which component of the neuromuscular (myoneural) junction is considered the presynaptic membrane
Neuromuscular junction The neuromuscular junction differs from chemical synapses between neurons. Presynaptic motor axons stop 30 nanometers from the sarcolemma, the cell membrane of a muscle cell. This 30-nanometer space forms the synaptic cleft through which signalling molecules are released. The sarcolemma has invaginations called postjunctional folds, which increase the surface area of the membrane exposed to the synaptic cleft.[2] These postjunctional folds form what is referred to as the motor endplate, which possess nicotinic acetylcholine receptors (nAChRs) at a density of 10,000 receptors/micrometer2 in skeletal muscle.[3] The presynaptic axons form bulges called terminal boutons (or presynaptic terminals) that project into the postjunctional folds of the sarcolemma. The presynaptic terminals have active zones that contain vesicles, also called 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.[2] AChRs are localized opposite the presynaptic terminals by protein scaffolds at the postjunctional folds of the sarcolemma. 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. Also present is the receptor tyrosine kinase protein MuSK, a signaling protein involved in the development of the neuromuscular junction, which is also held in place by rapsyn.[2]