Fatigue at the Mauthner fiber giant fiber synapse of the hatchet fish

PSPs in giant fibers were recorded intracellularly in response to caudal stimulation of both Mauthner fibers. Previously, prolonged high frequency stimulation (≥40/sec) was observed to reduce PSP amplitude to a level comparable to that of miniature (M) PSPs observed prior to stimulation; yet there were no failures, suggesting that decrease in PSPs was due to reduced quantal size or desensitization rather than reduced number of quanta. During PSP rundown produced by high frequency stimulation, MPSPs of normal size occur transiently early in the train even after PSP amplitude has become very small; normal MPSPs then cease as stimulation is maintained, but recover within a few minutes of cessation of stimulation. This suggests PSP reduction is due at least primarily to decrease in size of most quanta with a few normal quanta that are secreted late during rundown. Evidently the presynaptic store of transmitter is small and depleted relatively quickly, and it appears that after depletion the contents of partially filled vesicles can be released by stimulation.