The outer hair cell of the mammalian cochlea has a unique motility directly dependent on the membrane potential. Examination of the force generated by the cell is an important step in clarifying the detailed mechanism as well as the biological importance of this motility. We performed a series of experiments to measure force in which an elastic probe was attached to the cell near the cuticular plate and the cell was driven with voltage pulses delivered from a patch pipette under whole-cell voltage clamp. The axial stiffness was also determined with the same cell by stretching it with the patch pipette. The isometric force generated by the cell is around 0.1 nN/mV, somewhat smaller than 0.15 nN/mV, predicted by an area motor model based on mechanical isotropy, but larger than in earlier reports in which the membrane potential was not controlled. The axial stiffness obtained, however, was, on average, 510 nN per unit strain, about half of the value expected from the mechanical isotropy of the membrane. We extended the area motor theory incorporating mechanical orthotropy to accommodate the axial stiffness determined. The force expected from the orthotropic model was within experimental uncertainties.

The outer hair cell has a unique voltagedependent motility associated with charge transfer across the plasma membrane. To examine mechanical changes in the membrane that are coupled with such charge movements, we digested the undercoating of the membrane with trypsin. We inf lated the cell into a sphere and constrained the surface area by not allowing volume changes. We found that this constraint on the membrane area sharply reduced motor-associated charge movement across the membrane, demonstrating that charge transfer is directly coupled with membrane area change. This electromechanical coupling in the plasma membrane must be the key element for the motile mechanism of the outer hair cell.The outer hair cell changes the length of its cylindrical cell body in response to changes in the membrane potential (1-5), reaching a frequency of at least 20 kHz (6, 7). Because of its strategic location in the inner ear connecting the basilar membrane and the reticular lamina, this mechano-receptor cell presumably serves as an actuator in a feedback loop affecting vibrations of these tissues and thereby modulating the sensitivity of the mammalian ear (see refs. 8 and 9 for review). For this reason, the mechanism of this motility and its biological function have been a focus of intense research. For operating at high frequencies, a direct transduction mechanism would be advantageous. Indeed, the cell appears to be driven by a membrane-based motor that directly uses electrical energy available at the plasma membrane. The lateral membrane of the cell has charges that are transferable across the membrane (10-12), analogous to gating charges of voltage-gated ion transporters. These charges enable the cell to obtain electrical energy. If the electrical energy obtained by charge transfer across the membrane is directly converted into mechanical energy in a manner similar to piezoelectricity, the charge transfer must be reciprocally affected by an externally applied tension. That has indeed been experimentally verified (12-14), leading to estimates of the area change of a single motor unit (7,12,13).However, the simple characterization of the motor as described above is somewhat uncertain because of the structural complexity of the lateral membrane of the outer hair cell. The structural complexity obscures the molecular identity of the motor. In addition, the lateral membrane, where the motor resides, has an intricate structure of many elements (see ref. 15 for review) whose role in motility is uncertain. These factors leave uncertainties in the magnitude of tension that affects the motor.To characterize the elements of the motile mechanism in the plasma membrane, we examine a simplified system, in which the cortical cytoskeleton is digested with trypsin (13,16,17). The cells treated internally with trypsin can no longer maintain their ordinary cylindrical shape and become spherical when the internal pressure is elevated. The conspicuous length changes (up to 5%), which are elicited by changes in the membrane potential of ...

Due to the undesired impact of gravity, experimental studies of energy-dissipative gaseous systems are difficult to carry out on ground. In the past several years, we developed a series of experimental devices suitable for various kinds of microgravity platforms. The central idea adopted in our devices is to use long-range magnetic forces to excite all the particles within the system. Through the development of our devices, different component configurations, excitation protocols, and image-capturing methods have been tried and optimized to achieve best excitation and the maximum capability for data analysis.