Surface morphology of the bullfrog amphibian papilla.

The bullfrog amphibian papilla, which has been identified as the peripheral source of two of the three types of auditory axon in that animal, is shown to have a continual distribution of receptor types based on surface topography, but a dichotomy of types based on the adjacent tectorium and its apparent mechanical linkage to the receptors and their supporting cells and a second dichotomy based on receptor polarization patterns. Each dichotomy divides the receptors into two approximately equal populations and thus might correspond to the previously observed division of primary axons into two frequency-response types.