Action Potential In SA Node || Primary Pacemaker of the Heart

Do you know that heart can keep beating even when it is removed from the body? But why? We will find out the answer as we study the action potential in the SA node in this video. Welcome back to nonstop neuron.com where learning medical concepts is as easy as watching cartoons. Let's get started. The heart generates impulses on its own to trigger a heartbeat. For this, the heart has three pacemaker tissues: the SA node, the AV node and the Purkinje fibers. They all can generate impulse on their

own. However, their rate of impulse generation is different. Normally the SA node generates impulses faster than other tissues. Because of the slower rate in other tissues, an interesting thing happens. Let's say this is time zero and the SA node and AV node both start generating impulses on their own at the same time. Because of the faster rate in the SA node, impulse first appears in the SA node and quickly spreads to the AV node. This incoming impulse stimulates the AV node even before the AV

node has generated its own impulse. And this happens every time. So the AV node is stimulated at the frequency of the SA node. The same applies to the entire conductive system. So normally impulses from the SA node override all other tissues and the SA node controls the heart rate. But for any reason, if the SA node gets slower than the inherent frequency of other tissues, the other tissue will generate its own action potential before the impulse from the SA node reaches it. In that case that t

issue overrides all other tissue and becomes the pacemaker of the heart. Thus the fastest of the pacemaker tissues controls the heart rate. As I already said, normally the fasted one is the SA node. And that is the one that we will be studying in this video. The SA or sinoatrial node is also called the sinus node. It is located in the right atrium near its junction with superior vena cava. It is a small and flat, ellipsoid strip of specialized cardiac muscle. Now let's move towards action potent

ial in the SA node. But before that let's quickly see the basics behind it. So there are 3 main ions involved in the rhythmicity of the SA node. Na, Ca and K. Na and Ca concentration is higher outside the cell and they tend to diffuse in. As they are positively charged ions, their entry takes the membrane potential to electro positivity. K concentration on the other hand is higher inside. So it tends to diffuse out. And the exit of positively charged K ions makes the cell electronegative. The ba

lance between the movement of these ions controls the rhythmicity. With this basic cleared, now let's see the action potential in the SA node. Starting with the resting membrane potential. It is about -60 mV. However, this resting membrane potential is not so resting in pacemaker tissues. It is in fact unstable. The reason is slow sodium channels that allow an influx of Na even without any external stimulation. This slow Na entry slowly raises the membrane potential. This Na current is sometimes

referred to as funny current or If. The researchers did not expect this inward Na+ current at the end of repolarization. So when they found it, they found it funny. Anyway, along with Na, slow entry of Ca also contributes to the rise in membrane potential. The contribution of Ca increases gradually. Eventually, the potential reaches the threshold which is about -40 mV. So this was the phase of slow pacemaker potential. Next, at the threshold, more Ca channels open resulting in a rapid influx of

Ca. This produces upstroke which is also called phase 0. Then the Ca channels close and K channels open. So Ca influx ceases and K starts leaving the cell. Exit of positive ions brings the membrane potential towards electronegativity. Thus the action potential is terminated. This is phase 3. Wait... where is phases 1 and 2! Well, the phase 1 of initial repolarization and phase 2 is the plateau. But they are seen in ventricular myocytes. They are not seen in the SA node. So we directly have phas

e 3 of repolarization. The K efflux continues for some time even after this. So the membrane is hyperpolarized. Now we have reached a potential from where we started. At this point, the K channels start closing. So efflux of K decreases. And also the Na and Ca influx that we discussed earlier once again over-balances the K efflux. So there is a net entry of positive charge into the cell. So the membrane potential again moves toward the electro positivity. This phase is also called phase 4. The p

otential reaches the threshold and action potential is generated again. In this way the SA node keeps generating impulses on its own. Impulses thus generated spreads to the surrounding atrial fibers and to the AV node. This spread eventually triggers a heartbeat. Thus the SA node serves as the pacemaker of the heart. This property of being able to generate its own impulse is called automaticity. So that was all about the action potential in the SA node. Now let's have a quick summary. SA node is

a normal pacemaker of the heart. Action potential in SA node involves slow pacemaker potential due to slow entry of Na and Ca, upstroke due to rapid influx of Ca and Repolarization and hyperpolarization due to cessation of Ca entry and exit of K ions. Then the K efflux decreases and slow entry of Na and Ca again brings the potential to threshold to trigger the next action potential. Thus the SA node keeps generating action potential on its own. That's it for this video. If you feel this video w

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