Atwood H L, Govind C K, Wu C F
Department of Biology, University of Iowa, Iowa City.
J Neurobiol. 1993 Aug;24(8):1008-24. doi: 10.1002/neu.480240803.
The innervation of ventral longitudinal abdominal muscles (muscles 6, 7, 12, and 13) of third-instar Drosophila larvae was investigated with Nomarski, confocal, and electron microscopy to define the ultrastructural features of synapse-bearing terminals. As shown by previous workers, muscles 6 and 7 receive in most abdominal segments "Type I" endings, which are restricted in distribution and possess relatively prominent periodic terminal enlargements ("boutons"); whereas muscles 12 and 13 have in addition "Type II" terminals, which are more widely distributed and have smaller "boutons". Serial sectioning of the Type I innervation of muscles 6 and 7 showed that two axons with distinctive endings contribute to it. One axon (termed Axon 1) has somewhat larger boutons, containing numerous synapses and presynaptic dense bodies (putative active zones for transmitter release). This axon also has more numerous intraterminal mitochondria, and a profuse subsynaptic reticulum around or under the synaptic boutons. The second axon (Axon 2) provides somewhat smaller boutons, with fewer synapses and dense bodies per bouton, fewer intraterminal mitochondria, and less-developed subsynaptic reticulum. Both axons contain clear synaptic vesicles, with occasional large dense vesicles. Approximately 800 synapses are provided by Axon 1 to muscles 6 and 7, and approximately 250 synapses are provided by Axon 2. In muscles 12 and 13, endings with predominantly clear synaptic vesicles, generally similar to the Type I endings of muscles 6 and 7, were found, along with another type of ending containing predominantly dense-cored vesicles, with small clusters of clear synaptic vesicles. This second type of ending was found most frequently in muscle 12, and probably corresponds to a subset of the "Type II" endings seen in the light microscope. Type I endings are thought to generate the 'fast' and 'slow' junctional potentials seen in electrophysiological recordings, whereas the physiological actions of Type II endings are presently not known.
利用诺马斯基显微镜、共聚焦显微镜和电子显微镜对三龄果蝇幼虫腹侧纵腹肌（肌肉6、7、12和13）的神经支配进行了研究，以确定含突触终末的超微结构特征。如先前研究人员所示，在大多数腹节中，肌肉6和7接受“ I型”终末，其分布受限且具有相对突出的周期性终末膨大（“突触小体”）；而肌肉12和13还具有“ II型”终末，其分布更广泛且“突触小体”较小。对肌肉6和7的I型神经支配进行连续切片显示，有两条具有独特终末的轴突参与其中。一条轴突（称为轴突1）的突触小体稍大，含有大量突触和突触前致密体（推测为递质释放的活性区）。该轴突还具有更多的终末内线粒体，并且在突触小体周围或下方有丰富的突触下网状结构。第二条轴突（轴突2）提供稍小的突触小体，每个突触小体的突触和致密体较少，终末内线粒体较少，突触下网状结构发育较差。两条轴突均含有清亮突触小泡，偶尔有大的致密小泡。轴突1向肌肉6和7提供约800个突触，轴突2提供约250个突触。在肌肉12和13中，发现了主要含清亮突触小泡的终末，通常类似于肌肉6和7的I型终末，同时还发现了另一种主要含致密核心小泡的终末，伴有小簇清亮突触小泡。第二种终末在肌肉12中最常见，可能对应于光学显微镜下所见的“ II型”终末的一个子集。I型终末被认为产生电生理记录中所见的“快”和“慢”接头电位，而II型终末的生理作用目前尚不清楚。
