is the development of myelin

in the peripheral nervous system

and the propagation of the action potential

along a myelinated axon

You should already have

a good understanding

of the Schwann Cell

and action potential.

The Schwann Cell forms a protective coating

around the axon

Scwann cells start to develop in the embryo

and continue to increase the wrapping around

the axon through childhood.

This increases the thickness

which peaks in adolescence.

This is why teenagers have such quick responses.

The Schwann cell contains

typical cell organelles and membrane structure

Notice as the Schwann cell surrounds the axon

the nucleus is squeezed to the outside wrapping of the cell.

This outer wrapping of the Schwann cell is called the "neurolemma."

The inner lining is made up of

layers upon layers of cell membrane.

This inner wrapping is called the myelin sheath.

The cell membrane

called the fluid mosaic model

is made up of a bilayer of lipids

integrated with proteins

The thicker the myelin

in other words, the more layers

of cell membrane making up the myelin

the more advantageous it is to the axon.

One advantage is the regeneration

of severed axons

Another advantage is an increase

in the speed of propagation of the action potential

along the axon.

The rest of this presentation will concentrate

on the increased speed of action potential

down the length of the axon

Here is the neuron

And you can see the repeated Schwann cell

membrane forming the myelin

Note that there's a small space

between the Schwann cells

where the axon is not covered

by the cell

These spaces are called Nodes of Ranvier.

From what you already know

action potentials occur at the axon hillock and

continue to be repeated

away from the cell body

much like dominoes falling one after another

An action potential starts

on a polarized membrane

which is negative 70

A stimulus causes the sodium gates to open slightly

and sodium starts to trickle into the cell

If the cell reaches -60 or threshold

the sodium gates open wide

and sodium floods in

bringing the inside of the axon to +30

At this point the

sodium gates close and potassium gates open

Potassium starts to pour out of the cell

This allows the neuron to become polarized again

Then the sodium potassium pumps

starts to actively transport sodium out

and potassium back into the neuron

First look at the propagation

of the action potential in the unmyelinated axon

Propagation is the repeating

of action potentials down the axon.

The action potential is repeated

because as the sodium comes in

it diffuses to adjacent areas within the axon

As the sodium increases in this area

threshold is reached

Sodium gates open wide

sodium rushes in

causing depolarization

and an action potential

as the sodium enters this area

it diffuses through the ectoplasm

and another action potential is created

This continues down the length of the axon.

Now look at the myelinated axon.

The same process applies

An action potential develops

And as the sodium comes in

it diffuses through the axon

It continues to diffuse through the portion

of the axon wrapped in myelin

The increased sodium concentration

reaches the Node of Ranvier

increases the ectoplasm

to -60 and

depolarization occurs.

The sodium gates open wide

sodium floods in

and we have an action potential

Again. The sodium diffuse

through the ectoplasm, reaching the next node

an action potential develops

The process is continued down the myelinated axon

passing from node to node

Compare the unmyelinated axon with the myelinated

You can see that action potential reach

the end of the myelinated axon more rapidly

The speed of the propagation

is faster going node to node than

action potentials that develop adjacent

to the previous action potential