Laminar origin of projections from the superficial layers of the superior colliculus in the tree shrew, Tupaia glis.

The laminar origin of the efferent projections from the superior colliculus to the pulvinar and to the dorsal and ventral lateral geniculate nuclei has been studied using the retrograde axonal transport of horseradish peroxidase. Following injections in either the dorsal or the ventral lateral geniculate nucleus, cells heavily labeled with the horseradish peroxidase reaction product are restricted primarily to the upper stratum griseum superficiale. These cells have small, fusiform somas with dendrites which extend dorsally and ventrally, perpendicular to the pial surface. In contrast, following injections in the pulvinar, cells labeled with reaction product are restricted primarily to the lower stratum griseum superficiale and to the most superficial part of stratum opticum. These cells typically have larger somas than cells in the upper stratum griseum superficiale, and often have dendrites which emerge horizontally from the cell body. When taken together with previous electrophysiological and anatomical studies, the present findings suggest that there is a laminar subdivision of the tree shrew stratum griseum superficiale, and that these subdivisions project selectively to different thalamic targets.