What is the action potential of cardiac muscle?

From the Guidelines

The action potential of cardiac muscle is a complex electrical signal that enables the heart to contract in a coordinated manner, with a longer duration and characteristic plateau phase compared to skeletal muscle action potentials. The cardiac action potential is initiated by rapid depolarization (Phase 0) due to sodium channels opening, followed by early repolarization (Phase 1) as potassium channels briefly activate 1. The plateau phase (Phase 2) is unique to cardiac cells, resulting from calcium influx through L-type calcium channels balanced by potassium efflux, maintaining depolarization for an extended period 1. This prolonged depolarization ensures adequate calcium entry for contraction and prevents tetanic contractions by creating a long refractory period. Repolarization occurs in Phase 3 as potassium channels fully open and calcium channels close, returning the membrane to its resting potential (Phase 4) 1.

Some key aspects of the cardiac action potential include:

• The plateau phase, which is critical for ensuring adequate calcium entry for contraction
• The role of potassium channels in repolarization
• The importance of the action potential in enabling coordinated contraction of the heart
• The differences between primary and secondary repolarization abnormalities, which can be caused by various factors such as ischemia, myocarditis, and ventricular conduction disturbances 1

The action potential of cardiac muscle is also influenced by various factors, including:

• Electrolyte abnormalities, such as changes in serum calcium and potassium levels
• Drugs and toxins, which can affect the shape and duration of the repolarization phases
• Changes in heart rate and ventricular conduction, which can lead to secondary repolarization abnormalities 1
• The spatial-temporal characteristics of ventricular repolarization, which can affect the configuration of the T wave on the body surface ECG 1

Overall, understanding the action potential of cardiac muscle is crucial for diagnosing and managing various cardiac conditions, including arrhythmias, ischemia, and heart failure. The most recent and highest quality study on this topic is from 2009, which provides a comprehensive overview of the cardiac action potential and its clinical significance 1.

From the Research

Action Potential Phases

• The action potential of cardiac muscle is composed of five phases that represent the changing ionic fluxes and membrane potentials of the cardiac cell 2.
• These phases are designated as phases 0-4, with phase 0 being the rapid depolarization phase and phase 4 being the resting membrane potential phase 3.

Ion Channels Involved

• The cardiac action potential is generated by an orchestrated function of ion channels, including potassium, sodium, and calcium channels 3.
• Potassium channels, such as K(v)11.1 (hERG1), K(v)7.1 (KCNQ1), and K(ir)2.1 (KCNJ2), play a crucial role in phase 3 repolarization 3.

Arrhythmogenic Behavior

• Cardiac arrhythmias can arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms 4.
• Changes in intracellular and extracellular concentrations of potassium, calcium, and magnesium can have important effects on the electrophysiology of the heart and can contribute to the production of various arrhythmias 2.

Modeling Action Potential Dynamics

• Computational modeling based on experimental data can be used to develop and test mechanistic hypotheses about cardiac electrophysiological research 5.
• A "minimal" ionic model of the human action potential can be developed based on in vivo human monophasic action potential recordings obtained during clinical programmed electrical stimulation 5.

Calcium Transient and Action Potential Alternans

• Alternation of cardiac action potential duration and concurrent alternation of the amplitude of the calcium transient can be regarded as important arrhythmia mechanisms 6.
• These phenomena can be reliably evoked by an increase in beat frequency or by ischemia, and can be related to the physiology of action potential duration alternans and calcium-activated ion currents 6.