the threshold potential has also been shown experimentally to adapt to slow changes in input characteristics by regulating sodium channel density as well as inactivating these sodium channels overall. hyperpolarization by the delayed-rectifier potassium channels causes a relative refractory period that makes it much more difficult to reach threshold. the delayed-rectifier potassium channels are responsible for the late outward phase of the action potential, where they open at a different voltage stimulus compared to the quickly activated sodium channels. they rectify, or repair, the balance of ions across the membrane by opening and letting potassium flow down its concentration gradient from inside to outside the cell. they close slowly as well, resulting in an outward flow of positive charge that exceeds the balance necessary. it results in excess negativity in the cell, requiring an extremely large stimulus and resulting depolarization to cause a response.