Direct observations of sound-induced motions of the reticular lamina, tectorial membrane, hair bundles, and individual stereocilia

*C.Q. Davis, D.M. Freeman (Massachusetts Inst. of Technology, Cambridge)

Sound-induced motions of the reticular lamina, tectorial membrane (TM), hair bundles of hair cells, and individual stereocilia have been measured in an in vitro preparation of the alligator lizard cochlea. Two-dimensional video images from a light microscope are acquired at evenly-spaced planes of focus throughout the receptor organ to generate a three-dimensional image. Three-dimensional images are acquired at evenly-spaced phases of the hydrodynamic stimulus using stroboscopic illumination. Results are saved on disk and the motion of any structure in the measured volume is estimated using an algorithm based on optical flow (Horn and Schunck, Artificial Intelligence, 17:185-203, 1981). Precision in the motion estimates of a structure depend on its contrast, magnification of the microscope, light level, and number of observations averaged. Under typical conditions, displacements as small as 14 nm can be estimated without averaging. This presentation will include slow-motion video clips of cochlear structures moving at audio frequencies to illustrate the method and to demonstrate the following preliminary results.

1. The frequency dependence of the TM's gross motion differs from that of the reticular lamina.
2. Different points on the TM move differently; i.e., the TM does not move as a rigid body.
3. The distance between adjacent stereocilia changes as the hair bundle moves but is not simply proportional to hair bundle displacement. The transformation from bundle displacement to inter-ciliary distance depends on stereocilia position within the bundle as well as on stimulus parameters. This result suggests that if tip-links between neighboring stereocilia mediate mechanoelectric transduction, then tip-link stretch is not simply proportional to hair bundle displacement: another mechanical signal-processing stage exists in the conversion of sounds to the neural code.

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