Figure 1. Formation of the neuromuscular junction.

(a) The axons of motor neurons enter the differentiating muscle mass while continuously releasing acetylcholine and agrin. At this stage, myoblasts are aligning and have just started to fuse. They express low levels of acetylcholine receptors, muscle-specific receptor tyrosine kinase (MuSK), rapsyn and the dystroglycans, all of which are distributed diffusely over the myoblast surface. (b) The growing tip of the first axon contacts the differentiating myoblasts and stops growing. Myoblasts are still fusing to form the primary myofibres. They have started secreting a scant and disorganized basement membrane, composed mainly of laminin. The expression of acetylcholine receptors, rapsyn, MuSK and dystroglycans increases. MuSK can be activated by neural agrin at this stage, indicating that all the components of the MuSK receptor complex are now expressed and functional. Aggregation of acetylcholine receptors begins. (c) Each multinucleated myofibre is surrounded by a continuous basement membrane and contacted by one or more axons. The basement membrane at the synaptic region becomes biochemically specialized. The tips of the axons have differentiated into synaptic terminals with organized sites for neurotransmitter release. Schwann cells are enveloping the nerve terminals. Acetylcholine receptors are concentrated at the postsynaptic membrane, where they become anchored to the basement membrane and the cytoskeleton via the dystroglycans. Synthesis of acetylcholine receptors is maintained selectively at the synapse by neuregulin released by the nerve. (d) Competition eliminates supernumerary synapses, leaving one synapse per muscle fibre. The postsynaptic membrane is raised and junctional folds appear. Neurotransmitter release sites become aligned with the troughs of the folds.

Figure 2. Molecular interactions and signalling at the vertebrate neuromuscular junction (NMJ). Acetylcholine release sites in the nerve terminal are aligned precisely with the mouths of the synaptic folds in the muscle postsynaptic membrane. The presynaptic and postsynaptic elements are separated by a highly organized synaptic basement membrane, which includes laminin, collagen, heparan sulfate proteoglycan and acetylcholinesterase (not shown). Incorporated into this basement membrane are also found proteins released by the nerve, such as agrin and neuregulin, which participate in the formation and maintenance of the NMJ. Agrin is anchored to the synaptic basement membrane via laminin, and interacts with two receptors on the muscle surface, incuding -dystroglycan and muscle-specific receptor tyrosine kinase (MuSK). Activation of MuSK by agrin initiates clustering of acetylcholine receptors at the top of the folds, and sodium channels at the bottom of the folds. Acetylcholine receptors are aggregated via activation of rapsyn, which also self-associates and recruits - and -dystroglycan. The dystroglycans are believed to enlarge and stabilize the cytoskeletal network by interacting with utrophin and dystrophin, and indirectly with syntrophins. In addition dystrophin, utrophin, dystroglycans and laminin are required for the proper folding of the postsynaptic membrane. The syntrophins can interact with dystrophin at the bottom of the folds as well as utrophin and signalling proteins such as nitric oxide synthase. Neuregulin released by the nerve activates ErbB receptors, which are also trapped in the postsynaptic membrane, and activates the transcription of acetylcholine receptor genes in synaptic nuclei.

Figure 3. Model of some of the molecular interactions occurring at a central excitatory synapse. The nerve is in close apposition with a dendritic spine of the postsynaptic neuron. Some of the molecular interactions at the synapse involve cell adhesion molecules, such as cadherins and neurexin–neuroligan complexes. These link the presynaptic terminal to the postsynaptic membrane, ensuring contact, organizing the actin cytoskeleton and, possibly, aligning vesicle release sites with glutamate receptor clusters in the postsynaptic membrane. Neurexin in the presynaptic terminal interacts with the membrane-associated guanylate kinase (MAGUK) calcium/calmodulin dependent serine protein kinase (CASK), which recruits calcium calmodulin kinase II, a tyrosine kinase activated by calcium and involved in cytoskeletal remodelling. The nerve synthesizes a membrane-bound form of neuregulin which binds to the postsynaptic ErbB receptor and regulates N-methyl-d-aspartate (NMDA) receptor synthesis. NMDA and -amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptors interact with specific MAGUKs, which ensure their localization at the synapse, anchor them to the cytoskeleton and recruit signalling proteins, such as nitric oxide synthase (NOS) and the tyrosine kinase Fyn. GRIP, glutamate receptor interacting protein; mRNA, messenger ribonucleic acid; PSD, postsynaptic density.