The current model does not account adequately for the characteristics of miniature endplate currents (MEPCs). We do not understand their relatively slow rise, the shape of their rise, their variable and sometimes prolonged decay, and the correlation between amplitude and decay time. If we assume that ACh is released from the vesicle through a pore and that the vesicle enlarges as it takes on additional transmitter, the predictions are more like MEPCs. However, previous measurements showed that after quantal size was increased the vesicles in the terminal were not enlarged. This need not be a problem, because some of the ACh is added to vesicles positioned at the active zones, a process known as second-stage loading. By using the false transmitter precursor monoethylcholine we provide additional evidence for second-stage loading. The distribution of quantal sizes at the junction usually does not follow a normal probability distribution; it is skewed to the right. The skew can be accounted for by a model incorporating second-stage loading in which the vesicles are released randomly, without regard to their ACh content. If the vesicles increase in size when they contain more transmitter, only vesicles at the active zone need swell.

译文

当前的模型没有充分考虑微型端板电流 (mepc) 的特性。我们不了解它们相对缓慢的上升,其上升的形状,其可变的,有时是延长的衰减以及振幅与衰减时间之间的相关性。如果我们假设ACh通过孔从囊泡中释放出来,并且囊泡随着附加的递质而增大,则预测更像是mepc。然而,先前的测量表明,在定量大小增加后,末端的囊泡没有扩大。这不是问题,因为一些ACh被添加到位于活性区域的囊泡中,这一过程称为第二阶段加载。通过使用假的递质前体单乙基胆碱,我们为第二阶段加载提供了其他证据。交界处的定量大小分布通常不遵循正态概率分布; 它向右偏斜。可以通过结合第二阶段加载的模型来解释偏斜,在该模型中,囊泡随机释放,而不考虑其ACh含量。如果囊泡在包含更多递质时尺寸增加,则只有活动区的囊泡需要膨胀。

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