Voiceover: In this video, I want to introduce
the autonomic nervous system, autonomic nervous system,
which is part of the overall nervous system,
and this is a functional division of the nervous system,
not a structural division, like the central nervous system
and the peripheral nervous system.
The autonomic nervous system consists of
efferent neurons in the peripheral nervous system
that do specific jobs.
So, these are efferent neurons, and these neurons
control three different types of cells.
The first are smooth muscle cells,
which are in all sorts of structures all over our body,
like around our blood vessels,
and they control cardiac muscle,
the muscle that makes up our heart tissue,
so, cardiac muscle, and these muscle tissue types
are different than the skeletal muscle,
the muscle that's all over, attached to our skeleton
that moves us around, because those are controlled
by different efferent neurons
of the peripheral nervous system.
Those are controlled by lower motor neurons,
not autonomic neurons.
The last thing that autonomic neurons
control are gland cells.
Some gland cells are controlled by
the autonomic nervous system.
Now, the autonomic nervous system is called this
because it tends to control all these things
without conscious involvement.
It doesn't require the involvement
of consciousness to control these things.
So, it's kind of autonomous.
It kind of does this stuff on its own,
without our conscious selves having to be involved,
for the most part, and we divide
the autonomic nervous system into two big subsystems.
So, let me write two big arrows here.
And this part we call
the sympathetic nervous system,
which is the first big part of the
autonomic nervous system.
So, I'll just write SNS for short,
for sympathetic nervous system, and this other big part
we call the parasympathetic nervous system.
Parasympathetic, so I'll just write PNS for short,
for parasympathetic nervous system,
and there are a number of big differences
between these two parts of the
autonomic nervous system that we can talk about
in this kind of introductory talk.
The first big difference is kind of where they start
in the central nervous system.
The sympathetic nervous system starts
in the middle of the spinal cord,
and at the middle part of the spinal cord,
let me draw a bunch of somas here,
and I'll just take one of these here,
and I'll draw a little short axon on the first neuron
that's coming out of the central nervous system,
and then it's going to synapse with the second neuron
in a ganglia close to where the first neuron is,
and then the second neuron is going to send
a longer axon to reach its target cell.
So, let me just draw a big T, to represent
some kind of target cell that it's going to synapse on,
and this target cell will be a smooth muscle cell,
a cardiac muscle cell, or a gland cell.
And here's an illustration of kind of the entire
autonomic nervous system, and here they're showing
kind of the middle part of the spinal cord
that all these first neurons in the
sympathetic nervous system are starting,
and then there's a short axon until they synapse
in a ganglia that's pretty close to the spine.
Here's a set of ganglia, and here are a few other ganglia,
but they all tend to be pretty close to the spine.
This set of ganglia are actually often linked together
in kind of a chain, which we actually call
the sympathetic chain, and here's just a
different illustration of the same thing.
So, here it's showing in the middle part of the spinal cord
that first axon's coming out, synapsing at a ganglia
close to the spine, with a lot of these ganglia
linked together in a chain, and then the second neuron
sending a longer axon out to synapse on the target cell
in whatever tissue you're talking about that contains
smooth muscle cells, cardiac muscle cells, or gland cells.
Now, the parasympathetic nervous system
has its first neurons start in a different place
in the central nervous system.
They start either up here in the brain stem, or they start
way down here at the bottom of the spinal cord,
and then their first neuron tends to send
a long axon out to synapse with the second neuron
in a ganglion at a distance from the first neuron,
and then that second neuron usually sends out
a short axon to synapse on its target cell.
I'll just write a big T here for target cell.
And here this illustration is showing this as well,
where it's showing the first neurons of the
parasympathetic nervous system either up here
in the brain stem or down here at the bottom
of the spinal cord, and then it's showing
these long axons on the first neuron,
until it reaches a ganglia at a distance
from the first neuron's soma and then a shorter axon
on that second neuron, until it reaches its target cell.
And here's another illustration,
just showing the same thing.
So, here's these first axons coming out of
either the brain stem up high
or the bottom part of the spinal cord down low,
and then these first long axons go all the way
until they meet a ganglion at a distance
from the first neuron soma, and then the second neuron
sends a shorter axon to the target cells.
So, the similarities in the structure
of the different parts of the
autonomic nervous system are that they both
usually consist of a chain of two neurons
connecting the central nervous system to the target cell,
but the differences are where those first neurons
start and whether there's a short first axon
and a long second axon or a long first axon
and a short second axon.
But, more importantly than these structural differences
between the different parts of the
autonomic nervous system, are the functional differences,
and these neurons do so many different things
in so many tissues of the body,
that it's a little hard to talk about them in general,
but there are these great phrases that can kind of help
think through lots of the changes that these
different parts of the autonomic nervous system do,
and for the sympathetic nervous system,
the phrase is fight or flight, fight or flight,
that the sympathetic nervous system,
when it's activated, will cause lots of changes in the body
that'll prepare to either fight or run away,
which can kind of help you deal with
threatening or dangerous situations.
So, I'll write that in red here for the
sympathetic nervous system, whereas the
parasympathetic nervous system I'll write
in a nice cool green here, because its phrase
is rest and digest, rest and digest.
So, when it's active, it often causes lots of changes
in the body that are more important for homeostasis
and just maintenance of the body
in nonthreatening situations.
So, let's take a few examples of a few tissues
where these different responses happen,
to get a feel for what this means.
So, first let's look down here at the
gastrointestinal system, the intestines or the gut,
and both the sympathetic and the parasympathetic
nervous system play a role in a lot of activities
of the gastrointestinal system, but one is
blood flow to the intestines, because the amount of blood
flowing through the intestines plays a big role
in how much digestion the intestines can do.
Blood flow to intestines, and it also plays a big role
in how much blood is available for other parts of the body.
So, when the sympathetic nervous system is activated
in some kind of fight or flight situation,
blood flow to the intestines decreases,
and that blood is actually diverted away
from the intestines, often to skeletal muscle.
So, all our muscles all over our body
that can help us move to deal with
dangerous situations, the blood is going to
leave the intestines and go to that,
because during a dangerous situation
is not the time to be digesting food.
It's the time to be moving,
so the blood's flow decreases to the intestines
and is diverted to skeletal muscle,
whereas most of the time, when you're in
a nonthreatening situation and it's time to rest and digest,
the peripheral nervous system is activated,
and that increases blood flow to the intestines.
That'll divert blood away from skeletal muscle,
because now you're not in a fight or flight situation,
and you want to rest and digest.
So, it's going to bring the blood flow back
to the intestines, to increase your ability to digest food.
If we look at the heart,
both the sympathetic and parasympathetic
nervous systems innervate the heart,
and we look at the heart output, kind of how much blood
the heart is pumping out over any particular unit of time.
Heart output of blood.
When the sympathetic nervous system is activated,
the heart output increases.
The heart pumps harder and pumps faster
and pushes more blood out, so that things
like skeletal muscle can get more blood flow.
In addition to diverting blood flow from the intestines
to skeletal muscle, the heart's just going to push more out,
so there's more available for the skeletal muscle.
When the parasympathetic nervous system is activated,
the heart output goes down.
The heart is pumping less hard,
and it's beating less often.
It's just working less, because you don't need
as much blood flow to the muscles for movement,
so you go to kind of a baseline level that's sufficient
for activities that involve resting and digesting.
So, these examples of blood flow involve the activity
of smooth muscle, because smooth muscle
is around our blood vessels and determines
where the blood is going to flow to,
and cardiac muscle, because the cardiac muscle
makes up the heart, and then if we think about gland cells,
there's a bunch of different glands
that the autonomic nervous system controls,
and they tend to be activated
kind of differently at different times.
So, one type of gland that's activated
during fight or flight situations,
when the sympathetic nervous system is active,
are sweat glands out here in the skin,
and the sweat glands are activated to secrete sweat,
which helps cool us down, which increases our ability
to move faster and farther, if we're able to stay cool,
whereas some glands that are activated by the
parasympathetic nervous system include things like
the salivary glands that produce saliva in our mouth,
because saliva is very useful for digestion,
and it's part of a number of activities
that happen that help us digest food.
So, I find these phrases helpful when I'm thinking about
what effects the autonomic nervous system
will have on different tissues of the body
during different situations,
because like in all of these examples,
most of the things that the sympathetic nervous system
does when it's activated increase the body's ability
to turn stored energy into movement,
to deal with dangerous situations,
like moving blood from the intestines to skeletal muscle
and increasing the amount of blood
being pumped around from the heart
and increasing sweat production from sweat glands
to keep us cool while we're moving
to deal with a dangerous situation,
whereas all of these things that the parasympathetic
nervous system is doing, make sense in
nonthreatening situations, where we're actually
trying to conserve and store energy,
like diverting blood flow away from skeletal muscle
to the intestines, to increase digestion,
decreasing heart/cardiac output to conserve energy,
and increasing saliva production
from the salivary glands to help with digestion as well.
But the autonomic nervous system affects
many more structures and has many more functions
than I can cover in this little introductory video.
For instance, autonomic neurons play a role
in changing the size of your pupils in your eyes,
in sexual responses, and in secretion from
a whole bunch of other glands,
and because it does so many different things,
I find it best to actually not cover it all
in one sitting, but instead to cover these things
as you're studying each individual organ system.
Because almost any organ system
you're going to cover is going to have
autonomic neurons coming in
and affecting how that organ system functions.