An analysis of the number and composition of the synaptic populations formed by photoreceptors of the fly

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Abstract
The photoreceptor terminals of newly eclosed female flies, Musca domestica, have been sampled in the first optic neuropile (or lamina) in one of two ways: first, in large number (n = 760) from single sections and second, from serial electron micrographs of the six terminals within each of three cartridges. Both sampling methods concur in assessing the number of synapses established with the two principal monopolar relay interneurons, L1 and L2, within each cartridge. Each receptor is calculated to be presynaptic at about 200 ± 40 (2 SE) synapses. This value considerably exceeds previous estimates, primarily because we took careful account of the appearance of synapses in different section planes. The number of these synapses correlates highly with the area of receptor terminal presynaptic membrane, so that each synapse is allotted, on average, about 1.6 μm2. The synapses are evenly graded in their distribution with an unexplained 23% decrease in both membrane perimeter and synapse number halfway along their receptor terminal's length. The numbers of synapses per receptor did not vary systematically within two horizontal (3 × 20 cartridge) strips of frontal, equatorial lamina sampled. Individual synapses are elongate tetrads (Burkhardt and Braitenberg, ′76) with two pairs of postsynaptic elements. The first pair is invariably contributed by the interneurons L1 and L2 (one each). The second pair comes either from the α processes of an amacrine cell or from a glial cell. In the distal lamina, however, L3 contributes one of the two postsynaptic processes, the second being αor glial. The overall ratio of postsynaptic involvement at distal synapses (α:glial:L3) is 55%, 20%, and 12% respectively, the remainder being unidentified.