Direct connections between the R7/8 and R1?6 photoreceptor subsystems in the dipteran visual system

Abstract

Musca and related flies have three main photoreceptor subsystems. The R1-6 group has short axons that terminate in the cartridges of the first optic neuropile, the lamina. The cartridges are bypassed by the longer axons of R7 and R8, which run together to terminate at different levels in the underlying medulla neuropile. The present account describes a shallow, previously unidentified zone in the lamina within which R7/8 make glancing contact with R1-6. At the distal border of the cartridge over no more than 3-4 microns depth, the tangentially directed short axon of R6 squeezes between the pair from R7 and R8, forming quite large areas of mutual contact (approximately 7 microns2). Less frequently, R1 is contacted. At least some of these sites contain smaller membrane specialisations indistinguishable from the more numerous gap junctions found more proximally that interconnect the terminals of R1-6. The R7/8 junctions with R6 are of comparable size (0.15 micron 2) and likewise possess symmetrical membrane densities. They provide proposed pathways for direct electrical interaction to account for observed electrical input from R7/8 to the R1-6 subsystem. In two cases R7/8 was possibly postsynaptic to R1-6 at a multiple-contact synapse, but even if functional, these sites were so rare that they are unlikely to have much operational significance.

Cell recognition during synaptogenesis is revealed after temperature‐shock‐induced perturbations in the developing fly's optic lamina

“…Distinction between R7 and R8 was based on a difference in the pattern in distal and proximal retina compared with the normal retina. Because the axons of R7 and R8 usually form a pair of profiles that courses alongside a trachea (Shaw et al, 1989), forming a characteristic triplet pattern, corroboration of these results could be also found in the lamina, where the R7/R8/trachea triplet was reduced to either a single axon profile accompanying a trachea or to a trachea alone.…”

Neurotransmitter regulation of circadian structural changes in the fly's visual system

“…1B). Two central cells (R7 and R8) have axons that bypass the lamina (Braitenberg, 1967;Shaw et al, 1989), and terminate in the second neuropile, or medulla. Neurites of 15 classes of neuron are associated with each lamina cartridge (Shaw, 1981;Strausfeld and Campos-Ortega, 1977), which are broadly considered within five groups: (1) synaptic terminals of R1-R6; (2) axons of R7,R8; (3) the axons and their dendritic spines of monopolar cells L1-L5, three of which (L1-L3) are the cartridge's chief output neurons; (4) the terminals of three cells with somata in the medulla: C2, C3 and T1; and (5) arborisations from widefield amacrine and tangential neurons (Fig.…”

Electrophysiological Method for Recording Intracellular Voltage Responses of <em>Drosophila</em> Photoreceptors and Interneurons to Light Stimuli <em>In Vivo</em>

“…Furthermore, there are strong synaptic connections from Am to R1, R2, R4 and R5, and glia cells are also synaptically connected to the network and may thus participate in neural image processing 6 . Finally, axonal gap-junctions, linking neighboring R1-R6 and between R6 and R7/R8 photoreceptors in the lamina, contribute to the asymmetric information representation and processing in each cartridge 14,20,28 . Intracellular voltage recordings from individual photoreceptors and visual interneurons in nearly intact Drosophila provide high signal-to-noise ratio data at sub-millisecond resolution 3,5,[7][8][9][10]29 -100 msec resolution.…”