Partitioning of parameter space between depressing and facilitating synapses. The parameter space describing the one-step docking model is represented by a cube where all three parameters p, δ, and r vary from 0 to 1. The axis origin (coordinates 0, 0, 0) is located at the bottom rear corner of this cube. Facilitating synapses appear on the right in this representation (blue space, with a facilitation pole at p = 0, δ = 0, r = 1), and depressing synapses appear on the left (red space, with a depression pole at p = 1, δ = 1, r = 0). The partition between the two regions is a curved surface defined by the relation p = (1/δ − 1)r/(1 − r).