Efferent neocortical projections to sensory nuclei in the brain stem of the opossum ( Didelphys virginiana)

Abstract

The origin, the brain stem location and the terminal endings of the neocortical fibers to the brain stem sensory nuclei have been described for the opossum. The degenerating axons that were produced by various neocortical lesions were impregnated by the Nauta-Gygax (1954) technique in the brains of 18 opossums. Fibers originating mainly in the postorbital, the parietal and perhaps the peristriate cortices terminated in the principal sensory trigeminal nucleus, the dorsal one-half to two-thirds of the various subdivisions of the spinal trigeminal nucleus, the nucleus of the tractus solitarius, the commissural nucleus of Cajal and the nuclei of the dorsal columns. Two main neocortical pathways to the brain stem sensory nuclei, which have been previously described for the rat, were present in the opossum. In addition fibers which pursue an unusual course to the principal sensory nucleus of the trigeminal and to the nucleus of the tractus solitarius were described. The opossum, a primitive marsupial, may lack a cortico-spinal caudal tract to the cervical spinal cord. The phylogenetic significance of the neocortical projections to the nucleus gracilis, a sensory relay station for impulses originating in the caudal trunk and hind-limb is discussed in this light. The possible implications of the pattern of neocortical projections to the nucleus of the spinal fifth tract is also discussed with respect to recent neurophysiological research.