• Working Myocardium
• Electro-Mechanical Properties of the Heart
• Heart Conduction System
• Sinoatrial (SA) Node (1)
• Bachmann's Bundle (2)
• Internodal Pathways (3, 4, 5)
• Atrioventricular (AV) Node (6)
• His Bundle (7)
• Right Tawara Branch (8)
• Left Tawara Branch (9)
• Left Posterior Fascicle (10)
• Left Anterior Fascicle (11)
• Purkinje Fibers (12)

Working Myocardium

• The heart consists of 2 basic functional parts:
• Working myocardium
• It is the heart muscle
• Contains myofibrils (muscle fibers)
• Action potential of the myocardium has 5 phases
• Impulse spreads through the working myocardium slowly
• (because it lacks gap junctions)
• When the impulse reaches the myocardium, it triggers contraction
• Results in atrial systole and ventricular systole
  
  
• Heart conduction system
• Consists of "electrical wires"
• Along which the impulse travels from the SA node
• Action potential of the conduction system has 3 phases
• Impulse spreads through the conduction system quickly
• Because it contains gap junctions
• Through which ions quickly pass during impulse propagation
• It does not have contractile ability (lacks myofibrils)

Heart Conduction System

• Supraventricular conduction system:
• Sinoatrial (SA) Node (1)
• Bachmann's Bundle (2)
• Internodal Pathways (3,4,5)
• Anterior Internodal Pathway (James' Bundle) (3)
• Middle Internodal Pathway (Wenckebach's Bundle) (4)
• Posterior Internodal Pathway (Thorel's Bundle) (5)
• Atrioventricular (AV) Junction (6,7)
• Atrioventricular (AV) Node (6)
• His Bundle (7)
  
  
• Ventricular conduction system
• Right Tawara's Branch (8)
• Left Tawara's Branch (9)
• Left Posterior Fascicle (10)
• Left Anterior Fascicle (11)
• Purkinje Fibers (12)

Bachmann's Bundle (2)

• Emerges from the anterior internodal pathway
• and continues to the left atrium
• Transfers impulses from the SA node to the myocardium of the left atrium
• Ensures parallel activation of the atria
• If interrupted, it results in
• Interatrial Block

Atrioventricular (AV) Node (6)

• Atrioventricular (AV) Junction is the collective term for:
• AV Node
• His Bundle
  
  
• The AV node is the only electrical connection between the atria and ventricles
• The exception is an accessory pathway
• The AV node is a secondary pacemaker
• In case of SA node dysfunction
• It begins to generate impulses with a frequency of 40-60/min.
• Impulses from the AV node are seen on the ECG as an junctional rhythm
• The AV node has 2 pathways:
• Fast and slow
• Physiologically, impulses pass only through the fast pathway
• If impulses start to circulate (re-entry) through the pathways, AVNRT can occur
• Impulse conduction through the AV node is decremental (slowed)
• Conduction disturbances can lead to AV blocks
  
  
• The AV node consists of 3 functional parts:
• Transient Zone
• Envelops the AV node (like a shell)
• Composed of autonomic cells
• It is the most arrhythmogenic part of the AV node
• Compact Zone
• Forms the core of the AV node
• Histologically similar to the SA node
• Slows down the impulse transmission to the ventricles and also acts as a filter
• The secondary pacemaker is located in the compact zone
• Trigger Zone
• The impulse slows down in the compact zone
• When it reaches the trigger zone, it speeds up
• The impulse then quickly passes through the trigger zone to the His Bundle

Right Tawara Branch (8)

• Begins in the area of the bifurcation of the His Bundle
• Its terminal parts are Purkinje fibers
• The impulse from the right branch depolarizes the entire right ventricle
• A block can occur with right Tawara branch block (RBBB)

Left Posterior Fascicle (10)

• Begins at the bifurcation of the left Tawara branch
• Ends in the posterior part of the left ventricle
• Activates the posterior and upper parts of the left ventricle
• The left posterior fascicle is not a single entity
• It consists of a group of parallel fascicles
• In case of disruption, a left posterior fascicular block occurs
• It is very rare (because all fascicles need to be disrupted)
• A much more common condition is left anterior fascicular block

Purkinje Fibers (12)

• Form a dense network of fibers in the ventricles
• Are the last parts of the conduction system in the ventricles, primarily originating from
• The right bundle branch
• The left posterior fascicle
• The left anterior fascicle
• Fibers transmit impulses to the subendocardium
• Purkinje fibers act as a tertiary pacemaker
• Activate in the event of dysfunction in the SA and AV nodes
• Generate impulses with a frequency of 20-40/min.
• On the ECG, this generates a ventricular rhythm