Now, what controls SA node rate.
Because you know
that your heart rate can go slower than 60,
they 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.
What 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.
It seems like a fancy word for heart rate,
but you have to know it.
How do you get a faster heart rate?
There, it is a sympathetic
nervous system response.
This is your classic fight or flight;
or like what I like to call it,
or maybe it’s simply flight
and aggressive conflict mediation.
Whatever you do,
you know that it's a process
in which you’re speeding up the heart.
This is a beta-adrenergic response
where you have norepinephrine being released
from presynaptic terminals,
binding to beta-1 adrenergic receptors
to speed up the heart.
We call that positive chronotropy.
Positive means higher than normal.
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 4,
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 it some boxes
and we 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, let’s just start off with the
parasympathetic nervous system.
Again, you have cholinergic nerves
that release acetylcholine,
which is your neurotransmitter
that's going to cause the
parasympathetic vagal responses.
The first thing that happens is
this acetylcholine decreases another
molecule located within the myocyte
This decrease in cAMP does two things.
The first thing it does
is it decreases that funny current.
a funny current goes through these HCN channels.
It decreases the current,
meaning that less sodium is going out –
into the cell.
If less sodium travels,
you’ll have a slower rate of Phase 4.
What do I mean by slower rate?
Phase 4 will climb less rapidly.
It is going to –
there is going to be a less
rise overrun of the response.
It slows it down. It flattens out Phase 4.
The other thing that cAMP does
is it phosphorolyzes calcium channels.
What this does is decrease calcium influx
and moves an action potential away
from normal membrane potential.
This too 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 has a charge.
When it leaves the cell,
the cell becomes more negative.
As a cell becomes more negative,
it's going to take it a longer period of time
to reach threshold.
So, there’s different ways
in which the parasympathetic nervous system changes
or slows heart rate.
It does it through the
rate of Phase 4 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.
So, now that you have
those three mechanisms
down for the parasympathetic nervous system,
let's move right into the
sympathetic nervous system.
we want to speed up the heart
because either we want to run away
or we’re going to have some good conflict mediation.
The first thing that happens here
is you release norepinephrine.
Norepinephrine binds the
beta-1 adrenergic receptors
and this increases cAMP.
So, it does the opposite of what
acetylcholine did to those muscarinic receptors.
It works via two mechanisms.
The first one is that
it is going to phosphorylyze calcium channels
to change the calcium influx,
which moves membrane potential
away from the action potential.
Widening the distance needed to travel
before you reach threshold.
The other thing that cAMP does
is 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 4,
meaning that it steepens that slope.
So, you're going to now reach threshold quicker
because the slope of Phase 4 is steeper.
So, between moving threshold
and between having this Phase 4
depolarization process happen quicker,
you increase heart rate.
And you increase the conduction
through the AV node.
These are ways in which
and parasympathetic vagal nerve
control heart rate.