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.