Fröhlich A, Meinertzhagen I A
J Neurobiol. 1987 Jul;18(4):343-57. doi: 10.1002/neu.480180403.
At the anterior rim of the first optic neuropile, or lamina, of the housefly's (Musca domestica) compound eye, the terminals of photoreceptors (R) innervate postsynaptic neurons in variable numbers to provide a continuous range of natural hypo- and hyperinnervations. Frequencies of photoreceptor synapses have been measured from quantitative electron microscopy on single sections of the lamina's unit synaptic modules, called cartridges. These are normally innervated by six photoreceptor terminals (6R cartridges). At the lamina's edge hypoinnervated cartridges (2R-5R) are found, whereas hyperinnervated cartridges (7R, 8R) are located at the equator between dorsal and ventral eye halves. In 2R cartridges each presynaptic terminal forms up to 1.5 times the normal, 6R cartridge number of synapses, thereby offsetting the reduced number of terminals and partially conserving the input upon the postsynaptic neurons. Thus the terminals have a reserve synaptogenic capacity never normally revealed. By comparison, terminals in 8R cartridges form about the same numbers of synapses as in "normal" eye regions, so that their postsynaptic neurons have a synaptic input increased by the extra number of terminals. The number of synapses formed between input terminals and target neurons is therefore not fixed but changes as a function of the total receptor terminal complement. The size of a photoreceptor terminal covaries to a certain extent with the number of its presynaptic sites; the spacing density of presynaptic sites over the terminals' surface in a 2R cartridge compared with an 8R cartridge increases far less (only 17%) than the increase in the number of sites (43%). The pair of postsynaptic cell interneurons in each 2R cartridge also shows a decrease in axonal diameter compared with those in 8R cartridges. Thus both the pre- and postsynaptic cells show size changes correlated with changes in their synaptic engagement.
在家蝇（Musca domestica）复眼的第一个视神经节层（即视叶）的前缘，光感受器（R）的终末以不同数量支配突触后神经元，从而提供一系列连续的自然低支配和高支配情况。通过对视叶单位突触模块（称为小眼体）的单切片进行定量电子显微镜观察，测量了光感受器突触的频率。这些小眼体通常由六个光感受器终末支配（6R小眼体）。在视叶边缘发现了低支配的小眼体（2R - 5R），而高支配的小眼体（7R、8R）位于背侧和腹侧眼半部之间的赤道处。在2R小眼体中，每个突触前终末形成的突触数量最多可达正常6R小眼体突触数量的1.5倍，从而抵消了终末数量的减少，并部分保留了对突触后神经元的输入。因此，终末具有一种正常情况下从未显示出的储备突触形成能力。相比之下，8R小眼体中的终末形成的突触数量与“正常”眼区的大致相同，因此它们的突触后神经元通过额外的终末数量增加了突触输入。因此，输入终末与靶神经元之间形成的突触数量不是固定的，而是随总的感受器终末数量而变化。光感受器终末的大小在一定程度上与其突触前位点的数量相关；与8R小眼体相比，2R小眼体中突触前位点在终末表面的间距密度增加远小于位点数量的增加（仅17%，而位点数量增加了43%）。与8R小眼体中的相比，每个2R小眼体中的一对突触后细胞中间神经元的轴突直径也减小。因此，突触前和突触后细胞都表现出与它们的突触连接变化相关的大小变化。
