Interactions between Passive and Active Vibrations in the Organ of Corti In Vitro

Abstract

High sensitivity and selectivity of hearing require an active cochlea. The cochlear sensory epithelium, the organ of Corti, vibrates because of external and internal excitations. The external stimulation is acoustic pressures mediated by the scala fluids, whereas the internal excitation is generated by a type of sensory receptor cells (the outer hair cells) in response to the acoustic vibrations. The outer hair cells are cellular actuators that are responsible for cochlear amplification. The organ of Corti is highly structured for transmitting vibrations originating from acoustic pressure and active outer hair cell force to the inner hair cells that synapse on afferent nerves. Understanding how the organ of Corti vibrates because of acoustic pressure and outer hair cell force is critical for explaining cochlear function. In this study, cochleae were freshly isolated from young gerbils. The organ of Corti in the excised cochlea was subjected to mechanical and electrical stimulation that are analogous to acoustic and cellular stimulation in the natural cochlea. Organ of Corti vibrations, including those of individual outer hair cells, were measured using optical coherence tomography. Respective vibration patterns due to mechanical and electrical stimulation were characterized. Interactions between the two vibration patterns were investigated by applying the two forms of stimulation simultaneously. Our results show that the interactions could be either constructive or destructive, which implies that the outer hair cells can either amplify or reduce vibrations in the organ of Corti. We discuss a potential consequence of the two interaction modes for cochlear frequency tuning.

Figure 1

Preparation of excised cochlear turn. (A) Excised gerbil cochlea. The apical and the basal turns are removed to expose the target location (red section). (B) Excised cochlear turn placed in microfluidic chamber. (C) Schematic of prepared tissue in the microfluidic chamber. Cochlear section was exaggerated for illustration. The excised cochlear turn is sealed with glue to separate two fluid spaces. The tissue is stimulated mechanically through a stimulating port (red arrow) and/or electrically through a pair of electrodes. Resulting vibrations are measured using optical coherence tomography. (D) An enlarged view of the organ of Corti (OoC) in the chamber from (C).

Figure 2

Status of tectorial membrane attachment. Vibrations of the tectorial membrane (TL), reticular lamina (RL), and Deiters’ cell apex (DC) due to mechanical stimulations were measured. The status of tectorial membrane attachment was assessed from the size of the sub-TM gap. (A) Tissue shape at the beginning of a preparation 65 min from animal death. (B) The same preparation as (A) 3 h later. The sub-TM gap was widened toward its lateral edge. (C and D) The relative motion of TM w.r.t. RL was measured at different stimulating frequencies. Thick curves and shaded spans represent mean and one standard deviation, respectively. (E) Vibration phase of TM w.r.t. RL versus sub-TM gap size. (F) Vibration phase of DC w.r.t RL versus sub-TM gap size. In (E) and (F), the phase value was obtained from the frequency range between 0.3 and 0.5 kHz. To see this figure in color, go online.

Figure 3

Frequency response to mechanical stimulation. The stimulating port was vibrated at constant amplitude (30 nm) at different frequencies between 0.3 and 3 kHz. Three sets of measurements from different preparations were distinguished by different symbols. (A and B) Vibration amplitude and phase measured at the reticular lamina. (C and D) Pressure amplitude and phase measured by the pressure transducer beneath the slit. (E and F) Displacement amplitude and phase with respect to pressure.

Figure 4

OoC vibrations due to mechanical and electrical stimulation. (A) B-scan image of the OoC. Red vertical dashed line represents the optical axis where vibrations were measured. The horizontal distance between the curve from the vertical line (green curve) indicates signal strength along the optical depth. Two void circles indicate the measurement points presented in (B)–(E). (B) Vibrations of the reticular lamina (blue) and the basilar membrane (red) due to mechanical stimulation. (C) Vibrations due to electrical stimulation (100 μA at 1 kHz). (D and E) Relative motion (amplitude and phase) of the reticular lamina with respect to the basilar membrane at different stimulating frequencies. The measurement location was 9 mm from the basal end. The cross symbols (×) indicate data points with poor signal at which signal/noise ratio <8 dB. To see this figure in color, go online.

Figure 5

Two vibration patterns in the OoC. The amplitude and phase measurements of 50 and 40 A-scan lines are shown on top of corresponding B-scan image. The top and bottom row panels show amplitude and phase of vibrations, respectively. (A) Amplitude and phase of vibrations due to mechanical stimulation. The stimulating port was vibrated at 1 kHz with a 30-nm amplitude. (B) Amplitude and phase of vibrations due to electrical stimulation. Stimulating current was applied at 2 kHz with a 100-μA amplitude. The top panels show the red curves, which indicate vibrating shapes along the broken lines. Color bar units are nanometers. The bottom panels show the asterisk symbols, which indicate the extremities of three outer hair cells. The color rings at the left bottom corner indicate the color scale of phase angle, e.g., between the dark blue and yellow spots, there is a phase difference of 180°. Scale bars, 25 μm.

Figure 6

Electromotility of individual outer hair cells. Electromotility of outer hair cells was measured over time and frequency. Time zero is defined as the onset of experimental dissection. (A) Motility of individual cells from eight cochleae are presented. The lines indicate the trend of exponential decay for the three rows of outer hair cells distinguished by different symbols and colors (see legends). (B) Mean motility of the three rows of outer hair cells over the first hour of measurement (<2.25 h). The error bar indicates standard deviation (n = 19, 16, 7 for OHC1, 2, and 3, respectively). (C) Motility from different cochleae are distinguished by different symbols. (D) Sodium salicylate decreased outer hair cell electromotility. Shaded vertical columns indicate the span of sodium salicylate application. The different symbols indicate independent trials. (E and F) Amplitude and phase of motility w.r.t electrical stimulation at different stimulating frequencies. Data are from three different cochleae. To see this figure in color, go online.

Figure 7

OoC vibration patterns due to simultaneous stimulations. Mechanical and electrical stimulations (1 kHz sinusoids) were applied simultaneously, but with different phases between the two stimulations (φ_ME) so that electrical stimulation lags the mechanical stimulation by (A) 0°, (B) 180°, and (C) 90°. The upper and the lower row panels represent the vibrations in amplitude and phase, respectively. The scale bars shown only in the right column as the same scales were use in the other columns.

Figure 8

Constructive and destructive interactions between two vibrations. Measurements were made at two points of the OoC: the reticular lamina (RL) and the basilar membrane (BM). (A) Frequency response to mechanical stimulations. (B) Frequency response to electrical stimulations. (C) The top of the plot displays simultaneous stimulations. Electrical stimulation lags mechanical stimulation by φ_ME. The bottom of the plot shows, depending on φ_ME, responses due to the two stimulations add up (constructive) or cancel each other (destructive). (D) Normalized vibration amplitude at the RL (C) and BM (σquare) for different φ_ME-values. Results at three frequencies are shown. The constructive φ_ME is defined at the peak of fitted sinusoidal curve (arrows). (E) Constructive φ_ME versus stimulating frequency for the reticular lamina and the basilar membrane. The filled and void symbols represent two sets of data obtained from different locations of a preparation.

Figure 9

Synthesized frequency response. (A) Response of the reticular laminar to simultaneous stimulation. (B) Response of the basilar membrane to simultaneous stimulation. Mechanical stimulation level was adjusted to have a flat vibration amplitude of 10 nm at the reticular lamina. To see this figure in color, go online.

Figure 10

Consequence of destructive interaction. (A) Passive vibrations. The OoC vibrates in phase. The relative motion between the tectorial membrane and the reticular lamina results in hair bundle deflection, which activates mechnotransduction. (B) Active vibrations. Because of the change in transmembrane potential, the outer hair cells elongate or contract. When the outer hair cells are motile, fine structures of the OoC vibrate out of phase. (C) When active and passive vibrations interact constructively regardless of frequency, the responses are amplified, but tuning quality remains similar. (D) Tuning can be enhanced if destructive interaction occurs away from the CF. To see this figure in color, go online.