Synaptic patterns of the superficial layers of the superior colliculus of the rat

An electron microscopic study has been undertaken of the upper layers of the superior colliculus, into which run fibers from the retina and visual cortex. Study of the normal synaptic patterns shows large numbers of serial synapses, particularly near the surface of the colliculus. The presynaptic components usually contain round vesicles and the intermediate profiles always contain flattened vesicles. The latter may be dendrites, dendrite‐like or dilatations of such profiles. Degeneration studies indicate that optic terminals are especially dense near the surface and that they are often the presynaptic terminal of a serial synapse. Very few optic terminals occur in the stratum opticum. Cortical afferents from the visual area mostly end deep in the stratum griseum superficiale, and in superficial stratum opticum, usually on small clear profiles. Studies of the progress of degeneration indicate that optic terminals first show a predominantly neurofilamentous and associated glycogen reaction, and later a predominantly electron dense reaction. Cortical terminals show an initial dense reaction, with occasional neurofilamentous endings. These time course fit well with light microscope data from Nauta and Glees stained material. The synaptic patterns described here closely resemble patterns described in retina and dorsal lateral geniculate body. How these reflect functional similarities has yet to be resolved.