WEBVTT

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So let's talk about
pacemaker cells.

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I'm going to actually
draw out the action

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potential for a pacemaker cell.

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And remember, this
is time over here.

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And let's do it with millivolts.

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This is positive up here
and negative down here.

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Now, our pacemaker
cells, let's specifically

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talk about the ones
in the SA node.

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So this is our SA
node action potential,

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

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And that's mainly
because of sodium,

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sodium leaking into the cell.

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

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Now it gets up to
this point, right,

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where I'm drawing
kind of a threshold.

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And this threshold is for what?

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Well, this is kind
of this dashed line

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represents the point
at which calcium

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channels start to open up.

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And so they open up
and causes the cell

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to become even more positive.

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So it was already
going positive,

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it's going to go
even more positive.

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

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

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those voltage gated calcium
channels, close down

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and potassium channels open up.

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Which causes the
membrane to repolarize.

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

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

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This is phase 0,
and this is phase 1.

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These are the three
phases we discussed.

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

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Let's say that we
view this, and I

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think that's a pretty
reasonable thing to do,

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view this as the heartbeat.

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This is one heartbeat, right?

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

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basically you would get
another one of these

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

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

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

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three minutes, it
would basically

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be just one after another
of these kinds of action

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potentials kind of
stacked on each other.

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

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

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

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

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

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But what would
that mean exactly?

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

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

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

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

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if you make the heartbeat
itself a little bit quicker,

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meaning takes less
time to finish,

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then the next one can
start a little bit early,

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and then that one
will finish early,

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and the next one
will start early,

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and basically, at
the end of a minute,

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you'll have more
heartbeats fit in.

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

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saying is that you're
increasing the heart rate.

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The number of heartbeats
in a minute goes up.

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So that's actually
pretty powerful.

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Because we think about
heart rates all the time,

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but rarely do we think
about exactly what

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that means for each
individual heartbeat.

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And what it means is that
each heartbeat goes quicker.

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Now, the opposite
is true too, right?

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You could imagine actually
extending this out.

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

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You could extend
it out that way.

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And if the heartbeat
goes longer,

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

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And that means that
you're basically

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saying that you're
reducing the heart rate.

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

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

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or lengthening the heartbeat
so that's actually,

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I think, a pretty powerful idea.

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Now let's take it
a step further.

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Let's actually do a
little thought experiment.

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

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1/10 of a second.

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

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but let's just imagine it is.

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

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

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What would our cell look like?

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

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

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

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this is what's happening
in our pacemaker cell

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at 1/10 of a second.

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So at this point,
you have a cell.

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Let me actually draw
a blown up version

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of our cell that
might look like this.

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And this cell is going
to have ions flowing in,

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

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And we know that this
is the dominant ion.

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

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kind of gushing into our cell.

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And we also have some
other ions coming in.

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And you might think,
well, wait a second,

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I thought only sodium comes in.

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And that's definitely
not the case.

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Even though sodium
is the dominant ion,

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meaning the cell is mostly
permeable to sodium,

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

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might be leaking out.

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

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Even though, in
this case, sodium

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is the major contributor
to the membrane potential.

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

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at the membrane.

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Now let's take a look
at this membrane,

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and let me show you
what might be out here.

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You've got some
receptors on this side.

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And those receptors are
for a neurotransmitter.

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So there's actually
nerves that come down

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and land right on
our pacemaker cell.

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And these are the
sympathetic nerves.

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And these nerves are releasing
some neurotransmitter.

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And this
neurotransmitter, I'm just

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

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Norepi sometimes it's called.

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So norepinephrine comes and
lands on these receptors

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and is going to cause
a signal into the cell.

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And it's going to
basically tell the cell

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to be permeable to these ions.

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Allow these ions to flow
across the membrane.

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So they say, OK, fair enough.

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

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And, of course,
it's not actually

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divided by one
side and the other.

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I'm just doing it
to kind of represent

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an idea, which is that
on this other receptor,

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you've got other kinds of
neurotransmitters landing.

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And these right here,
are acetylcholine.

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

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and this signal is coming
from parasympathetic nerves.

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You might have heard of
sympathetic and parasympathetic

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nerves.

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These are both part of the
autonomic nerve system.

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And the parasympathetic
nerves are

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sending kind of an
opposite message.

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

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don't allow so
much permeability.

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

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across your membrane.

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So opposite messages
coming in, and as it

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

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And so you get what
I've shown you.

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

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and a little bit of
potassium leaving.

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

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

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paste.

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Imagine that this happens.

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Something like this.

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

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a little bit.

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

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Let's say you have more
sympathetics coming in

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than parasympathetics, then you
might get something like this.

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Where instead of just a little
bit of neurotransmitters

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

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

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

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from this receptor.

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

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and that really outbalances
the one receptor on the right.

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So your sympathetic drive
here, you could say,

00:08:21.800 --> 00:08:24.764
is much stronger than your
parasympathetic drive.

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

00:08:26.930 --> 00:08:29.860
much stronger, than
what's going to happen

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

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Because, again, the
sympathetics are

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trying to get more
ion permeability.

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So you have a lot
more sodium gushing in

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

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You'll get more
calcium here, too.

00:08:50.640 --> 00:08:53.270
And you'll get more
potassium leaving the cell.

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So basically sympathetics are
going to cause all of the ions

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to increase in the
direction of movement.

00:09:00.630 --> 00:09:02.827
So you're going to get
more sodium to come in,

00:09:02.827 --> 00:09:04.660
you're going to get
more calcium to come in,

00:09:04.660 --> 00:09:07.620
and you're going to get
more potassium to leave.

00:09:07.620 --> 00:09:08.560
So that's interesting.

00:09:08.560 --> 00:09:10.719
And let's actually
just keep that in mind.

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I'm actually going to
do this one more time

00:09:12.510 --> 00:09:14.885
and show you what could happen
if the opposite were true.

00:09:14.885 --> 00:09:19.800
Let's say that in this case, you
had more parasympathetic drive.

00:09:19.800 --> 00:09:23.835
So let's say here, you have
now, in this third scenario--

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remember the first scenario was
kind of the baseline scenario,

00:09:26.790 --> 00:09:28.590
and this third
scenario now, let's say

00:09:28.590 --> 00:09:33.390
you have more acetylcholine
filling up these receptors.

00:09:33.390 --> 00:09:37.670
And that's outdoing what the
sympathetic nerves are doing.

00:09:37.670 --> 00:09:41.240
So now you've got a lot more
parasympathetic stimulation.

00:09:41.240 --> 00:09:43.860
Well, now this cell is
going to think, OK, well,

00:09:43.860 --> 00:09:47.570
the parasympathetics don't
want as much ion movement,

00:09:47.570 --> 00:09:49.840
so less sodium.

00:09:49.840 --> 00:09:51.560
Again, this is all
in 1/10 of a second,

00:09:51.560 --> 00:09:54.240
so if you just catch the
cell at 1/10 of a second,

00:09:54.240 --> 00:09:55.900
less sodium has moved in.

00:09:55.900 --> 00:09:59.290
Maybe less calcium
has gotten in.

00:09:59.290 --> 00:10:02.340
And maybe less
potassium has left.

00:10:02.340 --> 00:10:05.080
So if you look at 1/10 of
a second, the pictures,

00:10:05.080 --> 00:10:07.680
the snapshots are
really, really different.

00:10:07.680 --> 00:10:10.140
So in both scenarios,
sympathetics

00:10:10.140 --> 00:10:12.990
and parasympathetics,
it's the same ions.

00:10:12.990 --> 00:10:15.110
They're moving in
the same direction,

00:10:15.110 --> 00:10:19.050
but what we're looking at
is the amount of charge

00:10:19.050 --> 00:10:22.157
that's flowing over
a period of time.

00:10:22.157 --> 00:10:24.240
And sometimes you might
even use the word current.

00:10:24.240 --> 00:10:25.720
You might say,
well, sympathetics

00:10:25.720 --> 00:10:28.870
are increasing the current,
and parasympathetics

00:10:28.870 --> 00:10:32.200
are decreasing the current,
the amount of charge that's

00:10:32.200 --> 00:10:34.830
moving over a period of time.

00:10:34.830 --> 00:10:37.280
So how would this actually
look on our figure?

00:10:37.280 --> 00:10:38.780
So we drew a figure at the top.

00:10:38.780 --> 00:10:40.810
How would this actually
look on this figure?

00:10:40.810 --> 00:10:43.470
Well, I'm going to use
the colors red and green

00:10:43.470 --> 00:10:47.920
because that's kind of what
we've gotten into using here.

00:10:47.920 --> 00:10:51.736
So green, remember that was
our sympathetic scenario, well,

00:10:51.736 --> 00:10:53.360
what that's going to
do is that's going

00:10:53.360 --> 00:10:59.160
to basically increase the
amount of charge rushing in.

00:10:59.160 --> 00:11:03.010
And at 1/10 of a second,
you've got more positive ions

00:11:03.010 --> 00:11:04.000
in the cell.

00:11:04.000 --> 00:11:05.730
So, let's say, at
that point, you've

00:11:05.730 --> 00:11:07.660
actually already hit threshold.

00:11:07.660 --> 00:11:10.930
And you might now fire
in an action potential.

00:11:10.930 --> 00:11:14.200
And it will come
down just as before.

00:11:14.200 --> 00:11:17.030
And your heart rate
is basically going

00:11:17.030 --> 00:11:21.090
to go up because you've
shortened the heartbeat.

00:11:21.090 --> 00:11:23.810
And the opposite's going to
happen with parasympathetics.

00:11:23.810 --> 00:11:25.430
So with parasympathetics,
you're going

00:11:25.430 --> 00:11:29.840
to have a longer time to
get to that threshold.

00:11:29.840 --> 00:11:32.740
Because, again, it's
at 1/10 of a second,

00:11:32.740 --> 00:11:35.920
only a little bit of sodium
and calcium were inside,

00:11:35.920 --> 00:11:38.310
and only a little bit
of potassium had left.

00:11:38.310 --> 00:11:42.230
And you're going to have
roughly the same looking

00:11:42.230 --> 00:11:45.530
action potential as before.

00:11:45.530 --> 00:11:48.710
And you've gotten a much
lower heart rate now

00:11:48.710 --> 00:11:51.060
because the heartbeat
is much longer.

00:11:51.060 --> 00:11:54.390
So you can see that the amount
of current that's flowing

00:11:54.390 --> 00:11:55.170
is changing.

00:11:55.170 --> 00:11:58.855
And so, really, we're tweaking
phase 4 with our sympathetics

00:11:58.855 --> 00:12:02.580
and parasympathetics to
change our heart rate.