The immediate trigger for muscular contraction is a sudden increase in the concentration of calcium ions in the cytoplasm of the muscle fiber, the sarcoplasm. To prevent the muscle from being in a continual state of contraction, the calcium is stored in a system of intracellular membrane-bound channels. This system, called the sarcoplasmic reticulum, permeates the entire muscle fiber, so that each sarcomere is surrounded by it.

The membranes of the sarcoplasmic reticulum contain a calcium pump that uses the energy stored in ATP to transport calcium ions from the sarcoplasm, where the calcium concentration is maintained at a very low level, into the sarcoplasmic reticulum, where the calcium concentration is very high. The pump contains an enzyme that catalyzes the splitting of ATP into adenosine diphosphate ADP and P_i. This converting enzyme requires calcium and magnesium ions for its operation and thus is called calcium-magnesium ATPase. During the cleavage of one ATP molecule, two calcium ions are transported into the sarcoplasmic reticulum. The capacity of the sarcoplasmic reticulum to store calcium is enhanced by the existence of a special calcium-binding protein called calsequestrin, which has been identified in purified preparations of sarcoplasmic reticulum. It is estimated that when the muscle is at rest, the calcium concentration in the sarcoplasmic reticulum is more than 100 mmol/kg of dry weight.

Maintenance of the steep concentration gradient for calcium across the membranes of the sarcoplasmic reticulum and activation of the contractile mechanism use up ATP, which must be replenished quickly. ATP is most efficiently replenished by the oxidative pathway. Because of their high energy requirements, muscle fibers are rich in mitochondria, which contain the enzymatic machinery for oxidative metabolism. Mitochondria are most heavily concentrated near the sarcolemma, close to the capillaries that supply them with oxygen.

The muscle action potential is propagated from the region of the neuromuscular junction along the entire length of the muscle fiber. The electric impulse of muscle is similar to that of most nerve fibers. The sarcolemma contains voltage-dependent sodium channels that open in response to an injection of depolarizing (positive) current into the muscle fiber. Because the action of acetylcholine is to depolarize the sarcolemma at the neuromuscular junction, sodium channels open in the neighboring area of sarcolemma. The concentration of sodium ions inside the muscle fiber is kept very low by a pump consisting of a sodium/potassium–activated ATPase. The cleavage of one ATP molecule into ADP and phosphate is accompanied by the transport of three sodium ions out of the fiber and two potassium ions into the fiber. Because the intracellular sodium concentration is so low (about 10 mmol/L), when sodium channels open, sodium ions move into the muscle fiber from the extracellular fluid, where the concentration is much higher (about 110 mmol/L). The inward movement of these positively charged ions further depolarizes the sarcolemma, opening more sodium channels in a cycle of depolarization and increase in sodium conductance until the membrane potential reaches almost +50 mV.