Nervous system function depends on the intricate molecular architecture of the synapse. The NMJ is composed of a presynaptic terminal and a postsynaptic muscle target (Figure 3.1). Like all chemical synapses, the NMJ presynaptic terminal is characterized by a presynaptic density surrounded by a cluster of synaptic vesicles (SVs). This is frequently termed the "active zone", referring to the "active" site of SV fusion and neurotransmitter release. Surrounding the active zone is the region that accommodates reserve SVs and endocytosis and may act to fH. Van Epps, Department of Molecular Cell and Developmental Biology, University of California, Santa Cruz, California, USA; [email protected]

*Y. Jin, Department of Molecular Cell and Developmental Biology, and Howard Hughes Medical Institute, University of California, Santa Cruz, California, USA; [email protected]

separate synaptic domains from nonsynaptic cytoplasm. This region is frequently termed the "periactive zone." The postsynaptic site is defined by its juxtaposition against the electron-dense presynaptic site. Clustered neurotransmitter receptors at the postsynaptic site detect and then transduce the neuronal signal.

Synapse formation therefore involves the recruitment of a specific ensemble of molecules at the active zone, periactive zone, vesicle pool, and postsynaptic site. Once mature, the synapse can be maintained for the life of the organism, and in some cases is capable of remodeling in response to input. A number of synaptic genes involved in synapse formation have been identified and characterized at the invertebrate NMJ. The NMJs in C. elegans and Drosophila provide a well-defined genetic framework in which synaptogenesis can be studied in vivo at single-cell and single-synapse resolution.

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