Action Potential of Cardiac Muscle

in our discussion on cardiac muscle cells we said that cardiac muscle cells have a slightly different action potential than do neurons so this is what we're going to focus on in this lecture we're going to discuss the generation of an action potential in cardiac muscle cells now cardiac muscle cells are also known as cardiac myocytes where the word myocyte simply means a muscle cell now cardiac myocytes can generate an action potential as a result of three things they can either be stimulated by a neuron they can either be stimulated by a nearby muscle cell so if a nearby muscle cell generates an action potential that action potential can propagate to adjacent muscle cells as a result of the gap junctions found between cells and the third reason is something called myogenic activity so certain cardiac myocytes can actually generate their own action potential they can contract on them by themselves without actually any type of outside stimulus so this is known as myogenic activity now generally speaking the action potential can be broken down into five phases and this graph describes these five phases so the y-axis is the membrane voltage difference it's the potential difference between the inside and outside of the membrane of the cardiac myocyte known as our sarcolemma so the sarcolemma is another word for the membrane of a cardiac myocyte so basically this is given in millivolts and as we go higher along the y-axis our voltage increases it becomes more positive now the x-axis is our time and that's given in milliseconds and as we go along the y-axis in this direction our time progresses our time increases so we have the brown region and that is phase four the red region is phase zero the purple region is phase one the blue region phase two and the green region is phase number three so before we actually examine each one of these phases let's discuss what this dashed line is so the LA dashed line basically is our threshold potential this is basically the voltage difference that must exist across the membrane of the cell for our action potential to actually generate so if our stimulus does not reach this value no action potential is generated but if the stimulus reaches or exceeds this value our action potential is generated so let's begin by describing phase number four so phase number four is shown in brown and notice that this line has a constant slope of zero and that means our membrane voltage is not changing in fact this is the voltage of the membrane when the cell is resting when it's not generating any action potential and this is known as the resting membrane potential so the brown section describes the resting membrane potential of the cell for a typical cardiac myocyte this is at around negative 90 millivolts now before we discuss anything else let's discuss the relative concentrations of four different ions found on the inside and on the outside of the membrane so when the cell is resting when it's not generating any action potential that is when we are at phase four these are the relative concentrations so we have a higher concentration of sodium calcium and chloride on the outside then on the inside portion of the cell and we have a lower concentration of potassium on the on the outside then on the inside and these are the relative concentrations given to us in milli molar so mmm is milli molar so basically this cell membrane is impermeable to ions and that's because these ions carry charge now if we have some type of protein channel that opens up inside our cell membrane then our sodium or any other ion can move across the cell membrane and they will always move down their electrochemical gradient so for example if our sodium channels open up that carries sodium channels across the membrane then the sodium channels will move down their electrochemical gradient from a higher concentration to a lower concentration from the outside to the inside the same thing is true for calcium the same thing is true for these other two but for potassium they will move in the opposite because potassium has a higher concentration on the inside then on the outside now when the cell is resting our sodium as well as the calcium voltage-gated channels are closed but the potassium is able to actually leak into this or actually outside of the cell and because our potassium can leak slightly to the outside of the cell that's exactly why our membrane is negative on the inside than on the outside because we have a leakage of these potassium to the outside and then basically creates a slightly negative membrane potential on the inside of the cell so phase four is the resting membrane potential now let's move on to phase zero phase zero is shown by the red region so let's suppose we have some type of outside stimulus either by a neuron or by an adjacent adjacent muscle cell let's suppose the stimulus basically reaches the threshold potential of around negative 70 millivolts if this takes place then we have the opening of our sodium voltage-gated channels found on the cell membrane so these voltage-gated sodium channels are known as time-dependent channels and what time dependent means is they only open fear for a very software very small fraction of a second and they closed immediately afterwards so as soon as our threshold potential is reached all these sodium voltage-gated channels open up very quickly and because they open up so quickly we have the movement of the sodium channels from the outside of the sodium ions from the outside to the inside down their electrochemical gradient and as they move into the cell they carry positive charge into the cell and as they carry positive charge that increases the positive charge on the inside of the cell compared to our outside so we see this increase as shown by this curve now when we reach about negative 40 millivolts when it becomes greater than negative 40 millivolts we also have the opening of these special l-type calcium channels and we'll see what those are in just a moment so as these sodium channels as the sodium ions rush in they create a positive charge on the inside and a negative charge on the outside and that's exactly why we have this positive charge and increase in our membrane voltage potential so we see that this will cause the movement of sodium ions into the cell which will lead to the depolarization of that membrane so depolarization simply means our polarity of the cell membrane switches so at the resting membrane potential we had an internal negative and an external positive but when we depolarize the inside becomes positive so it goes from negative to positive and the outside goes from positive to negative that's what we mean by depolarization so the red is our depolarization phase that is phase zero so we also have the opening of l-type calcium channels at about negative 40 millivolts now what exactly do we mean by l-type calcium channel so that basically means long opening calcium channels so they open up slowly and they remain open and basically this means we have a very slow and steady rate of calcium ions and they slowly move from a higher concentration to lower concentration from the outside to the inside and this is shown by this diagram and this basically contributes to our increase in positive charge on the inside of our cell now eventually we move on to phase number one so this is phase number one now just as quickly as these sodium voltage-gated channels open they close remember because they're time dependent they are only open for a very small fraction of a second so as soon as we reach this highest point of positive 30 millivolts these sodium voltage-gated channels basically KO a close very quickly and at the same time potassium voltage-gated channels begin to open but they begin to open very slowly and so this is described in the following diagram so notice that our sodium voltage-gated channels shown in red basically close but the calcium voltage-gated channels the l-type calcium channels are still open so because they're open we still have a movement of positive charge the calcium into the cell but now our potassium channels begin to open up and we have a movement of potassium to the outside and so what that basically means is because we have a movement of the potassium onto the outside initially the movement of potassium to the outside is higher than the movement of calcium to the inside and so we have this decrease in our membrane potential it becomes more negative slightly more negative that is shown by phase one so the movement of the positively charged ions out of the cell down their electrochemical greed from a high concentration to a low concentration that causes the inside of the cell to basically become more pop more negative now this continues to take place until we reach about zero millivolts when we reach about zero millivolts the rate of movement of our sodium to the of the potassium to the outside is equal to the rate of movement of the calcium inside and what that basically means because our rates are equal because we have the positively charged potassium leaving the cell but we have the positively charged calcium entering the cell and the rates are equal that means the constant of the charge on the membrane will basically be the same for a short period of time and this is shown by the phase number two the blue region so once the cell reaches a voltage difference of about zero millivolts the rate of influx of the calcium is equal to the rate of eflags it leaves the cell of potassium and this extends our depolarization period and is known as our plateau phase so this is our plateau phase because we essentially reach a point on the action potential where the membrane potential is not changing it remains around zero millivolts now some people say that this is an extension of depolarization because what depolarize depolarization does is it basically maintains a positive concentration on the inside of the cell and that's exactly what we see on this diagram now other people say that this is our depolarization shown in red this is our early repolarization because repolarization means we're basically in the process of returning our membrane voltage to normal so some people say this is early repolarization this is our plateau and this is our repolarization period so this is our repolarization when we basically return in fact repolarization when we returning back to normal and this is our depolarization while other people say this entire section is our depolarization because what this does is it maintains a positive concentration on the inside of the cell so in a way we can imagine that what the plateau phase does is it extends the time of contraction it extends the period of time where our inside of the cell is basically positive with respect to the outside so this is phase number two our plateau phase and phase number three is this phase this is known as our repolarization phase this is when our calcium voltage-gated channels essentially closed and that causes the opening of even more potassium voltage-gated channels and that creates a very high if re flux of our potassium to the outside so our potassium rushes to the outside and that causes the inside of the cell to basically become more negative and eventually it returns to the resting membrane potential so as the calcium voltage-gated channels begin to close the efj locks of potassium exceeds the influx of calcium so the movement of potassium to the outside exceeds the movement of calcium to the inside and that basically causes the opening of even more of these voltage-gated potassium channels shown in green and that causes the increase in the rate of movement of our potassium to the outside and that causes the inside of the cell to become more negative because we have the movement of positive charge to the outside and that basically continues until we reach our resting membrane potential until we reach our phase number four this brown section this is the action potential inside cardiac myocyte

Channel: AK LECTURES

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Keywords: cardiac muscle cell, cardiac muscle, cardiac muscle fiber, action potential of cardiac muscle, graph of action potential of cardiac muscle, plateau phase of cardiac muscle, phases of cardiac action potential, cardiac action potential, action potential generation, voltage-gated caclium channels, L-type caclium channels

Id: uUbgcqo_IOg

Length: 15min 33sec (933 seconds)

Published: Fri Sep 26 2014