Neural response directionality correlates of hair cell orientation in a teleost fish

The otolithic end organs in the ears of teleost fishes play important roles in hearing. Although previous studies have shown that afferent fibers inne rvating otolithic organs are directionally sensitive to acoustic stimulatio n, no study has demonstrated that directionality of the otolithic afferent neurons derives directly from morphological polarity of the hair cells that they innervate. In this study we investigated whether or not there exists such a structure and function relationship in one of the otolithic organs, the saccule, by using intracellular and extracellular tracing, histochemist ry, and confocal imaging techniques. We observed a variety of morphologies of dendritic terminals of saccular ganglion neurons. Arbor innervation area s of these saccular neurons ranged from 893 mum(2) to 21,393 mum(2), and th e number of dendritic endings fell into a range between 10 and 54. We found that the response directionality of saccular ganglion neurons correlates s ignificantly with the morphological polarization of the hair cells in the r egions that they innervate. Therefore, we provide direct evidence to suppor t the hypothesis that fish are able to encode directional information about a sound source, particularly in elevation, using arrays of hair cells in t he otolithic organs that are oriented specifically along the sound propagat ion axis.