Bennett M R, Jones P, Lavidis N A
J Physiol. 1986 Oct;379:257-74. doi: 10.1113/jphysiol.1986.sp016252.
The number of quanta secreted from selected sites along terminal branches at toad (Bufo marinus) neuromuscular junctions was determined. Terminal branches were visualized by prior staining with the fluorescent dye, 3-3 Diethyloxadicarbocyanine iodide (DiOC2(5)); neither impulse conduction nor quantal release were affected by DiOC2(5) at concentrations less than 10 microM. The evoked quantal release recorded with an extracellular micro-electrode (me) at different sites along the length of terminal branches was determined in an external calcium concentration, [Ca]o, of 0.35-0.45 mM. For short branches (40-80 microns), me remained approximately constant for over 60% of the branches; for the rest, me declined approximately exponentially with an average length constant of 17 +/- 2 microns (mean +/- S.E. of mean). For both medium (81-120 microns) and long branches (121-160 microns), me declined in nearly all cases approximately exponentially with length constants of 39 +/- 5 and 54 +/- 8 microns respectively. These changes in me were observed at synapses having a wide range of terminal branching patterns. Some DiOC2(5)-stained branches possessed discontinuous cholinesterase staining. In general, me declined along these branches in the same way as along DiOC2(5)-stained branches with continuous cholinesterase staining. It is suggested that because of the decline in me along most medium and long terminal branches, many release sites have a very low probability for secretion in low [Ca]o. Release sites near the point of nerve entry, which have a relatively high probability, therefore make the main contribution to secretion recorded with an intracellular micro-electrode. As a consequence, transmitter secretion from the whole terminal does not fluctuate from impulse to impulse as much as expected if there were a large number of release sites, each with a low probability of secretion. Transmitter secretion then follows binomial rather than Poisson statistics.
测定了蟾蜍（海蟾蜍）神经肌肉接头处沿终末分支选定部位分泌的量子数量。通过事先用荧光染料3,3 - 二乙基氧杂二羰花青碘化物（DiOC2(5)）染色来观察终末分支；在浓度低于10微摩尔时，DiOC2(5)既不影响冲动传导也不影响量子释放。在细胞外钙浓度[Ca]o为0.35 - 0.45毫摩尔的条件下，用细胞外微电极（me）测定沿终末分支长度不同部位诱发的量子释放。对于短分支（40 - 80微米），超过60%的分支中me大致保持恒定；其余分支中，me大致呈指数下降，平均长度常数为17±2微米（平均值±平均值标准误差）。对于中等长度（81 - 120微米）和长分支（121 - 160微米），几乎在所有情况下，me均大致呈指数下降，长度常数分别为39±5微米和54±8微米。在具有广泛终末分支模式的突触处观察到了me的这些变化。一些DiOC2(5)染色的分支具有不连续的胆碱酯酶染色。一般来说，沿着这些分支me的下降方式与沿着具有连续胆碱酯酶染色的DiOC2(5)染色分支相同。有人提出，由于大多数中等长度和长终末分支上me下降，在低[Ca]o时许多释放部位的分泌概率非常低。靠近神经进入点的释放部位具有相对较高的概率，因此对细胞内微电极记录的分泌起主要作用。结果，整个终末的递质分泌不像存在大量分泌概率低的释放部位时那样随冲动波动那么大。递质分泌随后遵循二项式而非泊松统计。
