The number of components of enhancement contributing to short-term synaptic plasticity at the neuromuscular synapse during patterned nerve stimulation progressively decreases as basal release probability is increased from low to normal levels by changing extracellular Ca²⁺

Presynaptic short-term plasticity (STP) dynamically modulates synaptic strength in a reversible manner on a timescale of milliseconds to minutes. For low basal vesicular release probability (prob₀), four components of enhancement, F1 and F2 facilitation, augmentation (A), and potentiation (P), increase synaptic strength during repetitive nerve activity. For release rates that exceed the rate of replenishment of the readily releasable pool (RRP) of synaptic vesicles, depression of synaptic strength, observed as a rundown of postsynaptic potential amplitudes, can also develop. To understand the relationship between enhancement and depression at the frog (Rana pipiens) neuromuscular synapse, data obtained over a wide range of prob₀ using patterned stimulation are analyzed with a hybrid model to reveal the components of STP. We find that F1, F2, A, P, and depletion of the RRP all contribute to STP during repetitive nerve activity at low prob₀. As prob₀ is increased by raising Ca²⁺_o (extracellular Ca²⁺), specific components of enhancement no longer contribute, with first P, then A, and then F2 becoming undetectable, even though F1 continues to enhance release. For levels of prob₀ that lead to appreciable depression, only F1 and depletion of the RRP contribute to STP during rundown, and for low stimulation rates, F2 can also contribute. These observations place prob₀-dependent limitations on which components of enhancement contribute to STP and suggest some fundamental mechanistic differences among the components. The presented model can serve as a tool to readily characterize the components of STP over wide ranges of prob₀.