So now let’s move on to cutaneous
innervation. Now you’re all aware
that the surface of the skin, the entire surface of the
skin throughout the entire body has cutaneous nerve supply.
That is, there are nerves that innervate the surface
of the skin, so that when we touch ourselves,
when somebody else touches us, we know that
is happening, we are consciously aware of it.
And we have this sensory somatic fibres, which are returning
this sensory information, be it touch, pain, temperature
or pressure, back to the central nervous
system.But, this isn’t just say one
nerve that does it, an upper limb nerve.
We have a whole series of what are known as
dermatomes, specific regions of the body
that receive a specific
spinal cord segment spinal nerve.
So remember the spinal cord is divided
into those various segments, we can have
C1, C2, C3, C4 etc spinal cord segments.
Each one of those segments is going to give rise
to two spinal nerves. Remember this happens bilaterally,
on the left and the right side. And running within
those spinal nerves are going to be sensory fibres
that are taking information from the surface of theskin back to that specific
spinal cord segment. So for example, we can see here
we have the cutaneous innervation. We can see here
of the upper limb, on anterior view of the right
and the posterior view of the right. And we can see
that the upper limb has a nerve supply from the
C3, C4, C5, C6, C7, C8
cervical spinal cord segments.
So these segments of the
spinal cord are going to receive sensory innervation comin
from these respective parts of the body. So if someone was to
hold your thumb, that information
would pass to the sixth cervical
spinal cord segment. If they have to touch your shoulder
it’ll go to either C5 or C4 depending on which
bit they touch. So we have a map on our upper limb which tells the
spinal cord where that sensation is coming from. See we have got C3
all the way down to T3 here in the axilla.
And here we can see C3 and C4 is the base
of the neck and the superior aspect of the shoulder. Here
we can see we have got C5, which is the lateral part
of the arm. We can see we have got C6, which is the
lateral forearm and the thumb. We can see this on
both the anterior aspects and we can see it
on the posterior aspect as well. If we look to C7,
we can see that is the middle three fingers
of digits 2, 3, parts of 4. And also
we can pick it up, follow it back all the way along
this posterior aspect of the upper limb as well.
We can then see C8 little finger and
medial forearm we can see it running up here.
T1, we can see its location, T2 and T3.
Now this may seem
some random arrangements. It may appear confusing. But actually
if you see, it actually follows quite a logical pattern
if you hold the arm in this orientation.
C3, 4, 5, 6, 7, C8.
And then we now move on to the
Thoracic spinal cord segments. So T1, T2, T3
and then running down the chest would be 4,5,6,7,8 etc.
And this all due to a developmental
process. And if we look at the development of the upper limb.I’m not
going to go into this in much detail. But we can see the developing embryo
on this side of the slide. We can see the
developing embryo in the pictures at various stages.
And what we can see is that we have here quite
superiorly on the lateral surface of the developing embryo,
we have a limb bud. And this limb bud is going to be your
upper limb. We can see here we have got the prominent spinal cord
which is forming and if we compare that
to this diagram, we can see that the
spinal cord segments have already
passed out to their respective dermatome.
But yet, we have just go this limb bud here.
We haven't actually got an arm yet.
We haven't got a forearm. We haven't got a hand.
We have just got this little bud. But we can see that we have got the C5,
C6, C7, C8 arrangements already in place.
And all is going to happen
is that the arm is going to develop.
The forearm is then going to develop and then
the hand is going to develop. And as these structures
developed, so they take with it their spinal cord
segments innervation. So we can see that a
the limb bud develops from this lateral protrusions,
we have a very predictable arrangement of the dermatomes.
We can see that the thumb positioned cranially here
pointing up towards the head region. This is forming the lateral surface,
and then the upper limbs medial
surface down here is more caudal.
So we have a cranial aspect of the upper limb,
we have a caudal aspect of the upper limb. We have a
superior aspect, we have an inferior aspect. And as the dermatomal
distribution is already lined out here, we can see that its just going to
follow that path. So its not just a random
arrangement, it is actually predictable
due to this developmental process.
So we can see C3, C4, C5,
C6, C7, without C8 here and then T1 and T2.
we correlate that to the diagram we saw previously.
So we have
looked at areas of the upper limb that receive
innervation from the spinal cord segment,
the dermatomes. But what actual nerves
supply the dermatomes because these are
named. The dermatomes are supplied by cutaneous nerves and
these are mostly derived from the brachial plexus. The brachial plexus
will cover in great detail in the next lecture.
It is coming from segments C5, C6, C7,
C8 and T1. Segments, spinal cord segments give rise
to a whole series of nerves that form the brachial plexus
and may have a whole series of named branches.
And what you can see is that here we’ve got a
very similar map to the dermatomal distribution
from the previous slides. Here we have got a right
upper limb anterior view, we have got a right upper limb
posterior view. And you can see that the various nerves that supply the
dermatomes are named and these can be quite confusing. So, we can see we have
up on this regions some supraclavicular nerves.
These supply this region
which is the region above the clavicle, so supraclavicular.
We have the radial nerve here. We have the musculocutaneous nerve.
We have the median nerve, the ulnar nerve, all supplying
their respective parts of the upper limb by from this
anterior surface and musculocutaneous nerve,
radial, ulnar supplying on the posterior surface.
So these are the actual nerves that supply
the dermatomes. They give the
dermatomes the sensory distribution.
We can see more details coming down on the screen on this side.
Posterior cutaneous nerve of the arm apart from the radial.
The lateral cutaneous nerve of the forearm.
Medial cutaneous nerve of the forearm.
Posterior cutaneous nerve of the forearm.
So all of the aspect of the forearm is covered via
this cutaneous nerves that are coming from specific places.
Coming from the musculocutaneous nerves, coming from
the brachial plexus, directly coming from
radial nerves here or the radial nerve
or the intercostal nerves.
So, all I think is important that I go through these
in painstaking detail. Its an important
reference to see that the dermatomes
that cover the body are receiving
their sensory information via
these specific cutaneous nerves.
And we will see these in more detail when we look at the
brachial plexus and see where they are come from.