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. 1999 Apr 13;96(8):4420–4425. doi: 10.1073/pnas.96.8.4420

Figure 1.

Figure 1

Experimental configurations for measuring the electromechanical properties of an isolated OHC partially inserted into a micropipette. (A) electrically induced displacement of the cuticular plate of the OHC, the so-called electromotility, XOHCSC. (B) The electromechanical force, FOHC, produced by the OHC under loaded conditions. (C) The mechanical impedance, ZOHC, of the OHC. Circuit diagrams are the electrical equivalents of OHC mechanics, with forces given as voltages and velocities as currents (12). In other words, FOHC and ZOHC are the Thévenin equivalent source force and impedance of the OHC, including extracellular fluid. For the electromotility experiment, the superscript SC is used to denote “short circuit” because, by definition, these measurements were made under unloaded conditions and we are using Thévenin equivalents. A command voltage, USP, applied to the pipette solution induces voltage drops, U1 and U2, across those sections of the cell membrane excluded from and contained in the pipette, respectively (8); U1 and U2 are of opposite phase. Voltage U1 driving the excluded length, qL, is U1 = (1 − q)⋅USP. The electromotility (A) was determined by focusing the beam of a laser Doppler vibrometer (LDV) onto the cuticular plate of the OHC and dividing the measured velocity, VOHCSC, by jω, where j = √−1 and ω is the radial stimulus frequency. FOHC (B) was determined by placing a high-impedance mechanical load, the reverse side of a lever used in atomic force microscopy (AFL), against the cuticular plate. The AFL was a silicon crystal of length 450 μm and thickness 2 μm. The velocity of the AFL, VAFL, in response to electrical induced length changes of the excluded section of the cell was measured with the LDV focused on the AFL. If the mechanical impedance of the AFL, ZAFL, is sufficiently large compared with ZOHC, so that effectively open-circuit conditions prevails, then FOHC can be approximated as ZAFL⋅VAFL. The ZOHC (C) was determined by mechanically vibrating the micropipette with a piezoelectric actuator with velocity VCAP and measuring the resulting velocity of the AFL; the impedance was calculated as ZOHC = ZAFL⋅VAFL/(VCAP − VAFL). Moreover, ZOHC can be estimated indirectly from the values of XOHCSC and FOHC determined in the electrical experiments under unloaded (A) and loaded (B) conditions, respectively: ZOHC = FOHC/jωXOHCSC.