Further evidence for two types of corticopulvinar neurons.

Retrograde tracing experiments suggest that corticopulvinar connections originate from at least two subpopulations: medium and giant pyramids in layer 5, each with distinctive dendritic and local axonal arborizations. The present study used extracellular injections of PHA-L to delineate extrinsic axon arbors and to assess the possibility of categories that might correlate with these specific neuronal subpopulations. Two distinct types of terminations were found. Type I have smaller caliber preterminal axons, are generally elongate, and are studded with spinelike terminal specializations. Type II have larger axons (> 1.0 microns), spherical arbors, and mainly beaded specializations, some of which are conspicuously large (approximately 3.0 microns). These two axon types may differ in conduction velocity and membrane properties, and the interactions of these different features may be important for pulvinar function.