Pierce J P, Mendell L M
Department of Neurobiology and Behavior, State University of New York at Stony Brook 11794.
J Neurosci. 1993 Nov;13(11):4748-63. doi: 10.1523/JNEUROSCI.13-11-04748.1993.
Three physiologically characterized spindle (group Ia) afferents were labeled by the intracellular injection of HRP and were processed for light-level reconstruction. Thirty-five boutons in the ventral horn were then selected for analysis. They were serially thin sectioned and characterized in terms of volume, total surface area and the surface area of apposition to postsynaptic neurons (apposed surface area), mitochondrial volume, vesicle and active zone features, relation to presynaptic contacts, postsynaptic profile size, and position within the terminal arbor. Virtually all of these characteristics were widely variable, both within the entire population and in the endings of a single fiber. Apposed surface area, mitochondrial volume, vesicle number, active zone vesicle number, active zone number, and total active zone area were highly correlated in a positive linear manner with bouton volume. This suggests a type of ultrastructural "size principle," in which the morphological features associated with synaptic release scale directly in proportion to bouton size. This pattern also extends to local circuit interactions: the extent of an Ia bouton's input from axoaxonal contacts (86% receive at least one axoaxonal contact) was directly proportional to its size. In addition, the characteristics of an Ia bouton were related to its position on the postsynaptic element and within the terminal arbor. Vesicle density, percentage mitochondrial volume, and active zone size increased as the postsynaptic process decreased in size, while volume, apposed surface area, active zone number and area, and vesicle number all decreased as one moved downstream within a terminal branch, with the exception of the terminal bouton. Vesicle density also decreased as one moved away from the dorsal root entry zone.
通过细胞内注射辣根过氧化物酶（HRP）标记了三根具有生理特征的梭内肌（Ia类）传入纤维，并对其进行光镜水平的重建处理。然后选择腹角中的35个终扣进行分析。对它们进行连续超薄切片，并从体积、总表面积、与突触后神经元的贴合表面积（贴附表面积）、线粒体体积、囊泡和活性区特征、与突触前接触的关系、突触后轮廓大小以及在终末分支内的位置等方面进行表征。实际上，所有这些特征在整个群体以及单根纤维的终末中都存在很大差异。贴附表面积、线粒体体积、囊泡数量、活性区囊泡数量、活性区数量以及总活性区面积与终扣体积呈高度正线性相关。这表明存在一种超微结构的“大小原则”，即与突触释放相关的形态特征直接与终扣大小成比例。这种模式也延伸到局部回路相互作用：Ia终扣从轴-轴突触接触获得的输入程度（86%至少接受一个轴-轴突触接触）与其大小直接成正比。此外，Ia终扣的特征与其在突触后元件上以及终末分支内的位置有关。随着突触后突起尺寸减小，囊泡密度、线粒体体积百分比和活性区大小增加，而随着在终末分支内向下游移动，体积、贴附表面积、活性区数量和面积以及囊泡数量均下降，但终末终扣除外。随着远离背根进入区，囊泡密度也降低。
