Conceptual diagram for the post-tetanic transient increase in the FRP size in a calyx of Held. The diagram shows a schematic model for an active zone in a calyx of Held (red box, inset), which consists of a cluster of calcium channels at the center and four docked SVs (gray spheres) with variable distance from the calcium source (Meinrenken et al. 2002). It is assumed that the proximity of a SV to the Ca²⁺ source (positional priming) is a primary determinant of release kinetics of the SV, and thus two docked SVs which belong to an FRP reside inside the calcium microdomain (red dome), while the rest (two SVs in an SRP) resides outside the microdomain. Results of the present and our previous studies (Lee et al., 2008) imply that presynaptic Ca²⁺ elevation in the presence of CaM during tetanic stimulation activates MLCK and then myosin II, which in turn facilitates translocation of slowly releasing SVs toward the Ca²⁺ source. The alternative possibility that MLCK may facilitate the final step of molecular priming is not illustrated.