Hillman D E, Chen S
Brain Res. 1984 Mar 19;295(2):325-43. doi: 10.1016/0006-8993(84)90981-8.
Plasticity in the size of postsynaptic membrane specializations (postsynaptic densities) was analyzed by quantitation following lesioning of parallel fiber afferent axons to Purkinje cells in the cerebellum. Double sectioning of parallel fibers in the same folium as well as longitudinal undercutting of the molecular layer to destroy granule cells and their parallel fibers were used to produce various levels of afferent reduction to Purkinje cells. The length of profiles of membrane densities was measured utilizing semi-automated, computer-electron microscopy and the number of synapses was determined from their volume density and changes in volume estimated from cortical thickness. Correlation between the number of synapses on Purkinje cells and their average contacts area revealed a reciprocal relationship throughout a range of 0-67% reductions in parallel fiber synapses. Larger reduction levels had a progressive decrease in average size of contacted postsynaptic densities and an accompanying increase in the number of vacant postsynaptic specializations. Total absence of parallel fibers resulted in nearly all vacant sites on spines (except for a few connections with boutons having irregular shaped vesicles). These vacant sites were, on the average, only half the size of controls but their number was approximately double the control amount. This study confirms that location and size of synapses are not permanent and that plasticity in size of contacts allows reorganization in circuitry to compensate alterations in the number of inputs following a number of perturbations. The finding of a reciprocal relationship indicates that total contact area on Purkinje cells remained relatively constant throughout the entire range of reductions in the number of afferents. A 'constancy principle for total postsynaptic contact area' is envisioned to stabilize functional aspects of circuitry during developmental organization and to direct compensation following reduction in pre- or postsynaptic neurons from environmental effects or attrition in aging. Constancy in target area provides a realm under which the size of individual synapses can be modified as functional adaptations in circuitry even without changes in the number of connections.
通过对小脑浦肯野细胞平行纤维传入轴突损伤后的定量分析，研究了突触后膜特化结构（突触后致密区）大小的可塑性。采用在同一小叶内对平行纤维进行双切以及对分子层进行纵向侧切以破坏颗粒细胞及其平行纤维的方法，对浦肯野细胞产生不同程度的传入减少。利用半自动计算机电子显微镜测量膜致密区轮廓的长度，并根据其体积密度和由皮质厚度估算的体积变化确定突触数量。浦肯野细胞上突触数量与其平均接触面积之间的相关性表明，在平行纤维突触减少0 - 67%的范围内，二者呈反比关系。更大程度的减少会导致接触的突触后致密区平均大小逐渐减小，同时空的突触后特化结构数量增加。完全没有平行纤维会导致几乎所有棘突上出现空位（除了少数与具有不规则形状囊泡的终扣的连接）。这些空位平均大小仅为对照的一半，但其数量约为对照数量的两倍。本研究证实突触的位置和大小并非固定不变，接触大小的可塑性允许神经回路进行重组，以补偿在多种扰动后输入数量的改变。反比关系的发现表明，在传入数量减少的整个范围内，浦肯野细胞上的总接触面积保持相对恒定。设想了一个“突触后总接触面积恒定原则”，以在发育组织过程中稳定神经回路的功能方面，并在由于环境影响或衰老损耗导致突触前或突触后神经元减少后指导补偿。目标区域的恒定提供了一个范围，在此范围内，即使连接数量不变，单个突触的大小也可作为神经回路中的功能适应而被改变。
