Dorsal column nuclei and ascending spinal afferents in macaques

Cell populations and thalamic projections of the dorsal column nuclei in macaques have been investigated in the medullae of normal animals and of animals with injections of horseradish peroxidase in the nucleus ventralis posterolateralis. In the same species, the course, distribution and origin of ascending non-primary pathways to the dorsal column nuclei have been demonstrated with the aid of degeneration methods, 3H-amino acid autoradiography and retrograde axonal transport of horseradish peroxidase. Non-primary afferents to the gracile and cuneate nuclei ascend mainly in the dorsal columns and, to a lesser extent, in the dorsal part of the lateral funiculus. Afferents originating from lumbar segments and ascending in the lateral funiculus terminate mainly in the rostral part of the gracile nucleus while those ascending in the dorsal columns distribute throughout most of the rostrocaudal extent of the same nucleus. Afferents from brachial levels terminate mainly in the cuneate nucleus and in the external cuneate nucleus. Degeneration and autoradiographic material concurrently demonstrate that non-primary afferents to the cuneate nucleus terminate preferentially within certain cytoarchitectonic subdivisions of this nucleus. Ascending spinal afferents to the dorsal column nuclei originate mainly from the ipsilateral dorsal horn, particularly from its medial part at upper cervical levels and from a band of gray, throughout the cord, largely corresponding to lamina IV and adjacent laminae. Large neurons along the lateral border of the ventral horn at lumbar levels may also contribute non-primary afferents to the ipsilateral dorsal column nuclei. These anatomical results provide some cues to a revised view of the organization of the dorsal column nuclei in monkeys and, taken together with recent electrophysiological and clinical data, contribute to a re-evaluation of some functional aspects of the dorsal column-medial lemniscal system of primates.