Figure 10.

Model for restriction of resting pool mobility by synapsin and its effect on vesicle distribution. Vesicles are divided approximately equally into the resting (green) and recycling (red) pool by a synapsin-independent mechanism. In this manner, the fraction of recycling vesicles of the total population is similar in WT (top) and synapsin TKO (bottom) neurons, although the number of vesicles within the presynaptic terminal is smaller by ∼30% in the TKO terminals. The mobility of most resting vesicles is restricted by the synapsins (blue circle), while the mobility of ∼30% of the recycling vesicles is restricted by a different mechanism or protein (black circle). Altogether, ∼65% of the vesicles are immobile. In the absence of the synapsins, the immobilization of the resting vesicles is lost, and the mobile fraction of vesicles increases to ∼85% (∼15% immobile). Due to the increased mobility, SVs are dispersed into the axon, causing the boundaries of the synapse to become indistinct and increasing the number of SVs in the axon by a factor of 3.