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How calcium ions contribute in the muscle contraction

How do calcium ions contribute in the muscle contraction? Why do both muscle relaxation and muscle contraction spend energy?

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In muscle cells, calcium ions are stored in the sarcoplasmic reticulum. Whenever a motor neuron emits stimulus for muscle contraction neurotransmitters termed as acetylcholine are liberated in the neuromuscular junction and sarcolemma is excited. The excitation is conduced to sarcoplasmic reticulum which then liberates calcium ions into the sarcomeres.

In sarcomeres, the calcium ions join to troponin molecules related to actin activating myosin joining sites of actin. The myosin, then capable to join to actin, pulls this protein and sarcomere shortens. The summation of simultaneous contraction of sarcomeres and myofibrils comprises the muscle contraction. Throughout muscle relaxation, the calcium ions return back to sarcoplasmic reticulum. For myosin to join 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 which spends ATP too. Therefore both muscle contraction and relaxation are energy-spending procedures.

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