Electron microscopic identification of superior colliculo-pontine axon terminals.

Synaptic boutons emanating from axons of superior colliculus origin were identified by electron microscopy in the neuropil of the basilar pontine nuclei. Such boutons were relatively small (0.6-2.0 microns) and exhibited electron-dense degeneration within a 1-2 day period following electrolytic lesions which involved much of the superior colliculus. Degenerating boutons were observed in synaptic contact with dendritic shafts and spines as well as neuronal somata. The reactive boutons were rapidly engulfed by phagocytic elements and were no longer visible in the neuropil after 6 days of survival.