Static length changes of cochlear outer hair cells can tune low-frequency hearing

The cochlea not only transduces sound-induced vibration into neural spikes, it also amplifies weak sound to boost its detection. Actuators of this active process are sensory outer hair cells in the organ of Corti, whereas the inner hair cells transduce the resulting motion into electric signals that propagate via the auditory nerve to the brain. However, how the outer hair cells modulate the stimulus to the inner hair cells remains unclear. Here, we combine theoretical modeling and experimental measurements near the cochlear apex to study the way in which length changes of the outer hair cells deform the organ of Corti. We develop a geometry-based kinematic model of the apical organ of Corti that reproduces salient, yet counter-intuitive features of the organ’s motion. Our analysis further uncovers a mechanism by which a static length change of the outer hair cells can sensitively tune the signal transmitted to the sensory inner hair cells. When the outer hair cells are in an elongated state, stimulation of inner hair cells is largely inhibited, whereas outer hair cell contraction leads to a substantial enhancement of sound-evoked motion near the hair bundles. This novel mechanism for regulating the sensitivity of the hearing organ applies to the low frequencies that are most important for the perception of speech and music. We suggest that the proposed mechanism might underlie frequency discrimination at low auditory frequencies, as well as our ability to selectively attend auditory signals in noisy surroundings

Finite element modelling of cochlear electrical coupling

The operation of each hair cell within the cochlea generates a change in electrical potential at the frequency of the vibrating basilar membrane beneath the hair cell. This electrical potential influences the operation of the cochlea at nearby locations and can also be detected as the cochlear microphonic signal. The effect of such potentials has been proposed as a mechanism for the non-local operation of the cochlear amplifier, and the interaction of such potentials has been thought to be the cause of the broadness of cochlea microphonic tuning curves. The spatial extent of influence of these potentials is an important parameter for determining the significance of their effects. Calculations of this extent have typically been based on calculating the longitudinal resistance of each of the scalae from the scala cross sectional area, and the conductivity of the lymph. In this paper, the range of influence of the electrical potential is examined using an electrical finite element model. It is found that the range of influence of the hair cell potential is much shorter than the conventional calculation, but is consistent with recent measurements

An in vitro model for acoustic overstimulation

Although many studies have been performed on the effects of acoustic overstimulation on the inner ear, our knowledge about the cellular processes underlying reduced hearing sensitivity and auditory cell death is still limited. In order to further our understanding of cellular processes occurring in conjunction with acoustic trauma, we designed an in vitro model to study the effects of overstimulation directly on sensory hair cells isolated From the low-frequency part of the guinea pig cochlea. The isolated outer hair cells were subjected to pressure jets delivered by a glass micropipette positioned close to the cell, in order to mimic the pressure changes occurring in the intact inner ear during sound stimulation. A second micropipette coupled to a piezoresistive pressure transducer was used as a probe measuring the pressure at precise locations at and around the cell. In a previous study, we found that such stimulation gave rise to increases in the intracellular calcium concentration. The present study characterizes the stimulus, describes the computer-controlled setup used for calibration, and gives examples of different modes of overstimulation at the cellular level. The peak pressure that could be generated using the pressure jet was around 325 Pa, or 144 dB (re 20 mu Pa) at 140 Hz. The pressure jet elicited large mechanical vibrations of the cell bodies of isolated cells. The vibration mode of the cells often changed over time, implying that the stimulation caused changes of the cellular stiffness. However, most cells appeared quite resistant to the high intensity mechanical stimulation