The response characteristics of vibration-sensitive saccular fibers in the grassfrog, Rana temporaria

Abstract

The response characteristics of saccular nerve fibers in European grassfrogs (Rana temporaria) subjected to dorso-ventral, 10-200 Hz sinusoidal vibrations were studied. Only 4 fibers out of a total of 129 did not respond to the vibrations. 70 fibers had an irregular spontaneous activity of 2-48 spikes/s. These fibers were very vibration-sensitive. The synchronization thresholds at 10-20 Hz varied from below 0.005 to 0.02 cm/s2. In contrast to earlier results, all these fibers had low-pass characteristics (with respect to acceleration) and responded maximally at 10 and 20 Hz. 55 fibers had spontaneous activities from 0-2 spikes/s. These fibers were less sensitive than the fibers with higher spontaneous activity. The spike-rate thresholds varied from about 0.04 to above 1.28 cm/s2, giving a considerable range fractionation. Most of these fibers also had low-pass characteristics with respect to acceleration, but 8 fibers showed band-pass characteristics with maximal synchronizations and spike-rates occurring at 40-80 Hz. At high acceleration levels, most spikes fell within 5-10 degrees of the stimulus cycle. The phase-locking of the saccular fibers is therefore very acute at low frequencies. The phase angles preferred by the fibers at 10 Hz were bimodally distributed with the two peaks about 180 degrees apart. This finding probably reflects the morphological observation that the saccular macula contains two oppositely oriented hair-cell populations. The results also indicate that the actual motion of the otolith relative to the macula is complex. No behavioral role of a vibration receptor has been demonstrated in the grassfrog.(ABSTRACT TRUNCATED AT 250 WORDS)