Why does a larger stimulus require during the relative refractory period?

The requirement for a larger stimulus during the relative refractory period is primarily due to the state of the voltage-gated sodium channels. During this period, while some sodium channels have reset and are capable of opening again, others remain in an inactivated state. This means that not all channels are available to respond to a new stimulus. As a result, a stronger stimulus is necessary to depolarize the membrane sufficiently to reach the threshold potential for an action potential.

Additionally, during the relative refractory period, the membrane is still hyperpolarized due to the continued activity of voltage-gated potassium channels, which can also make it more challenging to initiate another action potential. However, the overarching reason for needing a larger stimulus relates to the incomplete recovery of the sodium channels. Thus, the condition of voltage-gated sodium channels not being fully ready to respond plays a crucial role during this phase.