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- [Voiceover] The nervous system itself

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isn't just really your
brain or spinal cord.

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Let's start off with the
central nervous system,

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known as the CNS.

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Now, it's made up of two things,
the brain and spinal cord.

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But this central nervous system

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is really the main hub
for the nervous system.

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It's mean to receive,
it's meant to send out,

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but it's also meant to
process all the information.

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So whenever you're
thinking about anything,

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it's the central nervous system

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that's processing that information.

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Everything outside of the
central nervous system

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is the peripheral nervous system.

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Now, the peripheral nervous system

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is all the nerves that
are in your arms or legs,

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or in the torso that's
not the spinal cord.

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But this peripheral system is broken up

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into a sensory and motor division.

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Well, the term sensory division

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implies that's all the
information that you're gathering.

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It's all that touch and taste, temp,

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all those things that are
coming into your body.

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That's the sensory.

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They bring the information

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into the central nervous system.

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Well, the other division,
the motor division,

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is meant to carry the information

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away from the central nervous system.

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It's meant to send it out to the muscles

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to tell them what to do.

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It's meant to send
information down to your

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intestine and stomach
to help with digestion.

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So, this is all the information

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leaving your central nervous system.

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So, sensory is input, motor is output.

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Let's start with this graphic.

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We start with the
peripheral nervous system.

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This is how everything gets in your body,

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temperature outside,

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the fact that something's
touching you on your skin.

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It could be the muscles or tendons,

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are they sore, are they hurting?

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It could mean you have a stomachache.

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Whatever it is, it's the input information

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that goes to the central nervous system.

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Now, the central nervous system,

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spinal cord and brain, are gonna process.

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They're gonna figure out what's going on,

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and what an appropriate response is.

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The response is the motor output.

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Now, this motor output really
has two different parts

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called somatic and autonomic.

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Well, you can kinda think of the autonomic

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as an automatic control.

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The autonomic reacts without
even thinking about it.

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You can think of autonomic
as in your heart,

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as in breathing, as in digestion.

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They occur without you
even thinking about it.

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Now, that autonomic division

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has sympathetic and parasympathetic parts.

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Sympathetic is known as fight or flight.

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That's the situation where if

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all of the sudden you're in
this life-threatening situation,

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the sympathetic kicks in, puts
your muscles in overdrive.

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Your sense are heightened,
your breathing is increased,

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your heart rate's increased.

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The whole body's riled up,

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ready to either go or
fight, fight or flight.

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While the parasympathetic,

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you can think of it
more as rest and digest.

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Parasympathetic is working
after Thanksgiving dinner.

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After you have that big, huge meal,

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how many people wanna go run a marathon?

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Even run a race?

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No, you wanna sit down and relax.

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Parasympathetic is rest and digest.

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Your body gets slowed down and lulled.

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But both of those sympathetic
and parasympathetic

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are part of the autonomic.

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The other motor output is somatic.

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Somatic is your voluntary control.

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Somatic is when you
think, raise your hand,

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and your hand goes up.

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When you think, let's walk
forward, and your legs move.

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It's conscious thought moving the muscles.

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So does it all work?

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How does all the information
get from part A to part B?

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All the information is traveling through

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what are called neurons.

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Neurons are simply cells,

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but they're very specialized cells

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that can actually make or generate

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and conduct electrical impulses.

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So they can make an impulse in the brain,

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then send it down via neurons to your arm

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to tell your muscle to
move, and your arm goes up.

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Now, these neurons are
broken into two parts.

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Now, think about, we had a
sensory and motor division,

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sensory was input, motor was output.

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Well, not we have a sensory neuron,

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these are the ones that
receive the information

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and bring it to the
central nervous system.

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Your sensory neurons carry information in

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while your interneurons,
they're kind of in between,

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with the motor neurons heading out.

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So you had that sensory
and motor division,

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well, we have sensory neurons
bringing information in,

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we have motor neurons
carrying information out.

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But then these interneurons.

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The interneurons are in
between, hence the term inter.

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They're found in between
sensory and motor.

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They're meant to connect the two,

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make sure the input matches the output.

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So the neurons are basically
just a relay system,

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a way of getting information
from point A to point B.

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Now, a neuron itself has many
different parts and pieces,

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but there are three main parts,

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the cell body, and this
is where the nucleus is.

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Kind of important,
because that's what keeps

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the actual cell alive.

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We have dendrites, and we have axon.

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Dendrites will receive
incoming information.

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Axons send information
away from the neuron.

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So it's always a one-way path.

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The information always
comes in on a dendrite,

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goes into the cell body,

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and then leaves the neuron via the axon.

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So, three parts, to receive, dendrite,

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relay, cell body, and
then send along, axon.

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Now, here we have an
example showing a relay

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between a sensory neuron, an interneuron,

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and a motor neuron.

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What's showing is the skin was damaged,

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it looks like a sliver.

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That means the sensory neuron

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is going to receive that impulse.

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The receptor of that sensory
neuron will pick it up.

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So there's the cell body, here's the axon.

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We know axons will relay information away.

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That means we need a
dendrite to pick it up.

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So, there's the dendrite.

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Dendrites send information
through the axon

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and down into the brain or spinal cord.

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Once in the brain or spinal cord,

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you might need an interneuron.

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The interneuron is going
to have the cell body

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with several dendrites and an axon.

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That means it can receive,
and then send away.

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That axon will send
information to a motor neuron.

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We're gonna take the information

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from the central nervous
system, brain, spinal cord,

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and send it off to a destination,
in this case, a muscle.

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So we have our cell body,
dendrites to receive the signal,

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and axon to carry it along.

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So, even though the neurons can have

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different shapes and different sizes,

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they have the same basic structure,

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cell body, dendrite
receives, axon sends away.

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So the way this electrical impulse works

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is called an action potential.

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Pretty much think of an action potential

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as an electrical impulse.

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When you turn a light switch
on in your house, what happens?

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Well, that light switch connects a circuit

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allowing an electrical
impulse to reach the light.

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That's really what a neuron is.

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It's a wire that allows
an electrical impulse

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to pass through.

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So, these action
potentials are the signals.

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This really is how your
nervous system communicates.

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It's how the information travels

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from point A to point B in your body.

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Now, we have these other cells

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that are part of the nervous system.

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They're called neuroglial cells,

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and even though neurons are
what carry the impulses,

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the neuroglial cells
make up pretty much 80%

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of the nervous system.

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These neuroglial cells are helper cells.

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They're meant to support,
meant to protect.

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But they cannot transmit
an action potential.

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These neuroglial cells cannot
carry electrical signals,

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so their goal is to make sure
the neurons stay healthy.

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They're meant to help
and protect the neurons.

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Now, there are two types that function

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on the axon of a neuron.

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We recall Schwann cells and this one looks

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like a kind of weird word, but
it's called oligodendrocytes.

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Both of these types of cells

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will wrap the axon in insulation.

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They'll help protect it.

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You can kinda think of an electrical wire.

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Anything you plug in has
an outside plastic casing,

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or rubber casing.

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Well, why don't you just hold the wire,

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the bare metal wire itself?

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Why put this casing on it?

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Well, yeah, sure you don't
wanna get electrocuted.

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The same idea here.

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These types of neuroglial, Schwann cells

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and oligodenddrocytes,

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they're going to cover the axon,

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keeping the action potential,

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or keeping that electrical impulse inside,

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preventing it from leaking out.

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So, these Schwann cells
are found in the PNS,

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the peripheral nervous system.

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Their main goal, insulation.

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They're meant to save the neuron energy,

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making sure that all
the electrical impulse

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gets from point A to point B.

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So by insulating, they
speed up transmission,

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make it faster.

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There's this term called
saltatory conduction.

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So, you can think of it as the way

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the electrical impulse leapfrogs

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from space to space.

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So, imagine if you had a
necklace made of beads.

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Just normal rounds, there's
spaces in between each bead.

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Each one of those spaces

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is where the electrical
impulse can jump from,

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from space to space.

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The beads are the insulation.

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They can't come or go through a bead.

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They'll be jumping point
A to point B to point C,

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make it move faster.

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Oligodendrocytes, they're
only found in the CNS,

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central nervous system.

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Both both Schwann and oligodendrocytes

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have the same function.

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They're meant to form this myelin sheath

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which protects the axon

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as well as makes the signal travel faster.

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So here you can go see
an example of a neuron.

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This particular one is a motor neuron.

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You can notice how you have all of these

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kinda like oval shapes
coming down the long axon.

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Those oval shapes are
the myelin sheathing.

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Those are the Schwann cells.

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Each one's a cell, each one insulates.

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And the signal will just jump
like this purple arrows are

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from gap to gap to gap to gap.