 Cardiac muscle cells have several characteristics that are different from skeletal muscle cells. Let's make a list of those characteristics. First of all, cardiac muscle cells have one nucleus per cell. This would be an awesome compare and contrast question on an exam comparing contrast the structural functional characteristics of cardiac muscle and skeletal muscle. So one of those characteristics is the fact that cardiac muscles have one nucleus per cell. Cardiac muscle tissue is striated. Do you remember what caused the striations in skeletal muscle? It was the presence of the sarcomeres. So because cardiac muscle tissue is striated, there are sarcomeres inside the tissue. And if you look down here, we've blown up a cardiac muscle cell, a couple of them here. And you can see that, yeah, we do have the myofilaments organized in sarcomere fashion. That gives you the visual of striation. Cardiac muscle cells are even more filled with mitochondria than skeletal muscle cells. Why? Because dog pounds, your heart started beating when you were 22 days old in your mama's belly. 22 days old and you get a little heart beating. And then your heart beats forever till you die. And that's like 100,000 beats a day. What? That's incredible. You need a lot of energy to sustain that kind of activity. And so we have, I think it said, a third of the volume of a cardiac muscle cell is made up of mitochondria. That's incredible. Other, there are a couple more characteristics about cardiac muscle tissue. One of them, cardiac muscle cells are branched. And this is something that you can actually see if you look at the histology of cardiac muscle tissue. But take a look at this. This is a cell and here's an end of the cell and here's an end of the cell. And did you see how it totally, like, branched? And this is a structural characteristic that enables cardiac muscle tissue to be stronger. You want it to be strong because if your heart rips apart, oh, that's probably not a real great idea. And if you think about exercise or stress or bears chasing you around in the woods, it's definitely, you want your heart to be able to increase its heart rate and increase the amount of blood that it can pump through your body without tearing. You really don't hear about heart muscle tears that often. You definitely hear about skeletal muscle tears. You know, you're playing soccer and you tear your quadriceps group. That really stinks. But if you tear your cardiac muscle, it's going to be a bad situation. So this branching is one piece that makes it just stronger and less likely to tear. The other thing that makes it stronger are these structures called intercalated discs. And intercalated discs are a combination. Look, here's one right here. This is an intercalated disc. It's where two cardiac muscle cells connect to each other. And you can end up with this visual of, like, a literal line between two cardiac muscle cells. This is the other one of the characteristics that helps you identify cardiac muscle tissue in the microscope. But these intercalated discs have, number one, they're made of structures called desmosomes. And these are, like, super strong staple connections. Like, they weld the two cells together at the intercalated disc. The other thing that is found in an intercalated disc, doggies, oh my doggies, gap junctions. And remember, gap junctions are just, like, tunnels that allow cytoplasm. Okay, we're going to make a little bit of yellow cytoplasm. They allow cytoplasm to move between cells. So if you've got a whole bunch of gap junctions in your intercalated discs, you're serving two functions. Number one, they're holding the muscle cells together. And number two, they're allowing cytoplasm through. This is functionally critical. Your heart will not do its job if those gap junctions are closed. Another characteristic of cardiac muscle tissue that's different from skeletal muscle tissue, is that you do not need a neuron. Okay, I'm not sure why I'm drawing this neuron. You don't need neural input in order for cardiac muscle tissue to contract. Cardiac muscle tissue will keep contracting until it runs out of energy. And I just have to show you, I probably should turn off the sound here because really there's no point in listening to my Marvel about this turtle that he killed one day when he decided he was hungry for turtle meat. And so he cut out the heart of the turtle, was cleaning the turtle, and then realized, look at the heart. It's still beating. And hopefully you're like, dude, really? Why? How's that even possible? It's possible because even though we've taken this heart out of the turtle body and it has no neural input whatsoever. It's not connected to the brain. It's not connected to the blood supply. The cardiac muscle is able to generate its own action potential and that allows it to continue to beat. This is pretty phenomenal. Oh, that was a close call. I always get scared when we get to the end of a YouTube video because sometimes things come up that you really don't want in my YouTube video. Okay, so there's two different kinds of cardiac muscle cells. There are contractile cells and auto-rhythmic cells. And we're going to look at contractile cells first. Both of them respond to action potentials. Both of them contract when an action potential tells them to. So let's take a look at contractile cells first.