Synaptic connections of intrinsic cells and basket arborizations in the external plexiform layer of the fly's eye

The first synaptic region of the fly's visual system contains sets of climbing fibres enclosing each optic cartridge in a double basket-like arrangement of processes. The two sets of processes, termed α and β, can be separately identified by their fine structure. αfibres are more electron dense than β; they are postsynaptic to receptor endings from the retina and contain synaptic specializations apposed to epithelial cell membranes, to β fibres and to the distal processes of the L4 monopolar neurone. β is postsynaptic to receptors and to α. It has a lighter cytoplasm and contains peculiar glial invaginations called ‘gnarls’. Degeneration experiments, where the optic chiasma between the 1st and 2nd synaptic regions has been severed, demonstrate that α fibres are intrinsic to the lamina. Golgi impregnation and electron microscopy of Golgi-impregnated cells have demonstrated that β fibres belong to a type of medulla-to-lamina cell, T1, whereas α fibres belong to an intrinsic (amacrine) cell in the lamina. The amacrine cells have wide fields through several cartridges. However, Golgi—EM studies show that each cartridge must be invaded by the processes of at least two cells. The synaptology of these elements is discussed with respect to previous data about the lamina, and certain analogies with the vertebrate plexiform layer are drawn.