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,

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