Course and origin of the crossed parabigeminotectal pathway in rat. A retrograde HRP-study.

The contralateral efferent connections of rat parabigeminal nucleus (pbgn) were studied by means of retrograde transport of horseradish peroxidase (HRP) after focal HRP injection into the superior colliculus (SC), the optic tract, the dorsal part of the lateral geniculate nucleus (LGNd) and the lateroposterior nucleus of the thalamus (LP). The cells of origin of crossed parabigeminotectal pathway in the medial pbgn are divided into two morphologically distinct subgroups, which project in a different way to the SC. A group of larger cells (mean soma diameter about 16 micron) project to the contralateral SC running over the LP. A crossed parabigemino-LP pathway could not be excluded. The axons of smaller neurons of the medial pbgn (mean soma diameter about 12 micron) run over the contralateral LGNd to the SC or there exists even a small parabigemino-LGNd pathway. These differences are suggested as a hint for parallel wiring from pbgn to the SC in rat.