The lateral reticular nucleus of the opossum ( Didelphis virginiana ). II. Connections

Conformational, histochemical and histofluorescent studies reveal that the entire lateral reticular nucleus (LRN) of the Virginia opossum is positive for cholinesterase activity and that its rostral portion is rich in fluorescent varicosities of the catecholamine type. Although neocortical‐LRN connections are relatively meagre, projections to the LRN from the red nucleus are extensive and topographically organized. Rubral‐LRN fibers arise from large‐medium sized neurons and distribute to the trigeminal division of the LRN as well as to specific portions of its external and internal divisions. Certain areas of the midbrain and pontine reticular formation, as well as the vestibular nuclei, project to the LRN and there is some evidence that reticular neurons adjacent to the LRN provide additional input. Although a relatively small fastigial‐LRN projection exists, no evidence was found for a contribution from any of the other deep cerebellar nuclei. Spinal‐LRN connections are extensive and topographically organized. Each of the inputs to the LRN have specific terminal targets, but there are varying degrees of overlap. Most of the LRN projects in an organized fashion to the anterior lobe of the spinal cerebellum, whereas only relatively restricted areas relay to the paramedian lobule and the pyramis. Lateral reticular axons distribute to specific longitudinal zones in such areas and the available material suggests that both convergence and divergence exist. The LRN also relays to the lobus simplex, and perhaps to crus I, as well as to visual‐auditory areas of the cerebellar vermis. The distribution of the various afferent connections of the LRN is interpreted in light of LRN‐cerebellar connections. Although we have described details that have not been elucidated in other species, where comparisons can be made it appears that the connectivity of the opossum LRN is comparable in most respects to that of placental mammals.