Understanding Cardiac Action Potentials

Hello, so today we're going to talk about the cardiac action potential. So when we say cardiac, it is a type of muscle that is located in your heart. So the heart muscle is very remarkable. So at an average heart rate of 70 beats per minute, the heart needs to contract and relax more than 100,000 times a day. without stopping or tiring. So the rate and the strength of these contractions must vary to meet physiological and pathological challenges. So today we're going to talk about the degeneration and spread of the action potential and the process and excitation coupling of your heart jack muscles. So myocyte is the cells of your heart. So you call the cells of your heart as myocytes. And there are two cardiac action potentials that are happening in your heart. So the first one is a non-pacemaker action potential. And the other one is a pacemaker action. potential so the non-face maker the non-face maker these are also known as fast response action potential because of rapid depolarization it can be found all throughout your heart except for the pacemaker cells so all of the muscles that is located in your heart so these cells are responsible for the fast response response action potential while your pacemaker is also known as the slow response action potential because of slower rate of depolarization and it can be found in your sinoatrial and atrioventricular nodes so both types of action potential in your heart differ considerably from action potentials found in your neural and skeletal muscle cells So one major difference is in the duration of the action potentials. So in a typical nerve, the action potential duration is about 1 millisecond, while in your skeletal muscle cells, the action potential duration is approximately 2 to 5 milliseconds. So in contrast, the duration of cardiac action potential range from 200 to... 400 milliseconds. So as you can see or as you observe your heart contractions happens very fast. So now let's discuss how the action potential happens in your non-pacemaker cells. the work of cells have a large stable resting membrane here and it display a prolonged action potential that has a platus stage so the first one the main the main ions involved here are your potassium sodium and also calcium so firstly so let's have a look on the typical cell or your myocyte so let's get one cell of your heart so in the cells of your heart you have this gap junctions okay where where the exchange of of ions are are happening here and also we have your channels so that is in different colors because it depends on the specific ions that that will enter or or exit your cells okay so first one is you have your um potassium that licks out of your cell so this is normal okay so in your resting membrane potential this is normal calcium potassium is leaking out so the resting membrane potential here is negative 90 millivolts okay so you have potassium leaking out and then in your gap junctions you have your sodium leaking inside yourself so the movement of molecules is from high concentration to low concentration and we talked about these examples earlier that in the amount of sodium outside your cell is higher compared to the amount of sodium inside your cell. Now the movement is from high concentration to low concentration. That's why sodium leaks in inside your cell through your gap junctions. The next one. Since you are increasing sodium inside your cell, then you're going to increase the membrane potential of your cell. So it will reach now the threshold. So the threshold here is negative 60 millivolts. Once you reach the threshold, then you're going to open. your sodium gated ion channel. So you're going to increase more sodium inside your cell because there are also sodiums leaking inside your cell through your gap junctions. So more sodiums inside your cell, then you are making your membrane to become positive. Okay, so when When you reach positive 10, then it will close your sodium-gated ion channel and it will open another channel which is your potassium-gated ion channel. So when potassium-gated ion channel opens, you are leaking out more potassium outside your cells which makes your membrane to become near zero millivolts. So after some time, you will open another gate. So after some time, this ion channel will open and that is your calcium-gated. ion channel so you have more calcium going inside your cell and you're also leaking out more potassium outside your cell so there are ions going in and ions going outside your cell which makes your graph to become straight so this stage is called your flat two stage okay so remember the the types of land formation that a plateau is like a mountain but it is flat on top so that's why this this is what they call this stage okay so now um then after some time again um this calcium gated ion channel closes leaving potassium gated ion channel um opens now if you're going to leave your potassium-gated ion channel open, then you're leaking out more potassium outside your cell, which makes your graph to become more negative until you reach the resting membrane potential. Now, in your non-pacemaker action potential, we have four phases. So we have the resting, the resting potential or the resting membrane potential is phase four. So phase zero is depolarization. Phase one is early repolarization. Then phase two is your plateau stage and phase three is your repolarization. Okay. So as you observe, so it kind of resembles your neural action potential or the skeletal muscle action potential. So remember that a non-pacemaker action potential lasts about 300 milliseconds. For the vast majority of this time, the cell is absolutely refractory to further stimulation. In other words, a further action potential will not be generated until repolarization is virtually complete. So this prevents tetany from reoccurring. So if a supramaximal stimulus occurs during the relative refractory period, the resultant action potential has a slower rate of depolarization and is of smaller amplitude than normal. So producing a much weaker contraction than normal. Now let's see now your pacemaker action potential. So this pacemaker. action potential the responsible cells here are your pacemaker cells and pacemaker cells are unique because they um they make your heart beats in a unison so that property is what we call automaticity so your pacemaker cell has this type of property automaticity Okay, and your pacemaker cells, we have three types of pacemaker cells that is located in your heart, namely sinoatrial node, atrioventricular node, and bundle of yeast or carcinogen fibers. Now, these cells are located here. So, this part is your sinoatrial node, and then this one is your atrioventricular node that is located in between your... um atrium and your ventricle then we have uh in your septum we have your bundle of keys and then in your uh the apex of your heart at the apex of your heart we have your purkinje fibers and we also have the minor um cells here which is your backman's bundle and then internodal pathways and the left and right branch you that separates your left and right ventricles going to your purkinje fibers okay now how does the the pacemaker action potential happens so um let's have a look again on your uh cell of your pacemaker okay so here so zoom is licking inside your cell through your sodium gated ion channel okay so if you're going to increase more sodium inside your cell then you're going to make your membrane to become less negative so the the membrane potential here is also negative okay so when you reach the threshold you will open another channel but sodium-gated ion channel will also open and then on this stage you will open two channels okay so the sodium-gated channel and the calcium-gated channel causing the entry of sodium and calcium inside yourself so these ions will make your membrane to become positive so the graph will show up until you reach positive 10 and then when you reach positive then it will make your calcium gated ion channel closes and then opens your potassium gated ion channel so when you close your calcium gated ion channel automatically calcium cannot enter your cell so if you're going to open your potassium gated ion channel then potassium will leak out of your cell because we have high amount of potassium inside our cell and we have low amount of potassium outside our cells then the movement is from high concentration to low concentration so potassium is going out so if we're going to leak out of our potassium it will make our membrane to become more negative until you reach the negative 90 and this cycle continues okay so remember that this um uh face here is one heartbeat in one second. That is happening a one heartbeat in one second. Okay. And the phases here is 4, 0, and 3, which is your resting membrane that is going up, slightly going up there, and your depolarization stage. and your repolarization stage okay so um as you so as you observe the the pacemaker action potential differs from the non-pacemaker action potential in faces uh in their faces so faces one and two are absent here the heart displays automaticity which makes your heart to contract in a continuous manner so the pacemaker cells do not have a stable resting action potential and it is the spontaneous depolarization of the pacemaker potential that gives the heart its automaticity okay so the pacemaker potential is produced by a decrease in membrane permeability to potassium and a slow inward current because of calcium influx and then an increase in sodium current because of sodium calcium exchange then once the threshold potential is reached then the calcium channel opens then calcium ion enter depolarize a depolarization occurs so in contrast to the non-face maker cells or the non-pacemaker action potential, there is no inward movement of sodium ions during depolarization. So repolarization occurs because of an increase in potassium permeability. So at the sinoatrial node, potassium permeability can be further enhanced by vagal stimulation. So this has the effect of hyperpolarizing the cell and reducing the rate of firing. And sympathetic stimulation has the opposite effect, of course. Now, the question is, how does the reuptake of calcium in your sarcoplasmic reticulum? Okay, in order to bring back the lost calcium or the lost ions inside your cell. So, just like your... skeletal muscle cell then the sarcoplasmic reticulum will also reuptake the lost calcium the lost calcium with the help of catecholamines okay so this catecholamines activate beta so with the help of catecholamines phosphorylation of of myosin also occurs that increase the rate of cross-bridging cycle so it also increased the rate of pre-uptake of calcium into your sarcoplasmic reticulum and also aids for relaxation so catecholamine here has a greater role for re-uptaking and for increasing the rate of the cross-bridging and for the relaxation process of your cardiac muscles. So this ends our topic and I'm hoping that you learned something. Thank you for listening.

Hello, so today we're going to talk about the cardiac action potential. So when we say cardiac, it is a type of muscle that is located in your heart. So the heart muscle is very remarkable.

So at an average heart rate of 70 beats per minute, the heart needs to contract and relax more than 100,000 times a day. without stopping or tiring. So the rate and the strength of these contractions must vary to meet physiological and pathological challenges. So today we're going to talk about the degeneration and spread of the action potential and the process and excitation coupling of your heart jack muscles.

So myocyte is the cells of your heart. So you call the cells of your heart as myocytes. And there are two cardiac action potentials that are happening in your heart.

So the first one is a non-pacemaker action potential. And the other one is a pacemaker action. potential so the non-face maker the non-face maker these are also known as fast response action potential because of rapid depolarization it can be found all throughout your heart except for the pacemaker cells so all of the muscles that is located in your heart so these cells are responsible for the fast response response action potential while your pacemaker is also known as the slow response action potential because of slower rate of depolarization and it can be found in your sinoatrial and atrioventricular nodes so both types of action potential in your heart differ considerably from action potentials found in your neural and skeletal muscle cells So one major difference is in the duration of the action potentials. So in a typical nerve, the action potential duration is about 1 millisecond, while in your skeletal muscle cells, the action potential duration is approximately 2 to 5 milliseconds. So in contrast, the duration of cardiac action potential range from 200 to...

400 milliseconds. So as you can see or as you observe your heart contractions happens very fast. So now let's discuss how the action potential happens in your non-pacemaker cells.

the work of cells have a large stable resting membrane here and it display a prolonged action potential that has a platus stage so the first one the main the main ions involved here are your potassium sodium and also calcium so firstly so let's have a look on the typical cell or your myocyte so let's get one cell of your heart so in the cells of your heart you have this gap junctions okay where where the exchange of of ions are are happening here and also we have your channels so that is in different colors because it depends on the specific ions that that will enter or or exit your cells okay so first one is you have your um potassium that licks out of your cell so this is normal okay so in your resting membrane potential this is normal calcium potassium is leaking out so the resting membrane potential here is negative 90 millivolts okay so you have potassium leaking out and then in your gap junctions you have your sodium leaking inside yourself so the movement of molecules is from high concentration to low concentration and we talked about these examples earlier that in the amount of sodium outside your cell is higher compared to the amount of sodium inside your cell. Now the movement is from high concentration to low concentration. That's why sodium leaks in inside your cell through your gap junctions.

The next one. Since you are increasing sodium inside your cell, then you're going to increase the membrane potential of your cell. So it will reach now the threshold. So the threshold here is negative 60 millivolts.

Once you reach the threshold, then you're going to open. your sodium gated ion channel. So you're going to increase more sodium inside your cell because there are also sodiums leaking inside your cell through your gap junctions. So more sodiums inside your cell, then you are making your membrane to become positive.

Okay, so when When you reach positive 10, then it will close your sodium-gated ion channel and it will open another channel which is your potassium-gated ion channel. So when potassium-gated ion channel opens, you are leaking out more potassium outside your cells which makes your membrane to become near zero millivolts. So after some time, you will open another gate.

So after some time, this ion channel will open and that is your calcium-gated. ion channel so you have more calcium going inside your cell and you're also leaking out more potassium outside your cell so there are ions going in and ions going outside your cell which makes your graph to become straight so this stage is called your flat two stage okay so remember the the types of land formation that a plateau is like a mountain but it is flat on top so that's why this this is what they call this stage okay so now um then after some time again um this calcium gated ion channel closes leaving potassium gated ion channel um opens now if you're going to leave your potassium-gated ion channel open, then you're leaking out more potassium outside your cell, which makes your graph to become more negative until you reach the resting membrane potential. Now, in your non-pacemaker action potential, we have four phases. So we have the resting, the resting potential or the resting membrane potential is phase four. So phase zero is depolarization.

Phase one is early repolarization. Then phase two is your plateau stage and phase three is your repolarization. Okay. So as you observe, so it kind of resembles your neural action potential or the skeletal muscle action potential.

So remember that a non-pacemaker action potential lasts about 300 milliseconds. For the vast majority of this time, the cell is absolutely refractory to further stimulation. In other words, a further action potential will not be generated until repolarization is virtually complete. So this prevents tetany from reoccurring.

So if a supramaximal stimulus occurs during the relative refractory period, the resultant action potential has a slower rate of depolarization and is of smaller amplitude than normal. So producing a much weaker contraction than normal. Now let's see now your pacemaker action potential.

So this pacemaker. action potential the responsible cells here are your pacemaker cells and pacemaker cells are unique because they um they make your heart beats in a unison so that property is what we call automaticity so your pacemaker cell has this type of property automaticity Okay, and your pacemaker cells, we have three types of pacemaker cells that is located in your heart, namely sinoatrial node, atrioventricular node, and bundle of yeast or carcinogen fibers. Now, these cells are located here. So, this part is your sinoatrial node, and then this one is your atrioventricular node that is located in between your...

um atrium and your ventricle then we have uh in your septum we have your bundle of keys and then in your uh the apex of your heart at the apex of your heart we have your purkinje fibers and we also have the minor um cells here which is your backman's bundle and then internodal pathways and the left and right branch you that separates your left and right ventricles going to your purkinje fibers okay now how does the the pacemaker action potential happens so um let's have a look again on your uh cell of your pacemaker okay so here so zoom is licking inside your cell through your sodium gated ion channel okay so if you're going to increase more sodium inside your cell then you're going to make your membrane to become less negative so the the membrane potential here is also negative okay so when you reach the threshold you will open another channel but sodium-gated ion channel will also open and then on this stage you will open two channels okay so the sodium-gated channel and the calcium-gated channel causing the entry of sodium and calcium inside yourself so these ions will make your membrane to become positive so the graph will show up until you reach positive 10 and then when you reach positive then it will make your calcium gated ion channel closes and then opens your potassium gated ion channel so when you close your calcium gated ion channel automatically calcium cannot enter your cell so if you're going to open your potassium gated ion channel then potassium will leak out of your cell because we have high amount of potassium inside our cell and we have low amount of potassium outside our cells then the movement is from high concentration to low concentration so potassium is going out so if we're going to leak out of our potassium it will make our membrane to become more negative until you reach the negative 90 and this cycle continues okay so remember that this um uh face here is one heartbeat in one second. That is happening a one heartbeat in one second. Okay.

And the phases here is 4, 0, and 3, which is your resting membrane that is going up, slightly going up there, and your depolarization stage. and your repolarization stage okay so um as you so as you observe the the pacemaker action potential differs from the non-pacemaker action potential in faces uh in their faces so faces one and two are absent here the heart displays automaticity which makes your heart to contract in a continuous manner so the pacemaker cells do not have a stable resting action potential and it is the spontaneous depolarization of the pacemaker potential that gives the heart its automaticity okay so the pacemaker potential is produced by a decrease in membrane permeability to potassium and a slow inward current because of calcium influx and then an increase in sodium current because of sodium calcium exchange then once the threshold potential is reached then the calcium channel opens then calcium ion enter depolarize a depolarization occurs so in contrast to the non-face maker cells or the non-pacemaker action potential, there is no inward movement of sodium ions during depolarization. So repolarization occurs because of an increase in potassium permeability.

So at the sinoatrial node, potassium permeability can be further enhanced by vagal stimulation. So this has the effect of hyperpolarizing the cell and reducing the rate of firing. And sympathetic stimulation has the opposite effect, of course. Now, the question is, how does the reuptake of calcium in your sarcoplasmic reticulum?

Okay, in order to bring back the lost calcium or the lost ions inside your cell. So, just like your... skeletal muscle cell then the sarcoplasmic reticulum will also reuptake the lost calcium the lost calcium with the help of catecholamines okay so this catecholamines activate beta so with the help of catecholamines phosphorylation of of myosin also occurs that increase the rate of cross-bridging cycle so it also increased the rate of pre-uptake of calcium into your sarcoplasmic reticulum and also aids for relaxation so catecholamine here has a greater role for re-uptaking and for increasing the rate of the cross-bridging and for the relaxation process of your cardiac muscles. So this ends our topic and I'm hoping that you learned something.

Thank you for listening.

Transcript for:Understanding Cardiac Action Potentials

Transcript for:
Understanding Cardiac Action Potentials