Identification under the electron microscope of climbing fibers and their synaptic contacts

Summary An attempt is made to identify, under the electron microscope, the climbing fibers of the cerebellum (in the cat) and their synaptic contacts with Purkinje cells and other cortical neurons. — Two kinds of axonal profiles, having synaptic contacts with primary and secondary dendrites of Purkinje neurons, can be recognized: One being terminal fibers densely packed with neurofilaments, having mainly contacts de passage with the dendrite surface, with small accumulations of synaptic vesicles at the presynaptic side of the contact. The others are rather knob-shaped contacts filled with synaptic vesicles and poor in neurofilaments. In chronically isolated folia, in which only local neurons and their processes have survived, all filamentous profiles have disappeared while vesicular ones are not appreciably reduced in number. It is inferred from this, that the neurofilamentous profiles correspond to climbing fibers, whereas the vesicular ones could be the endings of outer stellate axons, recurrent Purkinje axon collaterals, or ascending basket axon collaterals. — Similar two kinds of axon-terminal profiles are found in synaptic contact with Golgi and basket cell bodies. As in chronically isolated folia only the vesicular profiles survive, it is inferred that the climbing fiber has axo-somatic terminals on Golgi cells and basket cells as well. Previous information of this kind, gained with the light microscope and with degeneration studies, is thus substantiated with the aid of the electron microscope. The vesicular presynaptic profiles on Golgi and basket neurons are in the first case certainly and in the second with high probability endings of recurrent Purkinje axon collaterals. — The few axosomatic synapses found on outer stellate neurons may also be terminals of climbing fibers, but degeneration evidence for this is not conclusive. — The observations are summarized and evaluated from the functional point of view in a diagram, with consideration to recent physiological information on the function of climbing fibers.