- [Voiceover] The peripheral nervous system is what
links your central nervous system, your brain, spinal cord,
with all the inputs, all the information
coming into your body.
Every single nerve you have in your body is either bringing
information into the central nervous system or it's carrying
information away from the central nervous system.
So what is a nerve really?
A nerve is simply a grouping of axons from many
different neurons all wrapped together and protected.
You could have dozens, you could have hundreds,
you can have potentially thousands in some major nerves.
They're all carrying information to and from
the central nervous system.
Now an interesting side note, a nerve,
you don't have any nerves in your brain or spinal cord.
Instead the nerve actually changes names.
They're called tracts inside your central nervous.
But nerves for us are good enough.
Nerves are fine for the rest of the body,
just not in the brain or spinal cord.
We have 12 cranial nerves.
Not actually 12 pairs of cranial nerves,
but referred to 12 cranial nerves.
These nerves come directly from the brain.
They mostly reach up in the head but a few of them
reach down into the neck and one actually goes down
into your abdominal region.
We have 31 pairs of spinal nerves.
They're coming off of every vertebral opening
all the way down to the sacrum.
So sensory.
Well we know sensory brings information in.
So the sensory neuron can bring information in for both
the somatic and autonomic motor divisions.
Well, somatic, that's conscious control.
You think about, you do it.
Raise hand, hand goes up.
Somatic.
Autonomic divisions, well think autonomic, think automatic.
Your body is subconsciously taking care of that
amount of information.
But both the somatic and autonomic are part of the PNS,
peripheral nervous system.
All the incoming information heads into the central
nervous system and brings in these action potentials
from all over the body.
You're continually getting stimuli right from
your head to your feet.
It's all going to your brain as thousands and thousands
of action potentials carrying information in.
So let's look at the somatic division.
Somatic is voluntary.
That means your skeletal muscles.
Now, if you're saying hold on,
not all skeletal muscles are voluntary.
If you're thinking of a muscle twitch,
also get your arm twitching a little bit,
or maybe your eye twitches a little.
Or get a muscle cramp.
Sure, now that's not a voluntary function of a skeletal
muscle but it's also not the typical or normal.
Most people don't walk around all day, everyday,
with a cramped muscle.
Most people don't walk around all day, everyday,
with a muscle spasming continually.
Now, sometimes spasms can go on for hours,
and they're usually not the most comfortable.
So we're looking at normal functionality.
Normally somatic is voluntary.
Now the involuntary part is the reflex.
It's still skeletal muscle.
You can still move these muscles in a voluntary fashion
but you have these spinal reflexes that
can override your conscious control.
So if you're thinking about,
you can move this particular muscle,
and there's many different spinal reflexes.
But if the reflex kicks in, it automatically moves it
without you thinking about it.
So you have the flexor reflex.
The crossed extensor reflex.
The stretch reflex.
Well I'm thinking about stretch reflex.
If you can stand up straight, that's what's happening.
It's constantly using all the muscles throughout
your waist, your torso, your shoulders,
all to keep an upright posture.
Well, the flexor reflex.
If you ever put your hand on the stove.
What do you do, just hold it there and sniff around
and wait for the smell of burning flesh?
No, you know it's hot!
You pull it right off.
That's a flexor reflex.
It quickly pulled it or withdrew it back.
Crossed extensor.
Well, if someone ever grabs your wrist,
what's the first thing you do if you don't know the person
or it catches you by surprise?
You pull that wrist back towards your body.
But a lot of times as you pull that wrist back,
your other arm is trying to push the person away.
So a crossed extensor reflex.
So here's an example we're looking at.
Now if you look at the cell body of the sensory neuron
we know that sensory is bringing information in.
So in this case, you can see that foot
stepped on a broken glass.
Information comes in, goes to the dorsal root,
which is leading into the spinal cord.
It goes to some interneurons.
Remember interneurons are meant to connect,
connecting sensory to motor.
That sends out via the ventral root,
the out portion of the spinal cord,
and down the motor neuron.
But it sends it down both the right and left side.
The right side you can see is the side that was injured,
what stepped on the glass.
It's trying to pick that foot up,
not wanting to step on through it.
While the left side is trying to balance the leg.
Those are your effector muscles.
The cross extensor reflex is how the other side of the body
reacts while the flexor reflex is how the injured side,
or the stimulated side, reacts.
So we have our flexor reflex,
the same side that was where the stimuli was responds,
cross extensor, the opposite side responds.
So on to this autonomic division.
We said somatic was going to be
voluntary control unless a reflex.
But autonomic, think of automatic control, it just happens.
You don't even need to think about it.
The autonomic division is the motor output,
or I should say one of the motor outputs,
for the peripheral nervous system.
So this autonomic division really controls a lot
of your internal organs.
Think stomach, small intestines, large intestines, heart.
But there's two main divisions
within the autonomic division.
You have the sympathetic and parasympathetic.
The sympathetic division is your fight or flight.
That sympathetic division will get your muscles,
your lungs, your heart, your senses,
all up and ready to react to whatever is going on.
While the parasympathetic just kind of rests and digests.
It's a calming down.
If the parasympathetic is functioning at a higher level
your body wants to calm down and relax.
If the sympathetic is functioning at a higher level
your body is very agitated.
It wants to get up and go, wants to move.
So now you have both sympathetic and parasympathetic
neurons to each organ.
You may be going, why would you want both,
they do the opposite thing.
Well, if you're in a fight or flight scenario
your heart's beating faster.
So the sympathetic stimulates it.
But if you're in a rest and digest, you just had a big meal,
you're sitting down relaxing,
the parasympathetic is gonna help slow the heart down,
allow it to relax.
So you have to have both sympathetic
and parasympathetic in each organ.
So we said already, sympathetic,
body is set for emergencies.
Norepinephrine really is the key neurotransmitter.
Let's get your body ready to go.
So you can increase heart rate,
you're increasing respiration, you're breathing faster,
blood pressure's higher, your pupils dilate,
more light in means a little better sense what's going on.
Digestion and urinary systems have slowed down.
If you're in a fight or flight response do you really
need your digestive system
and urine system functioning full out?
No.
Before the question comes up of, well, hold on,
why if you're in a life or death situation sometimes
do people urinate themselves?
Or even defecate themselves?
That's not because these systems are functioning,
it's because of how much the systems are not functioning.
Both the digestive and urinary systems have sphincters
or muscles that prevent involuntary release.
But if you're at such a high level of sympathetic
innervation your system might not be maintaining enough
stimulus to keep the muscles closed of digestive
and urine system.
Those muscles open up and relax, everything comes out.
Now usually the way this works is one unified response.
You don't have the heart rate increasing first,
then respiration, no.
Everything occurs at the same time and it all occurs
within seconds of a stimuli.
It's the exact opposite of the parasympathetic.
So do you think it's ever a good idea to have both
sympathetic and parasympathetic
stimulating at the same time?
Not really.
Because the parasympathetic relaxes the body,
does the exact opposite of the sympathetic.
Acetylcholine is a key neurotransmitter here.
It's meant to slower your heart rate down,
slower your respiratory rates,
but put more blood into your digestive system,
to increase digestion.
Defecation and urination will occur
during parasympathetic division stimulation.
When the body's relaxed that's typically when the GI tract,
the digestive tract, and the urinary system,
will release their components.
So defecation occurs, urination occurs.
Sympathetic and parasympathetic are always antagonistic.
They do the exact opposite.
And it's this antagonistic role that
allows homeostasis to be maintained.
It's what keeps your body in a balanced order.
So here we have the breakdown
of sympathetic and parasympathetic.
Now you don't need to memorize this diagram
but get an idea of roughly what's going on.
Because see, on the sympathetic side, the fight or flight,
pupils dilate, more light in.
But you decrease salivation,
you're not eating anything right now.
Increased respiration.
Sure, you're breathing heavier.
Increased heart rate.
Because constricted blood vessels, more blood pressure.
Inhibits digestive processes.
So by inhibiting all these digestive processes,
you're slowing it down.
Relaxes the bladder muscles.
Inhibits defecation.
All the things that are not necessary in a fight or flight,
life or death situation, are being impeded.
The exact opposite on the parasympathetic.
Now one interesting thing with the parasympathetic.
All the parasympathetic nerves come from either
cranial nerves or all the way down at the end
of the vertebral column, the sacral nerves,
while the sympathetic comes from everywhere in between.
So we have the cranial and sacral is parasympathetic,
so they don't even come from the same place.
So if you look at the divisions and how they line up,
let's focus on the right for a second.
Sympathetic, parasympathetic.
Sympathetic, fight or flight.
Parasympathetic, relaxed, rest and digest.
Neurotransmitter, sympathetic is norepinephrine.
Parasympathetic, acetylcholine.
And how many neurons to reach the targets?
Well both of them, sympathetic and parasympathetic,
need two neurons.
So that means the entire autonomic division
needs two neurons,
while the somatic division just needs one.
That means one neuron can run all the way from your
spinal cord down to your foot.
That could be a two or three foot long neuron.
That's huge!
You're always using acetylcholine in the somatic.
It's meant to move skeletal muscles.