Anatomical identification of a telencephalic somatosensory area in a reptile, Caiman crocodilus.

Telencephalic projections of a thalamic somatosensory area were investigated in a reptilian species, Caiman crocodilus, by means of horseradish peroxidase (HRP) neurohistochemistry. Injections of HRP into the medialis complex labeled axons that leave the ventral aspect of this nucleus to enter the dorsal peduncle of the lateral forebrain bundle where they ascend. These fibers course rostrally to enter the central portion of the lateral forebrain bundle. Here, axons turn dorsally, pass through, and probably synapse on, interposed neurons of the ventrolateral area, to end in a central portion of the dorsal ventricular ridge (DVR). These findings were confirmed by HRP injections into this somatosensory area of the DVR that retrogradely labeled neurons in the medialis complex. This telencephalic somatosensory area identified in transverse sections was then reconstructed onto a surface view of the DVR. The locus of this forebrain somesthetic region was then compared with the location of the DVR projection areas of auditory (nucleus reuniens) and visual (nucleus rotundus) thalamic nuclei. This analysis suggested two features. First, telencephalic terminal zones of dorsal thalamic nuclei were largely separate and non-overlapping. Second, the locus of termination in the DVR of each thalamic nucleus reflected each nucleus's topography in the dorsal thalamus. Additional parallels in the neural circuitry of auditory, visual, and somatosensory systems that synapse in the midbrain and project to the DVR are documented.