Heterogeneity of synaptic vesicles in the squid giant fibre system

Four synaptic contacts in the squid giant fibre system (from the afferent boutons on the first order giant cell in the brain through the giant synapses in the palliovisceral lobe and the stellate ganglion), as well as neuromuscular junctions in the fin, contain populations of agranular (electron transparent) synaptic vesicles. A classification of these vesicle populations is possible on the basis of the average vesicle diameter and the reactivity with zinc iodide-osmium (ZIO). We distinguish 4 synapses with significantly different average vesicle sizes, 3 synapses with ZIO-positive vesicles and two synapses with ZIO-negative vesicles. In two consecutive links of the giant fibre system there are always structurally different synaptic contacts.