Effect of Parasympathetic and Sympathetic Activation on SA Node: Autonomic Control of Heart Rate
Phase 4 (Slow Diastolic Depolarization) of Action Potential in SA Node.
During this phase, the potential raises from maximum diastolic potential (-60 mV) to the threshold (-40 mV).
At the end of this phase, the threshold is reached and action potential is generated.
Among all phases of action potential, this phase plays a critical role in deciding rate of impulse generation by SA node.
Mechanisms to control heart rate target this phase to do so.
Parasympathetic Stimulation
Parasympathetic supply to the SA (vagus nerve)
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Releases acetylcholine
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Stimulates M2 receptors on membrane of SA nodal cells
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Series of intracellular signaling events
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1. Decreased Na current during phase 4.
2. Increased Permeability of Membrane to K Ions.
3. Decreased Entry of Ca in the Cell.
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These events eventually decrease heart rate as explained below:
Decreased Na current during phase 4
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The potential rise becomes slower i.e. the steepness of phase 4 decreases
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It takes longer to reach the threshold
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Action potential is delayed
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Decreased heart rate.
Increased Permeability of Membrane to K Ions
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K moves out of cell
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Loss of positive charge from the cell
↓ Makes the maximum diastolic potential more negative i.e. hyperpolarization
↓ Distance between maximum diastolic potential and threshold potential is increased
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It takes longer for phase 4 to reach threshold
↓ Action potential is delayed
↓ Decreased heart rate.
Decreased Ca Entry in the Cell
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1. Moves the threshold in more positive direction
2. Reduces steepness of phase 4 depolarization
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1. Distance between maximum diastolic potential and threshold potential is increased
2. Phase 4 becomes less steep (i.e. slow)
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Action potential is delayed
↓ Heart rate is decreased.
Thus, parasympathetic stimulation decreases heart rate.
Sympathetic Stimulation
Effects of sympathetic stimulation are mostly opposite to that of parasympathetic stimulation
Sympathetic neurons release noradrenaline
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Stimulate β1 receptors on cell membrane
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Series of intracellular signaling events
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1. Increased Na entry in the cell.
2. Increased Ca entry in the cell.
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These events eventually increase heart rate as explained below:
Increased Na Entry in the Cell
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The potential rise during phase 4 becomes faster i.e. the steepness of phase 4 increases
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Threshold is reached earlier
↓
Action potential appears faster
↓ Increased heart rate.
Increased Ca entry in the Cell
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1. Increases slope of phase 4 depolarization → Phase 4 moves faster.
2. Makes the threshold more negative → distance between maximum diastolic potential and threshold is decreased.
↓
Threshold is reached faster
↓
Action potential appears faster
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Increased heart rate.
Thus, sympathetic stimulation increases heart rate.