Frequency analysis in the peripheral auditory system of anurans

Abstract

The two auditory organs—amphibian and basilar papillae—in an anuran's inner ear are anatomically distinct and both lack basilar membranes. Of fundamental interest is whether these organs nevertheless are capable of a systematic frequency analysis (i.e., tonotopic organization) and how the response properties of their auditory nerve fibers compare with those of other vertebrates. Anatomical and electrophysiological studies in our laboratories during the past several years have focused on these issues. Each auditory fiber in the anuran's eighth nerve is characterized by a “V”-shaped tuning curve which is fairly selective about its own best excitatory frequency (BEF). In primitive frogs, the fibers fall into two disjoint populations based on their BEF's (low-frequency sensitive and high-frequency sensitive) and susceptibility to two-tone suppression, with each population derived from a separate papillae. In more advanced species a third population arises, associated with the amphibian papilla and sensitive to the mid-frequency range. Electrophysiological studies with paired clicks and pure tones predicted that a place mechanism is operative in the amphibian papilla but not in the basilar papilla. The implication of a place principle in the amphibian papilla was further reinforced by the presence of an extension of this organ's sensor macula in advanced species. Recent Lucifer yellow dye injections in single auditory fibers confirm the tonotopy in the amphibian papilla which is not present in the basilar papilla. These combined anatomical and electrophysiological studies are currently aimed at elucidating further correlations between hair cell locus, morphology, and auditory nerve fiber response properties.