Cardiac Conduction System

Introduction to the Intrinsic Cardiac Conduction System

The intrinsic cardiac conduction system is a network of specialized cardiac cells that generate and conduct electrical impulses, ensuring the heart beats in a coordinated and rhythmic manner. This system operates independently of the nervous system, although it can be influenced by external factors.

• Intrinsic conduction: Refers to the heart's ability to initiate and transmit action potentials without nervous system input.

• Coordinated contraction: Cardiac muscle cells must contract in a synchronized fashion for effective pumping.

• Gap junctions: Specialized connections between cardiac cells that allow rapid spread of action potentials.

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

• Pacemaker cells: Cells that regularly initiate action potentials, setting the rhythm of the heartbeat.

Example: The sinoatrial (SA) node contains pacemaker cells that initiate each heartbeat.

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. It initiates the electrical signal and is known as the heart's natural pacemaker.

• Internodal Pathways: Conduct impulses from the SA node to the atrioventricular (AV) node.

• Atrioventricular (AV) Node: Located in the inferior right atrium, it receives the impulse from the SA node and introduces a delay before passing the signal to the ventricles.

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

• Right & Left Bundle Branches: Located in the interventricular septum; carry impulses toward the apex of the heart.

• Subendocardial Conducting Network (Purkinje fibers): Smallest fibers; spread throughout the ventricular walls and stimulate contractile cells.

Example: The AV node introduces a delay, allowing the atria to contract before the ventricles.

Conduction Pathway and Contraction

The cardiac conduction pathway ensures that the atria and ventricles contract in the correct sequence for effective blood flow.

• Step 1: Pacemaker cells in the SA node initiate the action potential.

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

• Step 3: Action potential reaches the AV node, where a brief delay occurs.

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

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

Example: The delay at the AV node allows the atria to finish contracting before the ventricles begin.

Steps of Cardiac Conduction (Summary Table)

Control of Heart Rate

Heart rate is regulated by both intrinsic and extrinsic factors, including pacemaker cells and nervous system input.

• Pacemaker cells: Set the intrinsic rate of action potentials.

• Chronotropic factors: Extrinsic factors that affect heart rate (can be positive or negative).

• Medulla oblongata: Brain region responsible for chronotropic control of heart rate.

• Sympathetic nervous system: Increases heart rate via the cardioacceleratory center; signals travel down the sympathetic nerve and innervate the SA and AV nodes.

• Parasympathetic nervous system: Decreases heart rate via the cardioinhibitory center; signals travel down the vagus nerve and innervate the SA and AV nodes.

Example: During exercise, sympathetic stimulation increases heart rate to meet the body's increased oxygen demand.

Key Terms and Definitions

• Action potential (AP): A rapid change in membrane potential that travels along cardiac cells, triggering contraction.

• Pacemaker cells: Specialized cells that spontaneously generate action potentials, setting the heart's rhythm.

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

• Chronotropic factors: Substances or influences that change the heart rate.

Relevant Equations

• Heart Rate (HR) Calculation:

• Cardiac Output (CO):

Summary Table: Cardiac Conduction Structures and Functions

Additional info:

• This material is primarily relevant to human physiology and anatomy, specifically the cardiac conduction system, rather than General Chemistry. However, the concepts of action potentials, ion movement, and cell-to-cell communication are related to chemical principles such as membrane potential and electrochemistry.