00:03
Well, I mentioned earlier that we are going to look
at the location of both motor neurons and
also sensory neurons in the spinal cord. So
let us move on and try and understand now
the structure of the spinal cord. Here is
a section on the left-hand side taken through
the spinal cor. And immediately you see that
there are two different stain components.
00:31
One called the gray matter is shaped a bit
like the letter H or it is bit like a butterfly.
00:39
From the outside of that butterfly, there
is a darker stain and darker blue stained component
we call the white matter. So the spinal
cord consists of two major components, gray
matter, and white matter. And it is important
to understand the difference between the two
and it seems a bit silly saying it gray and
white matter where in fact they are blue in
this section, but that basis, that naming
is because if you section through a spinal
cord in fresh tissue, then it appears
white and gray because the gray part, the
butterfly shape within the spinal cord is
where all the cell bodies are located of neurons,
interneurons and also the ventral horn or
motor neurons. Remember the sensory neuron
cell bodies are not located in the spinal
cord. And the reason why it looks gray relative
to the outer white region is because the white
matter consists of all the axonal processes
and some dendritic branches as well. And those
axons are myelinated. They are surrounded
by fatty tissue myelin that I will talk about
later on. So it looks white, fatty white material
compared to the less myelinated region in
the center of the spinal cord, the butterfly.
02:12
So that is where it get its name white matter
and gray matter. And of course if you stain
the spinal cord as we will see in subsequent
slides, you will see that this white and gray
matter are often blue, brown depending on
the stain used. But the first thing you should
understand now is that the internal butterfly
shape is the gray matter and the outside
area is called the white matter. If you look
very very carefully down in the bottom of
the section where I have labelled the ventral
horn, this is where the somatic motor neurons
live. If you look very carefully at this low
magnification picture, you might be able to
see some little bluey spots. They are the ventral
horn cells, the motor neurons you see on the
right-hand section in higher magnification.
Again they have a rather pale nucleus with
a prominent nucleolus, which was labelled
in the previous description a few slides back
and look also at the appearance of the soma.
Remember that dark clumped stained material
is nissil substance. Well, you can see the term
there dorsal horn and ventral horn.
03:41
They are referred to the wings of the butterfly, if
you like. Dorsal means the posterior part
of the spinal cord, ventral is the anterior
or front part of the spinal cord. And they
house two different components, two different
sorts of neurons. And it is very important
for you to understand what neurons are housed
in these different areas. The dorsal horn
is where the fibres, the axons from sensory
neurons enter into the spinal cord.
04:18
The cell body is located at some distance away in ganglia.
But when those axons receive information from
the periphery and it travels all the way
up into the ganglion cell, it is then transported
and enters the spinal cord through these dorsal
horn. Whereas the ventral horn houses cell
bodies of somatic motor neurons. Those that are
going to innervate skeletal muscles.
04:50
Those that we are consciously aware of. And it is very
important you know the differences of the dorsal
horn and the ventral horn and the sorts of
neurons that you find there. Of course if
you look under very high power, then you will see
cell bodies of other neurons in both of those
regions and they of course, will be the intercalated
or interneurons that we described earlier.
05:20
Well, if you look at some parts of the spinal cord
as well, you can just make out a very small
lateral horn. The lateral horn is a part of
the spinal cord, a part of the gray matter
that houses the cell bodies of the visceral
motor neurons, those ones that are going to innervate
smooth muscle. Those that are going to innervate
components in the body that we are not really
aware of. These visceral motor neurons are
part of the autonomic nervous system and I
will refer again to them later on. They are
housed in the lateral horn. So just have a
look at this image on the left hand side. Look
where the lateral horn is located and just
test yourself whether you have remembered
what sort of neurons are associated with the
dorsal horn and what sort of neurons are associated
with the ventral horn. As I said it is very
very important that we remember that because
it is going to be very important later on.
06:29
And on the right hand side you can see again
a very high magnification picture of one
of these visceral motor neurons, they are
not as big as the large ventral horns cells
we saw earlier. And just to the right of that
cell, you can see some little dark stained blue
areas. They are the axons, they are the myelinated
axons of the white matter and we will describe
that in more detail later on.