Tectorial membrane travelling waves underlie abnormal hearing in Tectb mutant mice

Abstract

Remarkable sensitivity and exquisite frequency selectivity are hallmarks of mammalian hearing, but their underlying mechanisms remain unclear. Cochlear insults and hearing disorders that decrease sensitivity also tend to broaden tuning, suggesting that these properties are linked. However, a recently developed mouse model of genetically altered hearing (Tectb(-/-)) shows decreased sensitivity and sharper frequency selectivity. In this paper, we show that the Tectb mutation reduces the spatial extent and propagation velocity of tectorial membrane (TM) travelling waves and that these changes in wave propagation are likely to account for all of the hearing abnormalities associated with the mutation. By reducing the spatial extent of TM waves, the Tectb mutation decreases the spread of excitation and thereby increases frequency selectivity. Furthermore, the change in TM wave velocity reduces the number of hair cells that effectively couple energy to the basilar membrane, which reduces sensitivity. These results highlight the importance of TM waves in hearing.

Figure 1. Longitudinally propagating waves of radial motion along the TM.

(a) TM segments were suspended between two supports separated by 390–480 μm in a wave chamber. Double-headed arrow indicates sinusoidal displacement of vibrating support at audio frequencies. Schematic waveform (green line overlaid on TM) is a displacement snapshot illustrating the wave-like nature of TM motion. (b) Schematic drawing of a decaying wave showing the peak amplitude (A), wave envelope (purple), wave decay constant σ and wavelength λ. (c, d) TM radial displacement versus longitudinal distance across basal and apical TM segments. Radial displacement was measured relative to a point on the TM, ∼30 μm from the edge of vibrating support. (c) Wave motion in response to 18-kHz stimulation for basal Tectb^−/− (red open circles) and wild-type (blue pluses) TMs. Best fitting wave parameter estimates were σ =110 μm and λ=330 μm for Tectb^−/− mice, and σ =200 μm and λ=415 μm for wild-type mice. (d) Wave motion in response to 3.5-kHz stimulation for apical Tectb^−/− (red open circles) and wild-type (blue pluses) TMs. Best fitting wave parameter estimates were σ =90 μm and λ=220 μm for Tectb^−/− mice, and σ =340 μm and λ=560 μm for wild-type mice.

Figure 2. Spatial extent of basal and apical TM waves.

(a, b) Wave decay constant versus stimulus frequency for basal and apical TMs. Spatial bandwidths of tuning at the 10- and 20-kHz best places calculated from neural tuning curves in Tectb^−/− (red square with asterisk) and wild-type (blue square with asterisk) mice are also included. (a) Basal Tectb^−/− TM segments (n=4 TM preparations; red open circles) had wave decay constant (σ) values between 65 and 180 μm for stimuli >10 kHz. In contrast, the σ -values of basal wild-type TM segments (n=4 TM preparations; blue closed circles) were all ≥185 μm over the same stimulus frequencies (P-value <3.85 × 10⁻⁸). Black arrow indicates that characteristic frequencies (CFs) of basal TM segments extend beyond 10 kHz. (b) Apical Tectb^−/− TM segments (n=3 TM preparations; red open circles) had σ -values between 45 and 220 μm for stimuli ≥2 kHz. The wave decay constants of apical wild-type TM segments (n=4 TM preparations; blue closed circles) were typically larger, ranging between 195 and 350 μm, and >350 μm for stimuli ≤7.5 kHz (P-value <1.83 × 10⁻⁶). Upward blue arrows indicate that σ -values were considerably larger than the length of TM in the wave chamber. Double-sided black arrows denote range of CFs corresponding to apical TM segments. The grey shaded region denotes the range of frequencies (>10.5 kHz) when TM motion was significantly attenuated (in the noise floor).

Figure 3. Phase vs stimulus frequency for basal and apical TMs.

(a) Basal Tectb^−/− TM segments (n=3 TM preparations; red open circles) accumulated more phase (>10 kHz) than did basal wild-type TMs (n=3 TM preparations; blue closed circles) at a location ∼250 μm from the vibrating support. (b) Phase accumulation was greater for apical Tectb^−/− TMs (n=4 TM preparations; red open circles) compared with apical wild-type TMs (n=3 TM preparations; blue closed circles) at frequencies above 1 kHz (at ∼250 μm location). Lines are linear regression fits to the data. The grey shaded region denotes the range of frequencies (>11 kHz) when TM motion was too small to measure phase differences.

Figure 4. Propagation velocity of basal and apical TM waves.

Vertical lines mark the interquartile range and circles mark the median wave velocity measured in Tectb^−/− (red open circles) and wild-type (blue closed circles) TMs at each stimulus frequency (horizontal axis). (a) In basal segments, the median velocities in Tectb^−/− TMs (n=3 preparations) were smaller than those of wild types (n=7 preparations). Even at high frequencies, when differences were small (10–40%), these differences were statistically significant (P<2×10⁻⁴). (b) In apical segments, the median velocities in Tectb^−/− TMs (n=3 preparations) were significantly smaller (P<0.043) than those of wild types (n=4 preparations), with differences at low frequencies (<5 kHz) of as much as a factor of 6. The grey shaded region denotes the range of frequencies (>11 kHz) when TM motion was too small to measure wave velocity.

Figure 5. Changes in TM wave properties account for abnormal hearing in Tectb^−/− mice.

(a) Light microscope images of typical Tectb^−/− (top) and wild-type (bottom) TM samples in the wave chamber. Superimposed waveforms illustrate associated TM wave displacements in response to 18 kHz stimulation (amplitudes exaggerated for clarity). Shaded regions illustrate the decay constant σ, which is the distance over which the wave amplitude decays by a factor of e (∼110 μm for Tectb^−/− in red; ∼200 μm for wild type in blue). Marginal (top black line) and limbal (bottom black line) boundaries for both TM samples are indicated. (b) Relation between TM decay constants (ordinate) and frequency tuning (abscissa). Distances between pairs of horizontal lines indicate the TM decay constants for Tectb^−/− (red) and wild-type (blue) TM samples from a. Distances between pairs of vertical lines indicate frequency bandwidths. The relations between spatial and frequency bandwidths are closely approximated by projection onto the sloping line, which indicates the map between best place and best frequency in mouse. (c) Frequency tuning curves (adapted from Russell et al.9) showing changes in frequency bandwidth in a typical Tectb^−/− (red line) and wild-type (blue line) mouse. Arrows indicate bandwidths 10 dB SPL above the most sensitive frequency (20 kHz).