Cardiac Conduction System: Anatomy & Physiology Study Notes part A

Cardiac Conduction System

Introduction to the Intrinsic Cardiac Conduction System

The intrinsic cardiac conduction system is a network of specialized cardiac muscle cells (myocytes) that generate and conduct electrical impulses throughout the heart, ensuring coordinated contractions and efficient pumping of blood.

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

• Coordinated contraction: Cardiac muscle cells are connected by gap junctions, allowing ions and action potentials to pass rapidly from cell to cell.

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

• Pacemaker cells: Generate action potentials at regular intervals, setting the heart's rhythm.

Key Terms:

• Nodes: Clusters of pacemaker cells that initiate and regulate action potentials.

• Gap junctions: Specialized intercellular connections that allow direct electrical communication between cardiac cells.

• Pacemaker: Specialized cells that set the pace of the 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.

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

• Internodal Pathways: Connect the SA node to the AV node and distribute the action potential through the atria.

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

• Atrioventricular (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.

Structures of the Cardiac Conduction System

Structure	Location	Function
Sinatrial (SA) Node	Right atrium (superior wall)	Initiates action potential; pacemaker
Internodal Pathways	Atria	Distribute action potential
Atrioventricular (AV) Node	Right atrium (inferior wall)	Delays and relays action potential to ventricles
AV Bundle (Bundle of His)	Interventricular septum	Conducts impulse to bundle branches
Right & Left Bundle Branches	Interventricular septum	Conduct impulse to ventricles
Purkinje fibers	Ventricular walls	Stimulate ventricular contraction

Conduction Pathway and Contraction

Proper heart function requires a precise sequence of electrical conduction and muscle contraction.

• 1. Atria contract first, followed by ventricles.

• 2. No gap junctions between atrial and ventricular cardiomyocytes; must use conduction system to stimulate ventricles.

Steps of Cardiac Conduction

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

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

3. Action potential reaches the AV node.

4. After a brief delay, the AV node initiates a new action potential.

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

6. Action potential spreads through the Purkinje fibers, stimulating ventricular contraction.

Order of Electrical Conduction in the Heart

The correct sequence of electrical conduction ensures efficient heart function:

1. Pacemaker cells initiate an action potential (SA node).

2. Action potential is passed through the atria.

3. Action potential reaches the AV node.

4. 100 ms delay at the AV node.

5. Action potential moves down the AV bundle.

6. Action potential moves down the right and left bundle branches.

7. Purkinje fibers distribute the action potential.

8. Action potential is passed through contractile cells of the ventricles.

9. Ventricles contract.

Control of Heart Rate

Heart rate is regulated by both intrinsic and extrinsic factors:

• Intrinsic factors: Pacemaker cells initiate action potentials.

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

• Medulla oblongata: Responsible for chronotropic control via autonomic nervous system.

Dual Innervation of the Heart

Nervous System	Effect on Heart Rate	Pathway
Sympathetic	Increases heart rate	Cardioacceleratory center; signals via sympathetic nerves to SA node, AV node, and heart muscle
Parasympathetic	Decreases heart rate	Cardioinhibitory center; signals via vagus nerve to SA and AV nodes

Practice Questions and Key Concepts

• Which tissue allows rapid spread of action potentials? Gap junctions between cardiac muscle cells.

• What describes intrinsic conduction? Cells within the heart can initiate and transmit action potentials without nervous system input.

• If the SA node fails, which structure takes over as pacemaker? AV node is the next in line to initiate action potentials.

• AV node conduction delay: Ensures ventricles have time to fill with blood before contracting.

• Primary function of SA node pacemaker cells: Rhythmic generation of action potentials.

• Purkinje fibers: Most directly responsible for allowing contraction of the ventricles to begin at the apex of the heart.

Summary Table: Effects of Autonomic Nervous System on Heart Rate

Effect of Severing	On Resting Heart Rate	On Heart Rate During Exercise
Sympathetic nerve fibers	Decreases ability to increase heart rate	Heart rate does not rise as much during exercise
Parasympathetic nerve fibers	Resting heart rate increases	Heart rate increases more rapidly during exercise

Key Equations

• Heart Rate (HR) Equation:

$HR = \frac{60}{\text{R-R interval (seconds)}}$

• Cardiac Output (CO) Equation:

$CO = HR \times SV$

Where SV is stroke volume.

Additional info: The above notes expand on the brief points and fill in missing context for a comprehensive review of the cardiac conduction system, suitable for Anatomy & Physiology students.