Dendritic arbors and central projections of physiologically characterized auditory fibers from the saccule of the toadfish,Opsanus tau

Abstract

Neurobiotin was injected iontophoretically into saccular afferents of toadfish (Opsanus tau) after intracellular recording to examine dendritic arbors and central projections with respect to the physiological and directional response properties of the cells. Dendritic arbors of 36 afferents were examined in detail. Maximum diameter of the arbor and the number of terminal points were positively correlated with each other, but neither was predictive of spontaneous activity or sensitivity. Best azimuths were centered around 30 degrees -40 degrees, which corresponds to the angle of the saccule with respect to the fish's midline. In general, best elevations for afferents corresponded to hair cell orientations in the region innervated; unexpectedly low elevations obtained from afferents innervating the middle saccule may reflect curvature of the sensory epithelium against the otolith. Three efferent cells were filled partially. The location and large size of the efferent projections indicate that activity along the saccule could be modulated by a single efferent. All afferents projected to the dorsal zone of the descending octaval nucleus (dDON); many afferents bifurcated to terminate in the anterior octaval nucleus, and a few of those also had terminal fields in the medial zone of DON. All afferent projections into the dDON consisted of multiple axon collaterals projecting to numerous sites along the rostral-caudal extent of the nucleus. Variation in terminal field sites also was noted in the medial to lateral axis of the dDON; however, there were no consistent correlations between terminal field locations, physiology, and best directions of the saccular afferents.