If you could look inside a pre-synaptic axon terminal, you would find one of the characteristics is that it's loaded with these vesicles and they're just sitting there waiting to receive an action potential so that they can immediately dump their contents.

如果你能够一窥突触前的轴突末梢内部,你将会发现一个共同特征,神经递质都装载在小泡中,时刻准备着等待动作电位激活,以便迅速释放其内含物

耶鲁公开课 - 生物医学工程探索课程节选