Final answer:
An action potential is caused by changes in membrane permeability, specifically when a stimulus makes the cell membrane permeable to Na+ ions. This leads to depolarization and the influx of Na+ ions, resulting in a change in membrane potential. Repolarization occurs as the membrane becomes impermeable to Na+ ions and K+ ions move from high to low concentration.
Step-by-step explanation:
An action potential is ultimately caused by changes in membrane permeability. When a stimulus makes the cell membrane permeable to Na+ ions, depolarization occurs. This allows Na+ ions to enter the cell, resulting in a change in membrane potential. Repolarization follows as the membrane becomes impermeable to Na+ and K+ ions move from high to low concentration. In the long term, active transport through the Na+/K+ ATPase pump maintains the concentration differences necessary for the generation of action potentials.