A depolarization (action potential) at a neuromuscular junction travels along the muscle cell membrane (sarcolemma) to transverse tubules (T-tubules). From there the depolarization is propagated along the T-tubules and along the sarcoplasmic reticulum, where Ca++ ion channels open to release the ions. The Ca++ ions bathe the sarcomeres, permitting the thick and thin filaments to slide and hence, the muscle to contract.

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