<prompt> Relaxation of skeletal muscle occurs byRemoval of Acetylcholine from synaptic cleftClosure of nicotinic cholinergic receptorBinding of calcium with troponinRemoval of sarcoplasmic calcium  <response>: Once the calcium ions have been released from the sarcoplasmic tubules and have diffused among the myofibrils, muscle contraction continues as long as the calcium ion concentration remains high. However, a continually active calcium pump located in the walls of the sarcoplasmic reticulum pumps calcium ions away from the myofibrils back into the sarcoplasmic tubules. This pump can concentrate the calcium ions about 10,000-fold inside the tubules. In addition, inside the reticulum is a protein called calsequestrin that can bind up to 40 times more calcium. Ref: Guyton 13th ed, p 94 