The distribution of axon terminals with flattened vesicles in the nuclei of the amygdaloid body of the cat.

The morphology of synapses in the amygdaloid nuclei was studied in 10 cats. On the basis of the percentage of axon terminals with flattened vesicles (F-type) nuclei were distinguished, in which these terminals are as sparsely distributed as in most areas of the central nervous system, from other nuclei in which they are abundant (about one-third to one-half of all synaptic boutons). The lateral, basal dorsal and basal ventral nuclei belong to the first, the medial and central nucleus and the anterior amygdaloid area--to the second group. The cortical nucleus, which generally has a small number of boutons of F-type has some parts seemingly belonging to the first, and others to the second group. In all amygdaloid nuclei axon terminals of F-type form symmetrical synaptic contacts. In nuclei with a low percentage of F-type terminals these boutons are predominantly small and synapse either with perikarya or with large dendrites. The amygdaloid nuclei having numerous F-type terminals contain not only small but also larger terminals with flattened vesicles. Both, the larger and smaller axon terminals form in these nuclei synaptic contacts with various parts of dendrites even with very small ones and with dendritic spines. The subdivision of amygdala into two parts, one with a low and another with a high number of F-type boutons would seem to support the hypothesis that amygdala may be subdivided physiologically into a dorsomedial--"excitatory" and basolateral--"inhibitory" portion.