The L4 monopolar neurone: a substrate for lateral interaction in the visual system of the fly musca domestica (L)

Optic cartridges of the first synaptic region of the fly's visual system are interconnected by a set of collaterals derived from the L4 monopolar cells. These neurones are the only efferent monopolar cells to contact elements in more than one cartridge. Electron microscopy of Golgi-impregnated cells has shown that sets of distal (outer) processes of L4 contact, and are postsynaptic to, the α profiles of lamina intrinsic cells at single cartridges. L4's proximal (inner) collaterals, situated at the inner face of the lamina, contact the receptor terminals and the radial monopolar cells (L1 and L2) of its own and of two adjacent cartridges. One L4 meets components of 6 other L4 cells of 6 surrounding cartridges. Conventionally fixed material shows that the proximal collaterals of L4 are presynaptic to radial monopolar cells and some receptor endings as well as being pre-and postsynaptic to the other L4 neuronal components to which they extend, or which extend to them. The numbers of the distal processes postsynaptic to α fibres vary across the eye. There is a graded increase from back to front and from the dorsal and ventral poles of the eye towards its equator.