pumps out blood so just recapping this is our heart as we know the heart works like a pump which pumps blood around the body let's have a closer look at the heart and the circulation of the blood coming to and from the heart first of all we'll look at a cross sectional view of the hot the heart consists of four chambers if you remember the right atrium the right ventricle the left atrium and the left ventricle these four chambers are important because blood passes through them within the heart the ventricles are important because the ventricles are the one which essentially pumps blood out of the heart and so they need to be strong and so they contain most of the cardiac muscle cells here in black after oxygen has been used by the body tissues the blood supply will be deep oxygenated will be in low oxygen and hot and it will have higher concentrations of carbon dioxide these dis this deep oxygenated blood supply will then travel back into the heart through the vena cava and it will enter into the right atrium the blood will then go into the right ventricle the right ventricle and pump out the blood through the pulmonary artery towards the lungs where it will be read excision ated so now the blood supply has oxygen in it this reoxygenate blood supply will travel through the pulmonary vein back towards the heart and enter the left atrium the left ventricle which contains most of the cardiac muscle cells will then pump this oxygen-rich blood through the aorta towards the rest of the body to other body tissues and so this circulation continues the circulation from the heart between the heart and the lungs is known as a pulmonary circulation and between the heart and the body tissues is known as a systemic circulation now in order for the heart to eject blood or pump blood out there is contraction of cardiac muscles and this is triggered by action potentials so let's have a look at the cardiac muscle cells the cardiac muscle cells are different from the smooth muscles or the skeletal muscles but they have some similarities for example the cardiac muscles are also striated like the skeletal muscles they contain one nucleus per cell usually so they're mono nucleated and it looks something like this this is a magnified view another important feature cardiac muscles possess that differs it from skeletal muscle for example is that they have intercalated discs junctions which separate each cardiac muscle cell and these intercalated discs these allow the cardiac muscle cells to contract in unison so if one coded muscle cells contract all cardiac muscle cells contract and this is important to pump blood to pump blood in unison around the body also carry muscle cells contain t tubules and second plastic reticulum for calcium release and so therefore calcium as we know is very important for muscle contraction now we know that the cardiac muscle cells contract in response to an action potential so let's look at this action potential of a cardiac muscle cell in a ventricle for example but before we look at an action potential we have to understand a membrane potential and how an action potential changes a membrane potential so let's have a quick look here we have a cell membrane of any type of cell but for now we'll just say it's a cardiac muscle cell or cells within the heart now the cells in the heart have a membrane potential a membrane potential is the difference in electrical potential between the inside and the outside of the cell then and this is due to the different concentration of ions across the membrane so different concentrations of ions in the outside in respect to the inside here we have the inside and here of the outside of this heart cell in the heart cells at rest essentially after just after the ha has finished pumping the blood out the inside of the cell membrane is negative in respect to the outside and therefore the membrane potential is usually negative and this is caused by the different distribution of ions across the membrane so let's look at these ions and there are three main ones look at in the cardiac cells so on the outside we have more calcium and sodium ha ions which are usually positively charged and at the inside we have more potassium ions which are also positively charged now when these different ions move from one side to the other they will change the membrane potential of course how do they move well they move through these protein channels protein pools and so therefore when these ions move through these protein channels specific protein channels they will change the membrane potential so for example here we have two protein channels for calcium ions calcium ions within the heart have two types of protein channels they have an L type and they have a T type so these calcium channels move inside the cell from the outside through these protein channels they will change the membrane potential so let's look at a graph and try to understand this membrane potential process so here we have a graph with telling the x axis and the potential in millivolts on the y axis remember that after the heart pumps out the blood the membrane potential is usually negative so here we have negative after just after the heart pumps out the blood but when these calcium channels open for example the t-type calcium channel opens calcium will come in and will change the membrane potential to become more positive and if the other calcium channel opens the l-type calcium channel this allows more calcium to come in and so change the membrane potential to become even more positive now what's important to know and understand is that when these ion channels closes this will cause the ions to move back to where they came from either through via pump or via some other means and so the movement of these ions back to its origin location will obviously change the membrane potential keep that in mind so I hope you understand how the million potential changes within the heart 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 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 it'll 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 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 a 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 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 contract okay this might sound confusing but the point at which 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 our 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 is 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 a 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 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 ization of ventricle here we have our main pacemaker cell known as a sign of atrial node or SA for short and then we have 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 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 be 70 to 80 beats per minute now the very interesting thing about the heart is that the high 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 found out experimentally with animal studies that if the AV node is also damaged the bundle branches them.he and the bundles of his 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 not 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 an 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 the 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 or an action potential optional potentials finished it will repolarize back down to negative 60 and then 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 cardiac muscle cells which will change their membrane potential which will then cause contraction I hope this makes sense hope this video was good hope you enjoyed 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