Our group focuses on the fundamental physics that enables the auditory system to detect extremely small, rapid mechanical movements. We study how it senses sub‑nanometer deflections, how it preserves this remarkable sensitivity despite noise, and how signals from higher brain centers regulate that sensitivity to protect the fragile hair cells from damage.

The image on the cover of the March 11 issue of Biophysical Reports was taken with a high-speed CMOS camera used to track the motion of hair bundles in the sensory epithelia of the American bullfrog inner ear. The ensemble of rod-like structures can be seen emerging from the hair cell bodies. These hair bundles are the organelles that perform the first stage of mechanotransduction, which is the way organisms convert mechanical inputs into electrical signals that can be further conveyed and interpreted by the brain. The image was obtained by using differential interference contrast, in an upright bright field microscope using a 60´ objective. The orientation is not the typical top-bottom view used for experiments but rather shows a slanted view for enhanced visibility. At the center, a custom-fabricated borosilicate glass probe approaches a hair bundle to make contact and to introduce a controlled mechanical displacement. This allows us to emulate the effect of incoming vibrations, and thus to study the response of an individual hair cell.

In our work, we used label-free imaging methods to identify underlying somatic activity during mechanotransduction evoked in the hair bundle by the glass probe. We studied hair bundle motion while imaging deeper layers within the tissue, to observe active areas within the soma. Activity hotspots were mainly located at the outer limits of the cell’s membranes, consistent with regions of ionic channels and synaptic contacts. Strong mechanical stimulation of the bundle produced phase-locked somatic responses.

Elucidating the active nonlinear response of these exquisite biological detectors can help us to better comprehend the fundamental mechanisms involved in the vestibular and auditory systems responsible for our ability to balance and hear. In particular, label-free and minimally invasive techniques of studying hair cell dynamics can pave the way to better tackle hearing pathologies, such as sensorineural hearing loss, and vestibular disorders.    

If you are interested in learning more about our research, please visit https://bozoviclab.physics.ucla.edu.

— Martín A. Toderi, Dzmitry Vaido, and Dolores Bozovic