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. 2000 Apr 1;20(7):2480–2494. doi: 10.1523/JNEUROSCI.20-07-02480.2000

Fig. 8.

Fig. 8.

Expected EPSC variance in two models of release-independent STD. A, Depression attributable to decreasing release probability (PR) at each vesicle release site. Variance fell monotically with decreasing PR when initial PR was constrained to be <0.29 (see Results). Curve plotted according to the binomial variance formula, ς2 = NPR(1 − PR).B, Depression attributable to decreasing action potential conduction probability (PC) through axonal branches. Variance could fall monotically, or rise and then fall with decreasing PC, depending on the number of release sites per axonal branch (SB). Curves plotted according to Equation 2, with PR = 0.29 and initial conduction probability (PC1) constrained to 1. N and q do not affect the normalized variance. C, Depression attributable to decreasing PC with heterogeneity in axonal branch properties. Populations of 500 axonal branches with heterogeneous numbers of sites per branch (SB). SB values were drawn from a normal distribution with mean 30 and SDs between 10 and 30 as labeled. In addition, the rate of decrease in PC was also heterogeneous across branches, with mean 0.2 and SD 0.05 (see Materials and Methods). Mean–variance relations were nearly parabolic, with clear rise and fall in variance with depression of the mean. D, Simplified illustration of depression attributable to reduced release probability. Similar format as in Figure 5D except that all release sites had the same initial release probability. E, Depression attributable to reduced axonal conduction probability at axonal branches. In this illustration, action potential failed to propagate through center branch, effectively removing three release sites. Release probabilities were unchanged at other sites. F, Schematic representation of heterogeneous axonal branch properties. Note range of numbers of release sites per branch and variety in axonal branching complexity. Axonal branching complexity represents one potential mechanism underlying heterogeneous conduction probabilities through terminal branches (see Materials and Methods).