Key Points on Cardiac Muscle Contraction

Cardiac Muscle Contraction

• Cardiac muscle is an excitable tissue that can generate action potentials.

Major Sequence of Events

• Phase 4: Resting Membrane Potential

  • The cell is at rest before any action occurs.

• Phase 0: Depolarization

  • Action potential (AP) opens voltage-gated Na⁺ channels, leading to a rapid increase in Na⁺ permeability.
  • These channels eventually close.

• Phase 1: Initial Repolarization

  • Fast K⁺ channels open, allowing initial repolarization.

• Phase 2: The Plateau

  • Voltage-gated Ca²⁺ channels open slowly, increasing Ca²⁺ permeability.
  • Fast K⁺ channels close during this phase.

• Phase 3: Rapid Repolarization

  • Ca²⁺ channels close, and slow voltage-gated K⁺ channels open.
  • Resting stage ion permeability is restored (Phase 4).

Key Characteristics

• Sustained Depolarization

  • Caused by the slow opening of voltage-gated Ca²⁺ channels, contributing to a prolonged action potential.

• Comparison of Action Potentials

  • Neuron/Skeletal Muscle: 1-5 msec
  • Cardiac Muscle: >200 msec

Importance of Prolonged Action Potential

• Prevents tetanus.
• Allows heart to relax between contractions.

Why Cardiac Muscle Cells Do Not Undergo Summation and Tetanus

• Due to a longer refractory period, cardiac muscle cells finish contracting before the next action potential occurs.