Hearing Sound with Auditory Hair Bundles

How is all that beautiful music converted from acoustic waves into spikes?  The key biological component are some very sensitive cilia, hair-like structures, that sit at the end of auditory nerve cells. They move in response to passing waves, and that unleases spikes the head toward the auditory brain.  It really is quite magical. 

Who: Wisam Reid (Stanford)
What: Relating the Cohesiveness of Auditory Hair Bundles in Mammals to their Function
When: Friday March 9th at 10:30AM
Where: CCRMA Seminar Room (Top Floor of the Knoll)
Why: These are really quite amazing structures.

Bring your cilia to CCRMA and Wisam will talk about how they do their important job.



Relating the Cohesiveness of Auditory Hair Bundles in Mammals to their Function
Wisam Reid, Anthony J Ricci, and Dáibhid Ó Maoiléidigh
Department of Otolaryngology – Head & Neck Surgery Stanford University School of Medicine


Hair bundles are comprised of a set of stereocilia connected by links that ensure their cohesion in vestibular systems and nonmammalian auditory organs. In contrast, bundles from the mammalian cochlea exhibit weak coherence in response to experimental stimuli. Although coherence between stereocilia determines how hair cells transduce stimuli, we do not fully understand the relationship between a bundle’s cohesiveness and its function.

To connect a hair bundle’s structure with its response to stimulation, we construct a mathematical model of hair-bundle mechanics describing inner and outer hair cells. The model consists of a set of stereocilia connected by tip links and top connectors and includes the viscosity of the surrounding fluid. We vary the geometry and material properties of the bundle and analyze its responses to sinusoidal and step stimuli. These simulations are compared to the observed displacements of real hair bundles in response to experimental stimuli applied using a fluid jet or a stiff probe.

There are several consequences arising from a lack of bundle cohesion. We find that the measured stiffness and drag of a bundle depends on how the stimulus is applied and that a bundle may not be accurately described by a single spring constant and drag coefficient. Moreover, in response to a stimulus, tip links between different stereocilia do not extend the same amount and their extensions are not in phase with one another.

We conclude that the responses of a weakly coupled hair bundle to stimuli are considerably more complex than those of a tightly coupled bundle. To more completely characterize the exact role of cohesion, intrinsic noise, mechanotransduction channels, and adaptation need to be taken into account. Nonetheless, our reduced description reveals that bundle cohesion strongly affects how a hair cell detects external signals.

Biography
Wisam Reid is currently a research assistant in Professor Anthony Ricci’s lab in Stanford’s School of Medicine. Wisam holds a bachelors in Electrical Engineering & Computer Science from UC Berkeley. Wisam is also a CCRMA alumnus, having obtained his masters degree in Music, Science & Technology from Stanford University. This Fall Wisam will continue his academic training at Harvard Medical School as a PhD candidate in Speech & Hearing Bioscience & Technology (SHBT). Wisam’s research involves biological physics-based modeling of auditory neural processing. His current research project, in collaboration with Daibhid O Maoileidigh, involves modeling the evoked electrical responses of outer and inner hair cells in the cochlea by the opening and closing of mechanically sensitive ion channels. It is Wisam’s goal to contribute to a deepened understanding of the receptor cells of the inner ear and how the direct mechanical connections between the hair bundle and ion channels, contribute to hearing and hearing loss.