Cardiac Action Potential Flashcards
Describe the anatomy of the Cardiac Conduction System.
SA node –> AV node –> Bundle of His (common bundle branch) –> goes down septum through LBB and RBB –> Purkinje fibers
Cells that depolarize last…
Repolarize first
Which part of the surface of the heart receives AP first?
Endocardium of ventricles receives AP prior to Epicardium
Right ventricle epicardium before Left ventricle epicardium
What is the most important factor of conduction velocity?
Fiber size
Larger fibers have faster AP transmission
Rate the anatomy of the conduction system in terms of conduction velocity.
Purkinje > atrial and ventricular muscle > AV node
Why is the delay in AV node important?
It allows the atria to empty into ventricles before ventricle contract.
Why is the SA node the pacemaker?
AV node’s phase 4 depolarizes slower. It does not reach threshold until it receives a triggering signal from SA node.
Describe the SA and AV nodal APs with phases and ions.
Phase 4 - Opening of Funny voltage-gated Na+ channels
Phase 0 - Opening of slow Ca2+ channels and closing of special K+(b) channels
Phase 3 - Closing of slow Ca2+ channels and opening of special K+(b) channels
What does the steepness of phase 0 determine?
Velocity of conduction
If the SA node stops firing, what happens?
The AV node takes over, but at a slower rate
If the AV node stops firing, the Bundle of His or Purkinje fibers will begin to spontaneously depolarize without stimulation.
However, each is slower than the one previously. If SA node fires at a rate of 75 bpm, AV will be 40, BOH will be 20, etc.
Describe the APs in fast cardiac tissues with phases and ions.
Phase 4 - RMP, sustained by high K+(c) conductance
Phase 0 - rapid upstroke by crossing the threshold from voltage-gated Na+ (m) channels opening
Phase 1 - small depolarization caused by voltage gated Na+ (m) channels closing (inactivation gates closed) and some K+ (a) channels opening
Phase 2 - Plateau due to slow opening of voltage gated Ca2+ channels and closing of special K+ (b) channels
Phase 3 - complete repolarization caused by slow voltage-gated Ca2+ channels closing and K+(b) channels opening
What is a refractory period and why is it important?
When electrolyte gates have not reset to allow 2nd AP to be generated
Important b/c helps prevent arrhythmias and are longer in cardiac cells than neurons
What are the types of refractory periods?
Absolute refractory period - No depolarization at all
Relative refractory period - AP can be generated by will have abnormal conduction
Supranormal period - cell is more excitable than normal
What happens if AP is generated during a relative refractory period?
The subsequent AP conduction is weaker
Define Chronotropic effects.
Changes RATE of DEPOLARIZATION of SA node and therefore affects heart rate
Positive = faster
Negative = slower
