Name the 2 things that differentiate cardiac action potentials vs neuronal action potentials
- Cardiac action potentials are self-activating
2. Cardiac muscle has an absolute requirement for Ca influx through L-type Ca channels for action potential
Outline the cardiac conduction system of the heart
SA Node –> Internodal tracts (right atrium) –> AV Node –> Bundle of His –> R & L Bundle Branches –> Purkinjie Fibers
SA Node –> Interatrial tracts (left atrium)
Where do Pacemaker or “Fast Response” action potentials occur?
Atria, ventricles and Purkinje fibers
-Undergo rapid depolarization
Fast Response: Phase 0
Rapid Depolarization
- “Fast” Na channels open
Fast Response: Phase 1
Early/Initial Repolarization
- Transient outward current as K+ channels open
- Fast Na channels are closed
Fast Response: Phase 2
Plateau Phase
- L-type Ca2+ channels open leading to inward Ca movement
- Efflux of K+ through several types of K+ channels
T or F, The Ca entering the cardiac muscle cell during an action potential is enough to activate actin-myosin interaction
False, it is small and not enough, it acts as a trigger for the release of Ca from the SR which then promotes actin-myosin interaction.
- Occurs through Ryanodine receptors (RyR2)
Fast Response: Phase 3
Late or Final Repolarization
- Continual efflux of K+ through several types of K+ channels
- L-type Ca channels eventually close
Fast Response: Phase 4
RMP
- K+ channels remain open
- Calcium extrusion mechanisms become highly active
3 calcium extrusion mechanisms:
- SERCA - SR Ca2+ pumps
- Sarcolemmal Ca2+ pumps
- 3Na/1Ca2+ Exchangers (NCX)
Is it possible to produce a tetanus or summation in cardiac muscles?
No, it is impossible because the electrical and mechanical events overlap each other whereas in skeletal muscle electrical is done before the mechanical starts
- Tetany of the heart would lead to death
Where do “slow response” action potentials occur?
In the SA node and AV node
- Slow depolarization
Slow Response: Phase 0
Caused by an increase in Ca conductance.
Different than Fast Response in that it is not Na
Slow Response: Phase 3
Repolarization
- Increase in K+ conductance
Slow Response: Phase 4
Slow depolarization
- caused by an increase in Na conductance, which results in an inward current called If
- Accounts of pacemaker activity of SA Node
Effective or Absolute Refractory Period means what?
a 2nd action potential absolutely cannot be initiated, no matter how large a stimulus is applied
Relative Refractive period means what?
A 2nd action potential may be evoked only when the stimulus is sufficiently strong
Compare Positive and Negative Dromotrophy effects
Positive: Increases conduction velocity through the AV node and therefore decreasing the PR interval
Negative: Decreases conduction velocity through the AV node and therefore increases the PR interval
What is the mechanism of positive and negative dromotropic effect
Positive: Increases inward Ca current
Negative: Decreases inward Ca current and outward K+ current
