Answer:
Unlike other types of cells, motor cells of the muscle depend not just on sodium and potassium for generating an action potential, and carrying out their main task, which is movement, basically, but also on calcium. The reason for this is that calcium ions will not only help in the production of the action potential, especially in heart muscle cells, but will, most importantly, be the ones responsible to allow actin and myosin to form the cross bridges that produce movement.
When an action potential is initiated in a muscle cell, the normal process happens: sodium and potassium channels open, and there is a reversal of the normal resting membrane polarity. This process charges the cell, and prepares it for action. But in muscle cells, since the action is movement, there is not just one cell that must react, and there is much more power that is needed than what is caused by the presence of just sodium and potassium during action potential. Because of this, the cells have in their sarcoplasmic reticulum a storage of calcium ions, which will be pumped out into the cytoplasm. The addition of positive charges produced by this influx of calcium into the cytoplasm, which in normal circumstances should be negative, reinforces the action potential and helps it spread throughout the various cells that conform a muscle fiber. Calcium, also, will bind to the myosin and actin filaments and form bridges that will shorten and move to pull the fibers. This will generate motion.
When the desired effect has been reached, and as the cells return to resting phase, the sarcoplasmic reticulum starts to re-absorb the calcium ions in the cytoplasm, so that the myosin and actin fibers can go back to their resting state. Calcium ions in the cytosol, therefore, will decrease as they go back to storage in the reticulum.
