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Impulse Conduction in AV Node & Effect of Parasympathetic and Sympathetic Activation on it



Action Potential in AV Node

  • is similar to that seen in SA node.

Slow entry of Na and Ca

Slow rise in potential (phase 4)

at threshold Ca channels open

Rapid entry of Ca

Upstroke in action potential (phase 0)

Opening of K channels

Exit of K ions

Repolarization (phase 3)


Impulse Conduction at AV Node

Impulse enters to AV node from its atrial side

Stimulates and produces action potential in the First line of cells in AV node

Action potential reaches the opposite end of first cells

Positive ions diffuse from first cells into next cells via gap junctions

Produce action potential in next cells

Action potential reaches the opposite end of next cells

In this way the impulse passthrough all cells in AV node from atrial side to ventricular side


Effect of Parasympathetic Stimulation

Parasympathetic nerve (vagus) releases acetylcholine

Stimulates M2 receptors on cell membrane of AV node

1. Opens of K channels → K efflux → hyperpolarizes the cell

2. Closes Ca channel → Ca entry decreases → makes phase 4 less steep and also rises the threshold for action potential

Phase 4 takes longer to reach threshold

Appearance of action potential is delayed

Similar delay occurs in all the cells of AV node as the impulse moves ahead

delays overall impulse conduction speed in AV node.

  • Thus parasympathetic stimulation slows down impulse conduction in AV node.

  • Very strong stimulation of AV node can even completely block the conduction.


Effect of Sympathetic Stimulation

Sympathetic neurons release noradrenaline

Stimulates β1 receptors on the cell membrane

Opens Ca channels

Ca entry increase

1. makes phase 4 steeper

2. decreases threshold

Phase 4 reaches the threshold in a shorter time

Action potential appears in lesser time

Impulse crosses the cell in lesser time

Similar effect in all cells of AV node combined increases overall speed of conduction through AV node

 

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