Cochlear outer hair cells are a class of mechano-sensory cells in the ear that convert sound-induced mechanical vibration into electrical signal. These cells also act as a fast motor capable of responding at auditory frequencies. The sensitivity and the sharp frequency discrimination of the mammalian ear is achieved by outer hair cells' activity in pumping energy into mechanical resonance of the basilar membrane. We have previously established that the hair cell motor uses electrical energy based on piezoelectricity. This is achieved by the coupling of electric charge transfer across the membrane with membrane area changes. We determined the membrane area changes of prestin, a membrane protein that is an essential component of the motor. Prestin does have area changes coupled with charge transfer. Thus it is a piezoelectric membrane protein. However, prestin's area changes determined were less than half of those of outer hair cell motor. This result suggests that prestin's action in hair cells is likely amplified by interactions with its associated proteins. Force produced by outer hair cells depends on voltage oscillation (receptor potential) in the cells elicited by transducer current in hair bundles. Studies on electric properties of these cells indicate that the receptor potential is highly attenuated because the most part of the transducer current is used for charging up and down the membrane that acts as a capacitor. As the result, receptor potential has been regarded as too small to affect vibration in the cochlea. We found that the receptor potential at the cells' characteristic frequency should not be heavily attenuated up to 10 kHz because of piezoelectric resonance. The frequency limit of 10 kHz arises from the condition that force produced by outer hair cells needs to cancel out viscous drag. This suggests that outer hair cells in apical turns which sense vibrations lower than 10 kHz do not require any ion currents to help those cells' function. This prediction is consistent with the existing data on ion currents. In addition, outer hair cells in the basal turn, which operate at 10 kHz and higher, need an additional mechanism to boost their effectiveness. We recently found that basal outer hair cells have a fast potassium current, which could enhance receptor potential by reducing capacitive current at the cell membrane. More experimental data are needed to determine whether or not this current is indeed sufficient in extending the effective frequency range of outer hair cells.
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