Action Potentials in Myocardial Contractile Cells

Introduction

• Action potentials in myocardial contractile cells are crucial for heart function.
• Tetany prevention in cardiac muscle cells is vital, as sustained contraction would be life-threatening.

Key Properties

• Long Refractory Period:
  • Special properties of action potentials in myocardial cells lead to a significantly longer refractory period.
  • This prevents re-excitation before muscle contraction is over.

Resting Potential

• Resting potential is around -90 millivolts.
• Maintained by potassium ion efflux through delayed rectifier potassium channels.

Action Potential Trigger

• Unlike other cells, action potentials aren’t triggered by transmitters but by electrochemical stimulation.
• Depolarization from a neighboring cell causes positive ions to move through gap junctions.
• Membrane potential increases when it exceeds around -65 millivolts:
  • Potassium channels close.
  • Voltage-gated sodium channels open.

Depolarization

• Rapid influx of sodium ions leads to depolarization:
  • Membrane potential reaches -40 millivolts.
  • Sodium channels close after ~1 millisecond.
  • Maximum potential is about +20 millivolts.

Repolarization and Plateau Phase

• Initial repolarization occurs through:

  • Efflux of potassium ions.
  • Influx of negatively charged chloride ions.

• Membrane potential plateaus at 0 millivolts for 200-400 milliseconds.

• Calcium Influx:

  • L-type calcium channels, activated at >-40 millivolts, maintain the plateau.
  • Calcium channels close before final repolarization.

Final Repolarization

• Dominated by ion currents:
  • Rapid and slow potassium channels.
  • Delayed rectifier potassium channels restore resting potential.
• No After Hyperpolarization in cardiomyocytes like in skeletal muscles.

Refractory Period

• Absolute Refractory Period:
  • Sodium channels inactive until the end of the plateau phase.
  • Channels return to active state during final repolarization.
  • Fully activatable once resting potential is re-established.

Conclusion

• Similarities exist in action potential formation across neurons, skeletal muscle cells, and myocardial contractile cells.
• Differences are covered in other parts of the course.

Further Study

• Refer to parts two and three of the course for more on action potentials in neurons and muscle cells compared to myocardial cells.