Action potentials are the really rapid electrical changes that occur across the membrane of certain cells, and often propagates from one cell to an adjacent cell. Cells in the heart communicate this way.
That signal’s gotta start somewhere, so some of these cells, called pacemaker cells, have the responsibility of setting the rhythm and the pace of the heartbeat.
They’ve got this really important job, but they’re a relatively tiny group -- only about 1% of the heart cells -- and they’re able to continually generate new action potentials that get conducted to the rest of the heart -- the other 99% -- and that’s what tells the heart to pump.
The cells that receive this 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, which get their action potential signals directly from neurons.
Okay, now let’s take a closer look at the chemistry that gets that action potential moving. Action potentials are initiated by depolarization, which is the opposite of polarization.
Polarization is when there’s a higher negative charge inside the cell relative to outside the cell, and that difference in charge is called the membrane potential.
So if the membrane potential is negative the inside of the cell is more negative than the outside, if it’s positive the inside is more positive than the outside, and if it’s 0mV, then the inside and outside have the same charge - there’s 0mV of difference.
Ok -- so, the key here is understanding how the membrane potential changes, and it all comes down to the movement of ions. Specifically, two factors -- which ion wants to move across the membrane, and how permeable the membrane is to that ion.
So, depolarization is when ions move across the membrane and the membrane potential becomes less negative or even slightly positive. Think of a really pessimistic negative cell throwing his hands up and enjoying a moment of joy.
When one cell depolarizes enough - it can cause some ions to flow into neighboring cells and trigger them to depolarize as well. If one cell after another depolarizes, then there’s a depolarization wave which you can imagine would look like a wave moving through a crowd at a football stadium.
Each depolarization wave causes heart muscle contraction, so the rate at which depolarization waves ripple through the heart actually sets the heart rate.
So if depolarization waves are going through about once per second, that means that your heart beats once per second, or sixty times in a minute.
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 at 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.