Now axons, as I mentioned earlier
can be myelinated or unmyelinated.
Here is a high magnification image on the left
hand side of a nerve that has being treated by
a fixed nerve where the myelin has been retained.
And you can see that the axon is a very clear
structure and it is surrounded by myelin sheaths
staining black here. And on the right-hand
side, you can see the myelin sheath taken
by an electron microscope viewing. It is made
up of little lamellae or sheaths and sheaths
of myelin and on the right-hand side of that
you can see an axon. And look very carefully
inside the axon, you can see some little dots.
Those little dots represent myotubules. The microtubules
are very important for transport of substances
particularly neurotransmitters up and down
the nerve axon. And that large stained structure
on the right-hand side of the myelin sheath
is the Schwann cell nucleus. Again if you
look very carefully at the image on the left,
you can see some myelinated axons, but also
you can see some that are not myelinated at
al. And on the right-hand side, you can see
the nucleus of a Schwann cell and you can
see that the Schwann cell has wrapped up axons
and also you can see little tiny dots between
these axons. That represents the collagen
fibers of the endoneurium. So the Schwann
cell does invest the axon, but it does not
myelinate these axons. So in the peripheral
nerve then, you have some axons that are myelinated
and some that are not myelinated. And you would
also noticed that axons vary in their
size, their diameter and that often reflects
their speed of conduction ability.
Sometimes that myelin also, in fact not sometimes, when
the myelin sheath wraps around these axons,
the axon is myelinated by a whole large number
of Schwann cells. So when you look right down
the pathway of the axon, right down the length
of the axon, they have myelin sheaths wrapping
all the way around them. And there's gaps between
these myelin sheaths and those gaps are called
the node of Ranvier. And they are very important
because they allow the nerve impulse to jump
from one node to another and speed up the right
of impulse conduction.
It is called saltatory conduction.