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
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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