- [Voiceover] Like in every other system,
the nervous system also has some diseases.
Now, there's a type of diseases referred to
as demyelinating diseases.
Multiple sclerosis is one that falls in this category.
What happens in these demyelinating diseases
is the myelin sheath, or that myelin cover
that's around the axon, the insulation itself
is being damaged, is being broken down and removed.
The more myelin or the more protective sheeting
that's removed, the weaker and less efficient
the neuron will be.
The person ends up with weakness, visual impairment,
incontinence, and the list goes on and on and on.
Because any time the myelin is being removed
it's causing a problem with transmitting a signal.
Well, another type of demyelinating disease is ALS.
Amyotrophic lateral sclerosis,
or more commonly known as Lou Gehrig's disease.
Now, these are specifically looking
at the motor area of your spinal cord.
Now if you remember, motor means output.
The motor area is what sends information out.
So if this is what's being effected,
your motor output is not as efficient.
That means your skeletal muscles,
the ones that move your body, move your arm, move your legs,
they're weakening, they're not working as efficiently
because the signals are not reaching them.
So we have multiple sclerosis, MS,
and we have amyotrophic lateral sclerosis, ALS,
or known as Lou Gehrig's disease.
So when we talk about these neurons,
as long as they're intact and not suffering from a disorder,
the neurons are meant to target something else.
It could be another neuron, just relaying a signal.
It could be a muscle cell, telling that muscle cell
to contract, to move.
It could be a gland saying secrete something.
Pretty much anything in your body
could be a target for a neuron.
Right where it meets a target, though,
is referred to as a synapse.
The term synapse, it's a special junction
between the axon terminus and its target cell.
There's always a small little gap
present here at the synapse.
As the information that that neuron's carrying
must go across the synapse.
And that's referred to as synaptic transmission.
This is how you get the information
from your presynaptic neuron,
or that neuron that's before the synapse,
going to your target, also known as the postsynaptic
or after synapse.
Now, because we know it's an electrical signal
coming down the neuron,
when electricity hits a gap, a space,
is it really efficient to crossing a space?
I mean, how many of you can predict
where lightning's going to strike
exactly when it starts in the clouds?
No, you can't.
Because electricity does not flow
very efficiently through space.
So what ends up happening is when you go through
a synaptic transmission, it takes the electrical signal
from that neuron and turns it into a chemical signal.
That chemical signal is called a neurotransmitter.
Neurotransmitters are only released
when that action potential or that electrical signal
arrives at the end of the axon,
known as the axon terminus.
Well, terminus, terminal, end.
Well, when that action potential arrives,
it causes calcium to enter into the axon.
The calcium causes the neurotransmitter to release.
It goes across that space called the synaptic cleft
and eventually binds to the target.
But to be able to bind to the target,
it has to find a receptor.
Every neurotransmitter is specific to a receptor.
Think of like of a key and a lock.
Only a certain key will open that particular lock,
while only certain neurotransmitters can bind
to certain receptors.
Once they bind, they can open a gate,
they can initiate a signal,
there's many different things they can do.
But they have to find that specific receptor first.
So here's this little graphic
kind of showing what's going on.
We start with the action potential heading down the axon.
It reaches the end of the axon, the terminus, the bulb,
calcium flows in.
Once calcium flows in, then we can have
the actual neurotransmitter released
across the synaptic cleft.
Once it goes across the synaptic cleft,
it binds the receptor, remember the receptor is specific,
and then once it binds, it can open a channel for instance.
Here, neurotransmitter bound opened a channel.
The channels allow sodium to flow through.
Na is sodium in this case.
Now, when these bind, these neurotransmitters bind,
they can either have an excitatory effect
or an inhibitory effect.
So it all depends on what type of neurotransmitter it is,
but also what type of receptor it is.
Some neurotransmitters are always excitatory.
Some are always inhibitory.
Some receptors are always inhibitory,
some are always excitatory.
I hear you saying "Well, hold on,
what is excitatory and inhibitory?"
The term excitatory is referring to the fact
that the actual binding of that neurotransmitter
will cause a positive reaction,
it will cause something to occur.
The opening of that sodium channel was an excitatory effect.
An inhibitory effect is preventing something from occurring.
So if a neurotransmitter was inhibitory,
on that previous slide where the sodium
went through the channel,
if the inhibitory had bound,
the channel would remain closed.
So this excitatory or inhibitory
depends on the neurotransmitter.
There's more than fifty different
types of neurotransmitters.
Depends on the receptor.
There's different receptors
for each different neurotransmitter.
And the gated ion channels, depending on the type,
can also be excitatory or inhibitory effect.
Now this chart, don't worry about learning
all these parts and pieces here,
what's important is looking at the names
of neurotransmitters.
Just wanted to make sure you can see
what some of the names are and say
"Hey, might recognize this one,"
or "Hey, I've heard of that one before."
Acetylcholine, norepinephrine, serotonin, dopamine,
glutamate, endorphins.
Now I'm pretty sure people
probably haven't heard of this one,
gamma-aminobutyric acid, and somatostatin.
Now those are just some of a few neurotransmitters.
There's tons more beyond that.
So now these postsynaptic cells,
the ones that receive the information.
It really depends on how many neurons
are forming the synapse.
It's not always a 1:1 ratio.
Sometimes you might have five
all bringing information in to one postsynaptic cell.
Well, what also depends is the formation,
excitatory or inhibitory.
So the way information's processed,
the way it's integrated and processed,
depends on how it was received
and what type of information it is.
So we have this one possibility called convergence.
Well, if you're converging, that's many coming into few.
This particular example, you see three neurons
all converging or coming together
to the one neuron in the middle.
So we have up top the three presynaptic,
the ones before the synapse,
going to one postsynaptic in the middle.
So you have three different bits of information
all converging at the same time.
The term convergence.
We also have divergence, one going to many.
So if we take that middle neuron,
the one right in the middle of the page here,
can see how it goes down and branches out
to four different postsynaptic neurons.
So that one neuron can send the same signal
out to four different axons at the same time.
So you're diverging the signal from one to many.