Ultrastructure of synapses in the central nervous system of lamellibranch molluscs

Fine-structural characteristics of synaptic contacts were investigated in the central nervous system of different species of lamellibranch molluscs. Neuropile of the ganglia is characterized by regular occurrence of ultrastructurally well-defined polarized chemical synapses resembling those described in other invertebrate species and vertebrates. In addition to the generally observed membrane thickenings, enhanced density of synaptic membranes, cleft material and vesicle clustering on the presynaptic membrane, synapses are occasionally characterized by other and pinocytotic invaginations. Synaptic connections were distinguished on the basis of the vesicle content of the presynaptic terminal. Different forms of synaptic configurations (divergence, convergence, presynaptic modification) were observed in the ganglia.