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Mechanical entrainment of saccular hair cell bundles


Mechanical detection of auditory and vestibular system displays exquisite sensitivity, with sub-nanometer detection threshold. The system is also highly nonlinear, exhibiting sharply tuned frequency selectivity and compression of dynamic range. Detection of sounds and vibrations is mediated by the sensory hair cells, which transduce mechanical inputs into electrical signals via hair bundles' deflections. Experiments have consistently shown that hair bundles are not just passive detectors, as they spontaneously oscillate and respond to mechanical stimulus in an active manner. A number of theories based on nonlinear dynamics have described the active hair bundle as a nonlinear system poised near a Hopf bifurcation. Prior studies of mechanical response of hair bundles were done in spontaneously oscillating hair bundles, with mechanical stimulus fluctuating around bundles' resting positions. These conditions, however, might not be true under in vivo conditions. In fact, hair bundles from the bullfrog sacculus are coupled to an overlying membrane, which imposes a steady state offset to the bundle position, and suppresses bundles' spontaneous activity. In this dissertation, we study entrainment of hair bundles from the bullfrog sacculus by sinusoidal stimuli under different mechanical manipulations: offsets and couplings. First, multimode oscillations are more frequently observed upon application of a small negative offset onto spontaneous oscillating hair bundles. Using a numerical model based on detailed physiology of hair bundle, this complex temporal profile requires an additional element - a variable gating spring - with a stiffness that varies with calcium concentration. The dynamics of the process are slow compared to other timescales in the bundle, i.e. gating of transduction channels and slow adaptation process. Second, oscillating hair bundles subject to weak mechanical stimuli are extremely sensitive, with response in the phase histogram already observed at 0.4-pN stimulus. Time-dependent phase-locking behavior at slightly higher signal amplitudes exhibits phase slips, indicating that the system undergoes phase-locking via a SNIC bifurcation. Study of hair bundle dynamics under mechanical offsets reveals a spiking regime, which is even more sensitive to stimulus compared to the oscillatory regime. Larger mechanical offset yields suppression of spontaneous activity, during which spikes can be evoked by stimulus. Evoked spikes occur at a preferred phase of the stimulus cycle, and exhibit a constant amplitude, regardless of signal amplitude and frequency, and leading to an amplifying movement. Finally, we study how coupling between hair bundles affects their mechanical response. Synchronization of bundles' spontaneous movements is always observed, regardless of the original characteristic frequencies of hair bundles prior to coupling. While some coupled bundles show an enhancement, we find that, in general, coupling only two bundles does not significantly improve the sensitivity and frequency tuning.

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