The tonotopic organization of the bullfrog amphibian papilla, an auditory organ lacking a basilar membrane

Summary

Intracellular dye-injection studies have revealed tonotopic organization of the bullfrog (Rana catesbeiana) amphibian papilla, an auditory organ lacking a basilar membrane or its equivalent. The best excitatory frequency (BEF) for auditory stimuli was identified in each of twenty-nine VIIIth-nerve afferent axons that subsequently were traced to their peripheral terminations at the sensory surface. Among those axons, the five with BEFs greater than 550 Hz all terminated in the caudalmost region of the papilla, the ten with the BEFs greater than 300 Hz and less than or equal to 550 Hz all terminated in the central region of the papilla, and the fourteen with BEFs equal to or less than 300 Hz all terminated in the rostralmost region of the papilla (Fig. 4). The tectorium is very much larger and presumably more massive under the low-frequency region of the papilla than it is under the high-frequency region (Fig. 1). Higher-frequency axons tended to innervate few (one to four) receptor cells, and low-frequency axons tended to innervate many (six or more). Higher-frequency axons often terminated in large claw-like structures that engulfed the basal portions of individual hair cells and in this way were morphologically similar to type I terminals in the inner ears of higher vertebrates.