Thalamic projections from a midbrain somatosensory area in a reptile, Caiman crocodilus

Ascending projections to the thalamus from a midbrain somatosensory area were investigated in a reptile, Caiman crocodilus. Connections were determined utilizing horseradish peroxidase (HRP) neurohistochemistry in which tetramethylbenzidine was the chromogen. Anterograde experiments, in which HRP injections were placed into the midbrain, revealed bilateral projections to the medialis complex of the dorsal thalamus. These observations were confirmed by HRP injections into the medialis complex which retrogradely labeled neurons in the midbrain somatosensory area bilaterally. An ipsilateral predominance of connections was observed in both anterograde and retrograde HRP experiments. This somatosensory target in the thalamus, the medialis complex, is located between the auditory and visual nuclei. This organization and neural circuitry of somatosensation is similar to that of auditory and visual systems that terminate in the midbrain of Caiman.