experimental manipulations, using video-enhanced microscopy. The fluorescence images were further processed off-line, using a computerized algorithm, to remove out-of-focus information. When the isolated temporal bone preparation was subjected to acoustic overstimulation, large increases of the calcium concentration of the outer hair cells were seen. In addition, overstimulation caused contractions of the hearing organ that were reversible after the termination of the stimulus. Both the calcium changes and the contraction response could be expected to have severe effects on the function of the inner ear.
A system was also developed to measure the pressures delivered to the cells. This system was based on a piezoresistive pressure transducer connected to a glass micropipette brought into the immediate vicinity of cells subjected to the pressure jet, allowing highly localized pressure changes to be measured. The peak pressure that could be generated by the stimulus system was found to be 144 dB SPL. Taking the middle ear transfer function into account, this level would correspond to approximately 120 dB SPL at the tympanic membrane during normal sound stimulation. Using laser heterodyne interferometry, the sound-induced vibrations of the low-frequency regions of the inner ear were investigated. The responses of the perfused isolated temporal bone preparation was found to be similar to that of living animals, both in terms of sharpness of tuning and the presence of nonlinearities. These characteristics were also reflected in the extracellularly recorded receptor potentials of the hair cells. The mechanical and electrical responses of the low-frequency regions of the cochlea were substantially different from the high-frequency regions, however.
Methods were developed to load the organ of Corti with fluorescent dyes and to measure the fluorescence after various experimental manipulations, using video-enhanced microscopy. The fluorescence images were further processed off-line, using a computerized algorithm, to remove out-of-focus information. When the isolated temporal bone preparation was subjected to acoustic overstimulation, large increases of the calcium concentration of the outer hair cells were seen. In addition, overstimulation caused contractions of the hearing organ that were reversible after the termination of the stimulus. Both the calcium changes and the contraction response could be expected to have severe effects on the function of the inner ear.
Issue date: 1997-04-04
Publication year: 1997
ISBN: 91-628-2353-1
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