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What you will see in this presentation
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
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
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