Optic tectum of the eastern garter snake, Thamnophis sirtalis. IV. Morphology of afferents from the retina.

The morphology of single retinal terminals in the optic tectum of the eastern garter snake was demonstrated by orthograde filling from extracellular injections of horseradish peroxidase (HRP) into the optic tract. HRP-filled terminals share a characteristic shape and structure. Their parent axons course caudally in the stratum opticum within fascicles of 200-300 fibers of varying diameters. Single axons exit a fascicle and course into either the stratum fibrosum et griseum superficiale, ventrally, or the stratum zonale, dorsally, where they bifurcate successively two or three times into preterminal branches. Each preterminal branch gives rise to many thin, terminal branchlets laden with boutons. The arbors are ellipsoidal with their long axes oriented mediolaterally and their short axes oriented rostrocaudally. Arbors vary in their overall size (from 45 to 150 micron), in the diameters of their parent axons (from less than 0.5 to 3.0 micron), and in the size of their terminal boutons (from 0.5 to 3.5 micron). Bouton size increased with increasing diameter of the parent axon. The great majority of arbors are confined to one of three retinorecipient sublayers in the superficial tectum. However, the full range of arbor sizes and axon diameters is present in each sublayer.