Action Potential In Cardiac Muscle || Heart || Cardiovascular Physiology

An interesting feature about the cardiac action potential is a plateau. But why exactly is it seen? We will find out the reason as we study the action potential in cardiac muscle in this video. Welcome back to nonstop neuron.com where learning medical concepts is as easy as watching cartoons. Before starting I recommend watching a separate video on the generation of action potential first. Although we will be covering all the basics in this video also, your understanding will be better if you ha

ve watched that video. Now let's get started. This is a ventricular muscle cell. The ionic composition around here is similar to all other cells in the body. The Na concentration is higher outside the cell so it tends to diffuse in. K concentration is high inside the cell so it tends to diffuse out. Finally Ca concentration is also higher outside, so it tends to diffuse in. Now this graph is showing membrane potential vs time. Resting membrane potential in ventricular muscle is about -90 mV. We

know that at the heart, impulses are generated at the SA node. As the impulse reaches this point, the action potential will appear here. It begins with voltage-gated fast sodium channels. The incoming impulse, opens this channel. This opening allows the rapid influx of a large number of sodium ions. As sodium is a positively charged ion, it makes inside of the cell electropositive and the potential reaches about +15 mV very rapidly. This phase is called phase 0, upstroke, or initial depolarizati

on. Next, the fast sodium channels close. This prevents the rapid influx of Na. Also... a small quantity of potassium leaks out through K channels. This leads to a little bit of repolarization. This is called phase 1 or initial repolarization. Now comes the interesting part. One, soon after the onset of the action potential, fast potassium channels close. So the efflux of potassium ions stops. And two, the cardiac muscles have L type of Ca channels. L means long-lasting but I like to call them l

azy channels. Yes, they are lazy because they are slow to open and slow to close. They are activated during the upstroke and opens slowly. Now they allow the influx of Ca into the myocyte. Because of their slow nature, they stay open for a longer time and close very slowly. This allows an influx of Ca for quite some time. Thus two things are going on at present. Potassium is not moving out and at the same time, Ca is continuously moving in. This loading of positive ions keeps the membrane depola

rized for some time and thus creates a plateau in the action potential. Due to this long plateau, the contraction time is also longer in cardiac muscle. So that was phase 2 or plateau. Eventually though, the Ca channels do close, so the influx of Ca stops. And slow potassium channels open so the efflux of potassium starts. Due to the exit of the net positive charge, the membrane returns to resting negative potential. This repolarization phase is called phase 3 or final repolarization. Then of co

urse we have resting membrane potential till the next action potential. This is also called phase 0. So this was the action potential in ventricular muscle. Now let's talk about, how the ionic balance is restored. We have seen that during an action potential, sodium moves in, K moves out and Ca moves in. To restore the balance, we have Na-K ATPase pump, Na-Ca exchanger and Ca ATPase pump on the membrane. Na-K ATPase pump moves sodium out and K in, using energy from ATP. Na-Ca exchanger actively

moves Ca out using the downhill entry of Na into the cell. Don't worry about the Na entering here. It will be moved out by Na-K ATPase pump. And finally we have Ca pumps that move Ca out using energy directly from ATP. So this was all about action potential in cardiac muscle. Now let's have a quick summary. The action potential in cardiac muscle has 5 phases. The phase 0 is due to the opening of fast sodium channels and rapid influx of Na. Phase1 or initial depolarization is due to the closure o

f fast sodium channels and efflux of a small quantity of potassium through open potassium channels. Phase 2 or plateau is due to the opening of slow or L type of calcium channels leading to Ca influx and closure of potassium channels which prevents efflux of K. The Ca channels stay open for quite some time so we get a long plateau. Then Phase 3 or final repolarization is due to closure of L-type calcium channels that prevent further Ca entry and opening of slow K channels leading to K efflux. P

hase 4 is resting membrane potential till the next action potential. To restore the ionic balance, Na-K ATPase pump moves Na out and K in, and Na-Ca exchanger and Ca pump moves the Ca out. We will talk about spread of action potential in cardiac muscle in a separate video. That's it for this video. If you feel this video will help your friends and colleagues, please share it with them too. And don't forget to subscribe because lots more to come. At nonstop neuron, learning medical concepts is as

easy as watching cartoons. Thanks for watching, see you in the next video.