Muscle contraction begins with a signal from the nervous system. When a motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction, it triggers an action potential in the muscle fiber. This action potential travels along the sarcolemma and into the T-tubules, prompting the release of calcium ions from the sarcoplasmic reticulum. The calcium ions bind to troponin, causing tropomyosin to move away from binding sites on actin, allowing myosin heads to attach and form cross-bridges. The myosin heads then pivot, pulling the actin filaments inward, resulting in muscle contraction.
