Abstract

Exceedingly sensitive structures for detecting airborne sound are found within the labyrinth of many vertebrates. The inner ear of the amphibian contains two sense organs that have long been regarded as auditory transducers because of their structures and locations, although there has been no supporting physiological evidence. One of these, the basilar papilla, is a phylogenetic precursor of the cochlea. The other, the amphibian papilla, is found only in amphibia. Single-unit recordings from the branches of the eighth nerve of the bullfrog (R. catesbeiana) demonstrate that both the basilar and the amphibian papillae are sensitive to auditory stimuli. Units arising from each organ exhibit sharply frequency-dependent sensitivity (tuning curves), but the frequency of maximum sensitivity (best frequency) for units from each papilla occurs in a different range. Basilar papilla units (termed “simple”) have best frequencies between 1000 and 1500 cps. It is noteworthy that these units cannot be inhibited by acoustic stimuli. Amphibian papilla units (termed “complex”) are more varied in type: among them are auditory units, with best frequencies below 700 cps; traits that respond both to low-frequency auditory and to vibratory stimuli; and units that respond only to vibration. Activity from all of these units can be totally inhibited by sufficiently intense tone bursts of frequency from about 300–1000 cps. It has been shown that this inhibitory interaction is not under efferent control.

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