Help understanding Vagus Nerve affect on HR.

[JohnnnyD]

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As I understand the Vagus Nerve's relationship with the heart:

The Vagus Nerve is part of the parasympathetic nervous system and reaches from the bottom of the medulla to where it acts on the sinoatrial node in the right atrium. Without the vagus nerve, the heart would beat at rates of almost 100 beats a minute with the SA node acting as the heart's pace maker resulting in increased BP...among other things...making the vagus nerve obviously important. This high BP is detected by pressure/mechano-receptors in the aorta and other arteries which relay a message to the medulla to slow the heart by use of the vagus nerve. The vagus nerve is stimulated ultimately leading to the release of acetylcholine on the SA node, slowing the heart rate, and thus dropping BP.

For some reason, the very last part of this is confusing to me. In my mind I want to think that stimulation of the SA node would cause an INCREASE in HR. Why isnt this true?? Am I missing some basic concept about nerve stimulation? The ACh released on motor end plates causes contractions of muscles, not relaxation.. what makes it different here?😕

Thanks if anyone knows. seems as google has failed me here haha

 

[toothhornet88]

As I understand the Vagus Nerve's relationship with the heart:

The Vagus Nerve is part of the parasympathetic nervous system and reaches from the bottom of the medulla to where it acts on the sinoatrial node in the right atrium. Without the vagus nerve, the heart would beat at rates of almost 100 beats a minute with the SA node acting as the heart's pace maker resulting in increased BP...among other things...making the vagus nerve obviously important. This high BP is detected by pressure/mechano-receptors in the aorta and other arteries which relay a message to the medulla to slow the heart by use of the vagus nerve. The vagus nerve is stimulated ultimately leading to the release of acetylcholine on the SA node, slowing the heart rate, and thus dropping BP.

For some reason, the very last part of this is confusing to me. In my mind I want to think that stimulation of the SA node would cause an INCREASE in HR. Why isnt this true?? Am I missing some basic concept about nerve stimulation? The ACh released on motor end plates causes contractions of muscles, not relaxation.. what makes it different here?😕

Thanks if anyone knows. seems as google has failed me here haha

You are partially correct. The stimulation of SA node does increase HR but it also can decrease HR when stimulated and this is where the vagus nerve comes into play.

Just for additional clarification:remember that the SA node is a pacemaker, which means it maintains a steady heartbeat + controls blood pressure. This means that the SA node speeds up the heart rate (increasing BP) as well as slows down the heart rate (dropping BP) with the help of the vagus nerve.
So the vagus nerve justs acts as a signal to lower the heart rate.

 

[LuckyTangerines]

The primary neurotransmitter that brings about a lot of the parasympathetic effects is acetylcholine (ACh), which comes from the vagus nerve for parasympathetic output. So the way ACh is able to decrease the HR in this situation is because it acts on muscarinic ACh receptors on the SA node. These receptors are associated with bringing about all parasympathetic effects like decreasing HR and stimulating digestion.

ACh is also used for muscle contraction, but the way it is able to make muscles contract is because they bind to a different receptor: nicotinic ACh receptors.

 

[JohnnnyD]

Got it. Thanks guys, i guess my book didnt want to tell me about the different types of receptors first haha