Spinally projecting neurons of the dorsal column nucleus in a reptile: locus of origin and trajectory of termination.

Interconnections between the dorsal column nucleus and the spinal cord were investigated in a reptile, Caiman crocodilus. After placement of an anterograde tracer into the dorsal column nucleus, descending fibers are seen to leave this nucleus to enter the dorsal funiculus where they course ventrally to terminate in lamina V of the spinal cord as far caudally as C2. Placement of a retrograde tracer into cut fibers of the cervical spinal cord identified the relay cells of the dorsal column nucleus that project to the spinal cord. These neurons were mainly clustered in a caudal and ventral part of this nucleus. The soma of these spinally projecting cells were small and were generally round or oval in shape. A number of these neurons had the long axis of their soma oriented dorsoventrally, with a primary dendrite extending dorsally. Fibers in the dorsal funiculus that originated from the spinal cord enter the caudal part of the dorsal column nucleus and turn ventral. In the dorsal column nucleus, these axons run parallel to the vertically oriented dendrites of these spinally projecting cells before termination in close relation to the cell bodies of these neurons. Quantitative observations (mean +/- standard error) were made on well labeled neurons and included several measurements: area, perimeter, and degree of eccentricity (greatest width/greatest length) in both the transverse as well as the sagittal plane. These spinally projecting neurons in Caiman are located in the dorsal column nucleus in a position similar to that of spinally projecting cells in cats.