Organization of projections from the gracile, medial cuneate and lateral nuclei in the North American opossum. Horseradish peroxidase study of the cells projecting to the cerebellum, thalamus and spinal cord

Using the horseradish peroxidase technique on the North American opossum, we were able to locate the neurons within the dorsal column and lateral cuneate nuclei which innervate the cerebellum and thalamus as well as those within the dorsal column nuclei which project spinalward. The medial and lateral cuneate nuclei supply axons to the anterior lobe, the paramedian lobule and the pyramis of the cerebellum and the lateral nucleus provides an additional projection to the uvula. The cerebellar projections from these nuclei arise from neurons located rostral to the obex. The thalamic projections from the gracile and medial cuneate nuclei originate from neurons throughout their rostral to caudal extent, although most of them are located just rostral to the obex. Neurons within the lateral cuneate nucleus which innervate the thalamus are found at intermediate rostrocaudal levels where most of them approximate the medial cuneate nucleus. The medial cuneate also projects to at least lumbar levels of the spinal cord in the opossum and neurons giving rise to such connections are found at the level of the obex and caudal to it. Neurons within the dorsal part of the dorsal column nuclei were labelled only after thalamic injections. Our results in the opossum are compared with those obtained in several placental mammals.