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Abstract
In mammals, audition is triggered by travelling waves that are evoked by acoustic stimuli in the cochlear partition, a structure containing sensory hair cells and a basilar membrane. When the cochlea is stimulated by a pure tone of low frequency, a static offset occurs in the vibration in the apical turn. In the high-frequency region at the cochlear base, multi-tone stimuli induce a quadratic distortion product in the vibrations that suggests the presence of an offset. However, vibrations below 100 Hz, including a static offset, have not been directly measured there. We therefore constructed an interferometer for detecting motion at low frequencies including 0 Hz. We applied the interferometer to record vibrations from the cochlear base of guinea pigs in response to pure tones. When the animals were exposed to sound at an intensity of 70 dB or higher, we recorded a static offset of the sinusoidally vibrating cochlear partition by more than 1 nm towards the scala vestibuli. The offset’s magnitude grew monotonically as the stimuli intensified. When stimulus frequency was varied, the response peaked around the best frequency, the frequency that maximised the vibration amplitude at threshold sound pressure. These characteristics are consistent with those found in the low-frequency region and are therefore likely common across the cochlea. The offset diminished markedly when the somatic motility of mechanosensitive outer hair cells, the force-generating machinery that amplifies the sinusoidal vibrations, was pharmacologically blocked. Therefore, the partition offset appears to be linked to the electromotile contraction of outer hair cells.
Highlights
In the mammalian hearing process, mechanical stimuli originating from a sound propagate through the middle ear and cause oscillatory pressure changes in the cochlea
Before examination of the partition’s motion in each cochlea, we focused the microscope on the round window membrane and confirmed that the voltage of the interference signal detected at the same frequency as that of acoustic stimulation did not exceed the limit of detection (LOD) [29]
We constructed an interferometer by combining two previously described methodologies: the sinusoidal phase modulating (SPM) technique, which is intended for the measurement of static offsets [46], and homodyne interferometry, which quantifies sinusoidal vibrations [25] (Supplementary Fig. 1a–e)
Summary
In the mammalian hearing process, mechanical stimuli originating from a sound propagate through the middle ear and cause oscillatory pressure changes in the cochlea. The vibration elicits a wave that travels apically along the cochlear partition, a structure comprising the basilar membrane and overlying sensory hair cells and supporting cells [15, 50] (Fig. 1a). Aside from triggering the release of a neurotransmitter by inner hair cells, the electrical excitation elicited by displacement of the hair bundles changes the length of cell bodies of outer hair cells. This phenomenon is called somatic motility or electromotility; it depends upon the motor protein prestin [14, 52] and can mechanically amplify cochlear partition oscillations [31, 33]
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