Action Potentials in Myocytes

Overview

• Action Potentials: Rapid electrical changes across cell membranes.
• Propagation: Often moves from one cell to adjacent cells; crucial in heart communication.
• Pacemaker Cells: Initiate and set rhythm of heartbeat, composing 1% of heart cells.
  • Continuously generate action potentials for heart contraction.
• Myocytes: Receive action potentials and make up the myocardium, the heart's muscular layer.
  • Also known as contractile cells.
  • Different from skeletal muscle cells that receive signals directly from neurons.

Myocyte Action Potential Phases

1. Phase 4: Resting Phase

   • Myocyte at rest with a membrane potential of -90 mV.
   • Gap junctions allow ion movement between neighboring myocytes.
   • Neighbor myocyte depolarization raises potential to -70 mV (threshold potential).

2. Phase 0: Depolarization Phase

   • Voltage-gated sodium channels open at -70 mV.
   • Sodium influx raises potential to +20 mV.
   • All-or-none process: Threshold must be hit for depolarization.

3. Phase 1: Initial Repolarization

   • Sodium channels close.
   • Potassium channels open at +20 mV, allowing potassium egress.
   • Outward positive current reduces potential slightly ("notch" in graph).

4. Phase 2: Plateau Phase

   • Voltage-gated calcium channels open.
   • Calcium influx counterbalances potassium egress, stabilizing potential.
   • Calcium influx crucial for myocyte contraction and heartbeat longevity.

5. Phase 3: Repolarization

   • Calcium channels close; potassium channels remain open.
   • Continued potassium egress leads to net outward positive current.
   • Ion pumps expel calcium, causing muscle relaxation.
   • Membrane potential returns to -90 mV.

Recap of Phases

• Phase 4: Myocytes at rest.
• Phase 0: Sodium influx, myocyte depolarization.
• Phase 1: Potassium efflux slightly reduces potential.
• Phase 2: Calcium influx leads to plateau.
• Phase 3: Calcium channels close; potassium efflux repolarizes cell to resting state.