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 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 manner for effective pumping.

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

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

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

Key Terms:

• Nodes: Clusters of pacemaker cells that generate and conduct impulses.

• Action potentials: Electrical signals that trigger muscle contraction.

Anatomy of the Intrinsic Cardiac Conduction System

The conduction system consists of specialized myocytes that initiate and conduct electrical signals, ensuring the heart beats in a coordinated fashion. The main components are:

• Sinatrial (SA) Node: Located in the superior right atrium near the vena cava; contains pacemaker cells that initiate impulses.

• 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; 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: Inferior portion of the interventricular septum; conduct impulses to the Purkinje fibers.

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

Structures of the Cardiac Conduction System and Their Locations

Structure	Location
Sinatrial (SA) node	Right atrium
Atrioventricular (AV) node	Inferior right atrium
AV bundle (Bundle of His)	Superior interventricular septum
Right & left bundle branches	Inferior interventricular septum
Purkinje fibers	Subendocardial network in ventricles

Conduction Pathway and Contraction

For effective blood pumping, the conduction system must ensure that the atria and ventricles contract in the correct sequence. The pathway of electrical conduction is as follows:

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

2. Action potential spreads across the 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 before the ventricles contract.

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

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

6. Action potential passes through contractile cells, causing the ventricles to contract.

Steps of Cardiac Conduction (Summary Table)

Step	Description
1	SA node initiates action potential
2	AP spreads across atria
3	AP reaches AV node (delay)
4	AP moves down AV bundle and bundle branches
5	AP spreads through Purkinje fibers
6	AP spreads through ventricular contractile cells

Control of Heart Rate

Heart rate is regulated by both intrinsic and extrinsic factors. The intrinsic rate is set by pacemaker cells, while extrinsic factors include nervous system input and hormones.

• Pacemaker cells: Set the basic heart rate by initiating action potentials.

• Chronotropic factors: External 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 by stimulating the SA node, AV node, and heart muscle.

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

Effects of Nervous System on Heart Rate (Table)

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 above table is inferred from standard physiology; severing sympathetic nerves reduces the ability to increase heart rate during exercise, while severing parasympathetic nerves increases resting heart rate.

Key Definitions and Concepts

• 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 depolarize to initiate each heartbeat.

• Chronotropic Effect: Any factor that changes heart rate.

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

Formulas and Equations

• Heart Rate Calculation:

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

• Conduction Velocity:

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

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 artificial pacemakers.

Short Comparison: Intrinsic vs. Extrinsic Control of Heart Rate

Intrinsic Control	Extrinsic Control
Set by pacemaker cells (SA node)	Modulated by nervous system and hormones
Independent of external input	Responds to physical and emotional stimuli

Practice Questions (Examples)

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

• Which statement best describes intrinsic conduction of the heart? 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 the pacemaker? AV node

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