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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)

↓

Releases acetylcholine

↓

Stimulates M2 receptors on membrane of SA nodal cells

↓

Series of intracellular signaling events

↓

1. Decreased Na current during phase 4.

2. Increased Permeability of Membrane to K Ions.

3. Decreased Entry of Ca in the Cell.

↓

These events eventually decrease heart rate as explained below:


Decreased Na current during phase 4

↓

The potential rise becomes slower i.e. the steepness of phase 4 decreases

↓

It takes longer to reach the threshold

↓

Action potential is delayed

↓

Decreased heart rate.


Increased Permeability of Membrane to K Ions

↓

K moves out of cell

↓

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

↓

It takes longer for phase 4 to reach threshold

↓ Action potential is delayed

↓ Decreased heart rate.


Decreased Ca Entry in the Cell

↓

1. Moves the threshold in more positive direction

2. Reduces steepness of phase 4 depolarization

↓

1. Distance between maximum diastolic potential and threshold potential is increased

2. Phase 4 becomes less steep (i.e. slow)

↓

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

↓

Stimulate β1 receptors on cell membrane

↓

Series of intracellular signaling events

↓

1. Increased Na entry in the cell.

2. Increased Ca entry in the cell.

↓

These events eventually increase heart rate as explained below:


Increased Na Entry in the Cell

↓

The potential rise during phase 4 becomes faster i.e. the steepness of phase 4 increases

↓

Threshold is reached earlier

↓

Action potential appears faster

↓ Increased heart rate.


Increased Ca entry in the Cell

↓

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

↓

Increased heart rate.

  • Thus, sympathetic stimulation increases heart rate.

 

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