Bennett M V, Model P G, Highstein S M
Cold Spring Harb Symp Quant Biol. 1976;40:25-35. doi: 10.1101/sqb.1976.040.01.005.
The Mauthner fiber giant fiber synapses of the hatchetfish are chemically transmitting axo-axionic synapses in the medulla. Tetanic stimulation at room temperature depletes the presynaptic Mauthner terminal of vesicles and leads to the appearance of large numbers of irregular membraneous compartments in the terminal. Stimulation during cooling to 12 degrees C depletes the terminal of vesicles and greatly increases the external surface, which forms large whorls of invaginating double membranes. Many coated vesicles are attached to the surface and the invaginating whorls. It is concluded that vesicles are discharged by exocytosis and fusion of their membrane with the external surface, and that at room temperature, membrane is reinternalized by coated vesicles and formed into irregular compartments. In completion of the cycle, these compartments disappear, and the vesicle population recovers over an hour or two of rest. When the Mauthner fibers are stimulated at low rates, the p.s.p.'s in the giant fibers are large and suprathreshold. Minature p.s.p.'s are generated spontaneously or can be evoked by subthreshold depolarization or tetanic stimulation of the Mauthner fiber. Stimulation of the Mauthner fibers at gradually increasing frequencies depresses p.s.p. amplitude to or below the level of miniature p.s.p.'s, but no failures are observed. Small p.s.p.'s without failures suggest that the quantum number remains high but that quantal size is greatly reduced, either by partial filling, as is supported by the morphological observation of vesicle depletion, or by desensitization. When stimulation is stopped, recovery of p.s.p. amplitude occurs in 1 or 2 seconds, but if tetanic stimulation is resumed immediately, p.s.p. amplitude decreases again and much more rapidly than in the initial rundown. This result suggests that depression of p.s.p. amplitude is not due to desensitization and leaves partial filling as the most likely explanation of small quanta. Calculated quantal size following a tetanus recovers in 200-500 ms, which probably largely reflects the time for filling since enough vesicles can be supplied to prevent failures with much shorter intervals between stimuli. Because quantal size appears to decrease gradually as stimulation frequency increases, it appears that release of vesicles can interrupt filling, leading to the conclusion that filling and release sites are very close together. This conlusion is consistent with other data in the literature obtained by different techniques.
斧鱼的莫氏纤维巨型纤维突触是延髓中化学传递的轴 - 轴突触。在室温下进行强直刺激会耗尽突触前莫氏终末的囊泡，并导致终末出现大量不规则的膜性小室。在冷却至12摄氏度时进行刺激会耗尽终末的囊泡，并极大地增加外表面，形成大量内陷的双膜大涡旋。许多有被小泡附着在表面和内陷的涡旋上。得出的结论是，囊泡通过胞吐作用以及其膜与外表面融合而释放，并且在室温下，膜通过有被小泡重新内化并形成不规则小室。在循环完成时，这些小室消失，囊泡群体在一两个小时的休息后恢复。当以低频率刺激莫氏纤维时，巨型纤维中的突触后电位（p.s.p.）很大且超过阈值。微小的突触后电位是自发产生的，或者可以通过亚阈值去极化或对莫氏纤维的强直刺激诱发。以逐渐增加的频率刺激莫氏纤维会使突触后电位幅度降低到微小突触后电位水平或以下，但未观察到失败现象。无失败的小突触后电位表明量子数仍然很高，但量子大小大大减小，这要么是由于部分填充，囊泡耗尽的形态学观察支持了这一点，要么是由于脱敏。当刺激停止时，突触后电位幅度在1或2秒内恢复，但如果立即恢复强直刺激，突触后电位幅度会再次降低，而且比初始衰减时快得多。这个结果表明突触后电位幅度的降低不是由于脱敏，而部分填充是小量子最可能的解释。强直刺激后计算出的量子大小在200 - 500毫秒内恢复，这可能主要反映了填充时间，因为可以提供足够多的囊泡以防止在更短的刺激间隔下出现失败。由于随着刺激频率增加量子大小似乎逐渐减小，看来囊泡的释放可以中断填充，从而得出填充位点和释放位点非常靠近的结论。这个结论与文献中通过不同技术获得的其他数据一致。
