In this project, we will investigate the biomechanics that enable high-frequency (JHF) sound, greater than 10 kHz, to be transduced from middle-ear to cochlea in a group of terrestrial mammals. Anatomical studies indicate that in larger mammals, the malleus may-assume a twisting mode of vibration, rather than the classical hinging mode. Twisting would transfer sound energy more efficiently at HF because relative mass inertia is greatly decreased. At lower frequencies, the hinging mode is utilized due to the stiffness of the connecting ligaments and tendons. For smaller mammals, the impedance due to mass is not prohibitive at HF, so a twisting mode is not necessary. Accordingly, features of their anatomy seem to preclude twisting. ~ Here we propose an interspecies (human, cat, chincilla, and gerbil) investigation of the hypothetical twisting mode using physiologic measurement, morphologic observation and mathematical analysis. ~ SA1: Empirically determine impedance as a function of frequency for the hinging and twisting vibrational modes by inducing these motions through the malleus and measuring the cochlear output. Two magnetic voice coil motors will drive the anterior and posterior quadrants of the eardrum with independent phase. This causes hinging and twisting with equal and opposite phase respectively. A novel cochlea preparation that introduces a 'third window" will be created to measure the spiral ligament, which moves with stapes pressure, by laser Doppler. This gives a relative indication of cochlear output. SA2: Morphometrically quantify the primary anatomical structures of the eardrum and ossicles that could contribute to a twisting vibrational mode by multimodal imaging. Middle-ear morphometry will be Imaged by micro-CT. Collagen fiber structure at select areas in the eardrum will be imaged by multiphoton and transmission electron microscopy. Image analysis software will be used to obtain quantitative data from these Images. SA3: Analytically deduce the conditions required for a twisting vibrational mode by developing simple and finite element models of the eardrum and ossicles that can incorporate the methods and data acquired from SA1 and SA2. A simple model for the hinging and twisting modes will be formulated based on micro-CT data. This will guide the construction of a finite element model that can incorporate data from the other specific aims. Assessment of the parameters derived from the four species studied will allow us to formulate the fundamental physical and physiological basis for a twisting mode. ~ This research is relevant to current medicine because an improved understanding of eardrum and ossicle biomechanics is required to develop more effective technologies related to myringoplasty. This may be particularly important for post-surgical HF hearing preservation.
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