Nicol D, Meinertzhagen I A
J Comp Neurol. 1982 May 1;207(1):29-44. doi: 10.1002/cne.902070104.
The photoreceptors terminals of newly enclosed 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 micrometer2. 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 X 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 alpha 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 alpha or glial. The overall ratio of postsynaptic involvement at distal synapses (alpha: glial: L3) is 55%, 20%, and 12% respectively, the remainder being unidentified.
新羽化的雌性家蝇的光感受器终末，已通过以下两种方式之一在第一个视神经纤维层（即板层）中进行取样：第一，从单个切片中大量取样（n = 760）；第二，从三个暗盒中每个暗盒内六个终末的系列电子显微照片中取样。两种取样方法在评估每个暗盒内与两个主要单极中继中间神经元L1和L2建立的突触数量上是一致的。经计算，每个感受器在约200±40（2个标准误）个突触处作为突触前成分。这个值大大超过了先前的估计，主要是因为我们仔细考虑了不同切片平面中突触的外观。这些突触的数量与感受器终末突触前膜的面积高度相关，因此平均每个突触分配到约1.6平方微米的面积。突触在其分布上是均匀分级的，在感受器终末长度的一半处，膜周长和突触数量均出现无法解释的23%的减少。在取样的额叶赤道板层的两个水平（3×20个暗盒）条带内，每个感受器的突触数量没有系统性变化。单个突触是细长的四联突触（伯克哈特和布赖滕贝格，1976年），有两对突触后成分。第一对总是由中间神经元L1和L2（各一个）提供。第二对要么来自无长突细胞的α突起，要么来自神经胶质细胞。然而，在远端板层，L3提供两个突触后过程中的一个，另一个是α突起或神经胶质细胞。远端突触处突触后成分参与的总体比例分别为55%、20%和12%，其余成分未明确。
