Cellular and synaptic organization in the lamina of the dragon-fly Sympetrum rubicundulum

Abstract

The cellular and synaptic organization within the cartridges of the lamina of the dragon-fly have been analysed by light- and electron-microscopy. The ommatidium contains eight retinular (photoreceptor) cells all of which project to a single cartridge in the lamina. Six of the retinular cells terminate within the cartridges of the lamina (retinular terminals R 1-5 and R 8) and two pass through to the medulla (long visual fibres R 6 and R 7). The retinular terminals are arranged in pairs, which are matched in size, position and length. One pair terminates in the distal portion of the cartridge, while the remaining two pairs terminate at its proximal border. The cartridge also includes two large and two small axons of monopolar cells (M I, M II and M III, M IV respectively) and several unidentified minor profiles. The spatial arrangement of the main elements within a cartridge is constant through most of the depth of the lamina. All synapses within the cartridges contain similar elongate presynaptic specializations and have either a dyadic or triadic arrangement of post-synaptic elements. Analyses of serial sections reveal three principal types of synaptic configurations: A is a triadic synapse in which retinular cells (R 1-5 and R 8) are presynaptic to monopolar elements, usually with M II as the central postsynaptic process and M I as the lateral processes; B, found only in the proximal lamina, is a dyadic feedback synapse from M I upon one pair of retinular terminals (R 5 and R 8); and C is a triadic synapse in which the long visual fibre (R 7) is presynaptic to M II, the central postsynaptic element. Thus each cell type in the lamina is involved in specific synaptic connections and this results in different synaptic relationships for the two large output axons of the lamina, M I and M II.