Brachial Plexus – Axilla and Brachial Plexus

00:01 Okay. So the remainder of this lecture, I want to talk about the brachial plexus. The brachial plexus is really, really important, and that it provides all of the somatic innervation and also some sympathetic innervation to the upper limb. It is a network of nerves that supply the upper limb.

00:21 So, motor fibres to make muscles contract, sensory fibres to receive sensation, and also it carries sympathetic fibres for the sweat glands for the arrector pili muscles. Importantly, the limbs do not receive power sympathetic innervation, only receive sympathetic. The brachial plexus originates in the posterior triangle of the neck, and it passes from here to the axilla.

00:49 It is formed from the anterior rami of spinal cord segments, C5 through to T1. And these are known as the roots of the brachial plexus. So here, we can see we’ve got C5, C6, C7, C8, and T1. And these are going to give rise to various branches that go on to the brachial plexus. So here we can see C5, C6, C7, C8, and T1. Okay? This one up here, we’ve got the C4 vertebra is coming down at C4, and then C5, C6, C7, C8, and T1. And these give rise to those nerves that go on to form the brachial plexus. And these are known as the roots of the brachial plexus. These roots pass towards the axilla by passing through a space between anterior and middle scalene muscles alongside the subclavian artery.

01:56 You might not need to fully appreciate the scalene muscles that’s really part of the head and neck anatomy course, but it passes between these muscles as it heads towards the axilla.

02:06 It enters the axilla like everything enters the axilla through the cervico-axillary canal alongside the subclavian artery. So here, we can see some anatomical relations of the brachial plexus. And this diagram again is relatively complicated. It is a lateral view on the left hand side. This is anterior down here, and this is posterior here. This is the clavicle.

02:37 The clavicle here which is being removed and this is going to be the first rib. So we can see we’ve got the subclavian vein here and we can see the subclavian artery passing down alongside these structures here which were in yellow, which are the brachial plexus.

02:53 The roots pass between the anterior and middle scalene muscle. So here is anterior scalene and then middle scalene behind. So these roots are passing down through here. And here is the point where they’re going to receive post-ganglionic sympathetic fibres via the grey rami. These are coming from the inferior and middle cervical ganglia. So these are giving rise to the sympathetic fibres that are destined for the upper limb. And this is one of the main reasons why I’ve actually included this diagram, just to show that this is where these fibres receive their sympathetic innervation. It’s then going to enter the axilla via the cervico-axillary canal, enter the axilla in here. Here, we can see the cervico-axillary canal. Here’s the first rib and here’s the cut clavicle that has been removed.

03:47 So, let’s now just concentrate on the contents of the brachial plexus, its constituent parts. So here, we can see a diagram showing the brachial plexus. And we need to go over various aspects of it. We remember the roots, C5 to T1. So here, we can see the roots, C5, C6, C7, C8, and T1.

04:12 These are those roots that are going to give rise to the brachial plexus. From these roots, they converge to form trunks. So we have three trunks. Three trunks are coming from these five brachial plexus roots. We can see we’ve got a superior trunk, a middle trunk, and an inferior trunk. We can see that C5 and C6 are going to join to form the superior trunk. C7 carries on without receiving anything to form the middle trunk. And C8 and T1 join to form the inferior trunk. Let’s look at this in the diagram. So, here we can see C5, C6, C7, C8, and T1. We can see C5 and C6 join to form the superior trunk. So C5 here and C6 join to form the superior trunk. C7 which is here carries on to form the middle trunk.

05:28 We then see C8 and T1. These two join to form the inferior trunk. And these are all above the clavicle. We call this the supraclavicular part of the brachial plexus because it’s above the clavicle. C5, C6, form the superior trunk. C7 forms the middle trunk. C8, T1 forms the inferior trunk. We’ve gone from five roots down to three trunks. This is all between the neck and the cervico-axillary canal. We’re still above the clavicle. If we go back, we can now see that each of these trunks gives rise to an anterior and posterior division.

06:21 It happens as we pass through the cervico-axillary canal. So as we pass through the cervico-axillary canal, these trunks, each one of them is dividing into an anterior and to a posterior division.

06:41 The anterior divisions are going to give rise to fibres that supply the flexor compartments of the upper limb, and these flexor compartments sit anteriorly. The posterior divisions are going to give rise to branches that supply the extensor compartments, and the extensor compartments are within the posterior aspect of the upper limb. So, each of these trunks is going to give rise to an anterior and to a posterior division. And this is happening as they pass through the cervico-axillary canal. So let’s have a look. We can see we’ve got the superior trunk here, and that is giving rise to an anterior division here. See here, the superior trunk is dividing into an anterior division. We can see it’s also dividing into a posterior division that’s going down here. If we look at the middle trunk, the middle trunk is giving rise to an anterior division. And this anterior division of the middle trunk is going to unite with the anterior division of the superior trunk. So let’s do that again. Superior trunk here gives rise to an anterior division, and that anterior division is joined by the anterior division of the middle trunk. So these two anterior divisions join. If we look at the posterior division of the superior trunk, that joins with the posterior division of the middle trunk. We can see that down here. We can also notice that it receives the posterior division of this inferior trunk. So we can now see that the posterior division of the inferior trunk is joining the posterior divisions of the middle and of the superior trunk. So, all three, posterior divisions from the superior, middle, and inferior trunks join. The anterior divisions of the superior and middle trunk join, and the anterior division, we can see here, runs down without running towards anything.

09:05 It doesn’t attach to other anterior divisions from the various trunks. It stays on its own.

09:13 Anterior division from superior trunk and from the middle trunk unite. Anterior division from the inferior trunk carries on. The posterior divisions from the superior, middle, and inferior trunks unite together. What this results in is a series of cords. The divisions coming from the trunks form three cords. So the anterior divisions of the superior and middle trunks form the lateral cord. The anterior division of the inferior trunk forms the medial cord, the posterior division from all three trunks from the posterior cord. These cords are named according to their position relative to the second part of the axillary artery. So let’s have a look at this in a diagram. We can now see we’ve gone from five roots, one, two, three, four, five, down to three trunks, superior, middle, inferior. Each one of those trunks gave rise to two divisions, an anterior and a posterior. And now, where the anterior divisions here are formed, they are formed this one. That is going to be known as your lateral cord. Here, the direct continuation of the posterior divisions is known as your posterior cord. And here, the continuation of the anterior division from the inferior trunk is known as the medial cord. And we can see with the axillary artery here, we have lateral to the axillary artery, the lateral cord; posterior to the axillary artery, the posterior cord; and medial to the axillary artery, the medial cord. Now these three cords, lateral, medial, and posterior, are going to give rise to the terminal nerves of the brachial plexus. Terminal nerves of the brachial plexus, specifically, the musculocutaneous, the median, the axillary, the radial, and the ulnar. We have three cords of the brachial plexus giving rise to a series of terminal branches. If we go back to the previous slide, we can now see here, we have the lateral cord that is going to split into two. It splits into two. One of them is the musculocutaneous. The other one joins with the splitting of the medial cord to form the median nerve. So we've got the musculocutaneous direct continuation of the lateral cord. We also have a union from the lateral cord and the medial cord to form the median nerve, and we have a direct continuation of the medial cord in the ulnar nerve. And this forms a very nice M-shaped arrangement. We can see we’ve got this M here. And this is characteristic of the terminal branches of the brachial plexus where the two cords, the lateral and the medial cord, give rise to three terminal branches: musculocutaneous, median, and ulnar. The posterior cord splits into two, a large one and a small one.

13:00 This is posterior to the axillary artery. It gives rise to the axillary nerve that passes out of the axilla via the quadrangular space, and the radial nerve which also passes out of the axilla via the triangular space. So we have these two coming from the posterior cord, and that gives us one, two, three, four, five terminal branches. So here we can see those terminal branches in a bit more anatomical detail. This is an anterior arm. It’s the right arm, and we can see the lateral cord here, we can see the medial cord here, and we can see the posterior cord which has been pulled out from underneath the axillary artery so we can see the axillary nerve passing out through here. But we can see this nice characteristic M-shaped again. So we've got musculocutaneous nerve here. We’ve got the median nerve here.

13:59 And we’ve got the ulnar nerve passing down here. Here, we can see from the posterior cord, we’ve got the axillary nerve and we’ve got the radial nerve. So, these five terminal branches coming from the brachial plexus.