Figure 2.

Summary of the key characteristics of the models discussed in this review. (A–D) Postsynaptic response for the different models during stimulation at different frequencies. (A) The vesicle depletion model (Equation 3) predicts exponential decay of the response and an inverse relation between stimulus frequency and steady-state amplitude. A higher release probability causes faster and stronger depression [compare upper and lower graph, see also panel (E)]. (B) The depletion model with facilitation (Equations 3, 4) predict a transient response increase during high-frequency stimulation. For a low basal release probability p₀ the response remains elevated (top graph), while for higher p₀ vesicle depletion masks facilitation [bottom graph, see also panel (E)]. (C) Use-dependent vesicle replenishment (Equation 6) increases the steady-state response. (D) As panel (C), but with added slow use-dependent suppression of release probability. Here the postsynaptic response continues to slowly decay when the depletion model reaches steady-state [compare (C) and (D)]. (E) Steady-state response magnitude as a function of input frequency for the depletion model (circles) and the depletion model with facilitation (dashed lines). (F) Same as (E), but for the depletion model with use-dependent replenishment (UDE, circles) and the UDE model with slow suppression of release probability (RS, dashed). Note that the latter increases depression in particular at low frequencies. (G) Occupancy of the releasable vesicle pool for the models in panel (F). It is less depleted for the RS model as steady-state depression is mediated by the reduction in release probability. Parameters: τ_r = 1 s, a_f = 0.3, τ_f = 0.2 s [no facilitation in (C,D)], a_e = 0.4, τ_e = 0.1 s, a_i = 0.01, τ_i = 10 s.