Student Theses and Dissertations


Omar Ahmad

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

RU Laboratory

Hudspeth Lab


The organs of the inner ear rely upon a population of several thousand sensory hair cells to amplify and transduce acoustic, seismic, and kinesthetic signals. Each hair cell detects mechanical disturbances by means of its hair bundle, a motile organelle consisting of actin-filled, villous projections (called stereocilia) endowed with assemblies (called adaptation motors) of mechano-sensitive ion channels and myosin molecules that power both spontaneous and evoked movements. Active hair-bundle motility serves two functions: it mechanically amplifies sensory stimuli; and it regulates their transduction into electrical signals that drive the hair-cell synapse. To characterize these two functions, we consider here a model of the mechanical and electrical dynamics of the hair bundle of the bullfrog sacculus. Under simplifying assumptions, we reduce this model of a muscle fiber, and outline a procedure for estimating its parameters from experiment. We delineate the bifurcation structure of this simplified model, and analyse by perturbation methods its behavior in various dynamical regimes, notably in the relaxation-oscillation regime that displays prominently the hair bundle's active process; and in the near-Hopf-bifurcation regime at which auditory hair cells are thought to operate in vivo. We find close similarities between the dynamics of the active hair bundle and those of simplified models of a spiking neuron. In light of this analysis, we offer an account of the biophysical mechanisms underlying the spontaneous oscillations, frequency specificity, nonlinear gain, and self-tuning predicted for auditory hair bundles poised near a Hopf bifurcation.


A Thesis Presented to the Faculty of The Rockefeller University In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

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