action potential generation in skeletal muscle cells the sarcolemma is the plasma membrane of the skeletal muscle. It separates the extracellular space from the intracellular space Sodium ions are highly concentrated in the extracellular space outside of the muscle cell whereas potassium ions are highly concentrated inside the cell in the intracellular space this concentration of ions is what is seen at rest. Embedded in the sarcolemma are voltage-gated sodium channels, potassium, sodium, ATP-ase pumps and voltage-gated potassium channels in order to initiate an action potential along the sarcolemma a stimulus from a neuron at the neuromuscular junction causes a wave of positive charge to reach voltage-gated sodium channels on the sarcolemma the increase in positive charge causes the voltage-gated sodium channels to open positively-charged sodium ion flood into the cell following their concentration gradient This causes the inside of the membrane near to the open sodium channel to become more positively charged. This process is the depolarization of the sarcolemma. The build-up a positive charge on the inside of the membrane eventually causes the voltage-gated sodium channels to close at the same time voltage-gated potassium channels open since they open only when the inside of the membrane is at its maximum positive charge Positively charged potassium ions flow rapidly out of the muscle cell following their concentration gradient this decreases the level of positive charge inside the sarcolemma. This return of the membrane to resting levels of charge is called repolarization. Once the sarcolemma has been fully repolarized the potassium channels close. A sodium potassium pump uses ATP energy to restore the concentration gradient of sodium and potassium to their normal levels