Cardiac Conduction System: Anatomy & Physiology Study Notes

Cardiac Conduction System

Introduction to the Intrinsic Cardiac Conduction System

The intrinsic cardiac conduction system is a network of specialized cardiac muscle cells that generate and conduct electrical impulses throughout the heart, ensuring coordinated contractions. This system operates independently of the nervous system, although it can be modulated by extrinsic factors.

• Intrinsic conduction system: Initiates and conducts action potentials through the heart.

• "Intrinsic" means it does not require nervous system input to function; it operates within the heart itself.

• Results in heartbeats that are regular and coordinated.

• Gap junctions: Connect cardiac muscle cells, allowing ions and action potentials to pass from cell to cell.

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

• Pacemaker cells: Specialized cells that initiate action potentials at regular intervals.

Key Terms:

• Nodes: Regions of specialized cardiac tissue that generate and conduct action potentials.

• Action potentials: Electrical signals that trigger heart muscle contraction.

• Regular contraction: Heart must contract at the right intervals for effective pumping.

Anatomy of the Intrinsic Cardiac Conduction System

Main Structures and Their Locations

The cardiac conduction system consists of specialized myocytes that initiate and conduct electrical signals. Each structure plays a specific role in the sequence of heart activation.

• Sinatrial (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: Conduct impulses from the SA node to the AV node and distribute action potentials through the atria.

• Atrioventricular (AV) Node: Located in the inferior right atrium, near the tricuspid valve. 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 and stimulate contraction.

Example Table: Structures of the Cardiac Conduction System and Their Locations

Structure	Location
Sinatrial (SA) node	Right atrium
Atrioventricular (AV) node	Right atrium (near tricuspid valve)
AV Bundle (Bundle of His)	Superior interventricular septum
Right bundle branch	Interventricular septum (right side)
Left bundle branch	Interventricular septum (left side)
Subendocardial conducting network (Purkinje fibers)	Ventricular walls

Conduction Pathway and Contraction

Sequence of Electrical Activation

For effective pumping, the heart's conduction system ensures that the atria contract first, followed by the ventricles. This sequence is essential for proper blood flow.

• 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.

• 6. Action potential spreads through contractile cells of the ventricles, causing them to contract.

Additional info: The delay at the AV node is crucial for allowing the atria to fully contract before the ventricles begin to contract.

Steps of Cardiac Conduction (Ordered Pathway)

Electrical Conduction Sequence

The following steps outline the correct order of electrical conduction in the heart:

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

2. Action potential is passed through the atria.

3. Action potential reaches the AV node (100 ms delay).

4. Action potential moves down the AV bundle.

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

6. Purkinje fibers distribute the action potential.

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

8. Ventricles contract.

Control of Heart Rate

Intrinsic and Extrinsic Regulation

Heart rate is regulated by both intrinsic and extrinsic factors. Intrinsic control is provided by pacemaker cells, while extrinsic control involves nervous system input and other factors.

• Pacemaker cells: Provide intrinsic initiation of action potentials.

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

• Medulla oblongata: Responsible for chronotropic control of heart rate.

• Sympathetic nervous system: Increases heart rate via the cardioacceleratory center; innervates the SA node, AV node, and heart muscle.

• Parasympathetic nervous system: Decreases heart rate via the cardioinhibitory center; signal travels down the vagus nerve and innervates the SA and AV nodes.

Example Table: Effects of Severing Nerves on Heart Rate

Effect of Severing	On Resting Heart Rate	On Heart Rate During Exercise
Sympathetic nerve fibers	Decrease	Decrease
Parasympathetic nerve fibers	Increase	Increase

Additional info: The typical resting heart rate is about 75 bpm, but the SA node sets a rate of about 100 bpm without extrinsic factors. During exercise, heart rate increases due to sympathetic stimulation.

Key Concepts and Definitions

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

• Pacemaker Cells: Specialized cells in the SA node that spontaneously generate action potentials.

• Gap Junctions: Protein channels that allow ions to pass directly between cardiac cells, enabling rapid electrical communication.

• Chronotropic Effect: Any factor that changes heart rate.

Formulas and Equations

• Heart Rate Calculation:

$\text{Heart Rate (bpm)} = \frac{60}{\text{R-R interval (seconds)}}$

• Conduction Velocity:

$\text{Velocity} = \frac{\text{Distance}}{\text{Time}}$

Comparisons and Classifications

Comparison of Cardiac Conduction Structures

Structure	Function	Location
SA Node	Initiates heartbeat	Right atrium
AV Node	Delays impulse	Lower right atrium
AV Bundle (His)	Conducts impulse to ventricles	Interventricular septum
Bundle Branches	Transmit impulse to apex	Interventricular septum
Purkinje Fibers	Stimulate ventricular contraction	Ventricular walls

Examples and Applications

• If the SA node fails: The AV node can take over as the pacemaker, but at a slower rate.

• Clinical relevance: Disorders of the conduction system can lead to arrhythmias, requiring medical intervention such as pacemakers.

Practice Questions (from notes)

• Which feature of cardiac tissue allows for the rapid spread of action potentials through the heart? Answer: Gap junctions.

• Which statement best describes intrinsic conduction of the heart? Answer: Cells within the heart can initiate and transmit action potentials without nervous system input.

• If the SA node malfunctions, which part of the conduction system is most likely to take over as pacemaker? Answer: AV node.

• Which structure matches the cardiac conduction structure to its location? Answer: SA node: left atrium; Right and left bundle branches: septum.

• How does slower conduction at the AV node help the heart function? Answer: Ensures that the ventricles have enough time to fill with blood before they contract.

• What is the primary function of pacemaker cells in the SA node? Answer: Rhythmic generation of action potentials.

• Which structures allow contraction of the ventricles to begin at the apex? Answer: Purkinje fibers.