Myelin在线视频

Hi, everyone. I am Dr. Ma,

from 1st affiliated hospital

of Shantou University Medical College.

Welcome to this session of clinical histology.

Today we will talk about myelin

in the nervous system

Here are the learning objectives for you.

At the end of this session,

students should be able to recognize

the microscopic sections of axon,

myelin and node of Ranvier in nervous tissue

compare the difference of myelinated nerve fibers

in the central & peripheral nervous systems

summarize the myelin changes in patients

with Guillain-Barré Syndrome

Let's start off with a case.

Miss Simmons,

a 28-year-old woman presented

to the Emergency Department at 4 am.

She woke up at night with tingling sensation

in her feet.

Gradually,

she became unsteady while climbing stairs,

then developed weakness in the legs.

Later,

she could not walk and started to lose

control of her body from the feet up.

Physical examination revealed sensory disturbance,

decreased power and loss of tendon reflexes

in both legs.

After lab tests,

the patient was diagnosed

as having Guillain-Barré Syndrome,

called GBS for short.

It is a relatively uncommon disorder

in which the body's immune system

mistakenly launches an attack

on the myelin of the peripheral nerves.

As we already know,

our nerve cells or neurons communicate

by using electrochemical impulses.

Axons of peripheral nerves can be several feet long,

based on whether the axons are covered with myelin,

there are basically 2 kinds of nerve fibers,

myelinated and unmyelinated.

So, what is myelin?

Myelin is formed by glial cells,

but the particular type of glial cell

responsible for myelinating an axon

is different in the peripheral

and central nervous system.

In the peripheral nervous system,

the glial cells,

also called Schwann cells,

produce myelin.

Each Schwann cell wraps around one segment

of an axon many times to form one internode.

In the central nervous system,

oligodendrocytes generate myelin.

One oligodendrocyte can produce dozens of

internodes on multiple axons.

Take Schwann cell for example,

the myelinated cell membrane of the Schwann cell

is very rich in lipid.

It wraps around the axon many times,

very thinly and very tightly.

Ok.

So how does myelin look like under the microscope?

Here

we are looking at the sections of peripheral nerve

prepared with H&E staining.

This is the cross section of a peripheral nerve

showing many round entities.

We can see some kinds of substances in the middle

which are surrounded by a clearer

material on the outside.

Well,

the darker substance in the middle

depicted by our yellow arrow are axons of the neurons.

The neuron body is never found

in a peripheral nerve.

This lighter color indicated by the green arrow

surrounding the axon is myelin

which produced by the Schwann cell.

Now,

we move to the longitudinal section

of the peripheral nerve.

Schwann cell and fibroblast can be identified

on this section.

Schwann cell can be distinguished

by their palely stained nuclei.

The yellow arrow points to the nuclei of Schwann cell.

Fibroblast nuclei are thin and dark.

The red arrow points to the nuclei of a fibroblast.

We can see the axon surrounded by myelin sheath,

except where the node of Ranvieris located.

The black arrow here points to an axon.

Its myelin sheath is denoted by the blue arrow.

A node of Ranvier is shown by the green arrow.

So, What are the differences between myelinated

and unmyelinated fibers in function?

For an unmyelinated axon,

action potential is propagated

by the sequential opening

of voltage-gated sodiumion channels.

So,

as shown on the slide,

when sodium ions rush into the cell,

the inside becomes positive,

and the outside becomes negative.

It opens up the adjacent channel,

allowing sodium ions to rush in

and hence the same process is repeated.

This allows the action potential to travel

from one part of the axon to the next part,

and eventually reaches its target.

However,

lots and lots of voltage-gated sodium channels

need to open up one by one

allowing the electrical signal to move along the axon.

Therefore,

the conduction of electrical signal

through unmyelinated axon

takes a little bit more time.

In contrast,

thanks to the myelin sheaths,

which act like the insulation around an electric wire.

It helps to prevent electrical signals

that are traveling along the axon from decay

due to electrical current leakage

through the membrane.

However,

myelin does not cover the whole axon entirely.

Instead,

there are intermittent gaps in the myelin

where the axon is exposed to

extracellular fluid directly.

These gaps are called nodes of Ranvier

and the sections of myelin

that are adjacent to these nodes

are called internodes.

The nodes of Ranvier are rich in sodium channels,

which open in response to an action potential

traveling down an axon,

allowing positive sodium ions to enter.

Because the internodes are myelinated,

the action potential speeds up as it travels

and then slows down at each node of Ranvier

This gives the appearance that an action potential is

jumping from node to node,

and we call this saltatory conduction

Now let's go back to the case of Miss Simmons.

In GBS,

the myelin coating around the nerves

is damaged by the autoimmune process,

nerves are not able to conduct

electrical impulses normally.

If the injury is severe,

the underlying nerve can even die.

Therefore,

we can see GBS leads to various degrees of

abnormal sensation and weakness

which typically begins at the feet

and gradually goes up the body

as in the case of Miss Simmons.

The weakness can be severe

and lead to paralysis of the respiratory muscle as well,

thus affecting breathing.

That's all for today.

Thank you and here are the references.

See you next time.