What is cardiac action potential?

Cardiac Action Potential Explained Simply

The cardiac action potential is a special electrical signal that happens in heart cells when they beat, created by the movement of tiny charged particles (ions) through channels in the cell membrane.

The Five Phases of Cardiac Action Potential

The cardiac action potential can be divided into five distinct phases that work together to make your heart beat properly:

Phase 0: Rapid Depolarization

• This is when the heart cell quickly changes from its resting state (negative charge) to an excited state (positive charge)
• Sodium channels open suddenly, allowing sodium ions to rush into the cell 1
• This happens very fast - like flipping on a light switch
• This is what makes the heart cell "fire" and start contracting

Phase 1: Early Repolarization

• Right after the quick excitement, some potassium ions start to leave the cell
• This creates a small "notch" in the action potential, especially visible in cells from certain parts of the heart
• This phase is short but important for setting up the next phase

Phase 2: The Plateau

• This is unique to heart cells and makes their action potential much longer than nerve cells
• Calcium channels open, letting calcium ions enter the cell 1
• Calcium entering triggers more calcium release inside the cell, which makes the heart muscle contract
• The plateau keeps the cell excited for longer, preventing it from being re-excited too quickly
• This prevents the heart from beating too fast or irregularly

Phase 3: Repolarization

• The cell returns to its resting state as potassium channels open wide
• Potassium ions flow out of the cell, making it negative again 1, 2
• Calcium channels close, stopping the inflow of calcium
• This phase corresponds to the T wave on an ECG

Phase 4: Resting Phase

• The cell is at rest, with a negative charge inside (about -85 mV) 1
• The cell maintains this resting state until the next heartbeat
• Special pumps work to reset the ion concentrations inside and outside the cell

Why This Matters

The cardiac action potential is important because:

1. It allows heart cells to contract in a coordinated way
2. The unique plateau phase prevents the heart from beating too quickly
3. Problems with any ion channels can cause irregular heartbeats (arrhythmias)
4. Many heart medications work by affecting these ion channels
5. Different parts of the heart have slightly different action potentials, which helps the heart beat in the right sequence

Common Problems

• If sodium channels don't work properly: The heart may not be able to start beats effectively
• If calcium channels malfunction: The strength of heart contractions can be affected
• If potassium channels have issues: The heart may not relax properly between beats, leading to arrhythmias

The careful balance of these ion movements is what keeps your heart beating regularly. When doctors talk about heart rhythm problems or certain heart medications, they're often discussing how these electrical signals are affected.