The pattern of neocortical projections to the mesencephalon of the opossum, Didelphis virginiana

Abstract

The results of this investigation suggest a pattern of neocortical projections to the mesencephalon in the opossum, a primitive marsupial. The more rostral neocortical areas (frontal and preorbital cortices) distribute to the more rostral, ventral and medial midbrain areas. These areas include the central, the ventral and the deep tegmental areas, certain restricted portions of the red nucleus, the medial substantia nigra and the more ventral and medial areas of the periaqueductal gray. Projections from intermediate cortical regions (postorbital, paramarginal and parietal cortices) contribute the greatest number of fascicles to the read nucleus (particularly pars magnocellularis), to the intermediate substantia nigra, the dorsolateral midbrain tegmentum and the more lateral regions of the periaqueductal gray. Such cortical areas also give rise to the neocortical pathways to certain brain stem sensory nuclei as well as to most, if not all, of the cortico-spinal bundles in this form. More caudal cortical areas (striate and peristriate cortices) give rise to fibers which terminate in the more dorsal portions of the midbrain at superior collicular levels (the superficial strata of the superior colliculus), whereas the temporal area projects the most caudally and dorsally at inferior collicular levels (the nucleus of the inferior colliculus). Further sampling of primitive metatherian and eutherian mammaals is needed to determine whether the pattern reported herein for the opossum is a basic mammalian configuration or is characteristics of only certain marsupials.