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

Mechanical Amplification Exhibited by Quiescent Saccular Hair Bundles

Spontaneous oscillations exhibited by free-standing hair bundles from the Bullfrog sacculus suggest the existence of an active process that might underlie the exquisite sensitivity of the sacculus to mechanical stimulation. However, this spontaneous activity is suppressed by coupling to an overlying membrane, which applies a large mechanical load on the bundle. How a quiescent hair bundle utilizes its active process is still unknown. We studied the dynamics of motion of individual hair bundles under different offsets in the bundle position, and observed the occurrence of spikes in hair-bundle motion, associated with the generation of active work. These mechanical spikes can be evoked by a sinusoidal stimulus, leading to an amplified movement of the bundle with respect to the passive response. Amplitude gain reached as high as 100-fold at small stimulus amplitudes. Amplification of motion decreased with increasing amplitude of stimulation, ceasing at ∼6-12 pN stimuli. Results from numerical simulations suggest that the adaptation process, mediated by myosin 1c, is not required for the production of mechanical spikes

Dynamics of Mechanically Coupled Hair-Cell Bundles of the Inner Ear.

The high sensitivity and effective frequency discrimination of sound detection performed by the auditory system rely on the dynamics of a system of hair cells. In the inner ear, these acoustic receptors are primarily attached to an overlying structure that provides mechanical coupling between the hair bundles. Although the dynamics of individual hair bundles has been extensively investigated, the influence of mechanical coupling on the motility of the system of bundles remains underdetermined. We developed a technique of mechanically coupling two active hair bundles, enabling us to probe the dynamics of the coupled system experimentally. We demonstrated that the coupling could enhance the coherence of hair bundles' spontaneous oscillation, as well as their phase-locked response to sinusoidal stimuli, at the calcium concentration in the surrounding fluid near the physiological level. The empirical data were consistent with numerical results from a model of two coupled nonisochronous oscillators, each displaying a supercritical Hopf bifurcation. The model revealed that a weak coupling can poise the system of unstable oscillators closer to the bifurcation by a shift in the critical point. In addition, the dynamics of strongly coupled oscillators far from criticality suggested that individual hair bundles may be regarded as nonisochronous oscillators. An optimal degree of nonisochronicity was required for the observed tuning behavior in the coherence of autonomous motion of the coupled system