113 Normal Cardiac Electrical Activity

Question 1

Nerst Equation

Answer 1

Question 2

The Goldman-Hodgkin-Katz Equation

Answer 2

Question 3

Cardiac Action Potential summary

Answer 3

Question 4

Cardiac Action Potential: Phase 4

Answer 4

PHASE 4: IK1 (the inward rectifier K current) and a small, background Na current (not shown in the figure)

Question 5

Cardiac Action Potential: Phase 0

Answer 5

• Upstroke of the action potential
• INa , fast channel
• ICa (L-type), slow channel

Question 6

Activation/opening of rapid Na and slow Ca channels in Phase 0ish

Answer 6

Question 7

Cardiac Action Potential: Phase 1

Answer 7

Question 8

Cardiac Action Potential: Phase 2

Answer 8

Phase 2 mainly due to (1) Slow inactivation of ICa allows Em to remain positive; (2) K conductances can’t make Em negative early in plateau because IK activates slowly; and (3) IK-1 is inwardly rectifying

Question 9

Cardiac Action Potential: Phase 3

Answer 9

Main repolarizing K Currents:

• •Repolarization mainly due to IK, the delayed rectifier K+ current which has 3 components…
• –IKs Slow component
• –IKr Rapid component: blocked by antiarrhythmic agents
• –IKur Ultra-rapid component (mainly in atria)
• • and by the inward rectifier, IK1, which activates late in the plateau as the membrane potential decreases (resulting in less inward rectification

Phase 3: ICa inactivates; IK (IKr, IKs) activates in latter phase 2; as Em becomes more negative there is less inward rectification, allowing IK-1 to contribute

Question 10

Inward Rectification

Answer 10

Question 11

3 main K+ currents

Answer 11

• •Know about the 1st three (you’ll learn about others in the future)
• •Distinguished by different voltage, time, ligand dependencies and whether rectification is present. Examples are:

1. –Inwardly rectifying K+ current, IK1: Responsible for the the resting membrane potential, repolarization.
2. –Delayed rectifier repolarizing K+ currents, IK: IK-s, IK-r
3. –Transient outward K+ current, Ito: Responsible (in part) for phase 1