Dorsal funicular projections to the dorsal column nucleus in a reptile, Caiman crocodilus

Projections to the dorsal column nucleus were investigated in the reptile Caiman crocodilus following application of horseradish peroxidase (HRP) crystals to the cervical spinal cord after dorsal myelotomy. Brains were processed for HRP histochemistry by use of tetramethylbenzidine as the chromogen. Injection of the dorsal funiculus alone labelled axons that terminated solely in the ipsilateral dorsal column nucleus. Application of HRP that extended beyond the dorsal funiculus labelled axons that terminated in brainstem areas in addition to the dorsal column nucleus depending upon the injection site. Retrogradely labelled neurons in the brainstem were seen only after HRP injections that extended beyond the dorsal funiculus and varied depending upon the injection site. Dorsal funiculus fibers travel rostrally and turn laterally to reach more anterior portions of the dorsal column nucleus. Fibers peel off in an oblique ventrolateral fashion to terminate in the posterior part of the nucleus where boutons are intermixed with fibers. The anterior portion of the nucleus contains a greater proportion of axon terminals as compared to fibers than does the posterior part. A graphic reconstruction of the dorsal column nucleus was made by superimposing serial sections to view the nucleus in transverse, sagittal, and horizontal planes.