Serial and triadic synapses in the cerebellar nuclei of the cat.

A quantitative electron microscopic investigation of the nucleus interpositus in cat cerebellum reveals that about 1.5% of all observed synapses are established between synaptic vescile-bearing profiles. It is shown by serial sections that 70% of these synaptic complexes are triadic arrangements and 30% are serial synapses. Further analysis discloses that the first presynaptic element in the triadic and serial synapses may be one of four different axonal types: (A) Purkinje-cell axons; (B) and (C) afferent fibers containing large round vesicles and originating from the brain stem (probably mossy and climbing fiber collaterals); and (D) axon terminals containing small round vesicles. Indirect evidence suggests that type D profiles are the recurrent axon collaterals of the projective neurons. The second, postsynaptic and presynaptic, vesicle-bearing process in these complexes is either a class D terminal, or a somewhat more "dendrite-like" profile (Class E) containing flattened vesicles, and identified as belonging to processes of local Golgi type II interneurons.