Structure of synapes in the supraoesophageal ganglion of the water beetle (Dytiscus marginalis).
1. Light- and electron microscopic investigations prove that synapses, without any exception, are confined to the neuropil. 2. Under the light-microscope, synapses display the shape of terminal boutons at the ends of the nerve fibres; in some cases they appear as smaller or larger plates. Electron microscopic investigations suggest that also varicosities of nerve fibres can be regarded as synapses, though these might have possible arisen from axoplasmatic peristalsis. 3. Electron microscopically the overwhelming majority of the synapses are axodendritic contacts; axo-axonic contacts occur less often. 4. The generally accepted characteristics of synapses are defective. Membrane thickenings and intersynaptic spaces are missing. Accordingly, synapses in the supraoesophageal ganglion of the water beetle differ markedly from those described in Vertebrates. 5. Synaptic vesicles sometimes fill the axoplasm of the nerve fibre completely. In other cases, clusters of synaptic vesicles can be seen, on both sides of the contact. 6. Synaptic vesicles are mixed with neurosecretory granules. Synaptic vesicles may appear also in the dendrites.
D000818:Animals; D001369:Axons / Q000648:ultrastructure; D001517:Coleoptera / Q000648:ultrastructure*; D003712:Dendrites / Q000648:ultrastructure; D004947:Esophagus / Q000294:innervation; D005724:Ganglia / Q000648:ultrastructure; D013569:Synapses / Q000648:ultrastructure*; D013572:Synaptic Vesicles / Q000648:ultrastructure