Cardiac Action Potential Overview

Cardiac Action Potential

• All living cells have a resting membrane potential (RMP), expressed in millivolts (mV). Inside the cell is negative relative to the outside.
• Membrane potential changes due to ion flow (ions moving into or out of the cell).
• Depolarization occurs when positive charge flows into the cell, making the membrane potential less negative.
• The sinoatrial (SA) node initiates the heart's excitation signal (action potential).
• The conduction system in the heart consists of the SA node, AV node, bundles of atrial fibers, the bundle of His, and Purkinje fibers.
• The SA node is located at the junction of the superior vena cava and the right atrium.
• The AV node is in the right posterior portion of the interatrial septum.
• Three bundles of atrial fibers connect the SA node to the AV node.
• The bundle of His and its branches are components of the conduction system.
• Purkinje fibers are specialized conducting cells.
• Modified cardiac muscle cells form the conduction system, with fewer striations and indistinct boundaries compared to typical cardiac muscle.
• Purkinje fibers are large, with fewer mitochondria, and specialized for conduction.

Phases of Cardiac Action Potential

• Phase 0 (Depolarization): Voltage-gated sodium channels open, allowing rapid sodium influx, depolarizing the cell. Membrane potential reaches approximately +20 mV.
• Phase 1 (Initial Repolarization): Sodium channels close; the cell begins repolarizing. Potassium ions leave the cell through open potassium channels.
• Phase 2 (Plateau): Calcium ion permeability increases, while potassium permeability decreases. A combination of decreased potassium efflux and increased calcium influx maintain the action potential plateau.
• Phase 3 (Rapid Repolarization): Calcium channels close; potassium permeability increases significantly allowing potassium ions to rapidly leave the cell, returning the membrane potential to its resting level.
• Phase 4 (Resting Membrane Potential): The cell returns to resting membrane potential, which averages approximately -90 mV.

Action Potential of Cardiac Muscle Cells

• Myocardial fibers have a resting membrane potential of about -90 mV.
• Stimulation from the SA node initiates a propagated action potential, responsible for muscle contraction.
• The AV node can serve as a secondary (latent) pacemaker if the SA node is damaged.
• Ventricular contraction occurs only after the completion of systole (contraction) of the cardiac muscle.

Pacemaker Cells

• The SA node is the primary pacemaker of the heart, initiating normal sinus rhythm (NSR).
• The normal resting membrane potential of the SA node is -55 to -60 mV (compared to -85 mV in ventricular fibers).
• Sodium channels in SA cells are slightly leaky ("less negative"). Sodium influx contributes to gradual depolarization.
• Calcium-sodium channels open when the membrane potential reaches a threshold (~40 mV), causing a rapid rise in the membrane potential to ~0mV.
• Potassium efflux during repolarization returns the membrane potential to -55 to -60 mV.

Refractory Period

• Absolute Refractory Period (ARP): A period during which the membrane cannot be re-excited, regardless of the stimulus strength.
• Relative Refractory Period (RRP): A period during which the membrane can be re-excited, but only by a larger-than-normal stimulus.

Benefits of the Plateau Phase

• Prolongation of action potential duration
• Exposure to high cytoplasmic Ca²+ concentration, increasing the force of contraction.
• Increased time for all cardiac cells to contract simultaneously.