Experimental Theoretical Studies of Cochlear Mechanisms

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Project Summary Based on its strategic location, the tectorial membrane (TM) has long been thought to play an essential role in hearing, but the important cochlear mechanisms remain unclear. We propose research to improve our understanding of the functional role of the TM in determining (1) the remarkable properties of normal hearing — including its exquisite sensitivity and frequency selectivity — as well as (2) the hearing loss associated with genetic mutations of the TM and other cochlear pathologies. Conventional models of cochlear mechanics represent the TM as a viscoelastic solid, and while the importance of both viscosity and elasticity is well established, these models do not account for the central role of water in this tissue. The TM is a gel: 97% water contained in a macromolecular matrix of proteins and sugar groups. Recent studies show that sound-induced motions of water through this macromolecular matrix plays a critical functional role in determining the timing of mechanical responses – and this timing is central to determining the frequency selectivity that is a hallmark of mammalian hearing. The proposed research will measure and characterize the important consequences of the gelatinous nature of the TM, i.e., its poroelastic properties. This work is organized in two related aims. The first investigates mechanical consequences of poroelasticity. We have developed a technique based on atomic force microscopy to measure mechanical properties of the TM at the level of single hair bundles. We will apply this technique to characterize poroelasticity in TMs from normal-hearing rodents, as well as mice with genetic disorders of hearing. Our second aim focuses on the role of ions (especially calcium) that are dissolved in the liquid phase of the TM. The TM has been shown to concentrate calcium at levels well in excess of those in the surrounding endolymph. Changes in ionic concentrations have been shown to alter the electro-mechanical properties of the TM, and will thereby also affect the closely apposed hair bundles of hair cells. The local concentration of calcium is known to affect not only the sound-induced receptor potentials of hair cells, but also adaptation processes that are necessary to maintain the remarkable sensitivity of hearing. Results from these studies will increase our understanding of the cochlear mechanisms that underlie both normal and impaired hearing. This knowledge has important practical applications for the delineation of inner-ear disorders (and concomitant suggestions for treatment) and for the design of speech-processing devices such as cochlear implants, hearing aids, and speech-recognition systems.