Which sequence correctly lists the phases of an action potential in a neuron, in order?

• A. Resting potential, depolarization, hyperpolarization, repolarization.
• B. Depolarization, resting potential, repolarization, hyperpolarization.
• C. Resting potential, depolarization, repolarization, hyperpolarization.
• D. Hyperpolarization, depolarization, resting potential, repolarization.

Answer: (C)

The action potential unfolds through four stages in a specific order: resting potential, depolarization, repolarization, and hyperpolarization. At rest, the inside of the neuron is negatively charged relative to the outside. A stimulus that reaches threshold opens voltage-gated Na+ channels, Na+ rushes in, and the membrane potential becomes more positive—that’s depolarization. As the peak is reached, Na+ channels inactivate while K+ channels open, allowing K+ to exit; the membrane becomes negative again, which is repolarization. The outflow of K+ can overshoot, making the inside briefly more negative than at rest—that’s hyperpolarization. Ion pumps and leak channels then restore the resting potential. This sequence is the only one that starts with a resting state and proceeds through depolarization, then repolarization, and finally hyperpolarization. The other options either place hyperpolarization before repolarization, start with depolarization without a prior resting state, or begin with hyperpolarization, which doesn’t occur until after depolarization and repolarization.