NSF (IBN-415901)
The mammalian inner ear is exceptional in that it can process sound with high sensitivity and fine frequency resolution over a wide frequency range. The underlying mechanism for this remarkable ability is the cochlear amplifier, which operates by modifying cochlear micromechanics. Although the exact mechanisms underlying mechanical modifications are unclear, it has been shown that the tectorial membrane plays an important role. In particular, it has been proposed that the tectorial membrane provides a second resonant system in the cochlea, in addition to that of the basilar membrane. The functional significance of a resonant tectorial membrane, however, is contradicted by the close correspondence between neural tuning and basilar membrane frequency selectivity at best frequency, which implies that no intermediary filtering (or additional resonances) between basilar-membrane motion and mechanical input to the inner hair cells is required. Nonetheless, other indirect evidence (e.g., notches in high-level mechanical responses, level-dependent phase reversals) suggests that a second resonant system residing in the tectorial membrane may play an important role in normal cochlear function. Experiments are proposed to explore whether the tectorial membrane is, in fact, a resonant system and to examine its role in the cochlear-amplifier feedback loop. These experimental results are crucial longterm for understanding how outer-hair-cell motility and the highly nonlinear cochlear amplifier are controlled, and ultimately, how sound is processed and encoded by the inner ear. Specifically, the experiments address two hypotheses (1) that there is a gradient in stiffness and inertia along the tectorial membrane, thus providing the basis for a resonant system, and (2) that the mechanical impedance of the tectorial membrane is matched to the impedance of the stereocilia bundles of the outer hair cells.
Although some data are available for examining these hypotheses, they are not homogenous or comparable. Current data are from animals of varying species or age, or from isolated tectorial-membrane tissue. To address that problem in an innovative way, we have established an in vitro preparation, the hemicochlea, which allows us to study the tectorial membrane in situ, in the gerbil. In addition, we are able to confirm our in vitro measurement with in vivo experiments in the same species. Thus, the specific aims for this study are (1) to determine the driving point stiffness of the tectorial at several locations along the cochlea in the hemicochlea, (2) to measure the bulk stiffness of the tectorial membrane in vitro, (3) to confirm the hemicochlea measurements by in vivo experiments at two locations in the basal and the middle turn, and (4) to determine the bending stiffness of the outer hair cell stereocilia bundles. It is expected that tectorial membrane and outer hair cell stereocilia stiffness are matched. Furthermore, the impedance and stiffness data will allow us to better describe the role of the tectorial membrane in the cochlear-amplifier feedback loop.
This research has broad impact because medical students, otolaryngology residents, and high school teachers (research experience for teachers, RET) will be able to participate in several ways. By making stiffness measurements of the tectorial membrane in the hemicochlea and in living animals, and by interpreting the mechanical impedance measurements of the tectorial membrane, students will have a better understanding of mechanical engineering. By performing inner-ear surgery, students will have a better understanding of cochlear anatomy and physiology.
In the field of micromechanics the new knowledge may prove useful in developing micromechanical transducers of sound and motion.
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