 In this video, we're going to be discussing myocardial action potentials. Now, you might be familiar with skeletal muscle action potentials, and even though myocytes are a type of muscle tissue, they have their own unique types of action potentials that make them uniquely suited for the heart. For example, these action potentials occur in five different phases, as opposed to only three in skeletal muscle. These are the kind of potentials that are going to cause the atria and ventricles of the heart to contract in a synchronized manner. There's a unique type of action potential that occurs in the SA and AV nodes of the heart, and we'll discuss those in a separate video on pacemaker potentials. And action potentials spread from myocyte to myocyte by gap junctions. Gap junctions are pores in between the myocytes that allow ions like sodium to flow from cell to cell directly without leaving the membrane. This is important in the heart because it allows for a synchronized contraction, as opposed to skeletal muscle in which muscle fibers are activated and contract individually. If you take a look on the right here, you can see some cardiac tissue under a microscope. You know it's cardiac tissue because the muscle fibers have this nice branching pattern. You can see that there's some staining here, and the staining in this case is for some proteins that make up gap junctions, and you can see that they're present on both ends of the myocyte, and they allow sodium and other ions to flow between the cells. Now let's discuss the five phases of the myocyte action potential. By convention, we call the first phase phase zero or the rapid upstroke. I keep in mind that myocytes have a negative resting potential as illustrated in this diagram here. During the rapid upstroke, there's an opening of the voltage-gated sodium channels in response to a stimulus, causing sodium to enter the cell, and as you can see in the diagram, this causes the potential of the cell to rapidly become more positive, reaching a positive peak at this point. This is when we start to experience phase one or the initial repolarization. This is a phase that's unique to myocardial action potentials, and it involves the closing of those voltage-gated sodium channels in the opening of voltage-gated potassium channels. At this point, potassium will begin to exit the cell, making the potential slightly more negative as you see here. This is when we start to experience phase two, another phase that's unique to myocardial action potentials, and we call this the plateau. During the plateau phase, those potassium channels will remain open, keeping potassium exiting the cell. At this point, calcium channels will also open, allowing calcium to enter the cell. The entrance of calcium will balance out the exit of potassium, causing a plateau in the potential, as you can see in the diagram here. This is an example of calcium-induced calcium release, in which the release of calcium initiates a positive feedback loop, causing the release of more calcium from the sarcoplasmic reticulum. This plateau phase is very important and unique to the myocardial action potential, and it corresponds with diastole, and causes a delay that allows for the ventricles to get filled effectively. This is when we move on to phase three, or repolarization. During the repolarization phase, those calcium channels close, while the potassium channels remain open. At this point, potassium will continue to exit, causing a repolarization, and the potential will become more negative, as you see in the diagram here. Finally, we reach phase four, which is the return to resting potential. At this point, the potassium channels close, and we're ready for a new action potential to start. Now let's take a moment to touch on the concept of a refractory period. You can see the refractory period labeled in the diagram here, and it represents the time between this rapid upstroke, or the initiation of the action potential, and the next possible time that another action potential could be initiated. And this is important, especially in cardiac physiology, because it ensures that a second action potential does not start before the first one has ended, because that could interrupt the rhythm of the heart. We can further divide the refractory period into an absolute segment and a relative segment. During the absolute refractory period, no matter how large a second stimulus is that comes along, a second action potential cannot be kicked off, whereas during the relative refractory period, if a larger than normal stimulus were to come along, it is possible to initiate a second action potential. But as you can see in the diagram, the relative refractory period is quite short, and it's close to the myocytes resting potential.