Postsynaptic dorsal column pathway of the rat. III. Distribution of ascending afferent fibers

The distribution in the dorsal column nuclei (DCn) of post-synaptic dorsal column (PSDC) fibers was examined in rats following injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the spinal cord. Lemniscal neurons in the DCn were retrogradely labeled in the same animals by injecting the thalamus with Fluoro-Gold. In some experiments, primary afferent fibers were also labeled by injecting dorsal root ganglia with choleragenoid-conjugated HRP. Injections of PHA-L into the cervical enlargement labeled many fibers and varicosities throughout most of the ipsilateral cuneate nucleus. Labeled fibers were also present in the external cuneate and internal basilar nuclei. Injections of PHA-L into thoracic cord labeled fibers and varicosities in the medial cuneate and lateral gracile nuclei, as well as the external cuneate nucleus. Injections into the lumbar enlargement labeled fibers and varicosities throughout most of the gracile nucleus. Injections in sacral cord labeled fibers in the most medial part of the gracile nucleus. Dense labeling of PSDC fibers was found in areas with high densities of retrogradely labeled lemniscal neurons and areas with high densities of primary afferent fibers. In all regions of the DCn and in the external cuneate nucleus, fibers and varicosities labeled for PHA-L were seen in apposition to retrogradely labeled lemniscal cells. The distribution of postsynaptic afferent fibers in the DCn of the rat and its relationship to lemniscal neurons and primary afferent fibers contrast sharply with these features in cats.