Optic tectum of the eastern garter snake, Thamnophis sirtalis. I. Efferent pathways.

Extracellular, iontophoretic injections of horseradish peroxidase were used to anterogradely fill axons efferent from the optic tectum in garter snakes. The tectal efferent pathways consist of six axon types with distinct projections and terminal morphologies. Tectogeniculate axons pass into the diencephalon via the optic tract, bearing collaterals that form spatially restricted, rodlike arbors in the pretectum, the ventral lateral geniculate nucleus, and the ventrolateral nucleus. Tectoisthmi axons exit the tectum as a thin-caliber component of the ventral tectobulbar tract. They form spatially restricted, spherical arbors within nucleus isthmi. Tectoisthmobulbar axons also give rise to small, spherical arbors within nucleus isthmi, but the parent axons continue caudally into the pontine and medullary reticular formation issuing many short collateral branches. Tectorotundal axons reach the diencephalon via the tectothalamic tract and give rise to fine terminal collaterals in the nucleus of the tectothalamic tract ipsilaterally and in nucleus rotundus bilaterally. Single axons form sheetlike terminal fields that span the rostrocaudal extent of nucleus rotundus. Ipsilateral tectobulbar axons descend into the midbrain tegmentum where they issue several thick collaterals that terminate widely throughout the nucleus lateralis profundus mesencephali. The parent axon continues caudally giving off several widely spreading collaterals within the pontine and medullary reticular formation. Crossed tectobulbar axons enter the dorsal tectobulbar tract and cross the midline to form the predorsal bundle. Single axons give rise to terminal collaterals in the nucleus lateralis profundus mesencephali bilaterally, the contralateral pontine and medullary reticular formation, and the intermediate gray of the cervical spinal cord.