at which the activity is going to take place.
So with that background knowledge about the
structure of the spinal nerve and the axons it
contains, let us now have a look at how the
spinal nerve is structured. It is wrapped
up by a number of connective tissue components.
They're the epineurium, the perineurium and the
endoneurium. The epineurium is on the outside.
The perineurium wraps around individual muscle
bundles or nerve fascicles, just like when
you look at the structure of muscle, individual
muscle bundles are wrapped by perimysium.
Within that nerve fascicle or nerve bundle
are collections of axons and each of those
axons are surrounded by the endoneurium. Here
is a section of a nerve cut in low magnification
on the left hand side and then at higher magnification
on the right hand side. And I think it is important
to know how to identify these coverings when
you are really looking at a real section of
the nerve. Again on the outside, this rather
green stained component is the epineurium.
And if you look within that, you can see a
number of greenish bundle circular profiles,
they represent nerve bundles or nerve fascicles.
And there are number of the making up this
peripheral nerve. Each of those nerve fascicles
remember, is surrounded by partly the epineurium,
but also the perineurium. The perineurium
divides these bundles into smaller components,
but also wraps up the outside of the bundle.
And again let me remind you that individual
axons are wrapped up by the endoneurium.
It is bit hard to see that in this section at
this magnification, but in the next
section, it will be clearer.
Here is an image of a nerve fascicle or a
nerve bundle at high magnification.
You can see on the very outside, a bright green stained
component, that is collagen. It is part of
the epineurium that penetrates from the outer
capsule wrapping around the whole nerve and
dividing the nerve into nerve bundles or nerve
fascicles as you see here. Each nerve fascicle
itself also has another connective tissue
component called the perineurium, labelled
here. It is a squamous type epithelial covering.
It is a very important covering around the
nerve bundle. It is a barrier. It protects
the nerve fibres and nerve axons. It is fluid
filled and this protection is very important
to make sure the pathogens and other toxins
etc do not access to the very vital structures
within the nerve fascicle, the nerve axons.
And around each axon is the endoneurium. You
can just see very small fibres, very small
green stained components wrapping themselves
around individual nerve axons. The axons shown
here are represented by little dark dots and
most of them have a white halo around them.
That white halo is where myelin is usually.
The myelin has been lost during processing
of this tissue so you do not see it. You just
see the space where that myelin should be, the
myelin sheaths around axons. So have a look
through this image and again see if you can see little
dots that represent the axons and the halo
of white around those axons, you are looking
there at myelinated axons. As we will see
later on, some of these axons are not myelinated
at all, we call them unmyelinated axons.
The other nuclei you see here are other
Schwann cells or they could be the nuclei
of endothelial cells belonging to capillaries.
But most of the nice little circular ones
you see in this section are the Schwann cells.
They are the supporting cells of the peripheral
nerve axons. They are the cells that lay down
in the myelin. Now axons, as I mentioned earlier
can be myelinated or unmyelinated.