Relative stereociliary motion opposes amplification at distortion frequencies

The hair cell, or sensory receptor of the inner ear, achieves high sensitivity by amplifying its mechanical inputs. The mechanism of amplification depends on the concerted opening and closing of mechanically sensitive ion channels in the hair bundle, a cluster of actin-containing rods that protrude from the cell's top surface. When a hair cell is stimulated simultaneously at two frequencies, channel gating also produces distortion products or responses at other frequencies. Using a sensitive dual-beam interferometer to measure the motions of stereocilia, we found that hydrodynamic forces act within the hair bundle to suppress these spurious signals. The hair bundle has evidently evolved an effective means of amplifying input signals while reducing the effect of distortions.

A schematic diagram of the double-beam differential interferometer shows two lasers that provide independent beams of coherent light. The abbreviations signify: IO, optical isolator; BE, beam expander; SF, spatial filter with a single-mode optical fiber; P, polarizer; Λ/4, quarterwave plate; L1, steering lens; DM, dichroic mirror; L2, fixed lens; TI, tungsten illuminator; DIC, differential-interference contrast system including objective lens and Wollaston prism; CCD, charge-coupled-device camera; BF, bandpass filter; BS, beam splitter; PD, photodiode with current-to-voltage converter and amplifier; A/D, analog-to-digital converter. At the right, the average values of the spectral-power density (PSD), coherence, and phase for six stimulated cells are shown in red and the standard deviations in blue. Stereociliary movements at the frequencies of stimulation, ƒ1 = 90 Hz and ƒ2 = 115 Hz, display a coherence across the hair bundle close to one and a phase close to zero; at those frequencies the bundle moves as a unit. In contrast, large phase lags and coherence drops are apparent at the distortion frequencies 2·ƒ1 = 180 Hz, ƒ2 + ƒ1 = 205 Hz, and 2·ƒ2 = 230 Hz.