Neuromuscular Junction - Structure - Neuromuscular Junction

The neuromuscular junction differs from synapses in the central nervous system. Presynaptic motor axons are demyelinated and stop 30 nanometers from the sarcolemma, the cell membrane of a muscle cell. This 30-nanometer space forms the synaptic cleft through which signaling molecules are released. The sarcolemma has invaginations called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft. These postjunctional folds form what is referred to as the motor end-plate, which possesses acetylcholine receptors (AChRs) at a density of 10,000 receptors/micrometer2 in skeletal muscle. The presynaptic axons form bulges called terminal boutons that project into the postjunctional folds of the sarcolemma. The presynaptic boutons have active zones that contain vesicles, quanta, full of acetylcholine molecules. These vesicles can fuse with the presynaptic membrane and release ACh molecules into the synaptic cleft via exocytosis after depolarization. AChRs are localized by protein scaffolds at the postjunctional folds of the sarcolemma opposite the presynaptic terminals. Dystrophin, a structural protein, connects the sarcomere, sarcolemma, and extracellular matrix components. Rapsyn is another protein that docks AChRs and structural proteins to the cytoskeleton. The receptor tyrosine kinase protein MuSK is a signaling protein involved in the development of the neuromuscular junction and is also held in place by rapsyn.