Mechanics of the inner ear of the bullfrog (Rana catesbeiana): the contactmembranes and the periotic canal

The frog inner ear consists of a complex of fluid-filled membranous sacs an d canals containing eight distinct clusters of sensory hair cells. In this study we attempt to delineate the potential pathways for acoustic energy fl ow toward two of these clusters located within the amphibian papilla and th e basilar papilla. Detailed morphological measurements of the periotic cana l based on internal casts of the inner ear in the bullfrog (Rana catesbeian a) revealed that it is divided into a wide, tapered section and a narrower section comprised of two branches - one short and blind projecting into the endolymphatic space and another longer, terminating in the round window. A dditionally, we used laser Doppler velocimetry to record the velocity respo nses of the contact membranes of the amphibian papilla and basilar papilla. We found that the acoustic energy flow through these two structures is fre quency dependent such that the amphibian papilla contact membrane displays a peak velocity amplitude at frequencies less than 500 Hz, whereas the basi lar papilla contact membrane velocity response exhibits a maximum above 110 0 Hz. Our data advocate a mechanical substrate underlying the frequency seg regation in the auditory nerve fibers innervating the amphibian papilla and the basilar papilla.