The cerebellopontine system in the rat. II. Electron microscopic studies

Cerebellopontine axonal boutons in the neuropil of the basilar pontine nuclei (BPN) were marked for ultrastructural identification by producing unilateral electrolytic lesions of the superior cerebellar peduncle (SCP) as it exited from the cerebellum and before its decussation in the caudal midbrain. Three varieties of degenerating boutons were distinguished on the basis of size, type of degeneration, and postsynaptic locus. A relatively large variety of bouton (2.5-6.0 microns) that exhibited filamentous degeneration throughout the range of survival times employed (1-14 days) was the most frequently observed type of degenerating cerebellopontine bouton. Such boutons formed synaptic contacts with several small, dendritic, spinelike profiles as well as the shafts of intermediate or proximal dendrites. A second, far less numerous and somewhat smaller type of bouton (1.5-4.5 microns) was distinguished by the fact that it exhibited advanced dark degenerative changes after a 2-day survival period, formed multiple spine contacts (but not shafts), and was no longer apparent in the neuropil after a postlesion survival time of 6 days. The third variety of degenerating bouton was small (0.8-2.0 microns), exhibited dark degeneration with a 2-6 day survival period, contacted primarily shafts of small-diameter dendrites, and was observed more frequently than the larger dark boutons but less often than the large filamentous boutons. All three types of degenerating boutons contained round, clear, synaptic vesicles and formed only asymmetric synaptic active sites. It is suggested that the three types of degenerating axon terminals arise from at least three varieties of neurons in the deep cerebellar nuclei. Further it is suggested that such boutons originate from cerebellar efferent axons which distribute in collateral fashion to the thalamus, red nucleus, and/or the inferior olive.