Following the repolarization phase of a neuronal action potential, the membrane potential briefly becomes more negative than the resting potential before returning to baseline. The immediate cause of this after-hyperpolarization is:

• A Residual sodium channel inactivation drawing the potential downward
• B Active transport of sodium out of the cell by the Na-K ATPase
• C Chloride influx through GABA-gated channels
• D Continued opening of voltage-gated potassium channels beyond the resting potential ✓

Explanation

Voltage-gated K+ channels open slowly and close slowly, so K+ efflux continues after Vm has returned toward rest, driving the membrane briefly toward the potassium equilibrium potential, which is more negative than the resting potential. Closure of these channels restores the baseline. Channel inactivation (option A) reduces current flow and cannot itself move the membrane potential, and pump activity is far too slow to account for the millisecond timescale.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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