Central projections of the sciatic, saphenous, median, and ulnar nerves of the rat demonstrated by transganglionic transport of choleragenoid‐HRP (B‐HRP) and wheat germ agglutinin‐HRP (WGA‐HRP)

AbstractThe central projections of the rat sciatic, saphenous, median, and ulnar nerves were labeled by injecting each nerve with 0.05 mg B‐HRP, or 0.5 mg WGA‐HRP, or a mixture of both. The B‐HRP labeled large dorsal root ganglion cells (30–50 μm) and, correspondingly, 98% of axons labeled in a rootlet were myelinated; although all sizes of myelinated axons were labeled, a greater proportion fell in the large ranges (2–6.5 μ axon diameter) than in the small ranges (0.5–2 μm). Primary afferents labeled with B‐HRP were distributed in laminae I, III, IV, and V of the dorsal horn and extended into the intermediate grey and the ventral horn; Clarke's column and the respective dorsal column nuclei were also densely labeled. Motoneurons of the nerve were densely labeled by B‐HRP, including extensive regions of their dendritic trees. In contrast, WGA‐HRP labeled small dorsal root ganglion cells (15–25 μm) and in the dorsal rootlets, 84% of the labeled axons were nonmyelinated; the small population of labeled myelinated afferents mainly fell within the smaller ranges (0.5–2.0μm). Terminal fields of WGA‐HRP labeled afferents were restricted to the superficial dorsal horn (laminae I–III), and to limited regions in the dorsal column nuclei.Sciatic nerve projections traced by labeling with B‐HRP alone or in combination with WGA‐HRP were more extensive than previously described when using either native HRP or WGA‐HRP. Afferents to the dorsal horn extended from L1–S1, to Clarke's nucleus from T8–L1, to the ventral horn from L2–L5, and extended throughout the medial and dorsal region of the gracilie nucleus. Motoneurons were found from L4–L6. Using the same tracers, saphenous projections extended in the superficial dorsal horn from caudal L1 to rostral L4, in the deep dorsal horn to mid L4 and along the length of the central part of the gracilie nucleus. The median nerve projected to the internal basilar nucleus from C1–C6, the dorsal horn from C3–T2, Clarke's nucleus from T1–T6, the external cuneate nucleus, and a large central area throughout the length of the cuneate nucleus. Motoneurons were located in dorsolateral and ventrolateral nuclear groups from C4 through C8. The ulnar nerve projections were less extensive but also included the internal basilar nucleus from C1–C6, the medial region of the dorsal horn from C4–T1, Clarke's nucleus from T1–T6, the external cuneate nucleus, and the medial part of the cuneate nucleus. Motoneurons were found in the dorsolateral nuclear groups from caudal C4 through C8.Substantial overlap of the dorsal horn terminal fields of the sciatic and saphenous and of the median and ulnar nerves was revealed by the more potent labeling. Electron microscopy demonstrated that labeled profiles were mainly terminals, including those in the overlap areas; this finding ruled out the possibility suggested by others that label in these areas might be mainly axonal. The rostrocaudal extent and terminal field overlap indicates an anatomical basis for physiological observations of overlapping receptive fields of dorsal horn cells.