A study of overlap and collateralization of bulbar reticular and raphe neurons which project to the spinal cord and diencephalon of the North American opossum.

Labeled neurons were found in most reticular and raphe nuclei of the opossum's medulla after horseradish peroxidase (HRP) injections into either the spinal cord or diencephalon. The labeling produced by spinal injections differed somewhat from that produced by diencephalic ones, but HRP-positive neurons were found in what appeared to be comparable areas. In order to study the intermingling of bulbospinal and bulbodiencephalic neurons more directly, we used fluorescent markers (true blue, nuclear yellow and diamidino yellow dihydrochloride) in double-labeling experiments. When injections of one dye into the spinal cord were combined with injections of another into the diencephalon, neurons labeled by one or the other marker were found to be intermingled in most reticular and raphe nuclei. The double-labeling experiments also showed that some neurons of the bulbar reticular formation and raphe project to both the spinal cord and diencephalon, presumably by axonal collaterals. The number of such neurons represents a somewhat larger proportion of those projecting to the diencephalon than to the spinal cord. It appears, however, that most neurons project either to the spinal cord or to the diencephalon and do not provide collaterals to both areas.