The heart's electrical system is like a well-orchestrated symphony. The cardiac conduction system, with its specialized cells, initiates and coordinates heartbeats. This intricate network ensures the heart pumps blood efficiently, maintaining our body's vital functions.

From the sinoatrial node to the Purkinje fibers, each component plays a crucial role. The system's design allows for backup pacemakers, ensuring the heart keeps beating even if the primary pacemaker fails. This redundancy showcases the heart's incredible resilience and adaptability.

Cardiac Conduction System Components

Specialized Cardiac Muscle Cells

• The cardiac conduction system consists of specialized cardiac muscle cells that initiate and coordinate the contraction of the heart
• These cells have unique properties that allow them to spontaneously generate and conduct electrical impulses

Anatomical Structures

• The components of the cardiac conduction system include:
  • Sinoatrial (SA) node located in the wall of the right atrium, near the entrance of the superior vena cava
  • Atrioventricular (AV) node located in the interatrial septum, near the opening of the coronary sinus
  • Bundle of His, a collection of cardiac muscle fibers that originate from the AV node and extend into the interventricular septum
  • Left and right bundle branches, continuations of the bundle of His that descend along the interventricular septum and into the walls of the respective ventricles
  • Purkinje fibers, specialized cardiac muscle fibers that arise from the bundle branches and extend into the ventricular walls

SA Node as Pacemaker

Primary Pacemaker Function

• The SA node serves as the primary pacemaker of the heart, spontaneously generating electrical impulses at a rate of about 60-100 times per minute
• The SA node consists of specialized cardiac muscle cells with unique ion channel properties that allow for spontaneous depolarization
• The rate of SA node depolarization is influenced by the autonomic nervous system
  • Sympathetic stimulation increases the heart rate
  • Parasympathetic stimulation decreases the heart rate

Backup Pacemakers

• If the SA node fails to generate impulses or its impulses are blocked, the AV node can act as a secondary pacemaker, typically at a slower rate of 40-60 beats per minute
• In the absence of both SA and AV node activity, the Purkinje fibers can generate impulses at a rate of 20-40 beats per minute, ensuring continued heart function
• This hierarchy of pacemakers (SA node, AV node, Purkinje fibers) provides redundancy and ensures the heart continues to beat even if the primary pacemaker fails

Electrical Impulse Propagation Sequence

Atrial Depolarization and Contraction

• Electrical impulses originate in the SA node and spread rapidly through the atrial muscle fibers, causing atrial depolarization and contraction
• The impulse reaches the AV node, where it is delayed for about 100 milliseconds, allowing time for atrial contraction to complete before ventricular contraction begins

Ventricular Depolarization and Contraction

• From the AV node, the impulse enters the bundle of His and then divides into the left and right bundle branches, which conduct the impulse to the respective ventricles
• The bundle branches give rise to Purkinje fibers, which rapidly distribute the impulse throughout the ventricular myocardium, causing ventricular depolarization and contraction
• Ventricular repolarization occurs after the contraction, allowing the ventricles to relax and prepare for the next cycle

Electrical Activity vs Cardiac Cycle

Atrial Systole and Diastole

• The cardiac cycle consists of systole (contraction) and diastole (relaxation) of the atria and ventricles, which are coordinated by the electrical activity of the heart
• Atrial systole occurs when the SA node generates an impulse, causing atrial depolarization and contraction
  • This corresponds to the P wave on an electrocardiogram (ECG)
• The PR interval on the ECG represents the delay of the impulse at the AV node and the time taken for the impulse to reach the ventricles

Ventricular Systole and Diastole

• Ventricular systole occurs when the impulse reaches the ventricles via the bundle branches and Purkinje fibers, causing ventricular depolarization and contraction
  • This corresponds to the QRS complex on the ECG
• The ST segment on the ECG represents the period when the ventricles are fully depolarized and contracting
• Ventricular diastole occurs when the ventricles repolarize, allowing them to relax
  • This corresponds to the T wave on the ECG
• The QT interval on the ECG represents the total duration of ventricular depolarization and repolarization