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 it’s 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..