Updated: 3/31/2018

Myocardial Action Potential

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https://upload.medbullets.com/topic/108015/images/ventricular-action-potential.jpg
Snapshot
  • A 60-year-old man presents to his cardiologist for a follow-up of newly diagnosed diastolic heart failure. He has a history of asthma and chronic obstructive lung disease. He reports that his pulmonologist does not want him to take β-blockers. However, his cardiologist wants to start him on a medication to slow the heart rate. He reassures the patient that this drug acts not on β-receptors but on a Ca2+ channel. (Verapamil)
Introduction
  • Ions responsible for the action potential of atria, ventricles, and Purkinje fibers are the same
    • upstroke
      • inward Na+ current
    • plateau
      • slow inward Ca2+ current via L-type voltage-gated Ca2+ channels
      • a sustained period of depolarization
        • allows for ventricular filling
Ventricular Action Potential
  • Resting membrane potential
    • -85 mV resting membrane potential is maintained by inward rectifier K+ channels
      • inward rectifier K+ channels open at rest and close with depolarization
  • Phase 0, upstroke
    • rapid depolarization
      • caused by the opening of voltage-gated Na+ channels and inward Na+ current
        • Na+ current depolarizes cell membrane, which closes inward rectifier K+ channels
  • Phase 1, initial repolarization
    • brief repolarization and net outward current
      • inactivation gates on Na+ channels close
      • Na+ current is decreased
      • outward K+ current down an electrochemical gradient
  • Phase 2, plateau
    • stable, long period of depolarized membrane potential
    • inward and outward current are equal, with no net current flow
      • inward Ca2+ current (slow inward current)
        • opened L-type Ca2+ channels
          • blocked by Ca2+ channel blockers
        • entry of Ca2+ also triggers the release of more Ca2+ from sarcoplasmic reticulum in the cell (Ca2+-induced Ca2+ release)
      • outward K+ current
        • driven by electrochemical driving force
  • Phase 3, repolarization
    • net outward current
      • L-type voltage-gated Ca2+ channels close
        • a decrease in inward Ca2+ current
      • delayed rectifier K+ channels open completely and repolarize the membrane
        • increase in outward K+ current
    • inward rectifier K+ channels re-open
  • Phase 4, resting membrane potential
    • inward and outward currents are equal
    • delayed rectifier K+ channels close
    • inward rectifier K+ channels are fully open, resetting resting membrane potential at -85 mV
 

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Questions (2)
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(M1.CV.13.119) A 48-year-old female comes into the ER with chest pain. An electrocardiogram (EKG) shows a heart beat of this individual in Image A. The QR segment best correlates with what part of the action potential of the ventricular myocyte shown in Image B? Tested Concept

QID: 100635
FIGURES:
1

Phase 0, which is primarily characterized by sodium influx

63%

(22/35)

2

Phase 0, which is primarily characterized by potassium efflux

0%

(0/35)

3

Phase 1, which is primarily characterized by potassium and chloride efflux

3%

(1/35)

4

Phase 1, which is primarily characterized by calcium efflux

17%

(6/35)

5

Phase 3, which is primarily characterized by potassium efflux

11%

(4/35)

M 1 D

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