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?

QID: 100635
FIGURES:
1

Phase 0, which is primarily characterized by sodium influx

67%

(29/43)

2

Phase 0, which is primarily characterized by potassium efflux

0%

(0/43)

3

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

2%

(1/43)

4

Phase 1, which is primarily characterized by calcium efflux

14%

(6/43)

5

Phase 3, which is primarily characterized by potassium efflux

12%

(5/43)

M 1 E

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