Neuromuscular Junction Muscle contraction begins at the neuromuscular junction, where a motor neuron communicates with a muscle fiber. When an action potential reaches the end of the motor neuron, it triggers the release of acetylcholine into the synaptic cleft. Acetylcholine binds to receptors on the muscle fiber's membrane, leading to depolarization and an action potential in the muscle fiber.
Sliding Filament Theory Once the action potential travels along the muscle fiber, it leads to the release of calcium ions from the sarcoplasmic reticulum. The calcium ions bind to troponin on the actin filaments, causing a conformational change that shifts tropomyosin and exposes the myosin-binding sites on actin. Myosin heads then bind to actin, forming cross-bridges, and pull the actin filaments toward the center of the sarcomere, resulting in muscle contraction.
