Final answer:
Hyperpolarization during an action potential is caused by potassium efflux, as K+ ions move out of the neuron and make the inside more negative, overshooting the resting membrane potential.
Step-by-step explanation:
During an action potential, hyperpolarization beyond (more negative to) the resting membrane potential is primarily due to potassium efflux. This occurs when voltage-gated potassium channels open and potassium ions (K+) move out of the neuron, causing the inside of the cell to become more negative than the resting membrane potential. This outward movement of K+ ions overshoots the resting membrane potential, resulting in a hyperpolarization of the membrane, which is a key part of the action potential's 'resetting' process. Eventually, the resting membrane potential is restored as the extracellular K+ ions diffuse away and other mechanisms, such as the sodium-potassium pump, help to re-establish the normal distribution of ions.