Tectal efferents in the banded water snake, Natrix sipedon

Publisher Website
Google Scholar
Abstract
Visual information reaches the dorsal thalamus by two distinct routes in most reptiles. Retinal efferents terminate directly in the dorsal lateral geniculate nucleus (DLGN). Retinal information is also channeled indirectly through the tectum to nucleus rotundus. Retinal projections to DLGN and tectum are also well esablished in snakes, but the status of the tecto‐rotundal link of the indirect visual pathway is uncertain.Thus, tectal efferents were studied with Fink‐Heimer methods in banded water snakes (Natrix sipedon). The tectum gives rise to crossed and uncrossed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and osterior commissures. tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mesencephali, lateral habenular nuclei, and posterodorsal nuclei. Ascending projections reach the ventral lateral geniculate and suprapeduncular nuclei. there is a diffuse projection to the central part of the caudal thalamus and a dense, bilaternal projection to the DLGN.These results indicate that the relation of the tectum to the dorsal thalamus is different in snakes than in other reptiles. Nucleus rotundus is either absent or poorly differentiated and there is a strong convergence of the direct and indirect visual pathways at DLGN.