Figure 7.

Relationships between innate synaptic molecule dynamics and synaptic size dynamics and distributions. A, Illustration of the mesoscopic model used here to explore the aforementioned relationships. The model includes 4000 simulated synapses, each of which consists of a postsynaptic membrane - a 50 × 50 matrix of potential binding sites (slots) and synaptic scaffold molecules that bind to and unbind from these slots with indicated probabilities (see text for further details). The dark gray area indicates the immediate 3 × 3 neighborhood that influences the binding and unbinding probabilities, in this case of the light blue molecule. At any given moment, scaffold molecules can be either bound (bright blue) or free (light blue). Free molecules are part of a global pool shared by all synapses. B, Limiting distributions of synaptic sizes (4000 synapses) at the last simulation step (cycle 799) for two values of unbinding constants indicated at bottom right. C, Mean synaptic size during last 72 simulation steps. D, Synaptic sizes at last stimulation step plotted against their sizes 24 steps beforehand for the same two unbinding constant values. Linear regression fits to the two datasets and their respective parameters are shown. E, Slopes of linear regression lines as in D for the 72 last simulation steps for the two unbinding constants; apparent <ϵ> values derived from these data are indicated. Fits using these values of <ϵ> are shown as black dashed lines. F, Coefficients of determination (R²) of linear regression fits as in D for the 72 last simulation steps and two unbinding constants. G, Size changes for the two unbinding constants after 24 simulation steps (cycles 776–799) plotted as a function of initial size (cycle 776). Lines are linear regression fits. Fit parameters are indicated accordingly. H–J, Magnitudes of synaptic size fluctuations in data simulated for the two unbinding constants. Averages of SDs, coefficients of variation, and range/mean values calculated for all synapses over the last 24 simulation steps. Indicated p values are for t tests assuming unequal variances. K, Simulated FRAP curves for the two unbinding constants (200 synapses, 24 simulation time steps).