The Intrinsic Cardiac Conduction System: Structure, Function, and Regulation

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Intrinsic 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 and efficient pumping of blood.

Intrinsic conduction system: Generates and conducts action potentials through the heart independently of the nervous system.
Coordinated contraction: Cardiac muscle cells are connected by gap junctions, allowing ions and action potentials to pass rapidly from cell to cell.
Pacemaker cells: Specialized cells that set the rhythm of the heartbeat by initiating action potentials at regular intervals.
Nodes: Clusters of pacemaker cells that generate and conduct action potentials.
Conducting fibers: Specialized cardiac cells with few myofibrils, insulated from contractile cells, that rapidly transmit impulses.

Example: The sinoatrial (SA) node acts as the primary pacemaker, setting the pace for the entire heart.

Anatomy of the Intrinsic Cardiac Conduction System

Major Components and Their Functions

The conduction system consists of specialized myocytes that generate and propagate electrical signals, ensuring the heart beats in a coordinated manner.

Sinoatrial (SA) Node: Located in the superior wall of the right atrium, near the opening of the superior vena cava. Initiates the heartbeat and is known as the heart's natural pacemaker.
Internodal Pathways: Conduct impulses from the SA node to the atrioventricular (AV) node and distribute action potentials through the atria.
Atrioventricular (AV) Node: Located in the inferior right atrium, near the tricuspid valve. Delays the impulse, allowing the atria to contract before the ventricles. Contains secondary 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 and 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 myocardium, stimulating contractile cells.

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

Table: Structures of the Cardiac Conduction System and Their Locations

Structure	Location in Heart
Sinoatrial (SA) node	Right atrium (superior wall)
Atrioventricular (AV) node	Right atrium (inferior wall, near tricuspid valve)
AV bundle (Bundle of His)	Interventricular septum (superior portion)
Right and left bundle branches	Interventricular septum
Subendocardial conducting network (Purkinje fibers)	Ventricular walls (subendocardium)

Conduction Pathway and Cardiac Contraction

Sequence of Electrical Conduction

Proper heart function depends on the precise sequence of electrical conduction, which ensures that the atria and ventricles contract in a coordinated manner.

Pacemaker cells in the SA node initiate the action potential.
Action potential spreads across the atria via conducting fibers and contractile cells, causing atrial contraction.
Action potential reaches the AV node, where it is delayed to allow complete atrial contraction.
After the delay, the action potential moves down the AV bundle (Bundle of His) and into the right and left bundle branches.
Impulse travels through the subendocardial conducting network (Purkinje fibers), stimulating ventricular contraction.
Action potential spreads through contractile cells, causing the ventricles to contract.

Example: The delay at the AV node ensures that the atria have time to empty their blood into the ventricles before ventricular contraction begins.

Control of Heart Rate

Intrinsic and Extrinsic Regulation

Heart rate is regulated by both intrinsic pacemaker activity and extrinsic factors such as autonomic nervous system input.

Pacemaker cells: Set the basic rhythm of the heart (about 100 bpm in the absence of external influences).
Chronotropic factors: External factors that affect heart rate, which can be positive (increase rate) or negative (decrease rate).
Medulla oblongata: Contains the cardiac control centers that regulate heart rate via autonomic nerves.
Sympathetic nervous system: Increases heart rate and force of contraction by releasing norepinephrine.
Parasympathetic nervous system: Decreases heart rate by releasing acetylcholine, primarily via the vagus nerve.

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

Table: Effects of Severing Autonomic Nerves on Heart Rate

Effect of Severing	On Resting Heart Rate	On Heart Rate During Exercise
Sympathetic nerve fibers	Decreases or remains unchanged (since sympathetic tone is low at rest)	Blunted increase in heart rate during exercise
Parasympathetic nerve fibers	Increases (loss of vagal inhibition)	Heart rate can still increase, but baseline is higher

Practice and Application

Key Concepts and Example Questions

Gap junctions allow rapid spread of action potentials between cardiac cells.
Intrinsic conduction means the heart can beat independently of nervous system input.
If the SA node fails, the AV node typically takes over as the pacemaker, but at a slower rate.
The AV node slows conduction, allowing the ventricles time to fill before contracting.
Purkinje fibers are responsible for rapid conduction to the ventricular myocardium, ensuring coordinated contraction from the apex upward.

Example: The primary function of pacemaker cells in the SA node is the rhythmic generation of action potentials, setting the pace for the heart.

Summary Table: Cardiac Conduction System Structures and Functions

Structure	Function
SA node	Initiates heartbeat; sets heart rate
AV node	Delays impulse; allows atrial contraction before ventricular contraction
AV bundle (Bundle of His)	Conducts impulse from atria to ventricles
Bundle branches	Transmit impulse through interventricular septum
Purkinje fibers	Distribute impulse to ventricular myocardium

Additional info:

The heart's conduction system ensures that contraction begins at the apex and moves upward, efficiently ejecting blood from the ventricles.
Disorders of the conduction system can lead to arrhythmias, which may require medical intervention such as artificial pacemakers.