doi:10.1016/S0166-2236(97)01192-2 
Copyright © 1998 Elsevier Science Ltd. All rights reserved
How well do we understand the cochlea?
Renato Nobilia, Fabio Mammanob and Jonathan Ashmorec
a Dipartimento di Fisica, Università di Padova, via Marzolo 8, Padova, Italy,
b Biophysics Laboratory and INFM Unit, International School for Advanced Studies, via Beirut 2–4, 34014 Trieste, Italy, and
c Dept of Physiology, University College London, Gower Street, London, UK WC1E 6BT.
Available online 29 May 1998.
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Abstract
As sensory cells, hair cells within the mammalian inner ear convert sounds into receptor potentials when their projecting stereocilia are deflected. The organ of Corti of the cochlea contains two types of hair cell, inner and outer hair cells, which differ in function. It has been appreciated for over two decades that although inner hair cells act as the primary receptor cell for the auditory system, the outer hair cells can also act as motor cells. Outer hair cells respond to variation in potential, and change length at rates unequalled by other motile cells. The forces generated by outer hair cells are capable of altering the delicate mechanics of the cochlear partition, increasing hearing sensitivity and frequency selectivity. The discovery of such hair-cell motility has modified the view of the cochlea as a simple frequency analyser into one where it is an active non-linear filter that allows only the prominent features of acoustic signals to be transmitted to the acoustic nerve by the inner hair cells. In this view, such frequency selectivity arises through the suppression of adjacent frequencies, a mechanical effect equivalent to lateral inhibition in neural structures. These processes are explained by the interplay between the hydrodynamic interactions among different parts of the cochlear partition and the effective non-linear behaviour of the cell motor.
Author Keywords: hearing; chochlea; hair cell; chochlear mechanics; sensory transduction
Fig. 1. Travelling waves on the basilar membrane of the cochlea. (A)
A longitudinal section of the uncoiled cochlea is represented with vertical dimension expanded by about three times. A travelling wave elicited by a 3 kHz tone is shown as a red line displacing the basilar membrane (unbroken black line) from its resting position (the wave amplitude has been magnified about 106 times for clarity). Arrows around wave peaks indicate the direction of local fluid flow. The fluid mass affects the dynamics of the basilar membrane, loading its different parts by amounts that depend upon the local wave length. Notice the progressive shortening of the wave length up to a critical point beyond which both the basilar membrane and the fluid remain at rest. (B) Cross-section of the cochlear duct, showing that the basilar membrane is laterally clamped across the duct and supports the organ of Corti that hosts two types of sensory hair cells: inner hair cells, that transmit signals to the acoustic nerve, and outer hair cells (OHC), that provide mechanical amplification to the basilar membrane motion. (C) Two travelling waves, produced by a low-level input, are shown for different amplification levels (broken lines). Solid lines are wave-amplitude envelopes. (D) Basilar membrane velocity (in dB relative to 1 μm sec–1) versus input sound pressure level (in dB SPL) at fixed input characteristic frequency for the same two amplification levels as in (C). When the OHCs function properly (red line), the basilar membrane motion at low-input levels (below 20 dB SPL) is linear but greatly enhanced (40–60 dB) compared with the passive case (blue line). At higher input levels transducer currents saturate, limiting the undamping action provided by OHCs, and producing a compressive non-linearity in the basilar membrane response between 30 and 90 dB SPL. Above 90 dB SPL OHC forces are negligible compared with the intrinsic viscous forces and the response approaches the linear, passive case.
Fig. 2. Organ of Corti mechanics. (A)
The basilar membrane (BM) supports a rigid structure formed by the pillar cells (PC) and the reticular lamina (RL). One inner hair cell (IHC) sits at the left of the pillars with its stereocilia (St) close to, but not inserted in, the overlying tectorial membrane. A triplet of outer hair cells (OHCs), firmly anchored at their apex within the reticular lamina and cupped by Deiters' cells (DC), have their tallest stereocilia inserted in the tectorial membrane (TM). Deiters' cells provide visco-elastic coupling between the motile OHCs and the elastic basilar membrane. (B) The organ of Corti distorts under hair-cell contraction: the lever effect associated with cell-length change forces the arch structure formed by PCs to pivot around the inner attachment of the basilar membrane. The outermost basilar membrane segment keeps almost at rest. (C) Functional representation of the organ of Corti with the OHCs represented as a displacement generator and the visco-elastic components added as shown. The tectorial membrane is coupled visco-elastically to the reticular lamina through the cell stereocilia and the interposed fluid layer. Viscosity plays an important role in organ of Corti dynamics: the viscosity of the organ of Corti itself acts as a mechanical high-pass filter that compensates for the electrical low-pass filtering of the OHC receptor potential. An analogous compensation for the IHC capacitance might be provided by the high-pass filtering properties of the coupling between the stereocilia and the tectorial membrane.
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