Afferent connections to neocortex in the opossum (Didelphis virginiana)

Afferent axons to the opossum's neocortex were carefully traced to their areas of termination using the Nauta‐Gygax silver technique for degenerating axons. The two major systems of incoming fibers arise primarily from contralateral neocortex (commissural fibes), and ipsilateral thalamus (thalamocortical fibers). Following complete transection of the anterior commissure‐corpus callosum amalgam, degenerating fibers could be traced to almost all areas of neocortex. The same pattern resulted from total removal of neocortex from one cerebral hemisphere. Commissural fibers arising from neocortex cross the midline in teh posterior two‐thirds of the anterior commissure before terminating in contralateral neocortcial areas. The vast majority of fibers terminated in the deep cortical layers IV, V, VI. There was degeneration in layer III and occasionally in layer II, but essentially never in layer, I. A small central portion of striate cortex receives very few commissural fibers. Following thalamectomy, degenerating fibers could be traced to all areas of neocortex. One system of thalamocortical fibers terminates coextensively with commissural fibers in the deep layers IV, V, VI. A second system of thalamic projection arises from thalamic cells, traverses the cortical layers in parallel fascicles at 900 to the pial surface, bifurcates and ends in the thick outer plexiform lamina (layer I) of neocortex. These fibers run for some distance parallel to the pial surface before terminating among apial dendrites of neurons from all cell layers. This system of layr I thalamocortical connectiions appears in all neocortical areas, even in primary receptive cortex in teh opossum. There is little evidence of the highly specialized axonal arborizatiions that characterize layer IV thalamic terminations in the primary receptive cortex of eutherian mammals.