In the muscle cells calcium ions are stored within the sarcoplasmic reticulum. When a motor neuron emits stimulus for the muscle contraction neurotransmitters called acetylcholine are released in the neuromuscular junction and the sarcolemma is excited.
The excitation is conduced to the sarcoplasmic reticulum that then realeases calcium ions into the sarcomeres.
In the sarcomeres the calcium ions bind to troponin molecules associated to actin activating myosin binding sites of actin. The myosin, then able to bind to actin, pulls this protein and the sarcomere shortens. The summation of simultaneous contraction of sarcomeres and myofibrils constitutes the muscle contraction. During muscle relaxation the calcium ions return back to the sarcoplasmic reticulum.
For myosin to bind to actin, and thus for the contraction to occur, hydrolysis of one ATP molecule is necessary. During relaxation the return of calcium ions to the sarcoplasmic reticulum is an active process that spends ATP too. So both muscle contraction and relaxation are energy-spending processes.
