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# How Does Contraction Begin? Nerve impulse reaches an axon terminal and synaptic vesicles release acetylcholine (ACh). ACh diffuses to receptors on the sarcolemma, and Na+ channels open, causing Na+ to rush into the cell. A muscle action potential spreads over the sarcolemma and down into the tran...

# How Does Contraction Begin? Nerve impulse reaches an axon terminal and synaptic vesicles release acetylcholine (ACh). ACh diffuses to receptors on the sarcolemma, and Na+ channels open, causing Na+ to rush into the cell. A muscle action potential spreads over the sarcolemma and down into the transverse tubules. SR releases Ca+2 into the sarcoplasm. Ca+2 binds to troponin, causing the troponin-tropomyosin complex to move and reveal myosin binding sites on actin. The contraction cycle begins. # Contraction Cycle A repeating sequence of events that cause the thick and thin filaments to move past each other. There are four steps to the contraction cycle: 1. ATP hydrolysis 2. Attachment of myosin to actin to form crossbridges 3. Power stroke 4. Detachment of myosin from actin The cycle keeps repeating as long as ATP is available, and there is a high Ca+2 level near the filaments.