Harris K M, Stevens J K
Department of Neurology, Children's Hospital, Boston, Massachusetts 02115.
J Neurosci. 1988 Dec;8(12):4455-69. doi: 10.1523/JNEUROSCI.08-12-04455.1988.
We have used serial electron microscopy and 3-dimensional reconstructions of dendritic spines from Purkinje spiny branchlets of normal adult rats to evaluate 2 questions about the relationship of spine geometry to synaptic efficacy. First, do relationships between spine geometry and other anatomical indicators of synaptic activity suggest that spine size and shape might be associated with synaptic efficacy? Reconstructed spines were graphically edited into head and neck compartments; the area of the postsynaptic density (PSD) was measured; the volume of spine smooth endoplasmic reticulum (SER) was computed; and all of the vesicles in the axonal varicosities were counted. Spine head volume and the volume of SER contained in the head are well correlated with the area of the PSD and the number of vesicles in the presynaptic axonal varicosity. Spine neck diameter does not fluctuate with PSD area, head volume, or the vesicle number. These results suggest that the dimensions of the spine head, but not of the spine neck, are likely to reflect differences in synaptic efficacy. Second, does the geometry of cerebellar spine necks reduce the transfer of synaptic charge to the recipient dendrite from the theoretical maximum that could be transferred if the synapse were on a dendritic shaft? Comparison of volume to surface area showed that the spine heads are approximately spherical and the necks are approximately cylindrical. Application of results from a biophysical model that assumed these geometrical shapes for spines (Wilson, 1984) showed that the cerebellar spine necks are unlikely to reduce transfer of synaptic charge by more than 5-20% even if their SER were to completely block passage of current through the portion of the neck that it occupies. We suggest that the constricted spine neck diameter might serve to isolate metabolic events in the vicinity of activated synapses by reducing diffusion to neighboring synapses, without significantly influencing the transfer of synaptic charge to the postsynaptic dendrite.
我们利用连续电子显微镜技术和对正常成年大鼠浦肯野棘状小分支上树突棘的三维重建，来评估关于棘状结构与突触效能关系的两个问题。第一，棘状结构与突触活动的其他解剖学指标之间的关系是否表明棘的大小和形状可能与突触效能相关？将重建的棘状结构图形编辑为头部和颈部区域；测量突触后致密区（PSD）的面积；计算棘状平滑内质网（SER）的体积；并对轴突膨体中的所有囊泡进行计数。棘头体积和头部所含SER的体积与PSD面积以及突触前轴突膨体中的囊泡数量密切相关。棘颈直径不随PSD面积、头部体积或囊泡数量而波动。这些结果表明，棘头的尺寸而非棘颈的尺寸可能反映突触效能的差异。第二，如果突触位于树突干上，小脑棘颈的结构是否会使突触电荷向接受树突的传递比理论上的最大传递量减少？体积与表面积的比较表明，棘头近似球形，棘颈近似圆柱形。应用一个假设棘具有这些几何形状的生物物理模型（Wilson，1984）的结果表明，即使小脑棘颈的SER完全阻断电流通过其占据的颈部部分，也不太可能使突触电荷传递减少超过5 - 20%。我们认为，狭窄的棘颈直径可能通过减少向相邻突触的扩散来隔离激活突触附近的代谢事件，而不会显著影响突触电荷向突触后树突的传递。
