Fig. 1.

Representation of the synaptic cleft and perisynaptic region. A, Schematic representing the three different regions in the model. A disk-like synaptic cleft formed by the apposition of the presynaptic and postsynaptic elements contains a central PSD. A transition region extends from the cleft edge to a porous medium representing the bulk extracellular tissue.B, The extracellular volume, V, enclosed as a function of the radius, r, from the center of the model. Within the synaptic cleft the volume increases according to the law for the disk (cylindrical) geometry: V = πr²h (dashed line; h = 20 nm). A different law pertains for the three-dimensional porous medium that represents the distant extracellular space: V = α4πr³/3 (dotted line; α = 0.2). In the model a composite volume function (solid line) was constructed by joining an inner disk-like region to an outer porous medium-like region via a smooth transition region of 200 nm between the radii of 180 and 380 nm (indicated by the pair of vertical lines). C, In the porous medium, part of the model an effective diffusion coefficient (D) was used that was in general different to that used for the synaptic cleft (which was in most cases the diffusion coefficient for free medium). A smooth transition, analogous to that above for the volume function, was effected between these two regimes.