Structure of the piriform cortex of the opossum. III. Ultrastructural characterization of synaptic terminals of association and olfactory bulb afferent fibers

Terminals of olfactory bulb afferent (OB) and association (ASSN) fiberswithin the piriform cortex were characterized ultrastructurally. Identification was by electron microscopic (EM) autoradiography following injections of tritiated amino acids into the olfactory bulb and anterior piriform cortex. The results show that terminals of both fiber systems contain round vesicles and make asymmetrical synaptic contacts predominantly onto dendriticspines. Profiles with pleomorphic vesicles do not appear to be labeled from either site. Since there is strong evidence that both fiber systems generate excitatory postsynaptic potentials (EPSPs) in pyramidal cells, these results provide additional examples in the mammalian CNS of terminals with round vesicles and asymmetrical contacts that mediate an excitatory effect.

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