Cardiac Electrical activity and ECG Flashcards
What are the components of the electrical activity of the heart?
- The heart is a simultaneous dual pump.
- Each heartbeat is triggered by depolarisation of the membranes via an action potential.
- 1% of the heart is auto-rhythmic, which means it can generate its own action potentials.
What are regions of autorhythmicity?
- SA node
- AV node
- Bundle of His
- Purkinje Fibres
Where is the SA node?
In the top right of the right atrium.
- This is known as the pacemaker region, where most of the action potential is generated. It fires about 70x per minute on average.
How many milliseconds does it take for the action potential to pass across the right atria to the AV node?
30ms
Why does all the electrical activity have to pass through the AV node?
At the AV node, this is the only point of conduction between the atria and the ventricles, so all the electrical activity has to pass from the atrium to the ventricles through the AV node. This takes around 100ms.
What happens after the electrical activity gets through the ventricles?
Once across into the ventricles, the action potentials follows the bundle of his and stretches down the septum and around into the walls of the right and left ventricles. It then barges out to the Purkinje fibres which continues to pass the action potentials along. This takes around 30ms.
What are the SA nodes?
- They’re auto rhythmic cells
- They initiate and conduct the action potentials
- The electrical activity of the SA node pacemaker cells is very strange as there’s a slow drift to the threshold.
- They are known as the pacemaker cells and they initiate the electrical activity in the heart.
Why do SA nodes have this slow “drift”?
SA nodes have no resting potential, they’re either always depolarising or repolarising. Once the membrane potential is fully polarised, there’s a slow depolarisation step, which is a slow drift and a shallow incline. It will eventually reach the threshold and suddenly the action potential will jump and will repolarise and will start to slow drift again.
What’s causing this slow drift?
- Well there’s these iron channels known as funny channels, in these pacemaker cells and they allow the slow drift to happen.
- They start to let sodium ions into the cell causing an increase in the charge of the membrane potential and a slow depolarisation.
- As the approach the threshold potential, the transient calcium channels open and allows calcium in as well which starts to build the depolarisation. The transient type channels close and the long-lasting calcium iron channels open, letting lots of calcium in and there’s this rapid depolarisation at the end.
- At the peak the long-lasting channels close and the potassium channels open letting the potassium out therefore there’s a quick repolarisation.
- As there’s no resting potentials so as soon as they’re fully depolarised, the funny channels open and the process starts again.
How many action potentials are fired per minute from the AV node?
40-60 action potentials per minute
How many action potentials are fired per minute from the Bundle of His and Purkinje fibres?
20-40 action potential per minute
What can the cardiomyocytes do with action potentials?
Cardiomycoytes can carry action potentials. They can’t generate action potentials but they can pass the action potentials from the Purkinje fibres across the cardiomycyte to the next cariomyocyte.
What does the cardiomyocyte action potentials look like?
- They have a resting potential
- There is an influx of sodium ions which causes a rapid depolarisation.
- This is partially rectified by the opening of potassium channels to pump potassium out.
- There is then a plateau phase of action potential, where we see calcium starting to enter slowly into these cells.
- At the end of the plateau stage, there’s a rapid repolarisation caused by the potassium being pumped out again.
- This eventually reaches the resting potential, and will rest until you receive the next action potential to fire again causing this cycle again.
What is excitation-contraction coupling?
There is a plateau phase in the cardiomyocyte action potential and this is letting calcium into the cardiomyocyte and calcium is essential for cardiomyocyte contraction.
Therefore the action potential will increase the cytosolic calcium, and this calcium is coming from the extracellular space crossing into the cardiomyocyte via the long-lasting calcium channels. This will then bind to a receptor (ryanodine receptors) on the sarcoplasmic reticulum, which will let more calcium out of the sarcoplasmic reticulum and will increase intracellular free calcium. This will then activate troponin and cross bridge formation.
What are the two types of channels that are responsible for removing calcium from the cytoplasm and to switch off the contraction?
- Calcium ATPase Pump: active transport back into sarcoplasmic reticulum
- Sodium-Calcium exchanger: that removes Ca2+ from the cytosol back to the extracellular space.
What things can affect the action potential in the cardiomyocytes or the pacemaker cells?
- Abnormal levels of potassium in circulation.
- Increase or decrease in K+ results in decreased cardiac excitability and contractility. - Change in Ca2+ levels in entry will have bigger consequences in cardiac rhythm but also contraction of the heart.
- Use of Ca2+ blockers reduce force of contraction
- Use of Digoxin increases cytosolic Ca2+ and contractility
