Section 1D

Question 0

Name the 2 things that differentiate cardiac action potentials vs neuronal action potentials

Answer 0

1. Cardiac action potentials are self-activating

2. Cardiac muscle has an absolute requirement for Ca influx through L-type Ca channels for action potential

Question 1

Outline the cardiac conduction system of the heart

Answer 1

SA Node –> Internodal tracts (right atrium) –> AV Node –> Bundle of His –> R & L Bundle Branches –> Purkinjie Fibers

SA Node –> Interatrial tracts (left atrium)

Question 2

Where do Pacemaker or “Fast Response” action potentials occur?

Answer 2

Atria, ventricles and Purkinje fibers

-Undergo rapid depolarization

Question 3

Fast Response: Phase 0

Answer 3

Rapid Depolarization

- “Fast” Na channels open

Question 4

Fast Response: Phase 1

Answer 4

Early/Initial Repolarization

• Transient outward current as K+ channels open
• Fast Na channels are closed

Question 5

Fast Response: Phase 2

Answer 5

Plateau Phase

• L-type Ca2+ channels open leading to inward Ca movement
• Efflux of K+ through several types of K+ channels

Question 6

T or F, The Ca entering the cardiac muscle cell during an action potential is enough to activate actin-myosin interaction

Answer 6

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)

Question 7

Fast Response: Phase 3

Answer 7

Late or Final Repolarization

• Continual efflux of K+ through several types of K+ channels
• L-type Ca channels eventually close

Question 8

Fast Response: Phase 4

Answer 8

RMP

• K+ channels remain open
• Calcium extrusion mechanisms become highly active

Question 9

3 calcium extrusion mechanisms:

Answer 9

1. SERCA - SR Ca2+ pumps
2. Sarcolemmal Ca2+ pumps
3. 3Na/1Ca2+ Exchangers (NCX)

Question 10

Is it possible to produce a tetanus or summation in cardiac muscles?

Answer 10

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

Question 11

Where do “slow response” action potentials occur?

Answer 11

In the SA node and AV node

- Slow depolarization

Question 12

Slow Response: Phase 0

Answer 12

Caused by an increase in Ca conductance.

Different than Fast Response in that it is not Na

Question 13

Slow Response: Phase 3

Answer 13

Repolarization

- Increase in K+ conductance

Question 14

Slow Response: Phase 4

Answer 14

Slow depolarization

• caused by an increase in Na conductance, which results in an inward current called If
• Accounts of pacemaker activity of SA Node

Question 15

Effective or Absolute Refractory Period means what?

Answer 15

a 2nd action potential absolutely cannot be initiated, no matter how large a stimulus is applied

Question 16

Relative Refractive period means what?

Answer 16

A 2nd action potential may be evoked only when the stimulus is sufficiently strong

Question 17

Compare Positive and Negative Dromotrophy effects

Answer 17

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

Question 18

What is the mechanism of positive and negative dromotropic effect

Answer 18

Positive: Increases inward Ca current

Negative: Decreases inward Ca current and outward K+ current