Optic tectum of the eastern garter snake, Thamnophis sirtalis. II. Morphology of efferent cells.

Tectal efferent neurons were retrogradely filled from extracellular injections of horseradish peroxidase (HRP) into pathways efferent from the tectum. Tectorotundal neurons have cylindrical dendritic trees, 80-100 microns in diameter, that extend vertically across the central and superficial tectal layers. Apical and basal dendrites are laden with complex appendages. The axon gives rise to an intratectal, collateral arbor that extends horizontally into the stratum griseum centrale beyond the cell's dendritic tree. The parent axon exits the tectum laterally in the tectothalamic tract. Tectogeniculate neurons also have narrow, radially oriented, and highly branched apical dendrites, but their basal dendrites are infrequently branched and lack appendages. An intratectal axon collateral forms a small, spherical arbor overlapping the apical dendrites in sublayer c of the stratum fibrosum et griseum superficiale. The parent axon ascends vertically and just below the stratum opticum turns rostrad to follow the optic fibers to the diencephalon. Tectoisthmi neurons have small somata and thin, radial dendrites that arborize below the pial surface in the stratum zonale. An intratectal axon collateral forms a spatially restricted arbor ventral to the soma in register with the dendritic tree. Tectoisthmobulbar neurons have dendrites that arborize extensively in sublayer a of the stratum fibrosum et griseum superficiale. The axon exits the tectum without collateralizing and joins a small-caliber component of the ventral tectobulbar tract. Ipsilateral tectobulbar neurons have stellate dendritic fields, 150-250 microns in diameter, that are restricted to the deep layers of the tectum. Sparsely branched dendrites are appendage-free but bear many short, fine spicules. The axon initially ascends from the soma and recurves into the stratum album centrale without collateralizing before joining a medium-caliber component of the ventral tectobulbar tract. Crossed tectobulbar neurons have large, stellate dendritic trees with diameters ranging from 200 to 500 microns. Like ipsilateral tectobulbar neurons, their dendrites are appendage-free but bear spicules. Their thick-caliber axons exit the tectum without collateralizing and course deep in the stratum album centrale to reach the dorsal tectobulbar tract.