Dorsal Column Nuclei in a Prosimian Primate (Galago senegalensis)

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Abstract
The morphology of the gracile nucleus and its relationship to the distribution of the primary afferent fibers from hindlimb and tail spinal segments were studied in the lesser bushbaby. Individual rhizotomies were performed on 10 bushbabies at spinal cord levels Coc2, S3, S2, L6, L3, L1, T12 and T7. The gracile nucleus was subdivided into a caudomedial component, the pars medialis, and a rostrolateral portion, the pars lateralis. The pars medialis had cytoarchitectural specializations which were related to the distribution of caudal versus rostral spinal segments. Preterminal debris from all levels studied was present within both ipsilateral components of the gracile nucleus. Sparse contralateral degeneration was limited to the pars medialis and seen only after coccygeal lesions. In the caudal three-fourths of the pars medialis, a segmentotopic pattern of distribution was present with the caudal levels projecting to the extreme dorsomedial part of the nucleus. Upper lumbar and lower thoracic spinal segments projected to partially overlapped terminal fields in the lateral and ventrolateral regions of the pars medialis. At levels rostral to the pars medialis fibers from spinal segments studied terminated in a mediolateral pattern within the pars lateralis. Caudal levels projected to medial regions while rostral levels projected successively to the more lateral nuclear areas. The terminal zone for the L6 lesion occupied the largest transverse nuclear area of the pars lateralis. The relative location, shape, orientation and size of the terminal fields for the upper lumbar and lower thoracic segments were related to the location and basic organization of the respective fiber laminae in the gracile fasciculus. Only fibers from the coccygeal, sacral and seventh thoracic segments projected to the cuneate nucleus. Except for a sparse projection of nucleus z (Coc2, S3, S2) and the reticular formation (L3), no degeneration was observed in any other medullary nucleus.