Dendritic morphology of central auditory neurons correlates with their tonotopic position.

We have investigated the morphology of dendritic arbors in a central auditory nucleus, the lateral superior olive, of the Mongolian gerbil. Morphometric observations were obtained directly from Golgi-impregnated material by using a microcomputer-based three-dimensional data acquisition system. In particular, measurements were made to determine the dendritic arborization across each of three axes: the tonotopic axis, the rostrocaudal axis, and the isofrequency axis (i.e., perpendicular to the tonotopic axis). The tonotopic position of each cell was computed on the basis of a topographic map that has been constructed for the gerbil LSO (Sanes et al.: J. Comp. Neurol. 279:436-444, 1989). It was found that the span of a dendritic arbor along the tonotopic axis was directly correlated with the neuron's tonotopic position: Low frequency neurons had much broader arborizations than high frequency neurons. Moreover, the distribution of frequency bandwidths to which single LSO neurons responded showed a striking similarity to dendritic arborizations across the tonotopic axis. Lower frequency neurons responded to a larger number of octaves than higher frequency neurons. There was no correlation between tonotopic position and dendritic arborization in the isofrequency or rostrocaudal axis. Nor was there any correlation between frequency and total dendritic length, number of primary dendrites, or soma area. However, there was a small but significant difference between the primary dendrite diameter of low and high frequency neurons. Low frequency neurons had significantly greater diameters. These results suggest that the frequency selectivity of central auditory neurons may employ, as one morphological substrate, the distance over which their dendrites arborize along the tonotopic axis.