Event Archive
Jim Hudspeth - Making an effort to listen: mechanical amplification by ion channels and myosin molecules in hair cells of the inner ear
Wednesday, April 13, 2022 at 10:45am - 11:45am
Jim Hudspeth - Rockefeller University
Wednesday, April 13, 10:45AM (Zoom meeting starts at 10:30)
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“Making an effort to listen: mechanical amplification by ion channels and myosin molecules in hair cells of the inner ear”
As the gateway to human communication, the sense of hearing is of enormous importance in our lives. Hearing commences with the capture of sound energy by hair cells, the ear's sensory receptors, which convert that energy into electrical signals that the brain can then interpret. Each hair cell is a cylindrical epithelial cell surmounted by a hair bundle, an erect cluster of 20-300 rigid, actin-filled rods termed stereocilia. Mechanical force deflects the hair bundle and thereby excites the hair cell and its associated nerve fibers.
Uniquely among our sensory receptors, the hair cell is not a passive recipient of stimuli, but instead uses an active process to enhance its inputs. This active process amplifies mechanical stimuli by as much as a thousand fold, greatly increasing our sensitivity to weak sounds. Amplification is accompanied by frequency tuning, which yields a frequency resolution of less than 0.2 %, one thirtieth of the interval between piano keys. The active process produces a compressive nonlinearity that renders the ear sensitive to sounds over a million fold range of amplitude or a trillion fold range of power. Finally, the active process can be so exuberant as to become unstable; as a result, in a very quiet environment most normal ears spontaneously emit sound!
As a result of the cooperative gating of mechanically sensitive ion channels, a hair bundle is dynamically unstable: the relation between the bundle's displacement and the force required to accomplish it possesses two stable fixed points separated by a region of negative stiffness. This situation fosters amplification or oscillation when the hair bundle is pushed into its region of instability by molecular motors, specifically the myosin molecules associated with adaptation of the transduction apparatus to sustained stimuli. Experiments on individual hair bundles indicate that the bundle's operation near this instability—a Hopf bifurcation—accounts for the four characteristics of the active process.
