The action potential of a cell is a rapid change in membrane potential, which can be visualized on a graph with time on the x-axis (in milliseconds) and membrane potential on the y-axis (ranging from -90 millivolts to +30 millivolts). Initially, the inside of the cell is negatively charged, and as the action potential progresses, it undergoes two key phases: depolarization and repolarization.

During depolarization, sodium channels open, allowing sodium ions (Na⁺) to flow into the cell. This influx occurs because sodium is in higher concentration outside the cell, leading to a positive charge inside as the membrane potential rises from negative values, crosses zero, and even overshoots slightly into the positive range. This phase is characterized by the increase in membrane potential, indicating a loss of polarization.

Following depolarization, the sodium channels close, and potassium channels open. Potassium ions (K⁺), which are in higher concentration inside the cell, begin to exit. As potassium leaves, it carries positive charges with it, causing the membrane potential to decrease and return towards its resting state. This phase is known as repolarization, where the membrane potential falls back to its original negative value, completing the action potential cycle.

Understanding these processes is crucial for grasping how action potentials propagate along the sarcolemma in muscle fibers, as the movement of sodium and potassium ions is fundamental to the transmission of electrical signals in both muscle and nerve cells.