Cardiac Conduction System

Introduction to the Intrinsic Cardiac Conduction System

The intrinsic cardiac conduction system is a network of specialized cardiac muscle cells responsible for initiating and distributing electrical impulses throughout the heart, ensuring coordinated contractions. This system operates independently of the nervous system, allowing the heart to beat rhythmically.

• Intrinsic conduction: Does not require nervous system input; functions autonomously.

• Gap junctions: Allow action potentials to pass rapidly from cell to cell.

• Conducting fibers: Specialized cardiac cells with myofibrils, insulated from contractile cells.

• Pacemaker cells: Initiate action potentials at regular intervals.

Anatomy of the Intrinsic Cardiac Conduction System

The conduction system consists of several key structures, each with a specific location and function in the heart:

• Sinoatrial (SA) Node: Located in the superior wall of the right atrium, near the opening of the superior vena cava. Contains pacemaker cells that initiate the heartbeat.

• Internodal Pathways: Connect the SA and AV nodes, distributing the action potential through the atria.

• Atrioventricular (AV) Node: Located in the inferior right atrium, initiates ventricular contraction and contains pacemaker cells.

• AV Bundle (Bundle of His): Superior portion of the interventricular septum; conducts impulses from the AV node to the bundle branches.

• Right & Left Bundle Branches: Located in the interventricular septum; conduct impulses to the right and left ventricles.

• Subendocardial Conducting Network (Purkinje fibers): Smallest fibers; connect to contractile cells in the ventricles, stimulating contraction.

Table: Structures of the Cardiac Conduction System and Their Locations

Conduction Pathway and Contraction

Coordinated contraction of the heart requires a precise sequence of electrical events:

1. Pacemaker cells in the SA node initiate the action potential.

2. Action potential spreads across atria via conducting fibers and contractile cells, causing atrial contraction.

3. Action potential reaches the AV node, where a brief delay allows the atria to finish contracting.

4. Action potential moves down the AV bundle (Bundle of His) and right & left bundle branches.

5. Action potential spreads through Purkinje fibers, stimulating ventricular contraction.

Steps of Cardiac Conduction (Summary Table)

Role of Gap Junctions

Gap junctions are specialized intercellular connections that allow ions and action potentials to pass rapidly between cardiac muscle cells, enabling synchronized contraction. The AV node has fewer gap junctions, resulting in slower conduction and allowing the ventricles time to fill before contracting.

Pacemaker Cells and Heart Rhythm

Pacemaker cells in the SA node generate rhythmic action potentials, setting the pace for the heartbeat. If the SA node fails, the AV node or other components can take over as pacemaker, but at a slower rate.

Control of Heart Rate

Intrinsic and Extrinsic Control

Heart rate is regulated by both intrinsic and extrinsic factors:

• Intrinsic control: Pacemaker cells initiate action potentials independently.

• Extrinsic control: Chronotropic factors (e.g., nervous system, hormones) affect heart rate positively or negatively.

Neural Regulation

The medulla oblongata in the brainstem is responsible for chronotropic control of heart rate via dual innervation:

• Sympathetic nervous system: Increases heart rate by stimulating the SA node, AV node, and heart muscle.

• Parasympathetic nervous system: Decreases heart rate by innervating the SA and AV nodes via the vagus nerve.

Table: Effects of Sympathetic and Parasympathetic Nervous Systems

Example: Effect of Nerve Severing on Heart Rate

Key Terms and Definitions

• Action potential (AP): A rapid change in membrane potential that propagates along excitable cells.

• Pacemaker cells: Specialized cardiac cells that spontaneously generate action potentials.

• Gap junctions: Channels that allow ions and electrical impulses to pass between adjacent cells.

• Chronotropic factors: Factors that affect the rate of heart contraction.

• Medulla oblongata: Brainstem region controlling autonomic functions, including heart rate.

Relevant Equations

• Heart rate (HR) calculation:

Summary Table: Cardiac Conduction System Structures and Functions

Example Application

If the SA node fails, the AV node can take over as pacemaker, but the heart rate will be slower. The conduction system ensures that the atria contract before the ventricles, and that the ventricles contract from the apex upward, efficiently pumping blood.

Additional info: The cardiac conduction system is essential for maintaining a regular and coordinated heartbeat, and its disruption can lead to arrhythmias or inefficient blood flow.