Final answer:
The after-hyperpolarization that follows an action potential occurs due to a rapid outflow of potassium ions.
Step-by-step explanation:
The after-hyperpolarization that follows an action potential occurs due to a rapid outflow of potassium ions. An action potential is an all-or-nothing event that starts with the depolarization of the membrane caused by the influx of sodium ions.
Once depolarization is complete, potassium ion channels open, allowing potassium ions to flow out of the cell and repolarizing the membrane. This repolarization phase of the action potential is responsible for the after-hyperpolarization.