Auerbach A A, Bennett M V
J Gen Physiol. 1969 Feb;53(2):183-210. doi: 10.1085/jgp.53.2.183.
The hatchetfish, Gasteropelecus, possesses large pectoral fin adductor muscles whose simultaneous contraction enables the fish to dart upwards at the approach of a predator. These muscles can be excited by either Mauthner fiber. In the medulla, each Mauthner fiber forms axo-axonic synapses on four "giant fibers," two on each side of the midline. Each pair of giant fibers innervates ipsilateral motoneurons controlling the pectoral fin adductor muscles. Mauthner fibers and giant fibers can be penetrated simultaneously by microelectrodes close to the synapses between them. Electrophysiological evidence indicates that transmission from Mauthner to giant fiber is chemically mediated. Under some conditions miniature postsynaptic potentials (PSP's) are observed, suggesting quantal release of transmitter. However, relatively high frequency stimulation reduces PSP amplitude below that of the miniature potentials, but causes no complete failures of PSP's. Thus quantum size is reduced or postsynaptic membrane is desensitized. Ramp currents in Mauthner fibers that rise too slowly to initiate spikes can evoke responses in giant fibers that appear to be asynchronous PSP's. Probably both spikes and ramp currents act on the same secretory mechanism. A single Mauthner fiber spike is followed by prolonged depression of transmission; also PSP amplitude is little affected by current pulses that markedly alter presynaptic spike height. These findings suggest that even a small spike releases most of an immediately available store of transmitter. If so, the probability of release by a single spike is high for any quantum of transmitter within this store.
胸斧鱼（胸斧鱼属）拥有大型胸鳍内收肌，这些肌肉同时收缩能使鱼在捕食者靠近时向上猛冲。这些肌肉可由任一莫特纳尔纤维激发。在延髓中，每条莫特纳尔纤维在四条“巨纤维”上形成轴-轴突触，中线两侧各两条。每对巨纤维支配同侧控制胸鳍内收肌的运动神经元。微电极可在靠近莫特纳尔纤维与巨纤维之间突触的位置同时刺入它们。电生理证据表明从莫特纳尔纤维到巨纤维的传递是化学介导的。在某些情况下可观察到微小突触后电位（PSP），提示递质的量子释放。然而，相对高频的刺激会使PSP幅度降低到低于微小电位的幅度，但不会导致PSP完全消失。因此量子大小减小或突触后膜脱敏。莫特纳尔纤维中上升过慢以至于无法引发动作电位的斜坡电流能在巨纤维中诱发似乎是异步PSP的反应。可能动作电位和斜坡电流作用于相同的分泌机制。单个莫特纳尔纤维动作电位之后会伴随传递的长时间抑制；而且PSP幅度受明显改变突触前动作电位高度的电流脉冲影响很小。这些发现表明即使一个小的动作电位也会释放大部分即时可用的递质储备。如果是这样，对于该储备中任何一个递质量子而言，单个动作电位引发释放的概率都很高。
