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How calcium ions participate in the muscle contraction

How calcium ions participate in the muscle contraction? Why do both muscle contraction and muscle relaxation use energy?

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In muscle cells calcium ions are stored in sarcoplasmicreticulum. When a motor neuron emits stimulus for muscle contraction neurotransmitters called acetylcholine are liberated in the neuromuscular junction and sarcolemma is excited. The excitation is conduced to sarcoplasmic reticulum that then releases calcium ions in sarcomeres.

In sarcomeres calcium ions bind to troponin molecules associated to actin activating myosin binding sites of the actin. The myosin, then capable to bind to actin, pulls this protein and sarcomere shortens. The summation of simultaneous contraction of the sarcomeres and myofibrils constitutes muscle contraction. Throughout muscle relaxation the calcium ions return back to sarcoplasmic reticulum.

For myosin to bind to actin, and therefore for the contraction to take place, hydrolysis of one ATP molecule is essential. Throughout relaxation the return of calcium ions to the sarcoplasmic reticulum is an active procedure that spends ATP too. So both muscle relaxation and contraction are energy-spending procedures.

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