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What happens at the neuromuscular junction when acetylcholine is released?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

When acetylcholine is released from synaptic vesicles at the neuromuscular junction, it diffuses across the synaptic cleft and binds to specific receptors embedded in the muscle end plate.12

This binding causes the receptor molecules to change configuration, opening ion channels for approximately one millisecond. This allows small positive ions, primarily sodium, to flow into the muscle cell. The influx of these ions creates a local depolarization known as the end plate potential.321

If this potential is sufficient to reach the muscle cell's firing threshold, it triggers a self-generating action potential that propagates across the muscle fiber membrane to initiate contraction. To prevent constant stimulation, the enzyme acetylcholinesterase rapidly breaks down the acetylcholine, or the neurotransmitter diffuses out of the cleft.31