Figure 1. Structure of the hair bundle and configuration of the experiments.

(A) A schematic illustration portrays a hair bundle, in this instance that from the bullfrog’s sacculus, at rest (left) and when deflected towards its tall edge (right). The bundle is formed by rows of stereocilia that increase in height along the axis of sensitivity and are interlinked by molecular filaments, the tip links, that stretch as the bundle moves forward. The tip links project the stimulus force onto mechanosensitive ion channels. (B) A tapered optical fiber with a spherical lens at its tip is brought within a few tens of micrometers of a hair bundle. The fiber's angle of approximately 20 from the horizontal allows it to pass beneath the microscope’s objective lens without impinging upon other nearby hair bundles. An image of the hair bundle is projected through the microscope and onto a dual photodiode, which permits measurement of bundle motion with a precision in the nanometer range. Note that the extent of hair-bundle movement in this and the subsequent figures is greatly exaggerated for didactic purposes: the largest displacements move the bundle’s top by less than the diameter of a single stereocilium.