Editing Cardiac action potential (section)

===The fast sodium channel===
{{main|Sodium channel}}
These [[sodium channel]]s are voltage-dependent, opening rapidly due to depolarization of the membrane, which usually occurs from neighboring cells, through gap junctions. They allow for a rapid flow of sodium into the cell, depolarizing the membrane completely and initiating an action potential. As the membrane potential increases, these channels then close and lock (become inactive). Due to the rapid influx sodium ions (steep phase 0 in action potential waveform) activation and inactivation of these channels happens almost at exactly the same time. During the inactivation state, Na<sup>+</sup> cannot pass through (absolute refractory period). However they begin to recover from inactivation as the membrane potential becomes more negative (relative refractory period).{{cn|date=May 2025}}