Dr. Andy Galpin

Another thing that differentiates the skeletal from the cardiac tissue is how and how long they contract.

In skeletal muscle, we actually want the ability to do what's called summation to reach tetany.

And so what happens is the muscle fibers in, say, your biceps brachii will contract with that electrical potential.

And then actually almost before it gets all the way back to baseline, it will contract again.

And then it'll contract again and contract again.

And so those mini contractions start to stack on top of each other or summate.

And in fact, if you do that long enough, you can reach what is called full tetany.

Think of this as a muscle cramp.

So this is the muscle fibers themselves contracting permanently instead of doing this kind of on-off, on-off rhythm.

Cardiac tissue doesn't do that, and I think you could probably imagine why.

It would be a very bad thing for you to reach tetany of your heart.

Remember, when we first started talking about the anatomy of your heart,

The primary job of the heart is to move blood from the atria, or the top of the heart, to the ventricles in the bottom, and then move that out to the body.

So if this thing were to reach tetany, blood wouldn't move anywhere.

You wouldn't be able to circulate any blood throughout your body, and of course you would die.

So while it's okay to have a cramp in your calf and it's painful and it's annoying and it's all those things, having a cramp in your heart would be far worse.

And so your body hedges against that.

And what it says is, all right, if I have this extremely fast, what's called refractory time in skeletal muscle, it's the ability to kind of contract multiple times within a single muscle fiber.

I want to extend the time of contraction in the cardiac tissue so that I don't have that repeat in summation.

So in addition to not wanting tetany, you also need to allow time for blood to fill up the ventricles.