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 call 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.