Cutaneous Innervation – Cutaneous Innervation and Venous Drainage of Upper Limb

00:00 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.

00:13 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.

00:26 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.

01:06 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.

03:10 We can then see C8 little finger and medial forearm we can see it running up here.

03:16 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.

03:38 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.

04:06 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.

04:34 But yet, we have just go this limb bud here. We haven't actually got an arm yet.

04:41 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.

05:21 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.

05:34 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. And obviously 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.

06:58 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.

07:37 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.

08:03 We can see more details coming down on the screen on this side. Posterior cutaneous nerve of the arm apart from the radial.

08:11 The lateral cutaneous nerve of the forearm. Medial cutaneous nerve of the forearm.

08:15 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.