Gap junctions coupling photoreceptor axons in the first optic ganglion of the fly.

The first optic ganglion of the fly, the lamina ganglionaris, was investigated with the transmission electron microscope for the purpose of demonstrating possible electronic junctions. Within a cartridge, the six short receptor cell axons R1--R6 are extensively coupled by symmetrical gap junctions. This is mainly seen in the distal third of the first synaptic region where none or only a few lateral branches of the centrally lying L-fibres (L1, L2) penetrate the ring of six short receptor fibre endings. Gap junctions as found between R1--R6 are distinguished morphologically from chemically-mediated synapses by the closely apposed cell membranes. They exhibit only a 2--4 nm extracellular cleft. Unlike the chemical synapse the gap junction in the neuropile of the fly appears structurally symmetrical. No such gap junctions are found either between R-fibres and glial cells, interneurons and glial cells, between glial cells and between interneurones themselves, nor between the parallel long receptor axons R7/8, which bypass the lamina outside the cartridge. In accordance with electrophysiological data, it can now be argued that the six short receptor axons R1--R6 are electrically interconnected by symmetrical gap junctions.