Changing the heart rate - chronotropic effect | NCLEX-RN | Khan Academy

So let's talk about
pacemaker cells.

I'm going to actually
draw out the action

potential for a pacemaker cell.

And remember, this
is time over here.

And let's do it with millivolts.

This is positive up here
and negative down here.

Now, our pacemaker
cells, let's specifically

talk about the ones
in the SA node.

So this is our SA
node action potential,

and you know it starts out
kind of negative and creeps up.

And that's mainly
because of sodium,

sodium leaking into the cell.

And other ions are present as
well, but that's the major ion.

Now it gets up to
this point, right,

where I'm drawing
kind of a threshold.

And this threshold is for what?

Well, this is kind
of this dashed line

represents the point
at which calcium

channels start to open up.

And so they open up
and causes the cell

to become even more positive.

So it was already
going positive,

it's going to go
even more positive.

And it's going to get
to about that point.

And then finally, at this
point, those calcium channels,

those voltage gated calcium
channels, close down

and potassium channels open up.

Which causes the
membrane to repolarize.

So these are the three
phases we've talked about.

This is phase 4, we
numbered it as phase 4.

This is phase 0,
and this is phase 1.

These are the three
phases we discussed.

So now let's think about
it a little bit harder.

Let's say that we
view this, and I

think that's a pretty
reasonable thing to do,

view this as the heartbeat.

This is one heartbeat, right?

And you know if we were to
keep this picture going,

basically you would get
another one of these

and another one of these, and it
would just basically continue.

And this is what our heart
rate then looks like, right?

If you were just to look at
a strip over, let's say, two,

three minutes, it
would basically

be just one after another
of these kinds of action

potentials kind of
stacked on each other.

So now if I was to take this
heartbeat and shorten it,

let's say I was to make
it instead of ending where

it does, let's say I
ended it right there.

So that this whole thing kind
of gets brought this way.

Well, it would crunch down on
my action potential in phase 4.

But what would
that mean exactly?

I mean you think, well, so
what, so it's a little bit

more crunched down, happens
a little faster, so what?

Well, what it means,
if you think about it,

is if the heart beats are
stacking on top of each other,

if you make the heartbeat
itself a little bit quicker,

meaning takes less
time to finish,

then the next one can
start a little bit early,

and then that one
will finish early,

and the next one
will start early,

and basically, at
the end of a minute,

you'll have more
heartbeats fit in.

So by having a shorter
heartbeat, what you're really

saying is that you're
increasing the heart rate.

The number of heartbeats
in a minute goes up.

So that's actually
pretty powerful.

Because we think about
heart rates all the time,

but rarely do we think
about exactly what

that means for each
individual heartbeat.

And what it means is that
each heartbeat goes quicker.

Now, the opposite
is true too, right?

You could imagine actually
extending this out.

Let's say the heartbeat actually
goes a little bit longer.

You could extend
it out that way.

And if the heartbeat
goes longer,

then that means that you can get
fewer packed into one minute.

And that means that
you're basically

saying that you're
reducing the heart rate.

So when I say I'm increasing
or decreasing the heart rate,

really what I'm trying to
say is that I'm shortening

or lengthening the heartbeat
so that's actually,

I think, a pretty powerful idea.

Now let's take it
a step further.

Let's actually do a
little thought experiment.

Let's imagine that this is
1/10 of a second right here.

1/10 of a second.

And it may not be
exactly 1/10 of a second,

but let's just imagine it is.

And let's say I wanted to take
a look at our cell at this point

because that's where
1/10 of a second has hit.

What would our cell look like?

Let me actually just make a
little bit of space on a canvas

and draw out what our cell might
look like at 1/10 of a second.

And just to make sure I keep
everyone on the same page,

this is what's happening
in our pacemaker cell

at 1/10 of a second.

So at this point,
you have a cell.

Let me actually draw
a blown up version

of our cell that
might look like this.

And this cell is going
to have ions flowing in,

it's going to have, let's
say, sodium coming in.

And we know that this
is the dominant ion.

So let me draw, let's
say, a few of them,

kind of gushing into our cell.

And we also have some
other ions coming in.

And you might think,
well, wait a second,

I thought only sodium comes in.

And that's definitely
not the case.

Even though sodium
is the dominant ion,

meaning the cell is mostly
permeable to sodium,

calcium is actually leaking in,
and a little bit of potassium

might be leaking out.

So you have other ions moving
back and forth, as well.

Even though, in
this case, sodium

is the major contributor
to the membrane potential.

So if that's the case, now
let's take another look

at the membrane.

Now let's take a look
at this membrane,

and let me show you
what might be out here.

You've got some
receptors on this side.

And those receptors are
for a neurotransmitter.

So there's actually
nerves that come down

and land right on
our pacemaker cell.

And these are the
sympathetic nerves.

And these nerves are releasing
some neurotransmitter.

And this
neurotransmitter, I'm just

going to try to label as
I go, is norepinephrine.

Norepi sometimes it's called.

So norepinephrine comes and
lands on these receptors

and is going to cause
a signal into the cell.

And it's going to
basically tell the cell

to be permeable to these ions.

Allow these ions to flow
across the membrane.

So they say, OK, fair enough.

Now on the other side, you've
got another set of receptors.

And, of course,
it's not actually

divided by one
side and the other.

I'm just doing it
to kind of represent

an idea, which is that
on this other receptor,

you've got other kinds of
neurotransmitters landing.

And these right here,
are acetylcholine.

Now, acetylcholine is also
going to send a signal down here

and this signal is coming
from parasympathetic nerves.

You might have heard of
sympathetic and parasympathetic

nerves.

These are both part of the
autonomic nerve system.

And the parasympathetic
nerves are

sending kind of an
opposite message.

They're saying to this
cell, well, wait a second,

don't allow so
much permeability.

Don't allow so many ions
to go back and forth

across your membrane.

So opposite messages
coming in, and as it

turns out, that they kind of
balance and offset each other.

And so you get what
I've shown you.

You get some sodium coming
in, a little bit of calcium,

and a little bit of
potassium leaving.

Now, if I was to actually show
you now what could happen.

Let me try to take a shortcut
here and do a little cut,

paste.

Imagine that this happens.

Something like this.

Let's show you, I'm going to
have to move this canvas up

a little bit.

But let's say now, you
have more sympathetics.

Let's say you have more
sympathetics coming in

than parasympathetics, then you
might get something like this.

Where instead of just a little
bit of neurotransmitters

here, let's say
you get a lot more.

And let's say now this
receptor is also firing,

and let's say you get
a little bit of firing

from this receptor.

Well, now you get all three
receptors on the left,

and that really outbalances
the one receptor on the right.

So your sympathetic drive
here, you could say,

is much stronger than your
parasympathetic drive.

And if that's the case, if
your sympathetic drive is

much stronger, than
what's going to happen

is you're going to have more
sodium coming into the cell.

Because, again, the
sympathetics are

trying to get more
ion permeability.

So you have a lot
more sodium gushing in

and you'll get a little
bit of extra calcium, too.

You'll get more
calcium here, too.

And you'll get more
potassium leaving the cell.

So basically sympathetics are
going to cause all of the ions

to increase in the
direction of movement.

So you're going to get
more sodium to come in,

you're going to get
more calcium to come in,

and you're going to get
more potassium to leave.

So that's interesting.

And let's actually
just keep that in mind.

I'm actually going to
do this one more time

and show you what could happen
if the opposite were true.

Let's say that in this case, you
had more parasympathetic drive.

So let's say here, you have
now, in this third scenario--

remember the first scenario was
kind of the baseline scenario,

and this third
scenario now, let's say

you have more acetylcholine
filling up these receptors.

And that's outdoing what the
sympathetic nerves are doing.

So now you've got a lot more
parasympathetic stimulation.

Well, now this cell is
going to think, OK, well,

the parasympathetics don't
want as much ion movement,

so less sodium.

Again, this is all
in 1/10 of a second,

so if you just catch the
cell at 1/10 of a second,

less sodium has moved in.

Maybe less calcium
has gotten in.

And maybe less
potassium has left.

So if you look at 1/10 of
a second, the pictures,

the snapshots are
really, really different.

So in both scenarios,
sympathetics

and parasympathetics,
it's the same ions.

They're moving in
the same direction,

but what we're looking at
is the amount of charge

that's flowing over
a period of time.

And sometimes you might
even use the word current.

You might say,
well, sympathetics

are increasing the current,
and parasympathetics

are decreasing the current,
the amount of charge that's

moving over a period of time.

So how would this actually
look on our figure?

So we drew a figure at the top.

How would this actually
look on this figure?

Well, I'm going to use
the colors red and green

because that's kind of what
we've gotten into using here.

So green, remember that was
our sympathetic scenario, well,

what that's going to
do is that's going

to basically increase the
amount of charge rushing in.

And at 1/10 of a second,
you've got more positive ions

in the cell.

So, let's say, at
that point, you've

actually already hit threshold.

And you might now fire
in an action potential.

And it will come
down just as before.

And your heart rate
is basically going

to go up because you've
shortened the heartbeat.

And the opposite's going to
happen with parasympathetics.

So with parasympathetics,
you're going

to have a longer time to
get to that threshold.

Because, again, it's
at 1/10 of a second,

only a little bit of sodium
and calcium were inside,

and only a little bit
of potassium had left.

And you're going to have
roughly the same looking

action potential as before.

And you've gotten a much
lower heart rate now

because the heartbeat
is much longer.

So you can see that the amount
of current that's flowing

is changing.

And so, really, we're tweaking
phase 4 with our sympathetics

and parasympathetics to
change our heart rate.