Rapid regulation of vesicle priming explains synaptic facilitation despite heterogeneous vesicle:Ca channel distances.

Chemical synaptic transmission relies on the Ca-induced fusion of transmitter-laden vesicles whose coupling distance to Ca channels determines synaptic release probability and short-term plasticity, the facilitation or depression of repetitive responses. Here, using electron- and super-resolution microscopy at the neuromuscular junction we quantitatively map vesicle:Ca channel coupling distances. These are very heterogeneous, resulting in a broad spectrum of vesicular release probabilities within synapses.