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- [Voiceover] The peripheral
nervous system is what

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links your central nervous
system, your brain, spinal cord,

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with all the inputs, all the information

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coming into your body.

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Every single nerve you have in
your body is either bringing

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information into the central
nervous system or it's carrying

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information away from the
central nervous system.

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So what is a nerve really?

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A nerve is simply a
grouping of axons from many

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different neurons all wrapped
together and protected.

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You could have dozens,
you could have hundreds,

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you can have potentially
thousands in some major nerves.

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They're all carrying
information to and from

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the central nervous system.

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Now an interesting side note, a nerve,

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you don't have any nerves in
your brain or spinal cord.

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Instead the nerve actually changes names.

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They're called tracts
inside your central nervous.

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But nerves for us are good enough.

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Nerves are fine for the rest of the body,

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just not in the brain or spinal cord.

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We have 12 cranial nerves.

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Not actually 12 pairs of cranial nerves,

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but referred to 12 cranial nerves.

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These nerves come directly from the brain.

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They mostly reach up in
the head but a few of them

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reach down into the neck
and one actually goes down

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into your abdominal region.

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We have 31 pairs of spinal nerves.

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They're coming off of
every vertebral opening

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all the way down to the sacrum.


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So sensory.

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Well we know sensory
brings information in.

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So the sensory neuron can
bring information in for both

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the somatic and autonomic motor divisions.

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Well, somatic, that's conscious control.

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You think about, you do it.

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Raise hand, hand goes up.

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Somatic.

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Autonomic divisions, well think
autonomic, think automatic.

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Your body is subconsciously
taking care of that

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amount of information.

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But both the somatic and
autonomic are part of the PNS,

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peripheral nervous system.

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All the incoming information
heads into the central

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nervous system and brings
in these action potentials

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from all over the body.

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You're continually
getting stimuli right from

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your head to your feet.

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It's all going to your brain
as thousands and thousands

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of action potentials
carrying information in.

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So let's look at the somatic division.

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Somatic is voluntary.

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That means your skeletal muscles.

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Now, if you're saying hold on,

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not all skeletal muscles are voluntary.

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If you're thinking of a muscle twitch,

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also get your arm twitching a little bit,

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or maybe your eye twitches a little.

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Or get a muscle cramp.

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Sure, now that's not a
voluntary function of a skeletal

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muscle but it's also not
the typical or normal.

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Most people don't walk
around all day, everyday,

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with a cramped muscle.

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Most people don't walk
around all day, everyday,

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with a muscle spasming continually.

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Now, sometimes spasms can go on for hours,

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and they're usually not
the most comfortable.

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So we're looking at normal functionality.

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Normally somatic is voluntary.

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Now the involuntary part is the reflex.

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It's still skeletal muscle.

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You can still move these
muscles in a voluntary fashion

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but you have these spinal reflexes that

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can override your conscious control.

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So if you're thinking about,

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you can move this particular muscle,

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and there's many
different spinal reflexes.

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But if the reflex kicks in,
it automatically moves it

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without you thinking about it.

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So you have the flexor reflex.

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The crossed extensor reflex.

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The stretch reflex.

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Well I'm thinking about stretch reflex.

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If you can stand up straight,
that's what's happening.

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It's constantly using all
the muscles throughout

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your waist, your torso, your shoulders,

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all to keep an upright posture.

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Well, the flexor reflex.

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If you ever put your hand on the stove.

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What do you do, just hold
it there and sniff around

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and wait for the smell of burning flesh?

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No, you know it's hot!

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You pull it right off.

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That's a flexor reflex.

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It quickly pulled it or withdrew it back.

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Crossed extensor.

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Well, if someone ever grabs your wrist,

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what's the first thing you do
if you don't know the person

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or it catches you by surprise?

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You pull that wrist
back towards your body.

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But a lot of times as
you pull that wrist back,

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your other arm is trying
to push the person away.

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So a crossed extensor reflex.

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So here's an example we're looking at.

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Now if you look at the cell
body of the sensory neuron

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we know that sensory is
bringing information in.

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So in this case, you can see that foot

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stepped on a broken glass.

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Information comes in,
goes to the dorsal root,

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which is leading into the spinal cord.

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It goes to some interneurons.

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Remember interneurons
are meant to connect,

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connecting sensory to motor.

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That sends out via the ventral root,

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the out portion of the spinal cord,

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and down the motor neuron.

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But it sends it down both
the right and left side.

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The right side you can see
is the side that was injured,

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what stepped on the glass.

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It's trying to pick that foot up,

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not wanting to step on through it.

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While the left side is
trying to balance the leg.

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Those are your effector muscles.

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The cross extensor reflex is
how the other side of the body

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reacts while the flexor reflex
is how the injured side,

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or the stimulated side, reacts.

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So we have our flexor reflex,

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the same side that was where
the stimuli was responds,

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cross extensor, the
opposite side responds.


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So on to this autonomic division.

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We said somatic was going to be

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voluntary control unless a reflex.

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But autonomic, think of automatic
control, it just happens.

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You don't even need to think about it.

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The autonomic division
is the motor output,

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or I should say one of the motor outputs,

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for the peripheral nervous system.

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So this autonomic division
really controls a lot

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of your internal organs.

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Think stomach, small intestines,
large intestines, heart.

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But there's two main divisions

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within the autonomic division.

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You have the sympathetic
and parasympathetic.

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The sympathetic division
is your fight or flight.

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That sympathetic division
will get your muscles,

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your lungs, your heart, your senses,

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all up and ready to react
to whatever is going on.

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While the parasympathetic just
kind of rests and digests.

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It's a calming down.

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If the parasympathetic is
functioning at a higher level

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your body wants to calm down and relax.

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If the sympathetic is
functioning at a higher level

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your body is very agitated.

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It wants to get up and go, wants to move.

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So now you have both
sympathetic and parasympathetic

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neurons to each organ.

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You may be going, why would you want both,

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they do the opposite thing.

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Well, if you're in a
fight or flight scenario

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your heart's beating faster.

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So the sympathetic stimulates it.

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But if you're in a rest and
digest, you just had a big meal,

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you're sitting down relaxing,

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the parasympathetic is gonna
help slow the heart down,

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allow it to relax.

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So you have to have both sympathetic

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and parasympathetic in each organ.

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So we said already, sympathetic,

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body is set for emergencies.

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Norepinephrine really is
the key neurotransmitter.

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Let's get your body ready to go.

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So you can increase heart rate,

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you're increasing respiration,
you're breathing faster,

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blood pressure's higher,
your pupils dilate,

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more light in means a little
better sense what's going on.

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Digestion and urinary
systems have slowed down.

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If you're in a fight or
flight response do you really

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need your digestive system

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and urine system functioning full out?

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No.

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Before the question comes
up of, well, hold on,

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why if you're in a life or
death situation sometimes

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do people urinate themselves?

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Or even defecate themselves?

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That's not because these
systems are functioning,

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it's because of how much the
systems are not functioning.

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Both the digestive and urinary
systems have sphincters

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or muscles that prevent
involuntary release.

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But if you're at such a
high level of sympathetic

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innervation your system might
not be maintaining enough

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stimulus to keep the
muscles closed of digestive

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and urine system.

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Those muscles open up and
relax, everything comes out.

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Now usually the way this
works is one unified response.

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You don't have the heart
rate increasing first,

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then respiration, no.

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Everything occurs at the
same time and it all occurs

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within seconds of a stimuli.

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It's the exact opposite
of the parasympathetic.

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So do you think it's ever
a good idea to have both

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sympathetic and parasympathetic

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stimulating at the same time?

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Not really.

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Because the parasympathetic
relaxes the body,

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does the exact opposite
of the sympathetic.

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Acetylcholine is a key
neurotransmitter here.

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It's meant to slower your heart rate down,

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slower your respiratory rates,

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but put more blood into
your digestive system,

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to increase digestion.

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Defecation and urination will occur

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during parasympathetic
division stimulation.

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When the body's relaxed that's
typically when the GI tract,

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the digestive tract,
and the urinary system,

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will release their components.

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So defecation occurs, urination occurs.

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Sympathetic and parasympathetic
are always antagonistic.

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They do the exact opposite.

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And it's this antagonistic role that

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allows homeostasis to be maintained.

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It's what keeps your
body in a balanced order.

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So here we have the breakdown

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of sympathetic and parasympathetic.

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Now you don't need to
memorize this diagram

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but get an idea of
roughly what's going on.

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Because see, on the sympathetic
side, the fight or flight,

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pupils dilate, more light in.

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But you decrease salivation,

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you're not eating anything right now.

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Increased respiration.

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Sure, you're breathing heavier.

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Increased heart rate.

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Because constricted blood
vessels, more blood pressure.

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Inhibits digestive processes.

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So by inhibiting all
these digestive processes,

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you're slowing it down.

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Relaxes the bladder muscles.

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Inhibits defecation.

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All the things that are not
necessary in a fight or flight,

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life or death situation,
are being impeded.

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The exact opposite on the parasympathetic.

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Now one interesting thing
with the parasympathetic.

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All the parasympathetic
nerves come from either

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cranial nerves or all
the way down at the end

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of the vertebral column,
the sacral nerves,

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while the sympathetic comes
from everywhere in between.

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So we have the cranial and
sacral is parasympathetic,

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so they don't even come
from the same place.

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So if you look at the
divisions and how they line up,

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let's focus on the right for a second.

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Sympathetic, parasympathetic.

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Sympathetic, fight or flight.

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Parasympathetic, relaxed, rest and digest.

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Neurotransmitter, sympathetic
is norepinephrine.

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Parasympathetic, acetylcholine.

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And how many neurons to reach the targets?

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Well both of them, sympathetic
and parasympathetic,

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need two neurons.

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So that means the entire
autonomic division

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00:14:29,264 --> 00:14:31,385
needs two neurons,

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while the somatic division just needs one.

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That means one neuron can
run all the way from your

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spinal cord down to your foot.

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That could be a two or
three foot long neuron.

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That's huge!

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You're always using
acetylcholine in the somatic.

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It's meant to move skeletal muscles.