What are the similarities and differences between nodal action potentials and cardiac myocyte action potentials, including the ion channels involved?

From the Guidelines

Nodal and cardiac myocyte action potentials differ significantly in their mechanisms and functions within the heart, with nodal action potentials characterized by spontaneous depolarization during phase 4 and cardiac myocyte action potentials having more negative resting potentials and a prominent plateau phase. The differences between these two types of action potentials are crucial for understanding the specialized roles of each cell type in the heart. Nodal action potentials, found in the sinoatrial (SA) and atrioventricular (AV) nodes, are driven by the funny current (If) through HCN channels and T-type calcium channels, allowing them to act as the heart's pacemaker 1. In contrast, cardiac myocyte action potentials, found in atrial and ventricular muscle cells, have more negative resting potentials (-90 mV), rapid upstroke velocities due to voltage-gated sodium channels (Nav1.5), and a prominent plateau phase (phase 2) maintained by L-type calcium channels balanced by delayed rectifier potassium channels (IKr and IKs) 1.

Some key features of nodal action potentials include:

• Less negative resting membrane potentials (-60 to -40 mV)
• Slower upstroke velocities due to calcium-dependent depolarization via L-type calcium channels
• Lack of a distinct plateau phase
• Spontaneous depolarization during phase 4 (diastolic depolarization)

In contrast, cardiac myocyte action potentials have:

• More negative resting potentials (-90 mV)
• Rapid upstroke velocities due to voltage-gated sodium channels (Nav1.5)
• A prominent plateau phase (phase 2) maintained by L-type calcium channels balanced by delayed rectifier potassium channels (IKr and IKs)
• Repolarization via potassium channels, though the specific subtypes and timing differ

These differences reflect the specialized roles of each cell type: nodal cells as pacemakers that initiate the heartbeat, and myocytes as contractile units that respond to electrical signals to produce coordinated cardiac contraction 1. The most recent study 1 highlights the importance of understanding the electrophysiological mechanisms underlying atrial fibrillation, which is initiated by focal sources and maintained by reentrant and focal activities. The study also notes that age and cardiovascular diseases produce complex electrical and structural changes in the atria and/or pulmonary veins, creating a stable arrhythmogenic substrate that facilitates the perpetuation of atrial fibrillation.

From the Research

Nodal Action Potentials

• Nodal action potentials are generated by the atrioventricular (AV) node, which is a critical structure in the heart's conduction system 2, 3.
• The AV node has two pathways with different electrophysiological properties: a 'fast' pathway and a 'slow' pathway 2.
• The 'fast' pathway conducts faster but has a longer refractory period than the 'slow' pathway, which can lead to reentrant tachycardia 2.

Cardiac Myocyte Action Potentials

• Cardiac myocyte action potentials are generated by the working myocardium, which is responsible for contracting and pumping blood throughout the body 4, 5.
• The action potential in cardiac myocytes is propagated through gap junction channels, which allow for rapid and uniform conduction of electrical signals 4.
• The Luo-Rudy model is a computational model that describes the action potential in cardiac myocytes, including the membrane potential and action potential stages 5.

Comparison of Nodal and Cardiac Myocyte Action Potentials

• Nodal action potentials have a slower conduction velocity and a longer refractory period compared to cardiac myocyte action potentials 2, 4.
• Cardiac myocyte action potentials are generated by a different set of ion channels and have a distinct shape and duration compared to nodal action potentials 4, 5.
• Both nodal and cardiac myocyte action potentials are critical for the proper functioning of the heart, and abnormalities in either can lead to arrhythmias and other cardiac disorders 2, 3, 6.

Channels Involved

• The AV node expresses a unique set of ion channels, including calcium channels, potassium channels, and sodium channels, which contribute to the generation of nodal action potentials 2, 3.
• Cardiac myocytes express a different set of ion channels, including sodium channels, calcium channels, and potassium channels, which contribute to the generation of cardiac myocyte action potentials 4, 5.
• Gap junction channels, such as connexin, play a critical role in the propagation of action potentials between cardiac myocytes 4.