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- [Voiceover] Like in every other system,

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the nervous system also has some diseases.

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Now, there's a type of
diseases referred to

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as demyelinating diseases.

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Multiple sclerosis is one
that falls in this category.

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What happens in these
demyelinating diseases

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is the myelin sheath, or that myelin cover

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that's around the axon,
the insulation itself

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is being damaged, is being
broken down and removed.

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The more myelin or the
more protective sheeting

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that's removed, the
weaker and less efficient

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the neuron will be.

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The person ends up with
weakness, visual impairment,

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incontinence, and the list
goes on and on and on.

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Because any time the
myelin is being removed

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it's causing a problem
with transmitting a signal.

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Well, another type of
demyelinating disease is ALS.

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Amyotrophic lateral sclerosis,

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or more commonly known
as Lou Gehrig's disease.

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Now, these are specifically looking

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at the motor area of your spinal cord.

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Now if you remember, motor means output.

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The motor area is what
sends information out.

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So if this is what's being effected,

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your motor output is not as efficient.

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That means your skeletal muscles,

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the ones that move your body,
move your arm, move your legs,

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they're weakening, they're
not working as efficiently

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because the signals are not reaching them.

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So we have multiple sclerosis, MS,

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and we have amyotrophic
lateral sclerosis, ALS,

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or known as Lou Gehrig's disease.

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So when we talk about these neurons,

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as long as they're intact and
not suffering from a disorder,

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the neurons are meant to
target something else.

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It could be another neuron,
just relaying a signal.

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It could be a muscle cell,
telling that muscle cell

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to contract, to move.

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It could be a gland
saying secrete something.

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Pretty much anything in your body

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could be a target for a neuron.

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Right where it meets a target, though,

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is referred to as a synapse.

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The term synapse, it's a special junction

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between the axon terminus
and its target cell.

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There's always a small little gap

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present here at the synapse.

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As the information that
that neuron's carrying

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must go across the synapse.

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And that's referred to
as synaptic transmission.

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This is how you get the information

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from your presynaptic neuron,

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or that neuron that's before the synapse,

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going to your target, also
known as the postsynaptic

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or after synapse.

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Now, because we know
it's an electrical signal

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coming down the neuron,

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when electricity hits a gap, a space,

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is it really efficient
to crossing a space?

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I mean, how many of you can predict

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where lightning's going to strike

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exactly when it starts in the clouds?

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No, you can't.

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Because electricity does not flow

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very efficiently through space.

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So what ends up happening
is when you go through

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a synaptic transmission, it
takes the electrical signal

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from that neuron and turns
it into a chemical signal.

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That chemical signal is
called a neurotransmitter.

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Neurotransmitters are only released

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when that action potential
or that electrical signal

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arrives at the end of the axon,

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known as the axon terminus.

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Well, terminus, terminal, end.

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Well, when that action potential arrives,

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it causes calcium to enter into the axon.

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The calcium causes the
neurotransmitter to release.

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It goes across that space
called the synaptic cleft

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and eventually binds to the target.

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But to be able to bind to the target,

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it has to find a receptor.

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Every neurotransmitter is
specific to a receptor.

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Think of like of a key and a lock.

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Only a certain key will
open that particular lock,

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while only certain
neurotransmitters can bind

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to certain receptors.

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Once they bind, they can open a gate,

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they can initiate a signal,

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there's many different things they can do.

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But they have to find that
specific receptor first.

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So here's this little graphic

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kind of showing what's going on.

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We start with the action
potential heading down the axon.

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It reaches the end of the
axon, the terminus, the bulb,

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calcium flows in.

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Once calcium flows in, then we can have

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the actual neurotransmitter released

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across the synaptic cleft.

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Once it goes across the synaptic cleft,

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it binds the receptor, remember
the receptor is specific,

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and then once it binds, it can
open a channel for instance.

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Here, neurotransmitter
bound opened a channel.

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The channels allow sodium to flow through.

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Na is sodium in this case.

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Now, when these bind, these
neurotransmitters bind,

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they can either have an excitatory effect

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or an inhibitory effect.

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So it all depends on what type
of neurotransmitter it is,

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but also what type of receptor it is.

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Some neurotransmitters
are always excitatory.

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Some are always inhibitory.

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Some receptors are always inhibitory,

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some are always excitatory.

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I hear you saying "Well, hold on,

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what is excitatory and inhibitory?"

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The term excitatory is
referring to the fact

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that the actual binding
of that neurotransmitter

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will cause a positive reaction,

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it will cause something to occur.

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The opening of that sodium
channel was an excitatory effect.

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An inhibitory effect is preventing
something from occurring.

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So if a neurotransmitter was inhibitory,

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on that previous slide where the sodium

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went through the channel,

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if the inhibitory had bound,

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the channel would remain closed.

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So this excitatory or inhibitory

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depends on the neurotransmitter.

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There's more than fifty different

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types of neurotransmitters.

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Depends on the receptor.

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There's different receptors

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for each different neurotransmitter.

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And the gated ion channels,
depending on the type,

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can also be excitatory
or inhibitory effect.

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Now this chart, don't worry about learning

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all these parts and pieces here,

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what's important is looking at the names

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of neurotransmitters.

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Just wanted to make sure you can see

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what some of the names are and say

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"Hey, might recognize this one,"

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or "Hey, I've heard of that one before."

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Acetylcholine, norepinephrine,
serotonin, dopamine,

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glutamate, endorphins.

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Now I'm pretty sure people

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probably haven't heard of this one,

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gamma-aminobutyric acid, and somatostatin.

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Now those are just some of
a few neurotransmitters.

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There's tons more beyond that.



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So now these postsynaptic cells,

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the ones that receive the information.

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It really depends on how many neurons

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are forming the synapse.

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It's not always a 1:1 ratio.

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Sometimes you might have five

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all bringing information in
to one postsynaptic cell.

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Well, what also depends is the formation,

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excitatory or inhibitory.

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So the way information's processed,

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the way it's integrated and processed,

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depends on how it was received

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and what type of information it is.

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So we have this one
possibility called convergence.

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Well, if you're converging,
that's many coming into few.

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This particular example,
you see three neurons

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all converging or coming together

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to the one neuron in the middle.

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So we have up top the three presynaptic,

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the ones before the synapse,

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going to one postsynaptic in the middle.

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So you have three different
bits of information

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all converging at the same time.

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The term convergence.

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We also have divergence,
one going to many.

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So if we take that middle neuron,

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the one right in the
middle of the page here,

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can see how it goes down and branches out

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to four different postsynaptic neurons.

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So that one neuron can
send the same signal

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out to four different
axons at the same time.

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So you're diverging the
signal from one to many.