Action potentials are the really fast electrical changes that happen across the membrane of certain cells, and often propagate from one cell to an adjacent cell. And cells in the heart communicate this way. Now, that signal’s gotta start somewhere, so some of these cells, called pacemaker cells, have the responsibility of setting the rhythm and pace of the heartbeat. So they’ve got this really important job, but they’re a relatively tiny group, and make up only about 1% of the heart cells. But they’re able to continually generate new action potentials that get conducted to the rest of the heart, or the other 99%, and so these are what tell the heart to pump. The cells that receive that signal are called myocytes because they make up the myocardium, which is the muscular middle layer of the heart. Myocytes are also called contractile cells because they contract to allow the heart to pump blood. Myocytes are different from skeletal muscle cells though, which get their action potential signals directly from neurons. Cardiac myocytes receive signal from pacemaker cells causing them to contract.
Now let’s focus on a single myocyte cell going through a single action potential. The action potential of a myocyte is broken into five phases. Often they’re shown on a graph of membrane potential vs. time. We’re going to start with Phase 4, because why not.
In phase 4, or the resting phase, our little myocyte friend is at rest, hanging out with an overall charge or membrane potential of -90 mV. Now, the interesting thing is that it has gap junctions which are openings between two myocytes. So when the myocyte’s neighbour depolarizes, some ions - mainly calcium ions - start leaking through the gap junctions and that makes the membrane potential go up to about -70 mV. -70mV is called the threshold potential and it marks the start of phase 0.