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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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