The ion gates are protein channels that regulate ion flow into and out of the cell. There are three gates that are associated with the action potential: m, h, and n. The m and h gates control sodium flow, while the n gate controls potassium flow. In the resting phase of the action potential, the m gate is closed, while the h gate is open. Therefore, sodium is neither leaving or entering the cell. The n gate is also closed, so potassium can neither leave nor enter the cell. During depolarization, the m gate opens allowing sodium to diffuse down its gradient, while the n gate is still closed. During repolarization, the h gate closes, preventing sodium from coming into the cell. The n gate is open during this phase so potassium moves out. In the undershoot phase, the m gate closes, the h gate stays closed, and the n gate stays open. Finally, the h gate opens, the n gate closes and resting state is once again achieved. This is illustrated in the figure below.

where

(A) = Resting State

(B) = Depolarization phase

(C) = Repolarization phase

(D) = Undershoot

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