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Brachial plexus

Structure of the brachial plexus, including the roots, trunks, cords and branches.

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Transcript

For many, Saturday is a time to let your hair down and let loose. Have some fun and dance till your feet hurt. Those nights where you don't get home until the morning and just fall into bed. Until you wake later and can't feel your arm. Saturday night palsy? Compressed nerves? What’s it all about? Hmmm. Let's find out as we explore the brachial plexus.

Before we begin, I'd like to give you a quick overview of what we're going to be talking about in today's tutorial. Highlighted in green, we can see the brachial plexus, which is part of the peripheral nervous system. As you can see, the brachial plexus begins in the neck between the anterior and middle scalene muscles. It passes through the axilla or the armpit and courses through the arm to innervate the muscles, joints, and skin of the upper limb.

Now that we've seen the brachial plexus in situ, let's zoom in and look at it in isolation.

So this image looks pretty complicated, hey, but don't get stressed, we're going to take some time now to break it down. The first section of the brachial plexus known as the roots of the plexus are formed by the anterior or ventral rami of the spinal nerves C5 to T1 and these rami give rise to three trunks – the superior trunk, the middle trunk, and the inferior trunk. Each trunk divides into an anterior and posterior division which go on to form three cords of the brachial plexus – the lateral cord, the posterior cord, and the medial cord.

The nerves that we're going to be talking about in this tutorial arise from various levels of the brachial plexus. A simple mnemonic you can use to remember these levels is Read That Damn Cadaver Book – R for roots, T for trunks, D for divisions, C for cords, and B for branches. So, hopefully by the end of this tutorial, you'll go from this to this, as we learn about each section of the brachial plexus and its branches.

So let's start with the roots of the brachial plexus.

So the roots are the first section of the brachial plexus, and they arise from the anterior rami of the last four cervical spinal nerves and the first thoracic spinal nerve. Before we move on, I just want to show you this illustration of the spinal cord in the transverse plane to remind you of the origin of the anterior rami. At each vertebral level, a pair of spinal nerves arise from the spinal cord. The spinal nerves pass through intervertebral foramina on either side of the vertebral column before dividing into an anterior ramus and a posterior ramus. As we already know, the roots of the brachial plexus are formed by the anterior rami of C5 to T1.

Okay, so let's move on to look at these roots. The first root arises from the anterior ramus of C5, which emerges above the fifth cervical vertebra. The next two roots emerge above their respective vertebrae in a similar fashion. The second root arises from the anterior ramus of C6 and the third root arises from the anterior ramus of C7. Things change up slightly as we move to the fourth and fifth roots, and since there's no eighth cervical vertebra, the fourth, root which arises from the anterior ramus of C8 emerges above the first thoracic vertebra, whereas the fifth root arising from the anterior ramus of T1 emerges below the first thoracic vertebra. This transition is particularly important as the spinal nerves go from emerging above their respective vertebra to emerging below them.

So that's us covering the roots of the brachial plexus, which arise from the anterior rami of C5, C6, C7, C8, and T1.

Next, we'll talk about the trunks, which are the second section of the brachial plexus.

The three trunks of the brachial plexus originate from the roots past laterally over the first rib and enter the axilla. The first of the three trunks is the superior trunk, seen here highlighted in green, and this trunk, as you can see clearly here, is formed by the union of the anterior rami of C5 and C6. Two nerves arise from the superior trunk and they are the subclavian nerve and the suprascapular nerve, and we're going to look at these two nerves later on in our tutorial.

For now, however, let's have a look at the second trunk, which is the middle trunk. The middle trunk, unlike the superior trunk, originates directly from the anterior ramus of C7, and finally the inferior trunk like the superior trunk is composed of two nerves as you can see right here, the anterior rami of C8 and T1.

So that's us finishing the trunks of the brachial plexus, and we have a superior trunk that arises from the anterior rami of C5 and C6, a middle trunk that arises directly from the anterior ramus of C7, and an inferior trunk that arises from the anterior rami of C8 and T1.

So just to recap, so far, we've covered the roots and the trunks of the brachial plexus, and next, we'll move on to look at the divisions of this plexus.

The divisions are the bifurcations of the trunks and are the third section of our brachial plexus, and as you can see in this diagram, each of the trunks that we've described previously give off one anterior branch and one posterior branch. The three anterior divisions form part of the brachial plexus that ultimately give rise to nerves associated with the anterior compartments of the upper limb, whereas the three posterior divisions combine to form parts of the brachial plexus that give rise to nerves associated with the posterior compartments of the upper limb.

Okay, so time to move on to the cords, which follow on as the fourth section of the brachial plexus.

And as you can see here, the lateral cord is formed by the union of the two anterior divisions of the superior and middle trunk. The lateral cord gives rise to a branch known as the lateral pectoral nerve before it continues into the upper limb as the musculocutaneous nerve. The posterior cord is formed by the union of all three posterior divisions and the posterior cord gives rise to three branches before it terminates as the axillary nerve and the radial nerve – namely, the upper subscapular nerve, the thoracodorsal nerve, and the lower subscapular nerve.

The medial cord is actually the continuation of the anterior division of the inferior trunk, and like the posterior cord, it gives off three branches before it terminates as the ulnar nerve, and these branches are the medial pectoral nerve, the cutaneous nerve of the arm, and the cutaneous nerve of the forearm.

And before we move on, let's quickly summarize what we learned about the cords of the brachial plexus.

So the lateral cord is formed by the union of the two anterior divisions of the superior and the middle trunks, the posterior cord is formed by the union of all of the three posterior divisions, and lastly, the medial cord is the continuation of the anterior division of the inferior trunk.

And, finally, these cords give rise to the nerves of the upper limb, and first we're going to talk about the terminal or the major branches of the brachial plexus.

So, first up, we have the musculocutaneous nerve, which is the terminal branch of the lateral cord of the brachial plexus. And this nerve carries fibers from the anterior rami of spinal nerves C5, C6, and C7.

In our next illustration, we can see the bones of the shoulder joint and the musculocutaneous nerve from an anterior perspective, and this nerve provides motor innervation to the muscles of the anterior compartment of the arm, namely the coracobrachialis, the biceps brachii, and the brachialis muscle. It later becomes the lateral cutaneous nerve of the forearm which supplies sensory innervation to the lateral forearm.

Next, we have the median nerve, and as you can see in our illustration, this nerve is made up of fibers from the lateral cord and the medial cord of the brachial plexus. The median nerve carries fibers from the anterior rami of C6 to T1. The motor part of the median nerve innervates the flexor muscles of the anterior compartment of the forearm except for the flexor carpi ulnaris and part of the flexor digitorum profundus which are innervated by the ulnar nerve. It also supplies innervation to the thenar muscles and lateral to 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 and the digital cutaneous branch which innervates the lateral three-and-a-half fingers on the palmar surface of the hand.

Okay, so let's look at the terminal branches of the posterior cord, the axillary nerve, and the radial nerve. And here we have the axillary nerve, which is known as the circumflex nerve, and this nerve carries fibers from the anterior rami of C5 and C6. The axillary nerve passes posteriorly around the surgical head of the humerus and provides motor innervation to the deltoid and teres minor muscles. In addition, it gives off the superior lateral cutaneous nerve, which provides sensory innervation to the skin overlying the deltoid muscle.

The other terminal branch of the posterior cord is the radial nerve and it is the largest nerve in the upper limb and carries fibers from the anterior rami of C5 to T1. So it receives fibers from all five spinal nerves of the brachial plexus. The radial nerve supplies motor innervation to the muscles of the posterior compartment of the arm, namely, the triceps brachii and the anconeus. It also innervates the brachioradialis and the muscles of the posterior compartment of the forearm. In addition, it gives off the inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the arm which provides sensory innervation to the skin of the lateral lower arm and the skin of the posterior surface of the arm respectively.

And, lastly, we have the ulnar nerve, which is the terminal branch of the medial cord, and it carries fibers from the anterior rami of C8 and T1. The ulnar nerve provides motor innervation to the flexor carpi ulnaris, the flexor digitorum profundus, the two medial bellies, the intrinsic muscles of the hand except for the thenar muscles, and the medial two 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 now we've seen and described the sections of the brachial plexus as well as the main terminal branches off this plexus. But in addition to these nerves, the brachial plexus gives off some other nerves that arise from different sections of the plexus and let's have a look at some of these nerves now.

So we're going to begin with nerves that arise directly from the roots starting with the dorsal scapular nerve. The dorsal scapular nerve is a branch arising from the C5 root of the brachial plexus, and this nerve supplies motor innervation to the rhomboid and the levator scapulae muscles which, as you can see, are located in the posterior upper neck and back region.

Another important nerve that arises from the roots is the long thoracic nerve, and it originates from the anterior rami of C5 to C7. The long thoracic nerve passes vertically down the neck through the axillary inlet and supplies motor innervation to the serratus anterior muscle.

Now for some nerves arising from the trunks, specifically, the superior trunk. So the first nerve we're going to look at is the suprascapular nerve, which as I just mentioned, arises from the superior trunk of the brachial plexus. And this nerve carries fibers from the anterior rami of spinal nerves C5 and C6. Here we can see the bones and muscles of the shoulder joint from a posterior perspective and the suprascapular nerve provides motor innervation to the supraspinatus and the infraspinatus muscles. It also supplies sensory innervation to the acromioclavicular joint and the glenohumeral joint.

Next, we have the subclavian nerve which is also known as the nerve to the subclavius and arises from the superior trunk. The subclavian nerve carries fibers from the anterior rami of C5 and C6. It provides motor innervation to the subclavius muscle, which as you can see, is closely related to the clavicle.

Okay, so let's look at some nerves arising from the cords of the brachial plexus starting with the lateral cord.

The lateral pectoral nerve arises from the lateral cord and carries fibers from the anterior rami of C5 to C7. The lateral pectoral nerve provides motor innervation to the pectoralis major muscle and partly innervates the pectoralis minor muscle. In our image, we can see the pectoralis major with the pectoralis minor lying deep to this muscle.

Okay, so now we're going to look at some branches of the posterior cord of the brachial plexus starting with the upper subscapular nerve, also known as the superior subscapular nerve. This nerve carries fibers from the anterior rami of spinal nerves C5 and C6 and supplies motor innervation to the superior portion of the subscapularis muscle. As you can see, this muscle is associated with the anterior aspect of the scapula.

So, next, we have the thoracodorsal nerve, which is also known as the middle subscapular nerve. And this nerve arises from the posterior cord and carries fibers from the anterior rami of C6, 7, and 8. The thoracodorsal nerve provides motor innervation to the latissimus dorsi muscle, which is a muscle of the back. The lower or inferior subscapular nerve, also arising from the posterior cord, carries fibers from the anterior rami of C5 and C6. The lower subscapular nerve supplies motor innervation to the inferior portion of the subscapularis and the teres major muscle.

And, finally, let's look at the branches of the medial cord of the brachial plexus.

The medial pectoral nerve arises from the medial cord and carries fibers from the anterior rami of spinal nerves C8 and T1. The middle pectoral nerve provides motor innervation to the pectoralis minor muscle and partly innervates the pectoralis major muscle.

Next, we have the medial brachial cutaneous nerve which is also known as the medial cutaneous nerve of the arm, and this nerve arises from the medial cord and carries fibers from the anterior rami of C8 and T1. As the name suggests, the nerve provides sensory innervation to the skin of the medial arm.

We also have a medial cutaneous nerve of the forearm which originates just distal to the origin of the medial cutaneous nerve of the arm, and this nerve is also known as the medial antebrachial cutaneous nerve. Again, it arises from the medial cord and carries fibers from the anterior rami of C8 and T1. And as the name suggests, this nerve provides sensory innervation to the skin of the medial forearm.

Okay, so now that we're familiar with the brachial plexus, let's get clinical.

In today's notes, we're going to talk about thoracic outlet syndrome or TOS. So, TOS refers to a group of conditions that are caused by compression of the brachial plexus or subclavian vessels as they pass through the space between the clavicle and the first rib. There are three types of TOS – neurogenic TOS, venous TOS, and arterial TOS – and today, we're going to be focusing on neurogenic TOS as it is most relevant to our tutorial.

So the presentation of neurogenic TOS depends on the severity of compression of the brachial plexus; however, common symptoms include pain, weakness, numbness, or tingling in the neck, shoulder, and arm. Causes of neurogenic TOS include compression of the brachial plexus as it passes between the anterior and the middle scalene muscles and it can also be caused by bone abnormalities such as a congenital cervical rib, trauma, and repetitive activities that may be carried out in manual work and sports.

Investigations include a plain chest radiograph to look for bone abnormalities and an MRI to evaluate the brachial plexus and surrounding soft tissues. Treatment is initially conservative including physiotherapy to widen the thoracic outlet and medication such as anti-inflammatories, pain medications, and muscle relaxants. Surgery is carried out in patients with the debilitating symptoms despite previous interventions.

Okay, so before we bring our tutorial to a close, let's quickly summarize what we've learned today.

So, first, we took a look at the roots of the brachial plexus which are formed by the anterior rami of the spinal nerve C5, C6, C7, C8, and T1. Next, we spoke about the trunks of the brachial plexus – the superior trunk, the middle trunk, and the inferior trunk, which are derived from the roots of the brachial plexus. We saw that each trunk divides into an anterior and posterior division of the brachial plexus, which go on to form three cords – the lateral cord, the posterior cord, and the medial cord.

We moved on to talk about the branches of the brachial plexus starting with the terminal nerves namely the musculocutaneous nerve which arises from the lateral cord; the median nerve which arises from the lateral and medial cords; the axillary nerve and the radial nerve which arise from the posterior cord; and the ulnar nerve which arises from the medial cord.

We then looked at some other nerves that arise from the various levels of the brachial plexus. First, we saw the dorsal scapular nerve and the long thoracic nerve which are derived from the roots. Next, we looked at two nerves derived from the superior trunk – the suprascapular nerve and the subclavian nerve. Then we saw some nerves derived from the cords and we saw the lateral pectoral nerve derived from the lateral cord as well as the upper subscapular nerve, the thoracodorsal nerve, and the lower subscapular nerve which are derived from the posterior cord. And, finally, we saw the medial pectoral nerve, the medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm which are derived from the medial cord.

Then we finished up our tutorial with some clinical notes about neurogenic thoracic outlet syndrome, and that brought us to the end of our tutorial on the brachial plexus.

So I hope you enjoyed it. Thanks for watching and happy studying.

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