Response patterns and peripheral origin of auditory nerve fibers in the monitor lizard,Varanus bengalensis

Abstract

1. The activity patterns of primary auditory nerve fibers inV. bengalensis were studied with microelectrode recordings from the proximal and distal portions of the nerve. 2. Spontaneous firing rates varied from 0.65 to 52.1 spikes/s, and decay rates were exponential (Fig. 1). 3. A low characteristic frequency (CF) group of units had smooth, sharp tuning curves, whereas a higher CF group had less sharp, more complex tuning curves (Figs. 2 and 3). These tuning curves had no particular asymmetry (Figs. 4 and 5). 4. Recordings from fibers near the basilar membrane allowed for accurate mapping of the peripheral origin of different CF's (Fig. 6). The lower CF group was localized in the dorsal portion of the papilla, with the lowest CF's at the ventral end. The constricted-off ventral portion of the papilla had abnormally high CF's, and gave rise to the high CF group. Only the dorsal portion of the papilla was tonotopically organized. It is suggested that the constriction in the papilla allowed independent evolution of the ventral part of the papilla. 5. Click polarity reversal data (Fig. 8) and pure tone response pattern data (Fig. 7) indicate the ventral third of the dorsal papilla has unidirectional hair cell orientation. It is suggested that bidirectional hair cell orientation is a peripheral specialization for sound onset emphasis.