Morphologic evidence for fiber sorting in the fasciculus cuneatus

This report describes the medullary course and projection patterns of cervical dorsal root nerve fibers in five mammalian species (bushbaby, tree squirrel, raccoon, potoroo, and brush-tailed possum). After cutting a single cervical dorsal root and allowing appropriate postoperative survival, we studied serial, Fink-Heimer impregnated sections of the medulla from each individual to locate degenerated afferent fibers and endings. Fibers from those rostral (C2) or caudal (C8) cervical roots studied traverse the medulla as a single bundle, forming collaterals en route into the nucleus cuneatus and external cuneate nucleus at all levels. Fibers from most cervical roots (C4 through C7, sometimes C3), however, separate into two discrete bundles as they project rostrally. A more medial, superficially placed group of fibers projects predominately to the dorsal part of nucleus cuneatus, which seems to be somatotopically arranged. The second fiber group, larger, deeper, and more laterally situated, projects mainly to ventral and ventrolateral nucleus cuneatus (asomatotopically) and to the external cuneate nucleus. Based on segmental and species differences in the presence of this fiber separation, and on prior physiologic and anatomic evidence, we suggest that this form of fiber sorting in the cuneate fasciculus is modality based. It appears likely that fibers in the medial group are of primarily cutaneous origin, and that many if not most of those in the larger lateral population arise from deep receptors in muscle and joints.