We have studied the laminar position, morphology, and synaptic relationships of neurons in the cat superior colliculus which project to the interadjacent division of the lateral posterior nucleus (LPi), using the retrograde transport of horseradish peroxidase. The neurons which project to LPi are remarkably varied in depth, size, morphology, and synaptic density and appear to consist of at least four cell types. Labeled cells were found in four laminae. Forty‐seven percent were found in the superficial gray layer (50–550μm), all but a few within its deep subdivision. Forty‐seven percent were located in the optic layer (550–1,200μm), the majority of these being within the upper one‐half of the layer. Seven percent were found in the intermediate and deep gray layers (below 1,200μm). Cell body area varied widely, ranging from 37 to 768 μm2 (mean of 243μm2). Based on cell size, shape, and dendritic field orientation, we identified four distinct cell morphologies which were labeled. Thirty‐five percent were stellate, 32% were vertical fusiform, 19% were granule, and 12% were horizontal cells. Electron microscope analysis confirmed that neurons projecting to the lateral posterior nucleus are a morphologically diverse group. A sample of 71 labeled cells varied significantly in density of synaptic input as well as in size, shape, depth, dendritic distribution, and cytology.
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