In this video, I want to provide an overview
of neuron structure.
Neurons in adults have a soma.
It's also called a cell body-- soma.
And they have processes called neurites,
which are divided into dendrites and axons.
Dendrites are usually short, branched processes
that are often covered in small spines that
increase their surface area and perform some other functions.
So these are dendrites.
And then the other neurite they have
is called an axon, which is usually
long and unbranched until it reaches its end.
So this is the axon.
The area where the axon leaves the soma
is called the axon hillock.
The axon may be short or it may be very long, up to one meter
or more.
And it usually is unbranched for most or all of that length,
until it gets to the end, in these structures,
which are called axon terminals.
And at this point, it will often branch and create
multiple axon terminals.
The first part of the axon is called
the axon initial segment.
Or it's also called the trigger zone.
And we'll get into the reason for that in the next video.
Axons can be so long that they are
dependent on systems that transport substances
from the soma, which contains most of the organelles,
to the axon terminals, and vice versa.
Things have to be transported both directions.
And the axon is dependent on those systems.
Large axons are usually wrapped in a sheath
of a material called myelin.
And axons that have a myelin sheath
have little gaps between these segments
of myelin called nodes of Ranvier.
So the sheath I've drawn in yellow
is the myelin, each of these little segments of sheath here.
And these gaps that regularly interrupt the sheath
are called nodes of Ranvier, these little gaps in the myelin
sheath.
The axon terminals will come very
close to the target cells of the neuron.
And I'll just draw it here.
So these are the target cells.
And these targets cells may be another neuron,
they may be a muscle cell, or they may be a gland cell.
A few neurons even have axons that terminate on capillaries,
to secrete substances called hormones into the bloodstream.
The place where an axon terminal comes
close to touching the target cell is called a synapse.
This is a pretty typical structure for a neuron.
But there are multiple structural types
of neurons, each of which can be further divided into subtypes.
So let's go over some of the big categories
of structural types of neurons.
In the central nervous system, neurons
start as neural stem cells, which
turn into most of the cell types of the central nervous system.
And these neural stem cells then differentiate
into cells called neuroblasts.
And don't worry about the details here.
Because we'll go into a lot more detail
in other videos on development of the nervous system.
But neural stem cells and neuroblasts
look pretty similar.
They're basically just shapeless cells without processes.
Neural stem cells can become almost any neural cell
of the central nervous system, while neuroblasts can only
become neurons.
Neuroblasts will then migrate away
from the neural stem cells to the location
that their somas will have after development.
Neuroblasts then extend a process, which is an axon,
toward their target cell.
And that axon is tipped with this structure called a growth
cone-- growth cone.
The axon growth cone follows guidance cues
in the environment until it reaches
the target cell of the neuron.
A similar process occurs for neurons
in the peripheral nervous system.
But the original and the migrating cells
for those neurons are neural crest cells, instead of
neural stem cells and neuroblasts.
Neurons at this stage have only one process, which is an axon.
So they are now called unipolar neurons-- unipolar.
That's the structural type of this neuron
because there's one pole to the cell, one process giving
a sense of direction on this otherwise shapeless cell.
Unipolar neurons are present in humans,
mainly during development.
The next structural type of neuron has a soma.
And it has one axon.
But it also has one dendrite.
So since this structural type of neuron
has two processes, or two poles, it's
called a bipolar neuron-- bipolar.
The next structural type of neuron
has a soma, just like the others, and one axon.
But it has multiple dendrites.
And so since it's going to have multiple poles,
it's called a multipolar neuron-- multipolar.
And this is the most common structural type
of neuron in adult humans.
The last big category of structural types of neurons
is a little different.
It has a soma, like all the rest.
And then it has one a short process
coming out of the soma, that then divides
into two long processes going in different directions.
And these are both axons.
The axon bringing information in from the periphery
is called the peripheral axon.
And the axon bringing information
into the central nervous system is called the central axon.
The very end of the peripheral axon acts a lot like dendrites
do on the other structural types of neurons.
And we'll start to go over the function of dendrites and axons
in the next video.
And then this part of the peripheral axon near the end
is the axon initial segment, where the trigger zone, just
like this part is on a multipolar neuron
close to the soma.
And just like in these neurons where this is the trigger zone,
and then the end of the axon has the axon terminals,
in this type of neuron this is the trigger zone of the axon.
And then the axon terminals are all the way
at this end of the central axon.
So this type of neuron has a big, long, funny name.
It's called a pseudounipolar neuron-- pseudounipolar.
And the reason is that it's kind of, sort
of like a unipolar neuron, with only one process
coming out of the soma.
But that little short process immediately
splits into these two long axons.
So it's really a different shape than the unipolar neurons.