Contractile Cardiac Cell Action Potential

Phases of the Cardiac Action Potential

The action potential in contractile cardiac muscle cells is essential for the coordinated contraction of the heart. It consists of five distinct phases, each characterized by the movement of specific ions across the cell membrane.

• Phase 0 – Rapid Depolarisation: This phase is initiated by a rapid influx of sodium ions (Na+) into the cell through voltage-gated sodium channels, causing the membrane potential to become more positive.

• Phase 1 – Initial Repolarisation: A brief, partial repolarisation occurs due to the transient outward movement of potassium ions (K+).

• Phase 2 – Plateau: During this phase, calcium ions (Ca2+) enter the cell through L-type calcium channels while potassium ions continue to leave. The inward Ca2+ current balances the outward K+ current, resulting in a plateau in the membrane potential.

• Phase 3 – Repolarisation: Calcium channels close, and potassium efflux dominates, returning the membrane potential toward its resting value.

• Phase 4 – Resting Membrane Potential: The cell returns to its resting state, maintained primarily by the Na+/K+ ATPase pump and background K+ currents.

Example: The plateau phase (Phase 2) is unique to cardiac muscle and is responsible for the prolonged refractory period, preventing tetanus and ensuring rhythmic contractions.

Key Ion Movements During Each Phase

• Na+ influx: Responsible for rapid depolarisation (Phase 0).

• K+ efflux: Contributes to initial repolarisation (Phase 1), plateau (Phase 2), and final repolarisation (Phase 3).

• Ca2+ influx: Maintains the plateau phase (Phase 2).

Summary Table: Cardiac Action Potential Phases

Cardiac Electrical Conduction System

Pathway of Electrical Activity in the Heart

The cardiac conduction system ensures the heart beats in a coordinated and efficient manner. Electrical impulses follow a specific pathway through specialized cardiac tissues.

1. Sinoatrial (SA) Node: Located in the right atrium, the SA node acts as the heart's natural pacemaker, generating spontaneous action potentials that initiate each heartbeat.

2. Atrial Conduction Pathways: The impulse spreads through the atria, causing atrial contraction.

3. Atrioventricular (AV) Node: Located at the junction between the atria and ventricles, the AV node delays the impulse, allowing the ventricles to fill with blood before they contract.

4. Bundle of His (AV Bundle): The impulse travels from the AV node into the interventricular septum via the Bundle of His.

5. Right and Left Bundle Branches: The Bundle of His divides into right and left branches, conducting the impulse toward the apex of the heart.

6. Purkinje Fibers: These fibers distribute the impulse throughout the ventricular myocardium, resulting in coordinated ventricular contraction.

Example: Damage to the AV node can result in heart block, where the conduction of impulses from the atria to the ventricles is impaired.

Summary Table: Cardiac Conduction Pathway

Key Terms

• Depolarisation: A decrease in membrane potential (more positive inside the cell).

• Repolarisation: Return of the membrane potential to a more negative value.

• Plateau Phase: A period of maintained depolarisation unique to cardiac muscle cells.

• Pacemaker: A region of the heart that initiates electrical impulses (e.g., SA node).

Relevant Equation

The Nernst equation can be used to calculate the equilibrium potential for each ion:

Where: R = universal gas constant T = temperature (Kelvin) z = charge of the ion F = Faraday's constant [ion]outside and [ion]inside = ion concentrations outside and inside the cell