We found that hair bundles exhibit a spatial and temporal non-uniformity when stimulated with a fluid jet. Using high-speed imaging of the hair bundle during fluid-jet stimulation, we characterize the motion of the whole outer hair cell hair bundle. On the other hand, the fluid jet is argued to provide a more uniform hair bundle stimulation ( Corns et al., 2014) however, details of the hair bundle motion’s spatial uniformity have not been described. For mammalian hair bundles, stiff probes are susceptible to uneven stimulation of the hair bundle due to the decreased bundle coherence ( Nam et al., 2015). Two stimulations methods are most used in ex vivo experiments to interrogate the MET process: stiff probes and fluid jets. Thus, understanding the within bundle and between stereocilia variations in movement is critical to constructing the force sensed by the MET channels which cumulatively generates the receptor current.
#HAIR STIMULATOR FREE#
As hair bundles are stimulated in a variety of manners in vivo, from free standing to embedded in an overlying membrane and from sinusoidal modulation to static displacement, bundle coherence is important in shaping how these stimulations are translated to a force sensed by the MET channel. Reduced coherence makes hair bundles more susceptible to the mode of stimulation, as the bundles can conform to the temporal and spatial variations in the stimulus. Importantly, this asynchrony of stereocilia motion can alter the macroscopic manifestations of channel gating and adaptation ( Nam et al., 2015). Mammalian cochlear bundles lack coherence ( Langer et al., 2001 Nam et al., 2015 Scharr and Ricci, 2018). The stereocilia of low-frequency hair bundles are highly synchronous, where individual stereocilia move in unison in time, distance, and direction ( Kozlov et al., 2007 Karavitaki and Corey, 2010).
Mammalian cochlear hair bundles are unique in having fewer stereocilia rows, typically three, with the lower two rows possessing functional mechano-electric transduction (MET) channels ( Beurg et al., 2009). Hair bundle arrays in different species and organs vary in height, number of rows, stereocilia thickness, staircase step size, and coherence, defined here as how uniformly and reproducibly stereocilia will move relative to each other in time, direction and distance. Quantifying the mechanical properties of the hair bundle is essential to evaluating its role in cochlear amplification.
#HAIR STIMULATOR GENERATOR#
The hair bundle is hypothesized to be an active force generator and integral to cochlear amplification ( Howard and Hudspeth, 1987 Kennedy et al., 2005 Beurg et al., 2008 Hudspeth, 2008). The inner ear hair cell hair bundle is comprised of an array of graded length stereocilia organized in a staircase manner. The auditory system relies on a cochlear amplification process to achieve its high dynamic range and sharp frequency selectivity. These viscoelastic mechanisms are integral to describing the mechanics of the mammalian hair bundle. The creep is consistent with originating from a linear passive system that can be modeled using two viscoelastic processes. Additionally, in response to stimulation, the hair bundle exhibited a rapid motion followed by a slower motion in the same direction (creep) that is described by a double exponential process. Based on modeling, the splaying is predominantly due to fluid dynamics with a small contribution from hair bundle architecture. We find a spatially non-uniform stimulation that results in splaying, where the hair bundle expands apart.
We present data quantifying the displacement of the whole outer hair cell bundle using high-speed imaging when stimulated with a fluid jet. The elicited macroscopic current is shaped by the hair bundle motion so that the mode of stimulation greatly influences the cell’s output. Hair bundle deflection activates mechano-electric transduction (MET) ion channels located near the tops of the shorter rows of stereocilia. Hair cell mechanosensitivity resides in the sensory hair bundle, an apical protrusion of actin-filled stereocilia arranged in a staircase pattern. 6Department of Molecular and Cellular Physiology, School of Medicine, Stanford University, Stanford, CA, United States.5Department of Mechanical Engineering and Aeronautics and Astronautics, School of Engineering, Stanford University, Stanford, CA, United States.4Neuroscience Graduate Program, University of Colorado Anschutz Medical Campus, Aurora, CO, United States.3Neuroscience Graduate Program, School of Medicine, Stanford University, Stanford, CA, United States.2Department of Otolaryngology, Head and Neck Surgery, School of Medicine, Stanford University, Stanford, CA, United States.1Department of Physiology and Biophysics, University of Colorado Anschutz Medical Campus, Aurora, CO, United States.
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