In a previous blog post we covered the action potential of the cardiac pacemaker cells- sino atrial node, atrio ventricular node and the Bundle of His.
In this post we can go on to look at the action potential of the muscle cells of the heart itself and see how it differs. The mechanisms are very similar with some slight differences. Its all in the chemistry!
The above image represents the charge difference across the membrane. You can see that the charge goes from negative to slightly positive and then back again.
How does this happen? Lets go from points 1-8 to understand.
Point 1 and 2
You can see here the myocyte. Just like the pacemaker cells there is a steady leak of potassium out of the cell.
There are also gap junctions in the cell wall.
When a cell adjacent to this cell depolarises, calcium and sodium will leak in to this cell via these gap junctions starting a rise in the membrane potential.
The flow in of calcium and sodium means that the membrane potential will reach the threshold.
This is when the sodium voltage gated channels will open allowing the fast flow of more sodium into the cell causing a rapid rise in the membrane potential.
The membrane moves from a negative value to a positive value.
Depolarisation has occurred!
Point 4 and 5
At this point those voltage gated sodium channels will close- stopping the rise in the membrane potential.
At the same point voltage gated potassium channels will open.
So, as positively charged potassium ions flow out of the cell, the membrane potential will start to become more negative.
In order to slow this move towards a more negative potential calcium channels then open.
So there is an equalisation between potassium flowing out and calcium flowing in.
This results in a plateau, where the membrane potential remains positive for a time.
This is to allow the wave of contraction to move through the heart rather than all the cells contracting together.
The calcium channels will then close.
Now there is only potassium leaving the cell.
This then causes a rapid drop in the membrane potential, becoming much more negative, moving back down to where it started.
Repolarisation has occurred!
The final part of the process is the voltage gated potassium channels closing.
So now we just have the steady leak of potassium out of the cell again just as we had at the start.
The membrane is stable!
So the depolarisation wave moves through the myocytes casing contraction of the heart. Fab!