Lecture 6 - The Cardiac Ventricular Action Potential Flashcards
What are the major intracellular ions?
Cations: Potassium, Magnesium
Anions: Phosphates, Some amino acids
What are the major extracellular ions?
Cations: Sodium, Calcium
Anions: Chloride
Is a ventricular ap the same shape as a neural ap?
No, Cardiac action potential has a linger duration of around 200 milliseconds
What is phase 4 of a ventricular ap? and what is occurring during phase 4?
Resting membrane potential
High sodium outside, low sodium inside
High potassium inside, low potassium outside
Pumps maintain resting membrane potential by pushing 3 Na out and 2 K in
The resting membrane potential is maintained until the cell is depolarised
What is phase 0 of a ventricular ap? and what is occurring during phase 4?
Depolarisation
Neighbouring cell passes on electrical stimulus, causing changes in the ion channels e.g. S4 and S6 in K+ channel
When the cellis stimulated the sodium ion channels open, allowing influx of sodium into the cell
There are subsets of sodium channels, in phase 0, when sodium is moving into the cell rapidly, the ion channels are fast acting sodium channels
This causes the negativity of -80mV to start to become positive
Membrane potential goes from -80mV to +40mV, the inside becomes positive with respect to the outside
What is responsible for phase 0?
Sodium Influx
What is phase 1 of a ventricular ap? and what is occurring during phase 1?
Early Repolarisation
Fast acting sodium channels are closed
Cell now start to repolarise
The gives the turn around point at the top of the ECG
Sodium is no longer flooding into the cell, so rise doesn’t continue, so start to see recovery
No more sodium coming in, but potassium can leave
Where is calcium found?
Calcium is present in both the intra and extracellular fluid
Calcium inside heart muscle cells is mainly bound up, it is not free, some is free in the intracellular fluid, but the majority is stored in the intracellular organelles, in heart cells it is stored in the sarcoplasmic reticulum
We have relatively high calcium in the extracellular fluid, but low in intracellular fluid as it is bound up
What is phase 2 of a ventricular ap? and what is occurring during phase 2?
Plateau phase - Myocytes contracting
Calcium channels are opening in the cell membrane, the calcium outside will move down its diffusion gradient into the cell
The calcium that enters, triggers release of calcium from sarcoplasmic reticulum
So we get a build up of calcium in the intracellular fluid, which is going to change the charge in the cell
How is calcium release triggered from the sarcoplasmic reticulum?
The Sarcoplasmic reticulum which has ryanodine receptors, which become triggers and cause calcium induced calcium release from the sarcoplasmic reticulum
What is phase 3 of a ventricular ap? and what is occurring during phase 3?
Rapid repolarisation Fast acting sodium channels closed Calcium channels closed Potassium channels open We have high potassium in cells, so potassium moves through channels out of the cell down a concentration gradient So the charge becomes negative
What happens to the calcium in cells after phase 2 has finished?
Either used up in contraction or taken back into intracellular stores
How do sinoatrial node pacemaker cells aps differ to normal ventricular aps?
Pace maker ap is more spiked
In pacemaker cells what is Phase 0 due to?
Influx of calcium ions
What phases do pacemaker cells aps have?
Phase 4 (RMP), Phase 0 (Depolarisation) and Phase 3 (Repolarisation) Don't have Phase 2 (Plateau) as cells are not contracting
