Active mechano-sensitivity by hair cells in the inner ear

Keywords: amplification, adaptation motor, auditory system, hair bundle, hair cell, Hopf bifurcation, mechanoelectrical transduction, negative stiffness, oscillations, vestibular system

In vertebrates, hearing and the sense of balance are initiated in the inner ear by specialized mechano-sensory cells, the hair cells. Hair cells mediate transduction of sound-pressure waves (hearing) or head accelerations (balance) into electrical signals that then propagate along nervous pathways to the brain. The inner ear's structures are immersed in a viscous fluid that should heavily damp sound-evoked mechanical vibrations. The ear, however, behaves as a highly-tuned resonator. Uniquely among sensory receptors, hair cells amplify their inputs by actively producing mechanical work that compensates viscous friction, thereby enhancing the sensitivity and sharpening the frequency selectivity of hearing. Our research at the interface between physics and biology aims at shedding light on the amplificatory process that shapes the sensation of sounds at the periphery of the auditory system.

Hair cells are each endowed with a mechano-sensory organelle, the hair bundle, that projects from the cell's apical surface into the surrounding fluid (Fig. 1).

Fig. 1 

In the laboratory, we use flexible microfibers to measure the mechanical properties of the hair bundle. Our experiments demonstrate that this mechano-sensory antenna behaves as a sort of micro-muscle that can actively set a stimulus fiber under tension and even oscillate spontaneously (video 1).

The hair cell can harness spontaneous hair-bundle oscillations to amplify its responsiveness to sinusoidal stimuli (Martin, 2008 ; Martin et Hudspeth, 1999). This hair-bundle amplifier offers double benefit for auditory detection: it enlarges the range of sound intensities that can be heard by amplifying only the weakest sounds and sharpens frequency selectivity by filtering the input to the hair cell. The ear is sensitive to sounds within a range of 20-20,000 Hz in humans. Our results led us to propose that sensitivity to different frequencies might result from the operation of an assembly of active oscillators with characteristic frequencies distributed within the auditory range.
Calcium controls the occurrence of spontaneous oscillations and the kinetics of active hair-bundle movements (Tinevez et al, 2007 ; voir Fig. 2A). In addition, active hair-bundle movements are associated with an unusual mechanical property (Fig. 2B). The stiffness of a hair bundle can indeed vary with bundle position and even be negative within a limited range of positions!

Fig. 2 

By combining experimental observations with simulations, we have built a theoretical description of the various manifestations of active hair-bundle motility in different species, including mammals (Martin et al, 2003 ; Nadrowski et al., 2004; Tinevez et al., 2007; Martin, 2008). In this model, active hair-bundle movements are powered by molecular motors of the myosin type.

To complement our research at the cellular level, we study in vitro the mechanical properties of the acto-myosin system, both at the level of a single acto-myosin crossbridge and of a few tens of motor molecules. Our experimental set-up combines optical tweezers, fluorescence microscopy and photorelease of molecules that can regulate the mechanical activity of myosin (video 2).

Present and future research will clarify the effects of factors that influence the properties of the hair-bundle amplifier. These includes the viscosity of the fluid in which the hair bundles are immersed and mechanical coupling between cells of similar characteristic frequencies. In addition, we will seek correlates at the single hair-cell level of well-known psycho-acoustical phenomena, such as auditory illusions or masking of a sound by another. In parallel, we study the aptitude of a motor assembly to produce spontaneous oscillations in vitro.

Last update: January 2009

Key publications

2008

• Martin P.
  Active hair-bundle motility of the hair cells of vestibular and auditory organs
  Active processes and otoacoustic emissions (Manley GA, Popper AN, Fay RR, eds), pp 93-144. New York: Springer

2007

• J.Y. Tinevez, F. Jülicher, P. Martin
  Unifying the various incarnations of active hair-bundle motility by the vertebrate hair cell
  Biophys Journal, 93:4053-4067 - Abstract
  The dazzling sensitivity and frequency selectivity of the vertebrate ear rely on mechanical amplification of the hair cells' responsiveness to small stimuli. As revealed by spontaneous oscillations and forms of mechanical excitability in response to force steps, the hair bundle that adorns each hair cell is both a mechanosensory antenna and a force generator that might participate in the amplificatory process. To study the various incarnations of active hair-bundle motility, we combined Ca2+ iontophoresis with mechanical stimulation of single hair bundles from the bullfrog's sacculus. We identified three classes of active hair-bundle movements: a hair bundle could be quiescent but display nonmonotonic twitches in response to either excitatory or inhibitory force steps, or oscillate spontaneously. Extracellular Ca2+ changes could affect the kinetics of motion and, when large enough, evoke transitions between the three classes of motility. We found that the Ca2+-dependent location of a bundle's operating point within its force-displacement relation controlled the type of movement observed. In response to an iontophoretic pulse of Ca2+ or of a Ca2+ chelator, a hair bundle displayed a movement whose polarity could be reversed by applying a static bias to the bundle's position at rest. Moreover, such polarity reversal was accompanied by a 10-fold change in the kinetics of the Ca2+-evoked hair-bundle movement. A unified theoretical description, in which mechanical activity stems solely from myosin-based adaptation, could account for the fast and slow manifestations of active hair-bundle motility observed in frog, as well as in auditory organs of the turtle and the rat.
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2004

• Nadrowski B, Martin P, Julicher F
  Active hair-bundle motility harnesses noise to operate near an optimum of mechanosensitivity
  Proc Natl Acad Sci USA, 101(33):12195-12200 - Abstract
  The ear relies on nonlinear amplification to enhance its sensitivity and frequency selectivity to oscillatory mechanical stimuli. It has been suggested that this active process results from the operation of dynamical systems that operate in the vicinity of an oscillatory instability, a Hopf bifurcation. In the bullfrog's sacculus, a hair cell can display spontaneous oscillations of its mechanosensory hair bundle. The behavior of an oscillatory hair bundle resembles that of a critical oscillator. We present here a theoretical description of the effects of intrinsic noise on active hair-bundle motility. An oscillatory instability can result from the interplay between a region of negative stiffness in the bundle's force-displacement relation and the Ca2+-regulated activity of molecular motors. We calculate a state diagram that describes the possible dynamical states of the hair bundle in the absence of fluctuations. Taking into account thermal fluctuations, the stochastic nature of transduction channels' gating, and of the forces generated by molecular motors, we discuss conditions that yield a response function and spontaneous noisy movements of the hair bundle in quantitative agreement with previously published experiments. We find that the magnitude of the fluctuations resulting from the active processes that mediate mechanical amplification remains just below that of thermal fluctuations. Fluctuations destroy the phase coherence of spontaneous oscillations and restrict the bundle's sensitivity as well as frequency selectivity to small oscillatory stimuli. We show, however, that a hair bundle studied experimentally operates near an optimum of mechanosensitivity in our state diagram.

2003

• Martin P, Bozovic D, Choe Y, Hudspeth AJ
  Spontaneous oscillation by hair bundles of the bullfrog's sacculus
  J Neurosci, 23(11):4533-4548 - Abstract
  One prominent manifestation of mechanical activity in hair cells is spontaneous otoacoustic emission, the unprovoked emanation of sound by an internal ear. Because active hair bundle motility probably constitutes the active process of nonmammalian hair cells, we investigated the ability of hair bundles in the bullfrog's sacculus to produce oscillations that might underlie spontaneous otoacoustic emissions. When maintained in the normal ionic milieu of the ear, many bundles oscillated spontaneously through distances as great as 80 nm at frequencies of 5–50 Hz. Whole-cell recording disclosed that the positive phase of movement was associated with the opening of transduction channels. Gentamicin, which blocks transduction channels, reversibly arrested oscillation; drugs that affect the cAMP phosphorylation pathway and might influence the activity of myosin altered the rate of oscillation. Increasing the Ca 2+ concentration rendered oscillations faster and smaller until they were suppressed; lowering the Ca 2+ concentration moderately with chelators had the opposite effect. When a bundle was offset with a stimulus fiber, oscillations were transiently suppressed but gradually resumed. Loading a bundle by partial displacement clamping, which simulated the presence of the accessory structures to which a bundle is ordinarily attached, increased the frequency and diminished the magnitude of oscillation. These observations accord with a model in which oscillations arise from the interplay of the hair bundle's negative stiffness with the activity of adaptation motors and with Ca 2+-dependent relaxation of gating springs.
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1999

• Martin P, Hudspeth AJ
  Active hair-bundle movements can amplify a hair cell's response to oscillatory mechanical stimuli
  Proc Natl Acad Sci USA, 96:14306-14311 - Abstract
  To enhance their mechanical sensitivity and frequency selectivity, hair cells amplify the mechanical stimuli to which they respond. Although cell-body contractions of outer hair cells are thought to mediate the active process in the mammalian cochlea, vertebrates without outer hair cells display highly sensitive, sharply tuned hearing and spontaneous otoacoustic emissions. In these animals the amplifier must reside elsewhere. We report physiological evidence that amplification can stem from active movement of the hair bundle, the hair cell's mechanosensitive organelle. We performed experiments on hair cells from the sacculus of the bullfrog. Using a two-compartment recording chamber that permits exposure of the hair cell's apical and basolateral surfaces to different solutions, we examined active hair-bundle motion in circumstances similar to those in vivo. When the apical surface was bathed in artificial endolymph, many hair bundles exhibited spontaneous oscillations of amplitudes as great as 50 nm and frequencies in the range 5 to 40 Hz. We stimulated hair bundles with a flexible glass probe and recorded their mechanical responses with a photometric system. When the stimulus frequency lay within a band enclosing a hair cell's frequency of spontaneous oscillation, mechanical stimuli as small as ±5 nm entrained the hair-bundle oscillations. For small stimuli, the bundle movement was larger than the stimulus. Because the energy dissipated by viscous drag exceeded the work provided by the stimulus probe, the hair bundles powered their motion and therefore amplified it.
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