Cardiology - Heart Physiology II (Muscle contraction and Pacemaker activity)

Cardiology – Heart Physiology II (Muscle contraction and Pacemaker activity)



so I hope you understand how the million potential changes within the hot cells and so now we can go back into our main diagram and look at the action potential of a cardiac muscle cell and the changes in the membrane potential so here we have the x-axis with time and milliseconds and the membrane potential in the in millivolts in the y-axis and here we have negative 90 millivolts new to 60 in under 30 and zero and 30 just after the heart muscle contracts and releases blood out it will be it will be at rest just short let's have a short rest and after this short rest which is typically negative 90 millivolts there will be a big jump in the membrane potential to become more positive and it will shoot up to positive 30 millivolts and this process is known as depolarization where we have a fast influx of sodium ions from the outside to the inside so it changes the membrane potential to become more positive the depolarization does not go past positive 30 this is a peak after depolarization there is a plateau phase where the sodium ion stopped moving in so there's a close in the sodium channels but there's a slow influx of calcium ions through the l-type calcium channels from so calcium are moving from the outside to the inside and so there's still a sort of positively charged membrane potential so what is really happening is that there's no more sodium's moving in from the outside to the inside there is still slow movement of calcium ions from the outside to the inside and there's no movement of potassium from the inside to the outside and then after the plateau phase we have the repolarization phase where the calcium ions stopped going in and there's our TAS ium ions moving out which makes which causes the membrane potential to become more negative and so it goes back to the negative 90 millivolts so essentially what is happening is that from negative 90 it jumps up to 30 and this is the contraction if you've seen a skeletal muscle graph of a contraction okay this might sound confusing but the point at which a action potential changes the membrane potential of a heart cell to to where it finishes is a period where a second action potential cannot occur this period is known as the absolute refractory period to better explain this let's look at a graph looking at what happens when a heart muscle receives an action potential and when a skeletal muscle receives an action potential so for skeletal for a skeletal muscle here when it receives an action potential it will contract like so however a second action potential can be generated straight away which creates a stronger contraction like so and this create and this is essentially where we have a continuous stimulation of action potentials on the muscle so for skeletal muscle action potentials can be generated before the first action potential has finished which then creates a basically overlapping effect until it reaches a point where the tension can no longer pass and this point is known as the tetanus it's just a tetanus for heart muscles an action potential creates contraction but a second action potential cannot be generated it can only be generated after the first action potential has completely finished after the contraction has completely finished and so this is known as the absolute refractory period when the second action potential cannot be generated it can only be generated after the first action potential has finished okay so we know that the muscle cardiac muscle cells require a action potential to change its membrane potential but where do these action potentials come from well for skeletal muscle if you remember they receive the action potential essentially from a neuron which passes it down to skeletal muscle cardiac muscle cells are different in that they receive action potentials which are generated by a group of cells known as pacemaker cells and this is independently of the neurons innervation of the heart so it's a pacemaker cells which give which produce generates action potential for the cardiac muscle cells to pump so this red drug I'm drawing here on the right atrium is the main pacemaker cells which generates the action potential and causes the heart the heart muscles to contract pumping the blood out of the heart in unison now let's have a closer look at the pacemaker cells of the heart in more detail so here we have the hot nanometer heart again right atrium right ventricle left atrium left ventricle here we have our main pacemaker cell known as a sign of atrial node or SA for short and then we have the another pacemaker cell right next to it known as the atrioventricular node or AV so essentially the SA node will generate an action potential all throughout the right of the atrium of the heart and also pass it on to the atrioventricular node which will pass it on then to the bundle of hiss on to the bundle branches over here on on the muscle fibers and then all onto the Purkinje fibers and essentially the action potential will be generated all through the heart causing the ventricles then to pump the blood out of the heart I hope that makes sense so the main pacemaker cells of the SA node this is a usual pacemaker the main pacemaker and it's action potential causes the heart to beat 70 to 80 beats per minute now the very interesting thing about the heart is that I can still function if the SA node is damaged or is broken because then the AV node can take over however the AV node will only cause 40 to 60 beats per minute which is lower and then again it has been founded experimentally with animal studies that if if the AV node is also damaged the bundle branches to hit and the bundles of hiss can take over but this will only cause about 20 to 40 beats per minute so it's very slow indeed and you probably need some form of heart surgery to fix this up so knowing that knowing the pacemaker cells let's look at the pacemaker activity of cardiac autorhythmic cells so the activity of these pacemaker cells because they they function without the help of neurons they create action potentials without the help of neurons so here we have the same graph that we looked at for the cardiac muscle cells the X being the time in milliseconds and the y axis being the membrane potential in millivolts we have negative 60 negative 40 negative 20 millivolts and zero millivolts here negative 43 is the threshold for these pacemaker cells for the sinoatrial node in particular so what happens here in phase one we have our deep the pacemaker potential which is which is where we have closed potassium channels and we have open t-type calcium channels where we have a slow influx of calcium ions from the outside to the inside which create which causes the membrane potential to become more negative and once it reaches this threshold of negative 40 due to the pacemaker potential it will shoot up to zero millivolts this is depolarization where an action potential begins when if action potential is generated by these pacemaker cells and this is caused by the opening of l-type calcium channels allowing more calcium to come in and so this action potential will be generated which will then pass on to the cardiac muscle cells right following depolarization we will have repolarization of the pacemaker cell well the count will where both the calcium ion channels the L type and the T type will be closed but the K the potassium channels will will open up which will allow potassium ions to go from inside the cell to the outside which will then bring the other membrane potential back to negative 60 essentially and this process will keep repeating itself as you can see negative 60 we have the pacemaker potential reaching the threshold causing depolarization which will cause an action generate an action potential officers action potentials finished you will repolarize back down to negative 60 and then the pacemaker potential begins again and the cycle just continues on so this is how the pacemaker cells generate action potential which will then pass it on to the correct muscle cells which will change their membrane potential which will then cause contraction hope this makes sense hope this video was good hope you enjoyed it please like comment and share next hopefully we look at the cardiac cycle and the blood supply to the heart itself the coronary arteries etc thank you you

48 thoughts on “Cardiology – Heart Physiology II (Muscle contraction and Pacemaker activity)”

  1. i love your videos, they are so helpful however i was wondering about the SA node…i thought the SA node is 60-100. am i trippin

  2. great video and explanation
    regarding the refractory period of the heart muscle , it's mentioned that there is no refractory period to heart muscle . but in fact heart muscle has refractory period of course not like the skeletal muscle but it has

  3. Dang!!! This video was made in 2013 and I am watching it in 2016 only, all those years I wasted digging into books, not grasping a word. "Sighsss !!!!"

  4. I think most of your videos are very well done and informative. However, you have made several mistakes in this video with regard to phases and which channels are open during said phases. Please consider remaking this video for those who use your videos as their primary resource for their education.

  5. Might be a silly question but when there's a Na+ influx, the membrane potential becomes more positive, however when there's a slow influx of Calcium into the cell, why does the membrane potential become more negative? Great video btw

  6. the pacemaker cells don´t have phase 1, 2 and 3: they only have phase 0, 3 and 4 because phase 1 is the fast influx of sodium that occurs in the myocardium cells and phase 2 is the plateau.

  7. Thank you for your presentation. As always the visual effects help me understand the process. Keep it up.!!

  8. I'm pretty sure there are some mistakes in this video… For example… L-Type are slow and T-type are fast.  Additionally, during the upstroke (depolarization) it's becoming less negative, not more negative as is verbally stated here.  These issues that I heard are in the later part of the video, closer to 7-9min.    If mistaken on any of these, I do apologize, I am by no means claiming to be an expert.

  9. Hi, I have a question here. Why did you say that during phase 2 (plateau) there is no movement of potassium from inside the cell to the outside? As far as I am aware, you have this movement through slow rectified K+ channels, which slightly repolarize membrane up to -20mV and at this point L-type Ca2+ channels close. This is the end of phse 2. Phase 3 begins from opening rapid rectified K+ channels, which repolarize membrane to -85 – -90 mV.

  10. Thank you for your video. Just one question: Do the sodium ion channels close at any point in pacemaker cells? 

  11. I don't get how the membrane potential dropped at 1 if there was no eflux of positive ions out of the cell… you skipped over that completely. There are 5 phases. You put your zero as one. There is no phase 4 either… This is very confusing >.< 

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