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

Summary 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° 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. A use in predator avoidance is likely, and it is possible that the sacculus is used for detection of water surface-waves. The vibration sense could therefore be of importance in the detection and localization of conspecifics in the breeding ponds.