Morphological basis for tonotopy in the anuran amphibian papilla

The amphibian papilla of the more-recently derived frogs and toads is similar to the mammalian cochlea in at least three ways: 1. Its sensory surface is a slender, curved structure. 2. It exhibits tonotopy, with the highest-frequency sensitivity located at the end apparently closest to the source of acoustical excitation. 3. Its single-axon tuning curves exhibit extremely steep high-frequency slopes and gentle low-frequency slopes, consistent with selection by a distributed-parameter, low-pass filter with cutoff frequency that decreases as one moves farther from the source of acoustical excitation. The filter operation in the cochlea is centered around the basilar membrane, a structure whose profound taper is largely responsible for the decreasing cutoff frequency. Although the amphibian papilla lacks a basilar membrane, it does possess a conspicuously tapered tectorial membrane, which might serve a similar function. In this paper, after reviewing briefly an old and simple model of the cochlear filter and the morphology of the amphibian papilla and its tectorial membrane, we are unable to reconcile the structure of the latter with the topological requirements for realization of a filter analogous to the former. In fact, we are unable to deduce the principles of frequency selectivity in the frog auditory endorgan.