Reptilian somatosensory midbrain: identification based on input from the spinal cord and dorsal column nucleus.

A midbrain somatosensory area was identified in reptiles Caiman crocodilus, by orthograde degeneration techniques after midcervical spinal cord transections or after dorsal column lesions. Terminations of these somesthetic inputs were located in a caudal lateral part of the central nucleus of the torus semicircularis and contiguous dorsal mesencephalon. Dorsal column projections were contralateral while spinal connections were mainly ipsilateral. In order to confirm the results of the anterograde degeneration experiments independently, injections of horseradish peroxidase were placed in this midbrain somatosensory recipient area. Retrogradely labeled neurons were identified in the contralateral dorsal column nucleus. These findings indicated that the pattern of degeneration seen after dorsal column lesions originated from the dorsal column nucleus itself. The results of the present experiments when compared with similar studies in other amniotes emphasize the following two points. First, dorsal column and spinal cord projections to the midbrain are features common to reptiles, birds, and mammals. Second, since these pathways and midbrain somatosensory terminations are common to amniotes, dorsal column nucleus and spinal circuits to the midbrain and their mesencephalic projection areas are phylogenetically ancient.