Which sequence describes the phases of the action potential and the afterhyperpolarization?

Which sequence describes the phases of the action potential and the afterhyperpolarization?

• A. Na+ channels open to depolarize; Na+ channels inactivate; K+ channels open to repolarize; afterhyperpolarization occurs due to continued K+ efflux; channels reset.
• B. K+ channels open first; Na+ channels never inactivate; no afterhyperpolarization.
• C. Na+ channels open but never inactivate; K+ channels remain closed; no afterhyperpolarization.
• D. Cl- channels determine depolarization; GABA receptor activation.

Answer: (A)

The sequence being tested shows how voltage-gated Na+ and K+ channels drive the action potential and the subsequent afterhyperpolarization. First, depolarization occurs as Na+ channels open, allowing Na+ to rush into the cell and quickly raise the membrane potential toward a positive value. These Na+ channels then inactivate, stopping further Na+ entry even while the membrane is still depolarized, which helps end the rising phase. Next, K+ channels open in response to the depolarization, letting K+ exit the cell and bringing the membrane potential back down toward negative values—that’s repolarization. The K+ conductance often persists a bit longer than needed, so the membrane potential dips below its resting level for a short time, producing the afterhyperpolarization. Finally, the channels reset: Na+ channels recover from inactivation and return to a closed, activatable state, K+ channels close as the membrane returns to resting potential, and the ion gradients are restored by pumps so the neuron is ready for another spike. The other sequences don’t fit because they imply no Na+ inactivation, or initial K+ opening, or depolarization driven by Cl- or GABA, which would not produce the classic action potential with a subsequent afterhyperpolarization.

The sequence being tested shows how voltage-gated Na+ and K+ channels drive the action potential and the subsequent afterhyperpolarization. First, depolarization occurs as Na+ channels open, allowing Na+ to rush into the cell and quickly raise the membrane potential toward a positive value. These Na+ channels then inactivate, stopping further Na+ entry even while the membrane is still depolarized, which helps end the rising phase. Next, K+ channels open in response to the depolarization, letting K+ exit the cell and bringing the membrane potential back down toward negative values—that’s repolarization. The K+ conductance often persists a bit longer than needed, so the membrane potential dips below its resting level for a short time, producing the afterhyperpolarization. Finally, the channels reset: Na+ channels recover from inactivation and return to a closed, activatable state, K+ channels close as the membrane returns to resting potential, and the ion gradients are restored by pumps so the neuron is ready for another spike. The other sequences don’t fit because they imply no Na+ inactivation, or initial K+ opening, or depolarization driven by Cl- or GABA, which would not produce the classic action potential with a subsequent afterhyperpolarization.