Spino-bulbar, spino-thalamic and medical lemniscal connections in the American opossum, Didelphis marsupialis virginiana

Projections from the spinal cord and dorsal column nuclei to more rostral levels of the neuraxis were investigated in seventeen adult opossums by the Nauta-Gygax and Fink-Heimer techniques. In all cases with spinal cord lesions a greater number of degenerating fibers distributed to the medulla and pons than to the midbrain and diencephalon. Numerous degenerating fibers ended within the medial reticular formation of the medulla and caudal pons, and within the lateral reticular formation of the rostral pons and midbrain. Degenerating fibers were numerous in the reticular formation following cervical and thoracic lesions, but sparse in specimens with damage restricted to either the lumbar or sacral spinal cord. The dorsal column nuclei received afferent connections from the well known dorsal funicular pathway and, although to a much lesser extent, from the main ventrolateral spinal bundle. Although most of the latter fibers ended in the subnucleus dorsalis and spinal vestibular nucleus, some penetrated into the gracile and cuneate nuclei. Conspicuous terminal degeneration was present within the inferior olivary nucleus following cervical and thoracic lesions, but was lacking in cases of either caudal lumbar or sacral cord lesions. The location of terminal degeneration within the lateral reticular nucleus is dependent upon the level of the lesion in the spinal cord. Degenerating fibers ended within the lateral vestibular nucleus in all cases of spinal cord hemisection, and within the medial portion of the facial nucleus in cases with a lesion rostral to C-4. After cervical and thoracic hemisections terminal fiber degeneration was present within the midbrain tegmentum, the periaqueductal gray, the intercollicular nucleus (Mehler,'69), the posterior thalamic nucleus, the ventrobasal nucleus, the parafascicular nuclei and the caudal nucleus ventralis lateralis. All thalamic nomenclature was taken from Oswaldo-Cruz and Rocha-Miranda, '68. In animals with more caudal lesions, no fiber degeneration was evident within the nucleus ventralis lateralis and so little within the ventrobasal nucleus that it was impossible to ascertain a somatotopic pattern of spinothalamic projections. Lesions of the dorsal column nuclei caused terminal degeneration within the inferior olivary nucleus, the pars lateralis of the nucleus of the inferior colliculus, the zona incerta, the posterior thalamic nucleus, the caudal part of the ventral lateral thalamic nucleus and the ventrobasal nucleus of the thalamus. Diffuse connections with the reticular formation, periaqueductal gray, midbrain tegmentum and the parafascicular complex were also observed. The results from small lesions indicate that the input to the ventrobasal nucleus in the opossum is organized in the typical mammalian fashion.