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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

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it acts as an insulator

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does not allow passage of impulse between atrial and ventricular musculature

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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

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more resistance to impulse conduction

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Speed of impulse conduction inside AV node is slow

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

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ventricular contraction is delayed a bit

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

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impulse enters the ventricles after the atrial contraction is almost complete

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then the ventricles contract

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

...

Without delay in impulse conduction at AV node

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

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Both would contract simultaneously

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Would make the pumping inefficient.


Bundle of His

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

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Divides into right and left bundle branches

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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

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Transmission through them is very fast

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

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

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These muscle cells stimulate remaining muscle cells

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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

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Septum: from left to right

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Anteroseptal region of myocardium toward the apex

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Bulk of ventricular myocardium from endocardium to epicardium

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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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