Date of Award
Doctor of Philosophy (PhD)
In the hearing and balance organs of tetrapod vertebrates, mechanical signals are transduced by an elegant organelle called the hair bundle. Deflections of this structure apply forces to mechanically gated ion channels. Hair bundles are not passive receivers of stimuli, but are instead active participants in the process of sensory transduction. They expend chemical energy to exert mechanical work, and can harness this active process to amplify their mechanical response to stimuli. Furthermore, the active process is tuned, allowing a given hair bundle to preferentially amplify a particular frequency; this feature is valuable in the analysis of complex sounds. Hair bundles can also enter an unstable regime in which their active process drives spontaneous oscillations. Studying this epiphenomenon can reveal mechanisms underlying the amplifying abilities of hair bundles. Despite the importance of amplification in hearing, little is known regarding the evolution of the active process; it is unclear if the active process is exclusive to tetrapods. It would be instructive, for instance, to know whether the active process predates the array of auditory specializations seen throughout vertebrates. Here, we approach this problem by investigating the mechanical activity of the hair bundles from the inner ears of two jawless vertebrates, the sea lamprey Petromyzon marinus and the American brook lamprey Lampetra appendix. We observe spontaneous oscillations in both of these animals. In the latter species, we also show evidence that their oscillations stem from mechanisms similar to those driving the spontaneous oscillations of tetrapod vertebrates. Furthermore, we found that hair bundles exhibiting these movements can entrain to and mechanically amplify particular stimulus frequencies. Taken together, our findings from a group distantly related to the tetrapods suggest that the active process of hair bundles is trait ancestral to all vertebrate ears.
Leitch, Katherine J., "Hair Bundles of a Jawless Vertebrate Employ Tetrapod-Like Tuned Mechanical Amplification" (2015). Student Theses and Dissertations. 271.