title: Gap junctions coupling photoreceptor axons in the first optic ganglion of the fly.
abstract: 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.
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology / Q000648:ultrastructure*; D005260:Female; D006793:Houseflies / Q000033:anatomy & histology*; D007365:Intercellular Junctions / Q000502:physiology / Q000648:ultrastructure*; D008297:Male; D010786:Photoreceptor Cells / Q000502:physiology / Q000648:ultrastructure*