Synaptic organization of the Purkinje cells in the cerebellum of the cat

Summary An attempt at distinction between excitatory and inhibitory synapses is made in the cat cerebellum. The former are assumed to contain spheroid vesicles (S-type) of average diameter of 500 Å, while the latter flattened vesicles (F-type) of smaller size than the former. The elongation index (the ratio of the length of major versus minor axis of the vesicles) of S-type synaptic vesicles was about 1.2, while that of the F-type was more than 1.7. Parallel fibers of granule cells make S-type synaptic contacts (en-passant type or crossing-over synapse) mostly on the spines of the smaller branchlets of Purkinje cells. Climbing fibers make also S-type synapses on the smaller spines with short necks of the larger dendrites of Purkinje cells, but not frequently on the direct surface of them. It must be emphasized that almost no F-type synapse has been recognized which makes synaptic contacts directly on the spine of any type. It makes synaptic contacts usually on the direct surface of dendrites of Purkinje cells. Basket cell axons embrace directly the somas of the Purkinje cells. Their synaptic contacts were always of F-type and of en-passant character.The hypothesis is proposed that excitatory (E-type) synapses can be identified with synapses of S-type, whereas inhibitory (I-type) synapses would correspond to the F-type terminals.