Introduction

The action potential in a nerve fiber is characterized by several phases: depolarization, repolarization, and after-hyperpolarization. The after-hyperpolarization phase (also called the undershoot) is when the membrane potential temporarily becomes more negative than the resting potential. This phase plays an important role in controlling neuronal excitability. The following article discusses the mechanisms that contribute to after-hyperpolarization, evaluating key statements explaining its generation.

Understanding After-Hyperpolarization

After-hyperpolarization occurs due to the interplay between sodium and potassium conductances:

• During the action potential peak, sodium channels inactivate, and potassium channels open slowly but stay open longer.wikipedia+1

• The continued outward flow of potassium ions (K⁺) causes the membrane potential to become more negative than its typical resting value.wikipedia

• Potassium conductance remains elevated during after-hyperpolarization, prolonging the hyperpolarized state until these channels close and the membrane restores its resting potential.

• The membrane potential during after-hyperpolarization approaches closer to the potassium equilibrium potential (E_K), which is more negative than the resting membrane potential.wikipedia+1

Analysis of Proposed Statements

A. The increased conductance of Na⁺ has returned to the baseline level but the conductance of K⁺ remains elevated during the after-hyperpolarization phase

• Correct.
  Sodium conductance rapidly returns to baseline after inactivation, while potassium channels remain open, sustaining the hyperpolarization.ncbi.nlm.nih+1

B. The membrane potential is pulled even closer to the K⁺ equilibrium potential at the after-hyperpolarization phase

• Correct.
  Continued potassium efflux drives the membrane potential more negative, closer to E_K, causing the after-hyperpolarization.wikipedia

C. The conductance of Na⁺ is increased before any change of K⁺ conductance during after-hyperpolarization phase

• Incorrect.
  Sodium channels are closed/inactivated during after-hyperpolarization; potassium conductance dominates.

D. At the after-hyperpolarization phase, the membrane potential is driven closer to Na⁺ equilibrium potential

• Incorrect.
  Membrane potential moves away from the sodium equilibrium potential (which is positive) toward a more negative potential near the potassium equilibrium potential.

Summary Table

Conclusion

The combination of correct statements explaining the after-hyperpolarization phase is:

(1) A and B