Connections of the tectum opticum in two urodeles, Salamandra salamandra and Bolitoglossa subpalmata, with special reference to the nucleus isthmi.

Tectal connections were studied in two urodele species following horseradish peroxidase injections into the tectum opticum. In both species retrogradely labelled cells were observed: ipsilaterally in the corpus striatum, lateral amygdala, ventral and dorsal thalamus and nucleus of DARKSCHEWITSCH--bilaterally in the pretectal nucleus, dorsal tegmentum and nucleus reticularis medius--contralaterally in the tectum opticum and area octavo lateralis. Besides these nuclei the nucleus isthmi was bilaterally labelled. Rostral efferent projections of the tectum opticum terminated in the ipsilateral pretectal area and the ipsilateral dorsal and ventral thalamus ipsilaterally coursing to the contralateral tectum via the commissura postoptica. Caudal efferents formed the bilaterally organized tecto-bulbar tracts innervating the rhombencephalon. Comparison of the results of a series of tectal horseradish peroxidase injections differing in depth, tangential extension and location, indicated that tectal afferents from the telencephalon, the contralateral tectum opticum and the medulla were sparse and widely branching. Projections of the telencephalon and all diencephalic nuclei terminated deep in the rostral tectum opticum. Projections of the medulla terminated preferentially deep in the caudal tectum opticum. The tecto-isthmic projection was highly topographic forming a layered terminal field lateral to the nucleus isthmi. The isthmo-tectal projection innervated the whole tectum opticum on the ipsilateral side and was highly topographic. On the contralateral side the caudal part of the tectum opticum was not innervated. The isthmo-tectal fibers terminated superficially in the tectum opticum on both sides of the brain. The nucleus isthmi identified here is proposed to be homologe to that of other vertebrates.