Okay. So now, we’re going to look
at that -- we’re going to continue
to look at some of that
organization of the nervous system.
So, very quickly we have two
bins, the CNS, the PNS,
central nervous system,
peripheral nervous system.
This is not new terms
for you, I’m sure.
And I’m sure you’re familiar
with what they represent.
Simply put the central nervous
system is the brain and spinal
cord and the peripheral nervous
system is everything else.
So it’s referring to all
the nerves and the sensory
structures outside of the
brain and spinal cord.
So, we can break down the
PNS a little bit further.
We have the somatic system and the
autonomic system with the autonomic --
the autonomic containing the involuntary
control of glands and smooth muscle.
And the name autonomic refers to the
fact that it’s essentially automatic.
So you’re not sitting
there right now thinking,
“I need to think about stomach digesting
or I need to think about breathing.”
It just happens, right?
So that means it’s done automatically
and this is all involuntary control.
It includes the sympathetic
and parasympathetic systems,
which is the fight or
flight or rest and digest,
and we’ve talked about
those quite a bit.
And you recall from the
previous or upcoming lectures,
fight or flight is in
response to a stimulus.
Are you going to stay there and
address the stimulus that’s in
front of you fight or are you going
to turn around and run away.
So the analogy that’s always
used is caveman days being
presented with a tiger or a,
you know, something else.
So saber-toothed tiger is about to attack
you, are you going to stay and fight,
or are you going to turn around
and run back to your cave?
And that’s an automatic response
that happens quite quickly.
And it happens to you I’m sure.
Same thing, maybe not a saber-toothed
tiger but if confronted with
a threat, do you fight, do you address
it, or do you turn around and run away?
And then there’s the parasympathetic
which is the rest and digest.
Again, we’ve talked about that
in quite a bit of detail.
So, let’s take a look at how the
PNS is anatomically organized.
So, all the neurons entering and exiting
the central nervous system are carried by
sort of two major categories here,
cranial nerves and spinal nerves.
So, there are 12 pairs of cranial nerves
that convey sensory and motor information
to and from the brainstem
which is from down below.
And then we have 31 pairs of
spinal nerves which convey sensory
information and motor information
to and from the spinal cord.
So we have two broadband.
We should be pretty
familiar with that.
And here’s an example
of the vagus nerve.
So the vagus nerve is an
example of a cranial nerve and
it decreases heart rate and
increases GI function.
And you can see how sort of convoluted
it is and it’s going through --
it’s part of the parasympathetic
nervous system --
sorry, parasympathetic division
of the autonomic nervous system
and it travels up to the brain.
Okay? So, the point is, you have a lot of
info coming from outside and it goes via
these two means to those two different
spots, so the brainstem or the spinal cord.
Now, we’re going to compare sort of somatic
motor neurons versus sensory neurons.
Now, the motor neurons, they
innervate skeletal muscle cells
and they use acetylcholine as
their primary neurotransmitters.
And the cell bodies are
in the brain stem or
the ventral front portion
of the spinal cord.
Now, on the other hand,
the sensory neurons have a long
dendrite which extends from
a sensory receptor in the skin
or muscle, so anywhere outside
to bring information in and
it’s connected to the soma
or cell body through the -- the
DRG or the dorsal root ganglion.
Again, something that
we’ve seen before.
There’s a pair of DRG for every
segment on the spinal cord.
And these are protected by the vertebral
column but their outside of the meninges.
And we’ll take a blowup of the
spinal cord in just a moment.
But the point is, they’re outside
of the central nervous system.
So, sensory neurons bringing
information -- sensory information
to the brain via the DRGs and
it does so through the spine.
And we have our motor neurons which
bring -- sorry innervate muscle.
So we have that -- those
two modes of information.
Now, the autonomic peripheral nervous
system is organized in a certain fashion,
and we have information
going in and out.
So, all efferents of the
sympathetic and parasympathetic
system consist of two
types of neurons.
Preganglionic neuron which has a cell
body in the brainstem or spinal cord
and postganglionic, this sends
an axon to the effector muscle.
So in English, we’re basically saying
we have -- we sort of have a connection
with the preganglionic being stuff
coming from the brain going out.
And then your postganglionic is a stuff
that’s going to the actual intended target,
so the muscle that you’re
trying to move or the
organ and gland that
you’re trying to activate.
All autonomic preganglionic
acetylcholine as their
So acetylcholine is extremely
important when we’re talking
about this type of peripheral
nervous system activation.
All parasympathetic postganglionic
neurons also release acetylcholine.
So there are lots of terms that
we’re throwing, preganglionic,
parasympathetic, autonomic, and it
might be kind of hard to grasp.
So one way to get through this
is to have a little diagram,
that flowchart that we showed, and it allows
you to understand parasympathetic versus
autonomic and what the roles are and
preganglionic versus postganglionic, same thing,
understanding which is what and then
motor versus sensory information.
So, nearly all the sympathetic
release norepinephrine as
So, the primary players
here are acetylcholine
and norepinephrine between
these two systems.
So let’s look at this through diagrams
because pictures are always easier to grasp.
So, on the top, we have our
somatic efferent innervation.
And then below, we have our
autonomic efferent innervation.
So, somatic means that this
is something that we are
conscious about and
we’re actually --
we’re sending a signal out
to say your muscle to flex.
So you can see that a signal comes from
the spinal cord through the dorsal horn
and it’s going to travel
on the length of an axon,
and then it’s going to release acetylcholine
that’s going to activate that muscle.
And down below, we have the
autonomic efferent innervation.
So stuff that like
your heart beating.
Again, this isn’t something you’re
thinking about, it just happens.
And you see the signal being passed,
acetylcholine being released.
Now, we have the ganglion cells and
then that’s where the synapse happens.
And then the norepinephrine or acetylcholine
goes on and sent to the heart.
Okay, so two
There are some commonalities of course
but you can notice the difference
in transmitter and you can notice
the autonomic ganglion cells.
let’s talk a little bit more
about the autonomic PNS anatomy.
All sympathetic preganglionic
efferent, so things going out
have their cell bodies in the thoracic
or lumbar regions of the spinal cord.
So the spinal cord is a really
unique, basically, highway that’s
sending information up to the
brain and sending signals out.
And it has a very important
protective function as well.
So remember, the CNS is spinal cord
and brain and PNS is everything else.
So this is why the sympathetic system
is sometimes referred to as the
thoraco-lumbar system because we’re
involving these two components.
Now, the parasympathetic preganglionic
efferent neurons have their cell bodies
in the brainstem of the lower portion of
the spinal cord or the sacral portion,
which is why sometimes get
that name craniosacral system.
In the preganglionic axons for the parasympathetic
versus sympathetic system differ in
length and we’re going to compare those two
in a summary table in an upcoming slide.
Okay. So, let’s look at this
overly complicated figure.
So I don’t want you to get
frightened when you see this.
There’s a lot of
And what this is, is an overview
of sort of all the components,
structure and function of the
peripheral nervous system.
So what I really want you to get
out of this is we’ve separated
both the sympathetic and the
So, we’re looking at stuff
that is sort of, you know,
like the heart rate breathing
versus organs and muscle control.
And you can see from the diagram
different types of inputs,
and that’s kind of really all
that I want you to get to.
There are the two different systems
and we have all the different organs,
effectors, and you see the different
input and how it’s going to go --
how it interjects
into the spinal cord.
So you can see how the
sympathetic stuff is going.
All the inputs are going
up through the spinal cord
as opposed to the stuff that
is going to the brainstem.
So, you don’t need to necessarily memorize
every single aspect of this diagram,
but it’s just kind of strengthening some of
the points that we’ve already mentioned.
Now, here’s a summary table
that should hopefully help,
So the general function is the fight or
flight for sympathetic that we mentioned,
mobilized energy versus rest and digest
is where relaxing, taking it easy.
Location of the
it’s the thoracic and lumbar
spinal cord for the sympathetic
versus the brainstem for
In terms of the length of the
axon: short versus long.
And then you can imagine why because for
the sympathetic it’s going just to the
spinal cord, whereas for the parasympathetic
it needs to go up to the brainstem.
And then we have the ganglia,
we say close to cord, far
from target versus far from
cord, close to target.
And then we have the postganglionic axon which is long and -- versus short.
And the transmitters
involved here for the
sympathetic are norepinephrine
This is a nice kind
of summary table.
This should definitely help you
to put all those pieces together.