Intracellularly labeled fusiform cells in dorsal cochlear nucleus of the gerbil. II. Comparison of physiology and anatomy.

Fusiform cells represent the major class of dorsal cochlear nucleus (DCN) projection neuron. Although much is understood about their physiology and anatomy, there remain unexplored issues with important functional implications. These include interspecies differences in DCN physiology and the nature of the cell-to-cell variations in fusiform cell physiology. To address these issues, a quantitative examination was made of the physiology and anatomy of 17 fusiform cells from a companion study. The results suggest that the basal dendrites of gerbil fusiform cells may be electrotonically more compact than those of the cat. This relative decrease in the filtering of excitatory inputs might account for the lower incidence of type IV units in that species. These data also suggest that the gerbil DCN lacks the high-frequency specialization described in the cat, because the tonotopic arrangement of the gerbil fusiform cells quantitatively matches the place-frequency map for the gerbil cochlea. Certain physiological properties have anatomical correlates. First, the basal dendrites of low spontaneous rate cells are directed away from the soma only in the caudal direction, while the high spontaneous rate cells have basal dendrites extending rostrally and caudally. Second, input resistance was dominated by the surface area of the apical dendrite. Third, the discharge pattern was correlated with apical dendrite orientation. Finally, there was a spatial gradient of sensitivity to broadband noise organized at least partially within an isofrequency axis. Such trends indicate that neighboring fusiform cells are endowed with different signal processing capabilities.