When we measure the mechanical properties of individual hair bundles from the bullfrog's ear, we find that an oscillatory bundle displays negative slope stiffness at the heart of its region of mechanosensitivity. This bizarre property implies that a bundle is not stable, but instead hops back and forth between two extreme positions. Each time the bundle moves, an adaptation process shifts the region of negative stiffness along the displacement axis. Modeling indicates that the interplay between negative bundle stiffness and mechanical adaptation produces spontaneous bundle oscillations similar to those observed. Just as the negative resistance of an electrically excitable cell or of such an electrical component as a tunnel diode can amplify electrical signals, negative stiffness can be harnessed by the ear to amplify mechanical stimuli.
Genetic analysis provides an unbiased means of identifying novel molecules of functional importance.
Moreover, genetic study of a model organism may shed light on heritable human diseases, such as the hundred or more inherited forms of human deafness. Because hair cells are unique to vertebrates, however, they almost certainly contain constituents that cannot be identified by genetic screens of fruit flies, roundworms, or other invertebrates. During the past three years, our group has therefore used three morphological and behavioral screens to identify mutations affecting the ear and the related lateral-line organ in the most genetically tractable vertebrate organism, the zebrafish. We have used three procedures to screen young fish from more than 200 mutagenized families. On the second day of larval development, about 30 hours after fertilization of eggs, we examine larvae microscopically for abnormalities of aural development.