>> Hello and welcome to the penguin prof channel.
In today's video, I'm going to talk
about the autonomic nervous system.
Students look at this -- holy [bleep] this looks awful.
We are going to do this penguin prof style.
You want to stay tuned because we are going
to cover a tremendous amount of stuff
in a succinct and, dare I say, fun way.
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Information I am assuming today some basics
about neuron function.
Why ions wants to move in the direction they do
and the magical world of signal transduction.
If these are not familiar to you,
go ahead check those out first.
So, here's the overall view.
We've got the periphery.
We've got the peripheral nervous system which brings information
from the periphery into the brain and the spinal cord.
This happens via a pathway we refer
to as the afferent or sensory pathway.
Within the brain and spinal cord, we have interneurons
that lie entirely within the cns.
And then coming out from the cns back to the periphery,
we have what we call the "motor" or "efferent" pathway.
That's actually what we are going to be looking at today.
So, there are two different categories of efferent pathways.
We have the somatic motor neurons
and those innervate skeletal muscle.
The muscle that you voluntarily move.
The rest of your motor pathways are autonomic motor
and those innervate visceral effectors.
And these are the ones that give students a really hard time.
And those are the ones we're going to be focusing on.
So, the players in this story -- the parts, if you will --
we've got dendrites and cell bodies.
And then emanating from those we have axons.
At the end of the axon, we have the axon terminal.
This should be a review for everyone.
The axon terminal releases molecules.
We refer to these as "neurocrines."
Most commonly they are neurotransmitters
but neurocrine is a more general term.
And we'll also include things like neurohormones
which are released into the blood.
These molecules will be received by receptors on target tissues.
And the transmission of a signal across a space like this --
whether it's a synapse or through the blood --
we refer to that as signal transduction.
And if you recall my video on signal transduction,
what determines the meaning of a message is the ligand
and the receptor that the ligand binds to.
And this really is the key
to understanding the autonomic nervous system.
So, we're going to be looking
at the ligands including acetylcholine.
Now, when you see a neuron that produces acetylcholine
or you're looking at a receptor that acetylcholine binds to,
we use the term "cholinergic" for that.
So, you'll see a cholinergic neuron.
Right? That just means the neuron releases acetylcholine.
We also have the ligands epinephrine and norepinephrine
and those used to be called adrenaline and noradrenaline
and that's where we get this term adrenergic.
Let's start with acetylcholine.
So, here's the actual structure, what it looks like.
And we're going to look at the receptors
that acetylcholine can bind to.
The first are the nicotinic receptors.
There are actually two sub types.
The only real difference is where they are in the body.
But when acetylcholine binds to this nicotinic receptor,
the response is always excitatory.
So, here's what happens.
You see acetylcholine binding to this receptor
which is itself an ion channel.
It allows sodium and potassium to move
in the directions that they want to go.
Now, sodium is going to flood in.
Potassium is going to sort of trickle out.
And the effect is going to be excitation.
If you don't know why that is that's the video
that you want to look at.
Now, acetylcholine can bind to another group of receptors.
These are called "muscarinic receptors."
And there are actually five subtypes of these.
Some of them are excitatory and some of them are inhibitory.
So, it depends on where you are in the body
and which muscarinic receptor you're looking at.
So, for example, some
of the inhibitory muscarinic receptors you'll notice this is
a g-protein coupled receptor.
And the effect is that potassium channels will be opened.
This causes a hyperpolarization or an inhibition.
And this is the kind of receptor that we see in myocardial cells
so this will cause a reduction of heart rate.
On the other hand, in other tissues, acetylcholine will bind
to other muscarinic receptors.
For example, in the smooth muscle of the gi tract.
And you get instead a depolarization.
You notice it's still a g-protein coupled receptor
but you get the closing of potassium channels.
So, you see, again, the meaning of the message depends
on the ligand and the receptor type.
So, let's look at epinephrine and norepinephrine next.
These are catecholamines.
And there are five different receptors
that these guys can bind to.
And you'll notice that they vary depending upon
where they are in the body.
They vary in structure.
And they also vary in sensitivity.
So, this calm of sensitivity just means that, for example,
the alpha ones are more sensitive to norepinephrine
than they are to epinephrine.
For beta ones, it's the same.
Right? I just included that because some of you will have
to know the different sensitivities of these guys.
Okay. But what happens when epinephrine
or norepinephrine actually binds to these receptors.
So, when they bind to an alpha one,
what you get is an activation of phospholipase c.
And you get the production of the second messenger ip3
or inositol triphosphate.
This you'll see in a lot of signaling cascades.
The increase in cytoplasmic calcium levels will also cause
muscle contraction.
Things like that.
If epinephrine and norepinephrine bind
to alpha two receptors, you see a decrease
in the second messenger cyclic amp.
And when epinephrine or norepinephrine binds to any
of the beta receptors, you see an increase in cyclic amp.
So, let's put it all together and go back
to our little schematic of motor pathways.
So, somatic motor neurons release a neurotransmitter
onto receptors on skeletal muscle.
And the effect is always excitatory
because the signal is always for contraction of skeletal muscle.
Now you might think that we could just take an autonomic
motor neuron and do exactly the same thing.
Unfortunately, no.
The reason why the ans can be such a pain is
because it appears that it takes two neurons
to do the job of one.
You have neuron number one releasing a neurotransmitter.
Not on a target but on receptors that are far away
from the target in what we call "ganglia."
In the ganglion, there's a neuron Number two which comes
out and will release a neurotransmitter
onto the final target.
So, in other words, you have two neurons
in the autonomic motor division while
in the somatic motor you have only one.
That's confusing.
We call these neurons not neurons number one and two
but a preganglionic neuron and a postganglionic neuron.
Now, hopefully, that makes sense.
Right? Now, these ganglia -- the location of them depends
on which of the two divisions you are looking at.
So, sympathetic ganglia generally run right along the
spinal cord -- just parallel to the spinal cord.
Most parasympathetic ganglia are out closer to the effectors.
So, the secret formula
of the autonomic motor division is this --
you got to know which neurocrine is released
by the preganglionic neuron onto which receptor on the ganglion.
Then you have which neurocrine of the postganglionic neuron
and which receptor on the target.
And when you know the combination
of these four things, you -- you got it.
Right? These are the only neurocrines we have
and these are the only receptor types you have to choose from.
And that will be it.
Let's look at the parasympathetic division.
The preganglionic neuron
in the parasympathetic division is always cholinergic.
Which means it releases a acetylcholine.
The receptors on the ganglion are always nicotinic.
So, that means this signal is always excitatory.
All right.
Well, what about the postganglionic neuron?
Oh, my gosh.
It's also cholinergic.
That's cool.
So, it also releases acetylcholine.
Now, the receptor type
on the target cells these are different.
These are muscarinic receptors.
So, depending on the which type --
remember, there were five subtypes of those.
You'll get either an excitatory or an inhibitory response.
So, for example, in the heart, it would be inhibitory.
But in smooth muscle of the gut, it would be excitatory.
And that ladies and gentlemen is the parasympathetic
motor pathway.
The sympathetic motor pathway,
the preganglionic neuron is also cholinergic.
So, do you see they're both cholinergic.
They release acetylcholine.
The receptors are the same.
They're nicotinic as well.
So, the only difference between the two
in this view is this postganglionic neuron.
In the sympathetic division, it's adrenergic
and it releases the neurotransmitter norepinephrine.
And the targets are going to be those adrenergic receptors --
either the alpha or the beta.
So, again, the effect on the target will depend on which
of the adrenergic receptors you've got.
So, here we are comparing the two.
And by the way, I have drawn the lengths
to be anatomically representative.
So, the parasympathetic division --
that preganglionic neuron is long
and the postganglionic neuron is short.
The opposite is true in the sympathetic division.
Right? So, that relates to where the ganglia actually are.
Now, just for comparison, I'm throwing
in the somatic motor division here.
So, you see the difference between the somatic motor
where you have only one motor neuron
and the autonomic motor neurons where you have two.
Now, we're almost done.
There's one more kind of weird one
and that's called the adrenal sympathetic pathway.
So, we have still a preganglionic neuron
that is cholinergic so it releases acetylcholine
onto nicotinic receptors.
But this isn't a ganglion.
This is the adrenal medulla.
So, the adrenal gland is very strange
because the cortex is actually a gland but the medulla,
the inner part, is actually modified nervous tissue.
So, it acts like a postganglionic neuron in a way
but it's not a neuron.
So, it's very strange
but it releases epinephrine and norepinephrine.
It's actually, I think, 80% epinephrine, 20% norepinephrine.
Looks like it may be releasing some dopamine as well
which is kind of exciting.
But it releases those neurocrines --
you see now why we call them neurocrines?
Because they act like hormones.
They're released into the blood.
And they will bind to adrenergic receptors
on targets all over the body.
So, that is a little bit strange.
There is no postganglionic neuron
in the adrenaline sympathetic pathway.
So, now we compare all pathways together --
somatic motor, parasympathetic, sympathetic,
and the adrenaline sympathetic.
And, ladies and gentlemen,
I'm excited to tell you that that is it.
Those are all the possible combinations.
So, if you can understand these pathways, you got it.
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