Various types of corticotectal neurons of cats as demonstrated by means of retrograde axonal transport of horseradish peroxidase.

The retrograde labeling of cortical neurons with horseradish peroxidase (HRP) was used to investigate the morphological features of neurons in various cortical areas projecting to the superior colliculus in the cat. Corticotectal cells were found to be labeled in layer V of the entire cerebral cortex. The number of labeled cells and their locations varied according to the sites of injections of HRP in the colliculus. Most of the corticotectal cells identified in the present study were small (9--20 micrometer in diameter, 66%) and medium (20--40 micrometer, 30%) pyramidal neurons and only 4% of them were large (more than 40 micrometer). The labeled cells, 261 in total number, had somal diameters of 20.8 +/- 8.0 micrometer (mean and SD). The range of sizes of the labeled neurons was different in different cortical areas. For example, the labeled neurons in the Clare-Bishop area had a greater proportion of large diameter cells than in other areas. The present findings are largely in agreement with the previous data of anterograde degeneration methods with respect to the topographical correlation of the corticotectal projections. However, in some cortical areas, e.g., the sensorimotor and the first visual (area 17) cortex of the lateral surface of the hemisphere, relatively small numbers of corticotectal neurons appear to have been labeled by retrogradely transported HRP. The sparsity of the labeled neurons in certain cortical areas may reflect the existence of corticotectal neurons with axon collaterals supplying brain structures other than the superior colliculus.