Action Potential – How the Heart Beats
Action potential is the sudden changing of the cells membrane potential (the electrical charge on the inside vs the electrical charge on the outside of the cell). In this post you will learn how the action potential of our heart’s cells is directly responsible for the beating of the heart. If you have not already done so I would read ‘cells and electrical charge
‘ first to bring you up to speed.
This change in the cells electrical charge is important as it directly effects which ions (electrically charged particles) can pass in and out of the cell triggering the action potential – the movement of these ions will then also effect the electrical charge of the cell.
Calcium ions entering the cell are the trigger that make it contract. I will explain how in the next post, but first let us have a look at how the charge of the heart muscle cell (cardiomyocyte) is able to alter over and over again.
I have read and listened to so many explanations of action potential and it is never a pleasant topic, often dull and explained in a very similar way all the time. If you do want a more clinical explanation then just google action potential and you will be inundated.
I am going to attempt to use an analogy to explain the cycle the heart cells go through and hopefully this will make things a little more digestible and memorable…. wish me luck. The heart cell cycle has 4 stages.
I invite you to join me inside the Heart Cell Nightclub.
The Heartbeat Nightclub – Action Potential
The empty night club. There is the potential for a lot of energy and dancing and music to happen the building, bar and speakers are all in place. But without outside influences the nightclub would just sit there doing nothing.
Luckily there are some staff (cleaners, barman, dj’s) with keys that slowly filter into the nightclub via the staff entrances and slowly set about getting the nightclub ready for business.
Thanks to the staff preparing the nightclub the main doors are ready to be opened. This is a very popular nightclub and the hype is huge so there are crowds of people outside waiting to come in. So many in fact that several sets of main doors are required. The crowds flood in filling the club very quickly.
The club gets so full that the main doors are all closed and no more people are allowed to enter. In fact it is so busy that some back door exits are opened and some people have already had enough and start to leave.
The doormen notice that people are leaving and decide they can start to let a few people in at a time using a side entrance. For those that have ever queued at a nightclub, this is your classic “one in one out” scenario. As people leave more people come in so the head count of people in the nightclub remains pretty constant.
The club is ever so slightly starting to empty so the manager decides its time to close. No more people are allowed in the nightclub and those remaining continue to leave through the exits.
The process repeats every night;
Staff filter in and ready the nightclub.
I am going to draw the activity of the Heartbeat Nightclub on a graph.
This cycle of events that happens day in day out are all dependent on the preceding eventuality. Believe it or not this is uncannily like how heart cells operate.
The Heart Cell Contraction – Action Potential
Sodium and Calcium ions filter into the cardiomyocyte via gap junctions. These are doorways where only particular ions can pass and in relatively small numbers. Sodium and calcium ions have a positive electrical charge, therefore as they enter the cell they have an affect on the its overall charge, making it more positive.
Numerically, these ions alter the membrane potential of the cell from -90mV to -70mV.
When the membrane potential of the cell is -70mV the main doors swing open. These are voltage-gated ion channels, doors that are only open during particular voltages for particular ions. In this phase the doorways that open are specific to sodium ions. Through the main doors the sodium is able to flood into the cell. It does this because of diffusion, the concentrations of sodium are high outside the cell and low inside the cell.
This very quickly changes the overall charge of the cell even further from -70mV to around +20mV. We call this depolarisation because the cell has gone from a negative net charge to a slightly positive net electrical charge.
At around 20mV the sodium voltage-gated ion channels (main doors) close. At this point another voltage-gated door opens specific to potassium. However this time there is large amounts of potassium ions inside the cell so it flood outwards because of diffusion. Potassium ions have a positive charge so as it leaves through these doors the overall charge of the cell becomes less positive. 20mV to around 5mV.
As the charge of the cell nears around 5mV a third set of voltage-gated doors open. These allow calcium to enter the cell (because of diffusion). Calcium ions are positively charged so make the cell more positive.
The potassium ions leaving the cell and the calcium ions entering the cell cancel each other out. The overall charge of the cell hovers around 5mV for a relatively long period of time.
Eventually the charge of the cell does become less positive. This causes the calcium specific voltage-gated ion channels to close, and the calcium ions no longer pass through them. The potassium ions however continue to exit through their voltage-gated ion channels taking their positive charge with them. The cell gradually becomes more negatively charged once more reaching -90mV… seeing as the cell has regained its polarity, we call this repolarisation.
Once repolarised at around -90mV the potassium specific voltage-gated ion channels also close.
The process repeats every heartbeat;
Sodium and Calcium filter into the cardiomyocyte via gap junctions.
Let us have a look at this on a graph too.
I know from experience that learning about action potential, depolarisation and repolarisation of cardiomycytes is incredibly dull. More so the subject matter is often very alien and hard to get your head around and remember. I hope that this nightclub analogy shows that the change in ‘state’ of the cell is down to movement of ions (people), in and out of the cell (nightclub) via channels (different doorways) that are only open at specific times.
Next we will look at how the opening of the calcium voltage-gated ion channels causes the cell to shorten (contract)
Thanks for reading.