And because our potassium can leak slightly to the outside of the cell, that's exactly why our membrane is negative on the inside than on the outside, because we have a leakage of these potassium to the outside. And that basically creates a slightly negative membrane potential on the inside of the cell. So phase four is the resting membrane potential. Now let's move on to phase zero. Phase zero is shown by the red region. So let's suppose we have some type of outside stimulus, either by a neuron or by an adjacent muscle cell.