Conduction System of the Heart

Key Concepts

• Cardiac Conduction System: The electrical system of the heart responsible for initiating and coordinating heartbeats.
• SA Node (Sinoatrial Node): The primary pacemaker of the heart, initiating electrical impulses.
• AV Node (Atrioventricular Node): Relays electrical impulses from the atria to the ventricles, introducing a delay.
• Bundle of His (Atrioventricular Bundle): Conducts impulses from the AV node to the ventricles.
• Bundle Branches: Divisions of the Bundle of His that carry impulses to each ventricle.
• Purkinje Fibers: Distribute electrical impulses throughout the ventricular muscle.
• Cardiac Muscle Cells (Cardiomyocytes): Specialized muscle cells of the heart, including contractile and conductive types.
• Intercalated Discs: Junctions between cardiac muscle cells containing gap junctions and desmosomes.
• Gap Junctions: Allow for electrical communication between cells.
• Desmosomes: Anchor cells together during contraction.
• Membrane Potential: The electrical charge difference across a cell membrane.
• Depolarization: A decrease in the membrane potential, making the inside of the cell more positive.
• Action Potential: An electrical signal generated by a cell.
• Spontaneous Depolarization: The ability of certain cells (like SA node pacemaker cells) to depolarize without an external stimulus.
• Funny Current (HCN Channels): A current in SA node pacemaker cells that contributes to spontaneous depolarization.
• ECG/EKG (Electrocardiogram): A medical test that measures the electrical activity of the heart.
• Atrial Fibrillation (AFib): A heart condition characterized by irregular and rapid electrical signaling in the atria.

The Heart's Electrical System: A Progressive Contraction

The video begins by relating the concept of taking one's pulse to the underlying electrical activity of the heart. The pulse felt is a result of the left ventricle pumping blood into the aorta and arteries. This leads to a discussion of the heart's complex, progressive contraction, which involves coordinated electrical signaling. The heart's muscle tissue, known as the myocardium or cardiac muscle tissue, is composed of specialized cells called cardiac myocytes or cardiomyocytes. These cells are highly branched and interconnected by intercalated discs. These discs are crucial as they contain gap junctions, facilitating cell-to-cell communication, and desmosomes, providing structural integrity during contraction. A human heart beats approximately 100,000 times a day, highlighting the continuous and coordinated nature of its function.

The Cardiac Conduction System: From SA Node to Ventricles

The focus then shifts to the electrical aspect of a cardiac cycle, specifically the role of conductive cells rather than contractile cells.

1. Initiation at the SA Node: The electrical impulse originates in the SA node, a region in the right atrium containing specialized pacemaker cells. These cells are unique because they undergo spontaneous depolarization without external stimulation.

   • Membrane Potential of SA Node Cells: Unlike most cells, SA node cells do not have a stable resting membrane potential. Their most negative charge is around -60 mV.
   • The "Funny Current": At approximately -60 mV, a current known as the "funny current" (or pacemaker current) is activated. This involves the opening of HCN channels, allowing both sodium (Na+) and potassium (K+) ions to pass. Significantly more Na+ enters than K+ leaves, making the cell's interior more positive.
   • Depolarization and Action Potential: As the membrane potential becomes more positive, it approaches a threshold of around -40 mV. At this point, calcium channels open, allowing calcium ions (Ca2+) to rush in, causing a rapid depolarization from -40 mV to +15 mV. Subsequently, calcium channels close, and voltage-gated potassium channels open, allowing K+ to leave the cell, leading to repolarization. Once the potential reaches -60 mV, voltage-gated potassium channels close, and the cycle repeats. The less negative resting potential of SA node cells contributes to their ability to spontaneously depolarize.

2. Conduction to the Atria: The action potential generated in the SA node spreads to neighboring cardiac muscle cells in the atria, causing them to depolarize and contract.

3. The Role of the AV Node: A fibrous ring prevents the direct electrical conduction from the atria to the ventricles. Instead, the impulse propagates to the AV node, located inferior to the SA node in the right atrium.

   • AV Node Function: The AV node also contains conductivity cells, including pacemaker cells, which can spontaneously depolarize. However, in a healthy heart, it primarily relays the impulse from the SA node.
   • Backup Pacemaker: The AV node's ability to spontaneously depolarize makes it a potential backup pacemaker if the SA node fails, though at a slower rate.
   • Conduction Delay: Crucially, the electrical signal travels slower through the AV node than the SA node. This delay, though brief (a fraction of a second), is vital. It allows the atria sufficient time to contract and pump blood into the ventricles before the ventricles begin to contract. This slower conduction is attributed to the AV node's conductivity cells having a smaller diameter and fewer gap junctions.

4. Ventricular Conduction: From the AV node, the action potential is transmitted to the bundle of His (also known as the atrioventricular bundle).

   • Bundle Branches: The bundle of His divides into a right and left bundle branch, each supplying one ventricle.
   • Purkinje Fibers: These branches lead to the Purkinje fibers, which are conductive fibers that spread the impulse throughout the ventricular muscle cells. The impulse starts at the apex of the heart and moves towards the base, ensuring coordinated ventricular contraction.

The Cardiac Conduction System: A Summary

The entire pathway—SA node, AV node, bundle of His, bundle branches, and Purkinje fibers—constitutes the cardiac conduction system. These specialized conductive cells, making up about 1% of heart muscle cells, are responsible for the electrical signaling that drives the heart's mechanical pumping action. At rest, the SA node typically generates 60 to 100 action potentials per minute. The remaining 99% of heart muscle cells are contractile cells, responsible for the actual squeezing of the heart.

Evaluating the Electrical System: The ECG

In the medical field, the electrocardiogram (ECG or EKG) is a common tool used to assess the function of the heart's electrical system. An ECG measures the voltage of electrical signals in the heart over time. The patterns on an ECG are interpreted by health professionals to identify abnormalities. For instance, atrial fibrillation (AFib), where multiple sites in the atria depolarize irregularly, can lead to inefficient atrial contraction, increasing the risk of blood clot formation. These clots can travel to the brain, potentially causing a stroke. The ECG is invaluable for detecting such electrical signaling problems, enabling timely diagnosis and treatment plans.