Retinal and visual cortical projection to the superior colliculus of the rabbit

Abstract

The rabbit superior colliculus was examined using a variety of light- and electron-microscopic techniques. Golgi study showed that there is one principal type of neuron whose axon leaves the superficial gray—the vertical cell. The remaining varieties of neurons possess heavily spine-studded dendrites and local axons. Among these types are the marginal cell, the stellate cell, and the pyriform cell. Using ultrastructural techniques, we found contralateral optic input is confined to a zone 100 to 350 μm below the surface of the colliculus. Terminals are presynaptic to both axons and dendrites and enter into serial synapses. They degenerate by passing through a filamentous to a dense phase. Visual cortical axon terminals are 300 to 600 μm below the surface and usually contact only one postsynaptic profile. They do not participate in serial synapses. F axon terminals (symmetric thickening, flattened vesicles) are also numerous in the upper collicular layers, but do not degenerate after enucleation or visual cortex ablation. We conclude that considerable evidence exists for a large degree of horizontal and vertical organization in the upper layers of the superior colliculus. These data are compared to the tectum in other species and to our ongoing experiments on the development of the superior colliculus.