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.
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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