Ultrastructure of nerve plexus in flatworms. II. Sites of synaptic interactions.

Synaptic components from the peripheral nervous system of the polyclad flatworm. Notoplana acticola, are described from electron microscopic observations. Quasi-neuropile, defined as clusters of neurites containing synaptic vesicles, occurs as scattered islands among the peripheral nerve cords of the plexus. Some neurite clusters only contain one type of synaptic vesicle but others are mixed. The most usual synaptic configuration consists of a single presynaptic element and a pair of postsynaptic neurites sharing a common synaptic cleft. These synapses are polarized and contain clear, 420 A vesicles. GABA-type synapses are also found. At least two kinds of solid-core vesicles also occur.