Dec
10
2024
Mechanical amplification in a critical regime by hair cells of the inner ear
A. James Hudspeth
The Rockefeller University
hosted by Ulrich Schwarz
4:00 PM SR41
As the gateway to human communication, the sense of hearing is of enormous importance in our lives. Every sensory organ of every inner ear of every chordate possesses the same type of receptor: the hair cell. Derived from the placodal ectoderm of the embryo, each hair cell retains an epithelial character: it lies in a continuous sheet of cells and is separated from neighboring hair cells by intervening supporting cells. The characteristic feature of a hair cell is its hair bundle, a compact cluster of 20-300 upright rods, the stereocilia, that extends from the apical cellular surface. As enlarged microvilli, the stereocilia consist of cores of cross-linked actin filaments that are surrounded by tubes of cellular membrane. Every hair bundle is tapered at its top, so that the stereocilia on one edge are shortest, whereas those in adjacent rows grow progressively longer toward the opposite edge. When a hair bundle is moved in the direction of its tallest stereocilia, whether by sound in an auditory organ or by acceleration in a vestibular organ, mechanically sensitive ion channels open, K+ and Ca2+ enter the cell, and the ensuing depolarization releases synaptic transmitter at the cell's base and excites afferent nerve fibers of the eighth cranial nerve.
Uniquely among mammalian sensory receptors, the hair cell of the inner ear is not a passive recipient of stimuli. Hearing instead uses an active process to amplify acoustical stimuli by as much as a thousandfold, to tune each hair cell to a narrow frequency band, and to render the ear sensitive over a trillionfold range in power. As a result of the cooperative gating of ion channels, the mechanically sensitive organelle of the hair cell— its hair bundle—is dynamically unstable: the relationship between applied force and the bundle's displacement possesses two stable fixed points separated by a region of negative stiffness. Operation near this instability—a Hopf bifurcation—explains all four characteristics of the active process. After introducing hair cells and defining their mechanical and electrical responsiveness, this presentation will describe the physical and mathematical basis of criticality, review the signatures and physiological control parameters of critical oscillation in amphibian hair cells, and offer evidence that the phenomenon extends to the mammalian cochlea as well.