- [Voiceover] The nervous system itself
isn't just really your brain or spinal cord.
Let's start off with the central nervous system,
known as the CNS.
Now, it's made up of two things, the brain and spinal cord.
But this central nervous system
is really the main hub for the nervous system.
It's mean to receive, it's meant to send out,
but it's also meant to process all the information.
So whenever you're thinking about anything,
it's the central nervous system
that's processing that information.
Everything outside of the central nervous system
is the peripheral nervous system.
Now, the peripheral nervous system
is all the nerves that are in your arms or legs,
or in the torso that's not the spinal cord.
But this peripheral system is broken up
into a sensory and motor division.
Well, the term sensory division
implies that's all the information that you're gathering.
It's all that touch and taste, temp,
all those things that are coming into your body.
That's the sensory.
They bring the information
into the central nervous system.
Well, the other division, the motor division,
is meant to carry the information
away from the central nervous system.
It's meant to send it out to the muscles
to tell them what to do.
It's meant to send information down to your
intestine and stomach to help with digestion.
So, this is all the information
leaving your central nervous system.
So, sensory is input, motor is output.
Let's start with this graphic.
We start with the peripheral nervous system.
This is how everything gets in your body,
temperature outside,
the fact that something's touching you on your skin.
It could be the muscles or tendons,
are they sore, are they hurting?
It could mean you have a stomachache.
Whatever it is, it's the input information
that goes to the central nervous system.
Now, the central nervous system,
spinal cord and brain, are gonna process.
They're gonna figure out what's going on,
and what an appropriate response is.
The response is the motor output.
Now, this motor output really has two different parts
called somatic and autonomic.
Well, you can kinda think of the autonomic
as an automatic control.
The autonomic reacts without even thinking about it.
You can think of autonomic as in your heart,
as in breathing, as in digestion.
They occur without you even thinking about it.
Now, that autonomic division
has sympathetic and parasympathetic parts.
Sympathetic is known as fight or flight.
That's the situation where if
all of the sudden you're in this life-threatening situation,
the sympathetic kicks in, puts your muscles in overdrive.
Your sense are heightened, your breathing is increased,
your heart rate's increased.
The whole body's riled up,
ready to either go or fight, fight or flight.
While the parasympathetic,
you can think of it more as rest and digest.
Parasympathetic is working after Thanksgiving dinner.
After you have that big, huge meal,
how many people wanna go run a marathon?
Even run a race?
No, you wanna sit down and relax.
Parasympathetic is rest and digest.
Your body gets slowed down and lulled.
But both of those sympathetic and parasympathetic
are part of the autonomic.
The other motor output is somatic.
Somatic is your voluntary control.
Somatic is when you think, raise your hand,
and your hand goes up.
When you think, let's walk forward, and your legs move.
It's conscious thought moving the muscles.
So does it all work?
How does all the information get from part A to part B?
All the information is traveling through
what are called neurons.
Neurons are simply cells,
but they're very specialized cells
that can actually make or generate
and conduct electrical impulses.
So they can make an impulse in the brain,
then send it down via neurons to your arm
to tell your muscle to move, and your arm goes up.
Now, these neurons are broken into two parts.
Now, think about, we had a sensory and motor division,
sensory was input, motor was output.
Well, not we have a sensory neuron,
these are the ones that receive the information
and bring it to the central nervous system.
Your sensory neurons carry information in
while your interneurons, they're kind of in between,
with the motor neurons heading out.
So you had that sensory and motor division,
well, we have sensory neurons bringing information in,
we have motor neurons carrying information out.
But then these interneurons.
The interneurons are in between, hence the term inter.
They're found in between sensory and motor.
They're meant to connect the two,
make sure the input matches the output.
So the neurons are basically just a relay system,
a way of getting information from point A to point B.
Now, a neuron itself has many different parts and pieces,
but there are three main parts,
the cell body, and this is where the nucleus is.
Kind of important, because that's what keeps
the actual cell alive.
We have dendrites, and we have axon.
Dendrites will receive incoming information.
Axons send information away from the neuron.
So it's always a one-way path.
The information always comes in on a dendrite,
goes into the cell body,
and then leaves the neuron via the axon.
So, three parts, to receive, dendrite,
relay, cell body, and then send along, axon.
Now, here we have an example showing a relay
between a sensory neuron, an interneuron,
and a motor neuron.
What's showing is the skin was damaged,
it looks like a sliver.
That means the sensory neuron
is going to receive that impulse.
The receptor of that sensory neuron will pick it up.
So there's the cell body, here's the axon.
We know axons will relay information away.
That means we need a dendrite to pick it up.
So, there's the dendrite.
Dendrites send information through the axon
and down into the brain or spinal cord.
Once in the brain or spinal cord,
you might need an interneuron.
The interneuron is going to have the cell body
with several dendrites and an axon.
That means it can receive, and then send away.
That axon will send information to a motor neuron.
We're gonna take the information
from the central nervous system, brain, spinal cord,
and send it off to a destination, in this case, a muscle.
So we have our cell body, dendrites to receive the signal,
and axon to carry it along.
So, even though the neurons can have
different shapes and different sizes,
they have the same basic structure,
cell body, dendrite receives, axon sends away.
So the way this electrical impulse works
is called an action potential.
Pretty much think of an action potential
as an electrical impulse.
When you turn a light switch on in your house, what happens?
Well, that light switch connects a circuit
allowing an electrical impulse to reach the light.
That's really what a neuron is.
It's a wire that allows an electrical impulse
to pass through.
So, these action potentials are the signals.
This really is how your nervous system communicates.
It's how the information travels
from point A to point B in your body.
Now, we have these other cells
that are part of the nervous system.
They're called neuroglial cells,
and even though neurons are what carry the impulses,
the neuroglial cells make up pretty much 80%
of the nervous system.
These neuroglial cells are helper cells.
They're meant to support, meant to protect.
But they cannot transmit an action potential.
These neuroglial cells cannot carry electrical signals,
so their goal is to make sure the neurons stay healthy.
They're meant to help and protect the neurons.
Now, there are two types that function
on the axon of a neuron.
We recall Schwann cells and this one looks
like a kind of weird word, but it's called oligodendrocytes.
Both of these types of cells
will wrap the axon in insulation.
They'll help protect it.
You can kinda think of an electrical wire.
Anything you plug in has an outside plastic casing,
or rubber casing.
Well, why don't you just hold the wire,
the bare metal wire itself?
Why put this casing on it?
Well, yeah, sure you don't wanna get electrocuted.
The same idea here.
These types of neuroglial, Schwann cells
and oligodenddrocytes,
they're going to cover the axon,
keeping the action potential,
or keeping that electrical impulse inside,
preventing it from leaking out.
So, these Schwann cells are found in the PNS,
the peripheral nervous system.
Their main goal, insulation.
They're meant to save the neuron energy,
making sure that all the electrical impulse
gets from point A to point B.
So by insulating, they speed up transmission,
make it faster.
There's this term called saltatory conduction.
So, you can think of it as the way
the electrical impulse leapfrogs
from space to space.
So, imagine if you had a necklace made of beads.
Just normal rounds, there's spaces in between each bead.
Each one of those spaces
is where the electrical impulse can jump from,
from space to space.
The beads are the insulation.
They can't come or go through a bead.
They'll be jumping point A to point B to point C,
make it move faster.
Oligodendrocytes, they're only found in the CNS,
central nervous system.
Both both Schwann and oligodendrocytes
have the same function.
They're meant to form this myelin sheath
which protects the axon
as well as makes the signal travel faster.
So here you can go see an example of a neuron.
This particular one is a motor neuron.
You can notice how you have all of these
kinda like oval shapes coming down the long axon.
Those oval shapes are the myelin sheathing.
Those are the Schwann cells.
Each one's a cell, each one insulates.
And the signal will just jump like this purple arrows are
from gap to gap to gap to gap.