Editing Cardiac action potential (section)

===Phase 1===
This phase begins with the rapid inactivation of the Na<sup>+</sup> channels by the inner gate (inactivation gate), reducing the movement of sodium into the cell. At the same time potassium channels (called I<sub>to1</sub>) open and close rapidly, allowing for a brief flow of potassium ions out of the cell, making the membrane potential slightly more negative. This is referred to as a 'notch' on the action potential waveform.<ref name="santana 496" />

There is no obvious phase 1 present in pacemaker cells.