The synaptology of the L4 neuron in the first visual ganglion of the fly, musca domestica

Abstract

Results of optomotor experiments on Musca and Drosophila led to the formulation of a conceptual model which allows the prediction of the insect's response to defined moving stimuli. A basic requirement of the model is that there should exist in the ganglion chain, between the compound eye and the motor output, cross-connections between the channels of information flow in the ganglia. Further optomotor experiments provided evidence about the directions within the ganglion of the cross-connections. The first visual ganglion of Musca is built of sub-units called cartridges, each of which is composed of six retinular cell axons synapsing onto five different neurons; some of which are second order neurons, some of higher order. One of these five neurons, “L4" has two long collaterals at the base of the lamina, which run from the cartridge to the two neighbouring cartridges in the hexagonal array. This configuration is compatible with that suggested by the optomotor experiments for the cross-connections postulated by the conceptual model. Thus it was of interest to examine the synaptic connections made by the terminations or "endfeet" of these collaterals. This study reports the results of examination, by electron microscopy, of serial ultra-thin sections of the first visual ganglion containing L4 collaterals. The results show that the three collaterals which terminate in each cartridge (two long collaterals from neighbouring cartridges plus one short one from the L4 within the cartridge) do so in intimate contact with each other and with the two second-order neurons LI and L2. The three collaterals are presynaptic to each other, each to the other two. Each of them is also presynaptic to the L2 neuron in that cartridge, and it seems that each synaptic contact between two members of the three collaterals is also simultaneously a contact with L2. No contacts are made to the LI fibre, in contrast to the systematic pattern of contacts to the L2 fibre. In addition to this basic scheme of connections the collaterals are also presynaptic to some of the retinular cell axons, usually R3 and R5, apparently in cartridges from the more central parts of the ganglion, or there may be a gradient of connectivity across the ganglion. Since no studies, apart from the anatomical ones, exist on this network, the conclusions drawn about the significance of these results must be tentative. The results are not incompatible with the idea that these collaterals could be the cross-connections postulated in the conceptual model. One function possibly mediated by the network could be lateral inhibition, which has been shown to occur in Musca.