Now, when this change takes place, as a result of that change in the membrane voltage, the electric potential difference between the two sides of the cell, what happens is this opens up. And once it opens up, it allows the movement of the sodium ions down their electrochemical gradient. From this from the outside to this side, to the inside. And again we'll study the specifics of these types of voltage gate ion channels in electra to come. Now let's focus on Gap Junction. So what are Gap junctions?