Now what controls SA node rate?
Because you know that your heart rate can go slower than 60they can go faster than 100, so what controls that process?
Nerves control it to the greatest degree.
So we have things like the parasympathetic nerve system.
So here you have cranial nerve number IX, sorry, number X which is the Vagus nerve.
The Vagus nerve releases acetylcholine, acetylcholine then binds to a muscarinic receptor on the SA node.
When this happens at this point, is it slows down heart rate.
So the more vagal the response, the slower the heart rate, we call that negative chronotropy.
Where does that come from?
Negative just means less than normal, chronotropy is what we call heart rate.
Seems like a fancy word for heart rate but you have to know it.
How do you get to a faster heart rate?
There is a sympathetic nervous system response.
This is your classic fight or flight or like what I like to call it, fight, flight -
or maybe it’s simply flight and aggressive conflict mediation.
Whatever you do, know that it’s a process in which you are speeding up the heart.
This is a beta adrenergic response where you have norepinephrine being released from presynaptic terminals
binding the beta 1 adrenergic receptors to speed up the heart.
We call that positive chronotropy. Positive means higher than nomal, chronotropy again is our name for heart rate.
So you can see here how the nervous system both speeds up and slows down the heart,
all through its actions only on the SA node in this case.
So now let’s take a couple of minutes and talk through exactly how the sympathetic and parasympathetic nervous systems
actually control heart rate because this is a complex process, you know that it’s just going to affect phase IV,
you know it might affect threshold but you're not sure how it’s going to affect all these things,
let’s just talk through it one by one, we’ll break it down, we’ll make it really simple,
we’ll give you some boxes and we’ll make sure we get this all.
So you’ll have it down and you won't forget it once you know how this process works.
So to start off with the parasympathetic nervous system.
Again you have cholinergic nerves that release acetylcholine
which is your neurotransmitter that’s gonna cause the parasympathetic vagal responses.
The first thing that happens is this acetylcholine decreases another molecule locator within the myocyte called cyclic AMP.
This decrease in cyclic AMP does two things, the first thing it does is decreases that funny current,
remember that funny current goes to this HCN channels, it decrease the current meaning that least sodium is going out into the cell.
If less sodium travels, therefore, you'll have a slower rate of phase IV.
What do I mean by slower rate? Phase IV will climb less rapidly.
It is going to -- there is going to be a less rise over run of their response. It slows it down, it flattens out phase IV.
The other thing that cyclic AMP does is it phosphorolysis calcium channels.
What these does is decrease calcium influx and moves an action potential away from normal membrane potential.
These two slows down the heart rate.
The final thing that happens is that the muscarinic activity increases potassium so you get more potassium that leaves the cell.
If you get more potassium that leaves the cell, remember potassium is positive, it’s positive, it has a charge.
When it leaves the cell, the cell becomes more negative.
As the cell becomes more negative it’s gonna take it a longer period of time to reach threshold.
So there's three different ways in which the parasympathetic nervous system changes or slows heart rate.
It does it through the rate of phase IV depolarization, it moves the action potential threshold away from membrane potential
and it has a negative shift in the membrane potential all three of those decrease heart rate and decrease conduction through the AV node.
Great! So now that you have those three mechanisms down for the parasympathetic nervous system,
let’s move right into the sympathetic nervous system.
Remember here, we wanna speed up the heart because either we wanna run away or we wanna have some good conflict mediation.
The first thing that happens here is that you release norepinephrine.
Norepinephrine binds to beta 1 adrenergic receptors and this increases cyclic AMP
so it does opposite of what acetylcholine did to those muscarinic receptors.
It works via two mechanisms, the first one is that it is going to phosphorolyze calcium channels to change the calcium influx
which moves membrane potential towards the action potential, decreasing the distance needed to travel before you reach threshold.
The other thing that cyclic AMP does is that it allows for more current to travel through those HCN channels.
So now the HCN channels allow more sodium influx which increases the rate of rise of phase IV meaning
that it steepens that slope so you are going to now reach threshold quicker because the slope of phase IV is steeper.
So between moving threshold and between having these phase IV depolarization process happen quicker,
you increase heart rate and you increase the conduction through the AV node.
These are ways in which the sympathetic and parasympathetic vagal nerve control heart rate.