Neuromuscular junction

The synaptic vesicle exocytosis-endocytosis cycle. After an action potential and Ca²⁺ influx, phosphorylation of synapsin is activated by calcium-calmodulin activated protein kinases I and II. This results in the mobilization of SVs from the cytomatrix toward the plasma membrane. The formation of the SNARE complex is an essential step for the docking process. After fusion of SVs with the presynaptic plasma membrane, ACh is released into the synaptic cleft. Some of the released acetylcholine molecules bind to the nAChRs on the postsynaptic membrane, while the rest is rapidly hydrolyzed by the AChE present in the synaptic cleft to choline and acetate. Choline is recycled into the terminal by a high-affinity uptake system, making it available for the resynthesis of acetylcholine. Exocytosis is followed by endocytosis in a process dependent on the formation of a clathrin coat and of action of dynamin. After recovering of SV membrane, the coated vesicle uncoats and another cycle starts again.