In vitro cochlear preparation. (a) The excised middle cochlear turn, shown in black as transected through the modiolus (center) and outer bony wall, separates the two compartments of the experimental chamber. The apical and basal aspects of the organ of Corti (white) are immersed in artificial endolymph (AE) and artificial perilymph (AP), respectively. Pairs of recording electrodes (RE) and stimulating electrodes (SE) measure microphonic potentials and provide transepithelial electrical stimuli. Acoustic stimuli from an earphone (orange arrow) are delivered to the basilar membrane (red arrow) through the air-and-fluid-filled lower compartment. (b) A schematic drawing of the cochlear partition as mounted in the in vitro recording chamber shows the organ of Corti (white) suspended between the bone (black) of the modiolus (M) and outer cochlear wall. The hair bundles of inner (IHC) and outer (OHC) hair cells are stimulated by shearing motions between the basilar membrane (BM) and tectorial membrane (TM). Radial movement of hair bundles of the inner hair cells (horizontal red arrow) is measured with a photodiode; vertical movement (vertical red arrow) was detected with laser interferometry using a glass bead (orange) atop the tectorial membrane. (c) A micrograph taken through a dissecting microscope displays the upper surface of the preparation. Three rows of outer hair cells spiral around the cochlear modiolus. (d) A video micrograph shows the hair bundles of inner hair cells, one of which is marked (red arrow) to indicate the axis of movements detected with a dual photodiode. (e) A similar micrograph shows two of the three rows of outer hair cells and their V-shaped hair bundles. Scale bars: (c), 100 μm; (d) and (e), 10 μm.

Basilar-membrane resonance. (a) A computer-generated voltage stimulus in the form of a geometric frequency sweep (top) drove an earphone that produced a constant pressure at the basilar membrane (middle) but elicited a striking resonance in the displacement of a hair bundle of an inner hair cell (bottom). (b) The frequency spectrum of the applied pressure demonstrates the linearity of the stimulus over the range of frequencies of interest. (c) The spectrum of bundle-displacement magnitude reveals a resonance peak at 850 Hz that is well fit by a Lorentzian curve (gray) with a Q₀ of 4.7 and a peak sensitivity of 1.8 μm·Pa⁻¹. (d) The response amplitude of a passive hair bundle was linear over a wide range of stimulus intensities. The measured slope in this doubly logarithmic plot (black line) is 1.03 (r² = 1.00). (e) A plot of the inverse square of the resonant frequency (ω₀) against the fluid mass in the lower compartment can be fit linearly (solid line) to obtain the stiffness of the system, in this instance 120 N·m⁻¹ (r² = 0.98). Fixation of the same preparation with formaldehyde increased the system’s stiffness to 200 N·m⁻¹ (dashed line; r² = 0.99).

Microphonic potential. (a) Tectorial-membrane velocity (gray) and microphonic potential (black) were recorded simultaneously (top traces) in response to a 67-dB, 800-Hz acoustic stimulus (black, bottom trace) in the presence of a +80-mV transepithelial potential. In this and all subsequent traces, positive polarization of the lower compartment, which correlates with current flowing into hair cells, and vertically upward velocity towards scala media are represented by upward deflections. (b) Treatment with amiloride reversibly abolished the microphonic potential in response to sinusoidal stimulation at a resonant frequency of 700 Hz. (c) Replacement of the K⁺ in endolymph by channel-impermeant NMDG greatly reduced the microphonic response to stimulation at a resonant frequency of 400 Hz. (d) The peak-to-peak magnitude of the microphonic potential was dependent on the transepithelial potential, which mimicked the in vivo endocochlear potential. (e) The concentration-response relationship for amiloride’s block of the microphonic potential is fit with a Langmuir isotherm (black line), revealing a K_I value of 96 μM, consistent with the drug’s role as a blocker of mechanoelectrical-transduction channels.

Electrically evoked hair-bundle movement. The application of sinusoidal (left) or pulsatile (right) current stimuli (gray) across the sensory epithelium evoked hair-bundle movements. (a) Treatment with amiloride reversibly diminished both responses, leaving only small transients at transitions in the pulse stimulus. (b) In another preparation, substitution of NMDG-based endolymph affected neither the amplitude nor the shape of the responses to electrical stimuli.

Compressive nonlinearity and amplification. The amplitude of hair-bundle displacement at the resonance peak was measured over a range of stimulus levels. (a) When a +80-mV transepithelial potential was applied in the presence of K⁺-based endolymph (closed circles), the power-law slope of the bundle’s response (solid line) diverged from linearity (dashed gray line) at low stimulus levels. In contrast, when no transepithelial potential was applied (open circles) or when amiloride was added to the apical solution (crosses), the response became linear. (b) When the apical surface was bathed in NMDG-based endolymph, the compressive nonlinearity persisted: the response at low stimulus levels (solid line) diverged from linearity (dashed gray line). (c) The nonlinearity is demonstrated in the increasing sensitivity of the response in (b) with decreasing sound-pressure level. (d) The vertical displacement of the cochlear partition in the presence of a +80-mV transepithelial potential and NMDG-based endolymph (closed circles) also yielded a power-law slope (solid line) that diverged from linearity (dashed gray line) at low stimulus levels. Turning off the transepithelial potential immediately linearized the response in the same preparation (open circles).