Relationships between the nigrotectal pathway and the cells of origin of the predorsal bundle

The goal of this study was to define the anatomical relationships between the terminal field of the nigrotectal pathway and the tectal neurons which project to contralateral brainstem gaze centers by way of the predorsal bundle. The distribution and morphology of the cells of origin for the predorsal bundle were determined by using a modification of the retrograde horseradish peroxidase technique which homogeneously filled their somas and dendrites. The terminal distribution of the nigrotectal tract was determined using both anterograde horseradish peroxidase and autoradiographic procedures. The results indicate that, in the grey squirrel (Sciurus carolinensis), the predorsal bundle cells are a heterogeneous population whose dendritic fields form a well-defined band confined to the inner half of stratum griseum intermediate. This inner sublamina also can be identified in Nissl and myelin stains. The same sublamina is the major target of the nigrotectal tract. The striking anatomical correspondence between the distribution of nigrotectal terminals and the cells projecting in the predorsal bundle supports a proposal, based on recent physiological investigations, that the nigrotectal tract plays an important role in the initiation of the saccade-related activity of the deep tectal cells (Chevalier et al., '81; Hikosaka and Wurtz, '83a-d).