Excitability and refractory periods

Cardiac excitability refers to the amount of inward current needed by myocytes or myocardial cells, cells in the muscular middle layer of the heart, to depolarize or generate an action potential.

Whether or not it depolarizes depends on if its voltage-gated «sodium ion channels are excitable or not. A more excitable cell might have more of its Na+ ion channels in the ready state, and even if there were a relatively weak current of Na+ ions flowing in, the cell might still depolarize easily.

On the other hand, a less excitable cell might have most of its Na+ ion channels inactivated, where they won’t open in response to stimuli, represented by this little ball stuck in the opening, and only a few of them are ready, and it would require a strong current of Na+ ions to flow in before it depolarized.

So let’s say this is a myocyte in one of the ventricles,, And this is a graph of membrane potential over time.. First, a few positive ions like sodium and calcium travel through gap junctions and enter into the cell, raising the membrane potential to a threshold level—typically around 70 mV.

At that point, the voltage gated Na+ channels open up, and lots of Na+ ions rush into the cell, causing depolarization. Right after depolarizing, at about +20 mV, the channels become inactivated, making those channels unavailable for another depolarization.

After the upstroke, there’s the plateau, and then as the cell repolarizes the sodium channels start to recover, and even though they’re closed, they’re still excitable, and eventually the cell repolarizes back to it’s usual state around -90mV..

During most of the action potential, the myocardial cell is unable to depolarize again, and this is called the absolute refractory period.

In other words, during the absolute refractory period, pretty much all the myocyte’s sodium channels are inactivated, so , so even if a bunch of inward current comes from the neighboring cell, it literally can’t depolarize.There are many Na+ channels on each myocardial cell, and each Na+ channel operates independently, but overall, most of them remain inactivated after the upstroke, through, the plateau, and until the cell has repolarized to about −50 mV, at which point some channels start to recover, at which point the cell would respond to a stimulus..

Now the way that the absolute refractory period is measured is that an electrophysiologist delivers a current to a myocardial cell and in response it has an action potential.

While that action potential is happening, the electrophysiologist delivers multiple bursts of current to the cell at regular intervals.

If a burst of current causes nothing to happen, then that means the cell is still in the absolute refractory period. But if a burst of current causes the cell to depolarize, then that means that the cell has emerged from the absolute refractory period.

Since the bursts of current are given at intervals, we landed just outside the absolute refractory period, it’s possible that the burst of current was given at the exact moment that the cell was emerging from it’s absolute refractory period.

But it’s more likely that the cell emerged from the absolute refractory period at some point and then waited a tiny bit before the next burst of current was applied.

In some cases the burst of current that’s used isn’t very strong, and as a result the cell may not depolarize even though it’s no longer in the absolute refractory period.

So when electrophysiologists are measuring the absolute refractory period, what they end up identifying is called the effective refractory period.

Next comes the relative refractory period, which begins at the end of the absolute refractory period and keeps going until the cell membrane has almost completely repolarized.

During this period, some of the Na+ channels have recovered and are available for another depolarization, and the proportion of Na+ channels increases over time.

A cell in the relative refractory period can depolarize, but it needs a bigger stimulus than normal. Finally, we’ve got the supranormal period, where supra- means “more than,” and this is when the myocardial cell is more excitable than usual.

The supranormal period starts when the membrane potential is −70 mV, and it keeps going until the membrane is fully repolarized back to −85 mV.

At this point, the Na+ channels have completely recovered, and the inactivation gates are open again. But, the cell’s membrane potential is closer to the threshold than it is at rest, so it’s slightly easier for the cell to depolarize.

All right, as a quick recap. When the cell’s at -90 mV it’s resting, and all the sodium channels are closed, but ready to rock.

Once a stimulus comes in, the channels open for a short period during the upstroke. The absolute refractory period is when the myocardial cell’s Na+ channels are inactivated and it absolutely cannot depolarize in response to a burst of current.

The effective refractory period is often used interchangeably with the absolute refractory period, and represents the practical limitations of trying to measure the absolute refractory period and is therefore a tiny bit longer.

The relative refractory period is when some of the myocardial cell’s Na+ channels are open, and so the cell is able to depolarize in response to a larger than normal burst of current.

The supranormal period is when all of the myocardial cell’s Na+ channels are recovered and ready to depolarize, but the cell is not fully repolarized and therefore is closer to threshold than when it’s fully repolarized.