Contralateral projection of primary afferent fibers to mammalian spinal cord

Abstract

Contralateral projections of spinal primary afferent nerve fibers have seldom been recognized and have never been fully described. In the present study we have traced crossing primary afferent fibers to their central nervous system endings using the Fink-Heimer procedure to impregnate fibers that degenerate after dorsal rhizotomy. Dorsal roots were cut at several cervical, brachial (forelimb), lumbosacral (hind limb), and caudal (tail) levels in four different mammalian species (North American opossum, brush-tailed possum, cat, and bushbaby). Crossed afferent fibers were seen consistently in all species after dorsal rhizotomy at high cervical and caudal cord levels. They were rarely seen at lumbosacral levels in any of the four species and were present at brachial cord levels only in opossum. Wherever crossing fibers occur, they traverse gray matter between dorsal funiculus and central canal and arch dorsally to terminate in medial and/or lateral parts of the contralateral dorsal horn (laminae III and IV). They generate a longitudinal plexus of preterminal fibers, one to two spinal segments long. Based on their distribution and mode of termination they seem likely to be of cutaneous origin. The consistent localization of their endings suggests an ordered projection, probably from related skin areas, onto the dorsal horn.