Structure of the brachial plexus, including the roots, trunks, cords and branches.
Hello everyone! It's Megan from Kenhub here, and welcome to our tutorial on the brachial plexus. In today's tutorial, we'll talk about the anatomy and function of the brachial plexus which is this structure seen here highlighted in green. In stages, we'll describe all the parts of this important plexus as well as some interesting clinical facts relating to it.
So, the brachial plexus is a somatic plexus that is formed by the anterior rami of the four lower cervical nerve roots and the first thoracic nerve root. This plexus is of particular significance due to the fact that all the major nerves that innervate the muscles, joints and skin of the upper limb originate from the brachial plexus. So, as you can imagine, it's of great clinical importance to know how all these nerves are formed and why a lesion in a specific nerve root could cause a symptom to one of the nerves of the upper limb. Towards the end of this tutorial, we'll look at the symptoms that come about as a result of lesions of the brachial plexus.
Before we explore the different parts of the brachial plexus, first let's talk about where it's located. At its origin, the brachial plexus lies in the posterior triangle of the neck in the angle between the clavicle and the lower posterior border of sternocleidomastoid. In this image on the left, sternocleidomastoid has been removed to give us a clear view of the plexus. As we can see in this image on the right, the brachial plexus passes posterior to the medial 2/3 of the clavicle and is also located posterior to the suprascapular vessels. It then travels downwards and comes to lie on the subscapularis muscle which we can see here and the first digitation of serratus anterior which we can see here.
Now that we're familiar with the location of this plexus, let's look at the different parts that make up the brachial plexus. When we look at this illustration on the right, we need to imagine that a cut has been made to the lateral parts of the vertebral arches of the cervical and thoracic vertebrae in order to see the brachial plexus from an anterior view. So here we can see the transverse vertebral arch of C5, C6, C7 and T1. We can also see from this image that the brachial plexus has numerous branches and is actually very complicated. Due to this complexity, it's been theoretically broken down into five anatomical sections which include five roots, three trunks, six divisions, three cords, and five terminal branches. There are also several pre-terminal branches which leave the plexus at various regions along its length.
In order to give you a more simple view of the brachial plexus, I want to show you this sketch here which shows a basic line drawing of the brachial plexus. Again, this illustration can appear quite daunting so we're going to break it down into the five sections we discussed in the previous slide. So first we have the roots then we have the trunks then we have the divisions followed by the cords and finally we have the terminal branches.
The roots consist of C5 to 8 and T1. The trunks are the superior, middle and inferior trunks which then divide off to give anterior and posterior divisions. In this illustration, the anterior divisions are color-coded black and the posterior divisions are color-coded grey. These divisions then contribute towards the lateral, posterior and medial cords. And, finally, we have the five terminal branches – the musculocutaneous nerve, the axillary nerve, the median nerve, the radial nerve, and the ulnar nerve.
A simple mnemonic to help you remember the sections of the brachial plexus is Real Texans Drink Cold Beer. The R stands for roots, the T for trunks, the D for divisions, the C for cords, and the B for branches. Over the course of this tutorial, we're going to explore all five sections of the brachial plexus starting with the roots.
As we could see in the line diagram, the roots are the first section of the brachial plexus and they arise from the ventral rami of the last four cervical spinal nerves and the first thoracic spinal nerve. The first root arises from the fifth cervical spinal nerve which emerges above the fifth cervical vertebra. So in this illustration, we can see the fifth cervical spinal nerve and we can see that it's sitting above the fifth cervical vertebra. The next root is the continuation of the sixth cervical spinal nerve which also emerges above the sixth cervical vertebra. The same is also happening with the seventh cervical spinal nerve which gives rise to the next root of the brachial plexus.
The eighth cervical nerve which you can now see highlighted in green emerges above the first thoracic vertebra which we can see here. The final root of the brachial plexus arises from the first thoracic spinal nerve which emerges below the first thoracic vertebra. It's important to note here that all of the following spinal nerves starting with the first thoracic spinal nerve emerge from the vertebral canal below their respective vertebrae. This trend occurs because there's eight cervical spinal nerves but only seven cervical vertebrae. So let's look at the second section of the brachial plexus which is the trunks.
The three trunks of the brachial plexus originate from the roots and pass laterally over the first rib to enter the axilla. The first of the three trunks is the superior trunk which we can see here highlighted in green. This trunk is formed by the union of the fifth and sixth cervical spinal nerves. Two pre-terminal nerves arise from the superior trunk of the brachial plexus and they include the subclavian nerve or nerve to subclavius and the suprascapular nerve. So in this illustration, we can clearly see that C5 and C6 unite to give the superior trunk.
The second trunk of the brachial plexus we're going to see is the middle trunk. This trunk originates from the seventh cervical spinal nerve C7. The final trunk of the brachial plexus is the inferior trunk. Like the superior trunk, it's composed of two nerves, the eighth cervical spinal nerve and the first thoracic spinal nerve also known as C8 and T1. Just to recap, so far, we've covered the roots and the trunks of the brachial plexus. Next, we're going to look at the divisions of this plexus. These divisions are bifurcations of the trunks and are the third section of the brachial plexus.
As you can see in this diagram, each of the trunks which we've described previously give off an anterior and a posterior branch. The three anterior divisions form parts of the brachial plexus that ultimately give rise to the peripheral nerves associated with the anterior compartments of the arm and forearm whereas the three posterior divisions combine to form parts of the plexus that give rise to the nerves associated with the posterior compartments.
The cords follow on from the divisions as the fourth section of the brachial plexus. The cords are named in relation to their position around the axillary artery so the lateral cord is located lateral to this artery, the posterior cord posterior to this artery and the medial cord medial to this artery. So in this line drawing, we could see that the lateral cord is formed by the anterior divisions of the superior and middle trunk. We can then see that the posterior cord is formed by the posterior divisions of all three trunks. Finally, we have the medial cord which is basically a continuation of the anterior division of the inferior trunk. The lateral, medial and posterior cords now divide into their subsequent nerve branches and continue distally down the upper limb to innervate various anatomical structures.
So now let's look at the three cords individually. The first one which we can see on the left is the lateral cord. This cord is formed by the union of the two anterior divisions of the superior and middle trunks. In the middle, we have the posterior cord which is formed by the union of all three posterior divisions. And, finally, this illustration on the right is the medial cord which is the continuation of the anterior division of the inferior trunk. These cords give rise to very important nerves of the upper extremities namely, the musculocutaneous nerve, the axillary nerve, the median nerve, the radial nerve, and the ulnar nerve. So now let's briefly discuss these nerves.
The musculocutaneous nerve arises from the lateral cord and therefore contains fibers from the spinal roots C5, C6 and C7. It innervates the coracobrachialis muscle, the brachialis muscle and the biceps brachii muscle. It later becomes the lateral cutaneous nerve of the forearm. The next terminal branch of the brachial plexus we're going to talk about is the axillary nerve which arises from the posterior cord. It carries fibers from the fifth and sixth cervical spinal roots C5 and C6. It passes posteriorly around the surgical head of the humerus and innervates both the deltoid muscle and the teres minor muscle. In addition, it gives off the superior lateral cutaneous nerve which provides sensory innervation to the skin of the upper posterior arm.
In the next illustration, we can see the axillary nerve highlighted in green. As you can see in this illustration, deltoid isn't present, however, we can see the branches that would otherwise supply this muscle and we can also see branches supplying teres minor.
The next nerve we'll talk about is the median nerve. This nerve is formed by the union of the lateral and medial cords of the brachial plexus therefore it contains fibers from the spinal nerve C5 through to T1. It passes into the upper limb anterior to the brachial artery and through the arm into the forearm. The motor part of the median nerve innervates the majority of the flexor muscles in the anterior compartment of the forearm. The muscles it doesn't innervate in this compartment are flexor carpi ulnaris and part of flexor digitorum profundus – both of which are innervated by the ulnar nerve. The median nerve also supplies innervation to the thenar muscles and the lateral two lumbricals in the hand. The sensory part of the median nerve gives rise to the palmar cutaneous branch which innervates the lateral part of the palm. It also gives rise to the digital cutaneous branch which innervates the lateral three and a half fingers on the palmar surface of the hand.
The largest terminal branch of the posterior cord is the radial nerve. It contains fibers from the fifth cervical to the first thoracic spinal nerves. It passes out to the axilla into the posterior compartment of the arm where it innervates the following muscles – triceps brachii, supinator, anconeus, brachioradialis, and the extensor muscles of the forearm. In addition, it provides sensory innervation to the skin on the posterior aspects of the arm and forearm, the lower lateral surface of the arm and the dorsal lateral surface of the hand.
As we can see here, the last terminal nerve of the brachial plexus is the ulnar nerve. The ulnar nerve is a large terminal branch of the medial cord, however, near its origin, it often receives a communicating branch of the median nerve carrying fibers from the seventh cervical spinal nerve. So this nerve has fibers from the eighth cervical spinal nerve, the first thoracic spinal nerve and, frequently, from the seventh cervical spinal nerve. The ulnar nerve innervates the following muscles – flexor carpi ulnaris, the two medial bellies of flexor digitorum profundus, the intrinsic muscles of the hands apart from the thenar muscles, and the two most lateral lumbrical muscles. It also provides sensory innervation to the anterior and posterior surfaces of the medial one and a half fingers and the associated palm area.
So until now, we've seen and described the sections of the brachial plexus as well as the main terminal branches of this plexus. But in addition to these nerves, the brachial plexus gives off some other nerves that arise from different sections of the plexus. These are known as pre-terminal nerves. A pre-terminal branch that we won't cover in the next few slides is the branch to the phrenic nerve. This contributing branch originates from the roots of the brachial plexus and stems from the fifth cervical spinal nerve. Unlike other branches of the brachial plexus, it doesn’t supply the upper limb and that's why it won't be considered further in this tutorial. So let's have a look at the other pre-terminal branches of the brachial plexus.
The first nerve we'll look at is the dorsal scapular nerve which arises from the roots of the brachial plexus. It carries fibers from the fourth and fifth cervical spinal nerves C4 to C5 and innervates the rhomboid and levator scapulae muscles. Another important nerve that arises from the roots is the long thoracic nerve. It originates from the anterior rami of the fifth to seventh cervical spinal nerves. It passes vertically down the neck through the axillary inlet and innervates the serratus anterior muscle.
The next nerve we'll talk about is the suprascapular nerve which arises from the superior trunk. It passes laterally through the posterior triangle of the neck and enters the posterior scapular region. As we can see in this illustration, it carries fibers from the fifth and sixth cervical spinal nerves C5 to C6. It innervates both the supraspinatus muscle and the infraspinatus muscle. Also arising from the superior trunk is the subclavian nerve. The subclavian nerve is a small nerve we can see here that arises from the fifth and sixth cervical spinal nerves C5 to C6. As the name suggests, this nerve innervates the subclavius muscle.
So far, we've seen the musculocutaneous nerve arise from the lateral cord of the brachial plexus, however, there are also other branches that arise from this cord such as the lateral pectoral nerve. This nerve passes anteriorly together with the thoracoacromial artery. It mainly innervates the pectoralis major muscle and partially innervates the pectoralis minor muscle. On the other hand, from the medial cord, the corresponding nerve is the medial pectoral nerve. Conversely, this nerve mainly innervates the pectoralis minor muscle and partially innervates the pectoralis major muscle.
The next nerve we'll look at is the medial brachial cutaneous nerve otherwise known as the medial cutaneous nerve of the arm. This nerve originates from the medial cord and innervates the upper part of the medial surface of the arm and the floor of the axilla. It's the first of only two nerves of the brachial plexus that serve entirely as cutaneous nerves. The second of the two nerves that serve entirely as cutaneous nerves is the medial antebrachial cutaneous nerve which is also called the medial cutaneous nerve of the forearm. As we can see in this image, this nerve originates just distal to the origin of the medial cutaneous nerve of the arm which we can see here. It passes out to the axilla and then continues down the arm providing innervation to the skin over the medial surface of the forearm.
The next pre-terminal branch of the brachial plexus we'll look at is the superior subscapular nerve. This nerve originates from the posterior cord and carries fibers from the fifth and sixth cervical spinal nerves. It functions to provide innervation to the upper part of the subscapularis muscle. Another nerve that arises from the posterior cord is the inferior subscapular nerve. This nerve carries fibers from the fifth and sixth cervical spinal nerves and provides innervation to the lower part of the subscapularis muscle as well as teres major.
The next nerve we're going to look at is the thoracodorsal nerve which is also known as the middle subscapular nerve. In this illustration, we can see that the thoracodorsal nerve is located between the superior and inferior subscapular nerves, therefore, it makes sense that it's sometimes referred to as the middle subscapular nerve. This nerve originates from the posterior cord and it carries fibers from the last three cervical spinal nerves C6 to C8. It provides innervation to the latissimus dorsi muscle.
Before we finish this tutorial, I'd like to take a few minutes to talk about the lesions of the brachial plexus. Brachial plexus injuries are usually a result of blunt trauma producing nerve avulsions and distraction. When the brachial plexus is damaged, it requires meticulous clinical history taking and examination. The two most common clinical conditions that are related to the brachial plexus are the Erb's palsy and the Klumpke's palsy.
The Erb's palsy is a paralysis of the arm caused by injury to the superior trunk of the brachial plexus. This lesion affects the fibers from the fifth and sixth cervical nerve roots. The most common cause of the Erb's palsy is dystocia which means an abnormal or difficult childbirth. For example, this can occur when the infant's head and neck are pulled towards the same side at the same time as the shoulders pass through the birth canal. This produces a widening of the head-shoulder interval. The paralysis of the deltoid, supraspinatus, infraspinatus, and biceps brachii muscles produces a characteristic clinical sign called waiter's tip deformity. Under such conditions, the arm hangs by the side and is rotated medially and the forearm is extended and pronated.
The other condition we're going to talk about is Klumpke's palsy. This is caused by a lesion to the inferior trunk which affects the eighth cervical and the first thoracic nerve roots. This type of injury is also related to a difficult childbirth and can occur during a difficult vaginal delivery. The paralysis of the intrinsic muscles of the hand can cause the most severe physical manifestation of Klumpke's palsy which is known as claw hand. Claw hand refers to when the affected forearm tends to lie flat and the wrist and fingers are tightened.
Now that we've discussed these two syndromes, that brings us to the end of this tutorial on the brachial plexus. I hope you enjoyed it and thank you for listening.
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