Projections of cervicothoracic dorsal roots to the cuneate nucleus of the rat, with observations on cellular "bricks"

The distribution of cervicothoracic (C1–T2) dorsal roots to the cuneate and accessory cuneate nuclei was studied in the rat with the Fink- Heimer I technique following single extradural rhizotomies. Cytoarchitectural analysis of the cuneate nucleus revealed an anatomically distinct caudal and rostral region. The caudal region was characterized by discrete aggregates of cells arranged as “slabs” or “bricks” which continue vertically almost to the ventral limit of the cuneate nucleus. In contrast, the cells of the rostral region were organized in a non-focal manner. The projection of the cervicothoracic dorsal roots to the cuneate nucleus reflected the cytoarchitectural pattern observed; non-focal terminal fields of degeneration in the rostral region and discrete “terminal field bands” of degeneration isolated from one another by degeneration-free zones in the caudal region. However, the projections of dorsal roots C5 and T2 to the caudal region is non-focal. In both regions of the cuneate nucleus, the distribution of dorsal root degeneration was topographically organized with cranial roots terminating ventrolaterally and more caudal roots, dorsomedially. The amount of intersegmental overlap of dorsal root terminal fields was greater in the rostral than in the caudal region. Individual dorsal roots projected differentially to the two regions of the cuneate nucleus. Roots C3 and C4 distributed primarily to the rostral region whereas C5 to T1 distributed to both regions. T2 root projected primarily to the caudal region. Dorsal roots C1 and C2 did not terminate in either region of the cuneate nucleus. All roots studied projected heavily and topographically to the accessory cuneate nucleus. Extensive overlap of the very dense terminal fields characterized the dorsal root projections to the accessory cuneate nucleus. On the basis of cytoarchitectonics and dorsal root projections, a dual organization of the cuneate nucleus was revealed. This organization reflected previous anatomical and electrophysiological studies in the rat and paralleled the organization described in the dorsal column nuclei of the cat. The significance of the dual organization with respect to dorsal column function was discussed, as was the finding of the lack of C1 and C2 dorsal root projections to the cuneate nucleus.