Transcription

Structure and Location of the Brachial Plexus

This is a tutorial on the brachial plexus. The brachial plexus is the somatic plexus formed by the anterior rami of C5-C8 and most of the anterior ramus of T1.

 

The brachial plexus as you can see, it originates in the neck. And then it passes over the 1st rib. You can see that here. And then it goes underneath the clavicle to enter the axilla.  We’re going to take a look at some of the structures in relation to the brachial plexus. We're also going to take a look at the basic layout of the brachial plexus.

 

Immediately exiting the intervertebral foramen, you've got the parts of the brachial plexus which are referred to as the roots.

 

We're going to switch over to a diagram, which you'll see over and over again. This is used to explain and demonstrate the basic layout of the brachial plexus.

 

This is a diagram you need to familiarize yourself. It gives you the basic concept of brachial plexus.  proximally here, we're looking at the roots of the brachial plexus. You can see the numbers on the right. These number the spinal nerves which gives rise to these roots. Like I mentioned, the brachial plexus is formed from the ventral rami of the spinal nerve C5-C8 and most of the anterior ramus of T1. These are called roots.

 

And then these roots form trunks.  C5 and C6 converge to form the superior trunk. C7 continues to form the middle trunk. And the C8 and T1 form the inferior trunk.

 

Then next, each trunk gives rise to an anterior and a posterior division.  the superior trunk has an anterior and a posterior division. The middle trunk has an anterior and posterior division. And so does the inferior trunk, a posterior and an anterior division.

 

And then these divisions give rise to cords.  You’ve got the lateral cord, the posterior cord and the medial cord.  you've got roots, trunks, divisions and cords.

 

You'll notice that there are several different branches coming off the various parts of the brachial plexus. You've got nerves coming off the roots, you've got nerves coming off the trunk and you'll notice that there's no nerves coming off the divisions, but then you've got nerves coming off the cords and then you've got terminal nerves.

 

I remember the roots and trunks by just thinking of a tree. A tree starts from the root, then you've got the trunk.  the roots are the first thing of the brachial plexus and then you've got the trunk. And then a tree divides, you’ve got the divisions.  you've got the anterior and posterior divisions after the trunks. And then you've just got to remember that cords are at the ends. I don’t have any special way of remembering that.  you've got the lateral, posterior and medial cords at the end.

 

So just coming back to the 3D model, I've dissected away all the additional nerves that come off the brachial plexus and you can see the basic structure with the roots, the trunks, the divisions lie under the clavicle which I've removed ( the clavicle would run between this sternum and the acromion process of the scapula) and then you've got the cords further down –  around here.

 

The roots and the trunks lie in the posterior triangle of the neck. They pass between the anterior scalene and the middle scalene muscle and they run posterior and superior to the subclavian artery.

 

You can see this posterior triangle here. The posterior border is the anterior part of the trapezius, the anterior border is the posterior border of the sternocleidomastoid and the base is the middle third of the clavicle.  the brachial plexus, the roots and the trunks emerge from this posterior triangle.

 

If I just rotate the model around, I can just show you the anterior and middle scalene muscles. These muscles attach to the 1st ribs. We've got the anterior scalene here and the middle scalene here. The roots and the trunks run between the anterior and the middle scalene muscle.

 

Then the brachial plexus then passes underneath the clavicle and over the 1st rib.  Behind the clavicle, you get the divisions of the brachial plexus.  you get the anterior and posterior divisions of the trunks.

 

And then you've got the cords, which lie in the axilla – this region here. We've got the lateral, the posterior and medial cord lying in the axillary region. They've got an important relationship with the axillary artery.  the cords lie around the second part of the axillary artery.

 

This diagram here shows it quite nicely. You've got the axillary artery here. You've got the lateral cord running lateral to the axillary artery. The medial cord, you can just see here. It runs medial to the axillary artery. And the posterior cord runs behind the axillary artery.

 

Just to illustrate the point about the axillary artery, let's just take a look at this diagram.  you've got the C5, C6, C7, C8, T1 roots. C5 and C6 unite to form the superior trunk. C7 continues to form the middle trunk. And C8 and T1 unite to form the inferior trunk.

 

Then you've got these divisions.  Each trunk has an anterior and posterior division.  the superior trunk has an anterior division, which continues like this. The posterior division goes posteriorly to form the posterior cord. The middle trunk has an anterior division which contributes to the lateral cord.  this passes anteriorly. The posterior division contributes to the posterior cord. And then the  inferior trunk has a posterior branch which contributes to the posterior cord. It's got an anterior branch which continues to form the medial cord. But all of the posterior branches from the three trunks go towards forming the posterior cord.

 

Let’s just draw on the axillary artery.  the axillary artery likes this. I'm just roughly drawing it on. And then it goes up to form the subclavian.  The posterior cord is shown in brown because it lies behind the lateral and the medial cord posteriorly.  let's just kind of fill it in. You can see that the middle cord lies medial to the axillary artery and the lateral cord lies lateral to the axillary artery, whereas the posterior cord with its formed from the posterior divisions of the three trunks runs behind the axillary artery.

 

That should have given you a basic understanding of the structure of the brachial plexus and its location in relation to the other areas of the body. You know that the roots and the trunks pass through the posterior triangle of the neck. And then you've got the divisions which lie under the clavicle. And you've got the relationship with the posterior cord to the axillary artery and the lateral and medial cords sitting on either side, lateral and medial to the axillary artery.

 

Next, we'll take a look at the various branches of the brachial plexus, the terminal nerves and the branches which comes off the roots and the trunks and the cords.

 

Branches of the Brachial Plexus

In this tutorial, we're going to look at the branches of the brachial plexus. This follows on from the first tutorial I did which outlined the location and the anatomical relations for brachial plexus.

 

You can see here, I've got a dissected brachial plexus. We've got the roots, C5, C6, C7, C8, T1 here. We've got the trunks, the divisions lying under the clavicle. And then you've got the cords distally.

 

We’ve got this nice diagram here. We've got the roots, C5-T1 here. We've got the trunks – the superior, the middle and the inferior trunk. We've got the divisions.  each trunk divides into an anterior and a posterior division. And then we've got the cords.

 

Coming off the roots, we've got two nerves that you need to know. The one up here is called the dorsal scapular nerve. You can see its origin here on the C5 root.  the dorsal scapular nerve innervates the rhomboid muscles. You can see the rhomboid muscles here, the minor and major (rhomboid minor and major) attaching from the spinous processes to the medial border of the scapula. That's innervated by the dorsal scapular nerve.

 

The other nerve that you need to know that comes from the roots is called the long thoracic nerve. This originates from three roots – C5, C6 and C7. It innervates the serratus anterior muscle. This is the serratus anterior muscle, which attaches to the ribs and the costal surface of the scapula.

 

From the roots, you've got the dorsal scapular nerve innervating the rhomboids and you've got the long thoracic nerve innervating the serratus anterior muscle.

 

Next, we come onto the branches of the trunks of the brachial plexus.  just like the roots, there's only two nerves you need to remember and also, they come from the upper part of the brachial plexus. The two muscles you need to know both begin with S and they both come from the superior trunk. I just remember them as the S-nerves.  you've got 'superior trunk' and you've got two nerves beginning with S. You've got the suprascapular nerve and you've got the nerve to the subclavius.  Subclavius and suprascapular nerve.

 

The nerve to the subclavius comes from the superior trunk and it innervates the subclavius muscle as the name suggests. The subclavius muscle is this little muscle here which sits between the first rib and the clavicle. It originates on the 1st rib and inserts onto the lower part of the clavicle.

 

The other nerve is the suprascapular nerve which you can see coming off here. This nerve innervates the supraspinatus and infraspinatus.  The suprascapular nerve is called the suprascapular nerve because it supplies muscles which sits on top of the scapula,  they're supra+scapula.

 

You can see here you've got the supraspinatus above the spine of the scapula and the infraspinatus sitting below the spine of the scapula. These muscles are innervated by the suprascapular nerve and these two muscles are part of the rotator cuff group of muscles.

 

Next, we've got the divisions of the brachial plexus.  you've got the trunks dividing into anterior and posterior divisions and the divisions don’t have any branches coming off them.

 

After the divisions, you've got the cords which are formed from the divisions joining together.  the superior trunk gives off an anterior division which becomes the lateral cord when combined with the anterior division of the middle trunk.

 

The posterior cord is quite easy to remember because it's right at the back,  it receives all the posterior divisions of the three trunks.  you can see this division here coming off the posterior division of the superior trunk, the posterior division of the middle trunk and the posterior division of the inferior trunk coming together to form the posterior cord.

 

The medial cord is formed by the anterior division of the inferior trunk. The posterior cord is formed by the contribution of the posterior branches from all three trunks. And the lateral cord is formed by the anterior branches, anterior divisions of the superior and middle trunk.

 

It’s useful to get your hand around these anterior and posterior divisions because the anterior divisions give rise to cords which are associated with the anterior compartment and the posterior divisions are associated with the posterior cord which is associated with muscles of the posterior compartment.

 

Because the lateral and medial cords are formed by anterior divisions, they're associated with muscles to do with the anterior compartment. And because the posterior cord is formed from posterior divisions, it's associated with muscles of the posterior compartment.

 

What I'm going to do now is before talking about the nerves which come off the sides of the cords, I'm going to talk about the terminal nerves. You've got five terminal nerves, which is where these cords terminate. There's a simple way of remembering these terminal nerves with a mnemonic, which is fairly memorable. Its 'my auntie raped my uncle'. It's not the cleanest mnemonic, but it helps to remember things.

 

The first nerve is the musculocutaneous nerve, which comes off the lateral cord. Then you've got the axillary and radial nerve, which come off the posterior cord. You've got the median nerve, which is formed by two branches which comes off the medial and lateral cords. They unite to form the median nerve. And then you've got the ulnar nerve.  'my auntie raped my uncle' – musculocutaneous, axillary, radial, median, ulnar.

 

Now you know the terminal nerves of the cords. We can now look at some of the branches that come off the sides of the cords.

 

We'll start with the lateral cord. You know from the mnemonic that the first terminal nerve is the musculocutaneous nerve. You also know that the median nerve is formed by branches that come off the lateral and the medial cords.  as well as the musculocutaneous nerve, you've got the lateral root of the median nerve.

 

And then you've got this little nerve here coming off the lateral cord. This is the lateral pectoral nerve. This innervates the pectoralis major muscle. The pectoralis major muscle is this large muscle here.

 

Now we'll talk about the branches coming off the medial cord.  again, you know that the median nerve is formed by the union of branches that come off the medial and lateral cords. The lateral cord provides the lateral root of the median nerve; the medial cord therefore provides the medial root of the median nerve.

 

You also know from the mnemonic that the ulnar nerve comes is the terminal nerve of the medial cord.  you already know that we've got the medial root of the median nerve and we've got the terminal ulnar nerve.

 

We’ve got a few other nerves that you can see. We've got three other nerves. These are pretty straightforward to remember because they're the M-nerves. Remember the S-nerves on the superior trunk? Now we've got the M-nerves on the medial cord. It's quite easy because you know that there was a pectoral nerve on the lateral cord, the lateral pectoral nerve. We've got the medial pectoral nerve on the medial cord.  the medial pectoral nerve is the most proximal nerve coming off the medial cord.

 

And then you've got two nerves to do with sensation to the skin.  you've got cutaneous nerves. You've got a cutaneous nerve to the upper arm and you've got a cutaneous nerve to the forearm.

 

The arm comes before the forearm. It's more proximal than the forearm, the more proximal nerve is the medial cutaneous nerve of the arm. And then after that, you've got the medial cutaneous nerve of the forearm.  You might also hear these nerves referred to as the medial brachial cutaneous nerve and the medial antebrachial cutaneous nerve.  'Brachial' just refers to arm and 'antebrachial' means forearm.

 

This diagram here just shows the sensory distribution of these nerves.  The medial cutaneous nerve of the arm supplies the skin over the medial side of the arm.  you can see this thin strip of innervation here in yellow. The medial cutaneous nerve of the forearm has a larger distribution where it innervates the skin.

 

That should be quite straightforward. From the mnemonic, you know you've got the medial root of the median nerve and you've got the ulnar nerve, which are the terminal nerves. And then you've got the three M-nerves.  you've got nerves to do with the pectoral muscle and you've got nerves to do with the supply to the skin, the sensory supply to the skin, the cutaneous nerves.

 

With the posterior cord, there's a nice mnemonic for remembering the five branches. From the mnemonic before, 'my auntie raped my uncle', you know that the axillary and the radial nerves come off the posterior cord.  you've got the axillary nerve here and the radial nerve here.

 

The other mnemonic for remembering nerves from the posterior cord is ULTRA.  you've got the upper subscapular nerve, the lower subscapular nerve, the thoracodorsal nerve and then you've got the radial and axillary nerves.

 

First, we've got the upper subscapular nerve or the superior subscapular nerve. Then we've got the thoracodorsal nerve. Then distally here, we've got the lower subscapular nerve. And then the axillary and the radial nerves.

 

Remember we saw the suprascapular nerve coming off the superior trunk here. Now, we've got the subscapular nerves. The suprascapular nerves innervated muscles which sat on top of the scapula, now the subscapular nerves innervate muscles to do with the underside of the scapula.

 

The upper subscapular nerve supplies these subscapularis muscle. The subscapularis muscle is this muscle which sits underneath the scapula and internally rotates the humerus. And then you've got the teres major muscle, which also medially rotates the humerus.

 

The upper subscapular nerves supplies the subscapularis and the lower subscapular nerves innervates the subscapularis and the teres major muscle.

 

The thoracodorsal nerve innervates this large muscle, which I've just added on to the model. This is the latissimus dorsi muscle. This adducts, extends and medially rotates the humerus.

 

That’s the branch of the posterior cord. Remember the mnemonic, ULTRA. U is upper subscapular (that's the most proximal branch of the posterior cord); L is lower subscapular; that comes after the thoracodorsal, which is the T part (T is thoracodorsal); and then you've got RA, radial and axillary. That's ULTRA.

 

Terminal Branches

I'm going to just quickly run you through some of the functions of the large terminal nerves.  We'll start off with the musculocutaneous nerve which comes off the lateral cord.

 

This nerve is responsible for innervating the three flexor muscles of the anterior compartment of the arm.  We’ve got the biceps brachii muscle, the brachialis muscle and the coracobrachialis muscle.

 

The ulnar nerve is responsible for innervating all intrinsic muscles of the hand except for the thenar muscles and the lateral two lumbricals, which are innervated by the median nerve.

 

As the ulnar nerve passes through the forearm into the hand, it innervates two muscles in the forearm. It innervates the flexor carpi ulnaris muscle and the medial half of the flexor digitorum profundus muscle.  I've just isolated those muscles on this model. You can see the flexor carpi ulnaris muscle and you can see the flexor digitorum profundus muscle.

 

In the hand, the ulnar nerve is responsible for innervating all the intrinsic muscles except for the thenar muscles.  We’ve got the abductor pollicis brevis, the flexor pollicis brevis and the opponens pollicis. And also, you've got the lateral two lumbricals here.

 

You’ve got the thenar group of muscles here and you've got the lateral two lumbricals over here and these are innervated by the median nerve, but all the other intrinsic muscles are innervated by the ulnar nerve.

 

In terms of sensory innervation, the ulnar nerve provides innervation to the medial one and a half finger.  You’ve got the little finger and half of the ring finger. And you've also got innervation provided over the medial aspect of the hand and the wrist, on the palmar surface.

 

And on the dorsal surface of the hand, you can see the area of innervation provided by the ulnar nerve.  You’ve got the medial part of the dorsal surface of the wrist and hand innervated by the ulnar nerve.

 

The median nerve is formed by the union of these roots that come from the lateral and medial cords.  You’ve got the lateral root from the lateral cord and the medial root from the medial cord. They unite and the median nerve is formed anterior to the last part of the axillary nerve.

 

This diagram illustrates it quite nicely.  You’ve got the lateral cord and you've got the medial cord. You've got the lateral and medial roots of the median nerve uniting in front of the axillary artery to form the median nerve.

 

The median nerve innervates all the other muscles of the anterior compartment which the ulnar nerve doesn't innervate.  It innervates all the muscles except the flexor carpi ulnaris and the medial half of the flexor digitorum digitorum profundus muscle.

 

And in the hand, you know which muscles are innervated because it's the ones that are not innervated by the ulnar nerve.  You’ve got the thenar muscles and the lateral two lumbricals, which are innervated by the median nerve.

 

In terms of sensory innervation, you can see the distribution of innervation in pink here.  The median nerve innervates the lateral 3 ½ digits.  You’ve got the thumb, index, middle and half of the ring. And then you've got the lateral surface of the wrist and the hand.  This distribution of sensory innervation provided by the median nerve.

 

Coming to the terminal nerves of the posterior cords, you've got the axillary nerve and the radial nerve.  The axillary nerve provides innervation to three muscles.  You’ve got the deltoid muscle here. You've got the teres minor muscle. And also, it innervates the long head of the triceps brachii muscle.

 

And in terms of sensory innervation, you've got a branch which comes off the axillary nerve called the superior lateral cutaneous nerve of the arm. This nerve innervates the area of skin and the deltoid region.  You can see this area of innervation in blue here. And on the posterior surface, it has this distribution of sensory innervation.

 

Just while we're on this image here, I forgot to mention earlier that the musculocutaneous nerve actually terminates as the lateral cutaneous nerve of the forearm. You can see this area of sensory innervation laterally on the forearm as the name suggests.  This is an anterior view. Posteriorly, you can see this area of innervation.  That’s the lateral cutaneous nerve of the forearm.

 

The final nerve is the radial nerve.  This is the largest terminal branch of the posterior cord. This nerve innervates all the muscles in the posterior compartment of the arm and the forearm.

 

It also provides innervation to the skin on the posterior surface of the arm and forearm. You can see this distribution in pink here.  Branches of the radial nerve innervate the skin on the back of the arm and forearm. And also on the dorsal surface of the hand, you've got this area of innervation.