 Welcome to Physiology Open, there are two types of cardiac muscle cells. First type are specialized cells which generate their action potential spontaneously, these are known as autorhythmic cells and they constitute 1% of total cardiac muscle cells. These cells do not need any help from outside to generate action potential, that's the word auto means, that is on their own. Rest 99% of cardiac muscle cells are contractile cells. The autorhythmic cells are a part of conducting system of the heart. In the conducting system the cells having this automaticity are SA node, AV node, bundle of hills and perkinje pipers. The rate of automaticity is highest in SA node, so we will talk about SA node action potential. The reason as to why this rate is highest in SA node that we will discuss little bit late. Now these autorhythmic cells and contractile cells have different types of action potentials, that is action potentials of different shapes and also the ions which are responsible for the action potential are different in both the types. See here in this diagram left side is showing action potential of autorhythmic cells while right side is showing action potential of contractile cells. I think you can very well appreciate the difference in the shapes of two action potentials. Most importantly see the slope of change in membrane potential in autorhythmic cells. This is important for automatic generation of action potential. Also see that both the action potentials are numbered. The rising phase that is going towards positive side of both action potentials is phase 0 and the falling phase that is coming back to resting membrane potential is phase 3 for both action potentials. And this phase is phase 4 in both action potentials even though it is constant in action potential of contractile cells while there is a slope in action potential of autorhythmic cells. Now this action potential is not having numbers 1 and 2. Since both these phases of contractile cell action potentials are lacking in this action potential you see, right? Directly 3 is coming. Now let us see in detail the generation of SA node action potential. This is what this video is about. So let us draw two axes. Y axis representing voltage in millivolts and X axis representing time in milliseconds. Now we will start from a voltage of minus 60 millivolts. So this is an autorhythmic cell with a membrane potential of minus 60 millivolts. Now what happens is there are certain channels on the membrane known as HCN channels that is hyperpolarization gated cyclic nucleotide channels which open at this potential. Now these channels are permeable to sodium ions. Since the electrochemical gradient for sodium is from outside to inside because of this gradient sodium ions start entering the cell. This makes the membrane potential little positive. The potential reaches to minus 50 millivolts another voltage gated channels open that is T type calcium channels open. Here T stands for transients that is they open for a small time and then they close. Again since calcium is more outside the cells than inside calcium starts entering the cells making the membrane potential yet more positive to minus 40 millivolts. This minus 40 millivolts is the threshold for generation of action potential. At this voltage voltage gated L type calcium channels open where L stands for long lasting because they are open for a little longer time. This also causes calcium to move from outside to inside until the potential reaches to plus 10 millivolts. By this time calcium channels close and voltage gated potassium channels open. Now since the gradient for potassium ions is from inside to outside they move from inside of the cell to outside. This starts bringing the potential back towards negative until it reaches minus 60 millivolts. So now we have reached our starting point. If you remember we started with HCN channels which open at minus 60 millivolts. So at this point these HCN channels open and allow entry of sodium ions. So again the potential starts rising towards positive and the cycle repeats itself. I will just digress a little bit here and talk about HCN channels. See these HCN channels which we spoke about in the beginning these are hyperpolarization gated cyclic nucleotide channels that is as the name suggests they open due to hyperpolarization. This is a rare kind of voltage gated channel since it opens due to hyperpolarization. Most of other voltage gated channels open when membrane becomes less negative that is T polarizes. Because of this they are turned as funny and the current which occurs due to movement of sodium ions to them is this known as funny current. One other property of this channel is that they are also regulated by the concentration of cyclic nucleotides inside the cell. That is they may open faster or slower based on the concentration of cyclic nucleotides inside the cell. This is discussed in another video on mechanism of change in heart rate. Okay let's come back for the last bit on the video. So these are phases 4, 0 and 3 of action potential of autorethmic cell. Here we are talking SNO action potential. The total duration for this SNO action potential is around 200 to 250 milliseconds. The phase 4 is responsible for automaticity of the generation of this action potential and is known as pre-potential or pace maker potential. The initial part of which is due to HCN channels and the later part due to T type calcium channels. The slope of the pre-potential is responsible for changing the number of impulses which can be generated in one minute. If it becomes steeper more number of impulses will be generated per minute and if it becomes less steep like this, lesser number of impulses will be generated per minute. Different autorethmic cells have different slope of this pre-potential. That is the rate of generation of impulse differs for different autorethmic cells. Secondly since each impulse leads to one heartbeat, this slope will determine the number of heartbeats per minute. That is heart rate. The details of this concept we have discussed in another video which you can check out here. Okay. Thanks for watching the video. Do not forget to subscribe to the channel Physiology Open. Thank you.