Nigral and cerebellar synaptic terminals in the intermediate and deep layers of the cat superior colliculus revealed by lesioning studies

The presence of degenerating nigral and cerebellar synaptic terminals in the intermediate and deep layers of the cat superior colliculus was demonstrated by electron microscopy following lesions of the substantia nigra or brachium conjunctivum. The superior colliculus was taken for analysis 4–5 days after operation. Nigral terminals underwent a dark type of degeneration following kainic acid lesion of the pars reticulata of the substantia nigra. The majority of nigral degenerating terminals and axons were found in the stratum griseum intermediate with a few in the stratum griseum profundum. Two kinds of cerebellar terminals were distinguished by general appearances such as size, type of synaptic contact and type of synaptic vesicle and by the pattern of degenerative changes following electrical lesion of the brachium conjunctivum. Large elongated synaptic terminals 4–7 μm in diameter, were found mainly in the stratum griseum profundum. They often had double termination with conventional dendrites and with vesicles containing dendrites. This kind of terminal had a filamentous type of degeneration. A second type of degenerating cerebellar terminal, characterized by an electron-lucent type of degeneration, was predominantly located in the stratum griseum intermediale. These terminals were circular, about 4 μm in diameter, and did not have synaptic contact with vesicle-containing profiles. The finding of the two types of degenerating terminal after lesion of the brachium conjunctivum can be considered as evidence of the coexistence of at least two kinds of cerebellar terminals in the superior colliculus. The presence of nigral and cerebellar terminals in the intermediate and deep layers of the superior colliculus implicates the involvement of the substantia nigra and cerebellum in control of collicular visuomotor function.