Landis D M
Department of Neurology, Case Western Reserve University, Cleveland, Ohio 44106.
J Electron Microsc Tech. 1988 Oct;10(2):129-51. doi: 10.1002/jemt.1060100203.
Application of rapid freezing, freeze substitution fixation, and freeze fracture techniques to the study of synaptic junctions in the mammalian central nervous system has revealed new aspects of synaptic structure that are consistent with and partially explicate advances in synaptic biochemistry and physiology. In the axoplasm adjacent to the presynaptic active zone, synaptic vesicles are linked to large spectrin-like filamentous proteins by shorter proteins that resemble synapsin I in morphology. This mesh of presynaptic filamentous proteins serves to concentrate synaptic vesicles in the vicinity of the active zone. The affinity with which the vesicles are bound by the mesh is probably modulated by the extent of phosphorylation at specific sites on the constituent filamentous proteins, and changes in the binding affinity result in changes in transmitter release. The structural organization of the postsynaptic density in Purkinje cell dendritic spines consists of very fine strands with adherent, heterogeneous globular proteins. Some of these globular proteins probably correspond to protein kinases and their substrates. The postsynaptic density, positioned at the site of the maximal depolarization caused by synaptic currents, apparently serves as a supporting framework for a variety of proteins, which respond to and transduce postsynaptic depolarization. At least two classes of filamentous protein fill the cytoplasm of spines with a complex mesh, which presumably contributes to maintenance of the spine shape. Membrane bound cisterns are a ubiquitous feature of Purkinje cell dendritic spines. Studies of rapidly frozen tissue with electron probe microanalysis and elemental imaging reveal that these cisterns take up and sequester calcium, which is derived from the extracellular space, and which probably enters the spine as part of the synaptic current.
将快速冷冻、冷冻置换固定和冷冻断裂技术应用于哺乳动物中枢神经系统突触连接的研究，揭示了突触结构的新方面，这些方面与突触生物化学和生理学的进展一致，并部分解释了这些进展。在突触前活性区附近的轴浆中，突触小泡通过形态上类似于突触结合蛋白I的较短蛋白质与大的血影蛋白样丝状蛋白质相连。这种突触前丝状蛋白质网络有助于将突触小泡集中在活性区附近。小泡与网络结合的亲和力可能受组成丝状蛋白质上特定位点的磷酸化程度调节，结合亲和力的变化导致递质释放的变化。浦肯野细胞树突棘中突触后致密物的结构组织由带有附着的、异质球状蛋白质的非常细的链组成。这些球状蛋白质中的一些可能对应于蛋白激酶及其底物。位于突触电流引起的最大去极化部位的突触后致密物显然作为多种蛋白质的支撑框架，这些蛋白质对突触后去极化作出反应并进行转导。至少两类丝状蛋白质以复杂的网络填充棘突的细胞质，这可能有助于维持棘突的形状。膜结合池是浦肯野细胞树突棘的普遍特征。用电子探针微分析和元素成像对快速冷冻组织的研究表明，这些池摄取并隔离钙，钙来自细胞外空间，并且可能作为突触电流的一部分进入棘突。
