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Muscle contraction

Explain how do calcium ions participate in muscle contraction? Why do both muscle contraction and muscle relaxation spend the energy?

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In muscle cells, calcium ions are generally stored within the sarcoplasmic reticulum. Whenever, the motor neuron emits stimulus for muscle contraction neurotransmitters which is known as the acetylcholine are released within the neuromuscular junction and sarcolemma is excited. The excitation is conduced to the sarcoplasmic reticulum which then realeases calcium ions in the sarcomeres.

In sarcomeres, the calcium ions bind to the troponin molecules which are associated to the actin activating myosin binding sites of the actin. The myosin, then able to bind to the actin, pulls this protein and sarcomere shortens. The summation of simultaneous contraction of sarcomeres and myofibrils constitutes the contraction of the muscle. During the muscle relaxation calcium ions returns back to the sarcoplasmic reticulum. For myosin, to get bind to actin, and hence, for the contraction to takes place, hydrolysis of one ATP molecule is essential. While relaxation, return of calcium ions to the sarcoplasmic reticulum refer to an active process that spends ATP too. Thus, both the relaxation and muscle contraction are energy-spending processes.

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