High-Frequency Tuning Properties of Bullfrog Lagenar Vestibular Afferent Fibers

Abstract

A common property of vertebrate acoustic sensors, including otoconial acoustic sensors in lower vertebrates, is steep slopes on the high- and low-frequency band edges of the amplitude tuning curves. Bullfrog otoconial acoustic fibers are responsive to sound and exquisitely responsive to substrate vibrations in the frequency range from 20 Hz to 300 Hz. The sum of the absolute values of the two band-edge slopes of the amplitude tuning curve of such a fiber typically ranges from 100 dB/decade to 160 dB/decade (sometimes as high as 220 dB/decade), implying typical dynamic order of at least five to eight. We wondered if such steep slopes and the high dynamic order implied by them reflect special adaptations in acoustic sensors or if they are inherent in all lower-vertebrate otoconial sensors excited in this frequency range. To address this question, we examined the amplitude tuning characteristics of afferent nerve fibers from a bullfrog otoconial vestibular sensor in the same frequency range. In this paper, we report observations of tuning for bullfrog lagenar vestibular fibers in the frequency range from 10 Hz to approximately 500 Hz. To make these observations, we stimulated the frog with random dorsoventral motion that exhibited Gaussian amplitude distribution and that was flat in velocity from 10 Hz to 1.0 kHz. For each afferent fiber studied, we used discrete cross-correlation (between stimulus waveform and axon spike train) and discrete Fourier transformation to compute an amplitude tuning curve. In contrast with the amplitude tuning curves from saccular and lagenar acoustic fibers, those from the lagenar vestibular fibers typically had band-edge slopes whose absolute values summed to approximately 20 dB/decade, implying typical dynamic order of one. We conclude that steep band-edge slopes and high dynamic order are indeed special features of acoustic sensors, not shared by vestibular sensors.