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Conductive System of the Heart: AV Node, Bundle of His, Purkinje Fibers

  • The conductive system of the heart takes the impulse generated at SA node to throughout the heart for a synchronized contraction of atria and ventricles.

The Conduction Pathway

Interatrial pathway
  • Passes through the anterior walls of the atria to the left atrium.

  • Takes the impulse from SA node to left atrium.

  • Note: In right atrium, the impulse spreads directly from the SA node without involvement of any intermediate pathway.

Internodal Pathways
  • Starts from SA node and terminates on AV node.

  • There are 3 of them:

    • Anterior internodal pathway: run in anterior atrial wall.

    • Middle internodal pathway: run in lateral atrial wall.

    • Posterior internodal pathway: run in posterior atrial wall.

  • Take the impulse from SA node to AV node.

AV node
  • is located in the posterior wall of the right atrium immediately behind the tricuspid valve.

AV node is the only site from where impulse can pass from atria to ventricles.

A continuous fibrous atrioventricular ring separates atrial muscle from the ventricular muscle


it acts as an insulator


does not allow passage of impulse between atrial and ventricular musculature


AV node is the only site from where impulse can pass from atria to ventricles.

Impulse Conduction at AV Node is Slow

AV nodal cells have less number of gap junctions


more resistance to impulse conduction


Speed of impulse conduction inside AV node is slow


It takes some time for impulse to cross the AV node and enter the ventricles


ventricular contraction is delayed a bit

↓ This gives enough time for atria to contract and empty their blood into the ventricles


impulse enters the ventricles after the atrial contraction is almost complete


then the ventricles contract


Such coordinated contraction of atria and ventricles makes pumping more efficient.


Without delay in impulse conduction at AV node


Impulse would spread to atria and ventricles at almost the same time


Both would contract simultaneously


Would make the pumping inefficient.

Bundle of His

From AV node, it passes downwards in the ventricular septum towards the apex of the heart.


Divides into right and left bundle branches


Each bundle branch progressively divides into Purkinje fibers.

Purkinje Fibers
  • are very large.

Cells of Purkinje fibers have a high number of gap junctions in between them


Transmission through them is very fast


Once the impulse enters the Purkinje system, it spreads almost immediately to the entire ventricular muscle mass.


The fibers stimulate the muscle cells that are in direct contact with the fiber


These muscle cells stimulate remaining muscle cells


Fibrous ring prevents entry of impulses back into the atria.

Sequence of Depolarization in Cardiac Tissue

The Cardiac Tissues are Depolarized in Following Order:

Both atria


Septum: from left to right


Anteroseptal region of myocardium toward the apex


Bulk of ventricular myocardium from endocardium to epicardium


Posterior portion of base of the left ventricle

Time Taken for Impulse to Reach Respective Structures:
  • SA node: 0.00 sec

  • Stimulate all atrial muscle cells: 0.07 to 0.09 sec

  • Reach AV node: 0.03 sec

  • Exit the AV node: 0.16 sec

  • Stimulate all ventricular muscle cells: 0.19 to 0.22 sec

Velocity of Conduction

  • Atrial muscle cells: 0.3 m/sec

  • Interatrial & Internodal pathways: 1.0 m/sec

  • AV node: 0.05 m/sec

  • Bundle of His: 1.0 m/sec

  • Purkinje fibers: 4.0 m/sec

  • Ventricular muscle cells: 1.0 m/sec


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