Anatomical identification of a postsynaptic dorsal column system in a reptile, Caiman crocodilus

The presence of a postsynaptic dorsal column system was investigated in reptiles, Caiman crocodilus. Several experiments were undertaken. First, a cytoarchitectonic analysis of the spinal cord was performed to identify its laminar organization. Second, the termination of dorsal root fibers in the dorsal horn of the upper cervical cord, brachial enlargement, and lumbar intumescence was studied utilizing anterograde tracing techniques. Third, placement of retrograde tracers into the dorsal column nucleus identified spinal cord neurons that projected to the dorsal column nucleus. These experiments document that, in addition to direct primary dorsal root input, a second pathway exists whereby nonfacial somatosensory information can reach the dorsal column nucleus in this reptilian group. This neural path arises primarily from neurons in the ipsilateral lamina V and, to a much lesser extent, from lamina IV in the cervical area and, to a far smaller degree, from the thoracic and lumbar dorsal horn. Each of these laminae whose neurons project to the dorsal column nucleus receive direct dorsal root input. This neural circuit in Caiman is similar to a neural pathway described in pigeons and several mammalian groups. These observations suggest that a postsynaptic dorsal column system is phylogenetically ancient and may well be a feature common to all amniotes.