The Brachial Plexus

The brachial plexus is one of the most intimidating anatomical structures to learn in the human body. Nerves are often difficult for people to learn, and when combined into a group that twists and turns, it's easy to understand why some people dread studying this topic. However, even if you aren't enrolled in an anatomy course, understanding the brachial plexus and the nerves involved can really help you learn a lot about the functioning of your own body, especially if you have had upper body injuries in the past. This includes, but is not limited to, poor posture, shoulder injuries requiring bracing, elbow injuries, and wrist/hand problems. 

​I've experienced this difficulty personally as well. In my opinion, the biggest challenge is getting started. I always tend to find that learning in a systematic manner is the best for me, so in this case, I tried to have a little bit of knowledge about some reference points that would allow me to learn the brachial plexus more efficiently. This won't work for everyone, but hopefully you can get something out of the following tips!

Reference Points

Before diving into the nerves involved in the brachial plexus and understanding their general courses and branches, I found it helpful to pick out a few reference points around the brachial plexus to give me a better understanding of transitions between area and where individual nerves actually reside. 

For example, it could be helpful to understand some of the skeletal structure around the neck and chest. In particular, focus on the cervical vertebrae and where nerve roots exit with respect to the adjacent vertebrae. Also, knowing the general location and orientation of the sternum, clavicle, scapula, and humerus can also be beneficial. Understanding the boundaries of the axilla can help as well. ​I found knowing all of the lateral neck anatomy to be beneficial, but I found this almost just as difficult as learning the brachial plexus!

Big to Small​

At this point, I found it easy to work from big to small, but others may prefer a different learning style. What I mean by this is understanding the most general information about the brachial plexus first, and then working down to characteristics and paths of individual nerves. 

For example, in order from medial to lateral, the brachial plexus has 5 roots, 3 trunks, 6 divisions, 3 cords, and 5 branches (the latter are the actual nerve names). In terms of remembering how many of each section there are, all you really need to remember is the number 536. This gives you the first half. Simple flip it around and the 3 and 5 give you the second half. 

5 3 6 3 5

5 3 6 3 5

Now, you can also use mnemonics for knowing the order of sections from medial to lateral. In reality, you can make any mnemonic that you want as long as it fits with the correct order, but there are some popular ones that are easy to remember. For example...

Roots Trunks Divisions Cords Branches

Rugby Teams Drink Cold Beer

Read That Damn Cadaver Book

Really Tired, Don't Care Now

(*Nerves instead of Branches)

Being able to learn the material presented above is already a huge step towards a solid understanding of the brachial plexus. In terms of the exact nerves involved, which appear as the final branches of the brachial plexus, these also have a defined order. From superior to inferior (cranial to caudal), they order is: 

  1. Musculocutaneous Nerve
  2. Axillary Nerve
  3. Radial Nerve
  4. Median Nerve
  5. Ulnar Nerve​

For me, I found the acronym MARMU ("mar-moo") to be easy enough to remember, but there are more advanced mnemonics to also help you remember the root contribution to the nerves, which will definitely be helpful if your knowledge of the brachial plexus is going to be tests. We will cover those advanced ones in a different article.

Sections of the Brachial Plexus

Now that we have a general idea of the basic arrangement of the brachial plexus and the nerves involved, let's look a little more at how each individual section of the brachial plexus is arranged.

Roots​

This is pretty simple. The most important point to remember is that the brachial ple​xus includes nerve roots C5, C6, C7, C8, and T1. Aside from that, C5 gives rise to the dorsal scapular nerve, C5 and C6 provide contributions to form the subclavian nerve, and C5/C6/C7 all provide contributions to the branch known as the long thoracic nerve.

Trunks

The trunks form from the roots. There are three trunks:

  1. Upper/Superior Trunk: Formed by the union of C5 and C6. This gives rise to the suprascapular nerve.
  2. Middle Trunk: Simply a continuation of C7.
  3. Lower/Inferior Trunk: Formed by the union of C8 and T1.

Divisions

This may look like a confusing and difficult part of the brachial plexus to learn, but it's arguably the most simple. There are 6 divisions - ​3 anterior divisions and 3 posterior divisions. Each trunk gives rise to both an anterior and posterior division.

Generally speaking, the anterior divisions supply the muscles that flex the shoulder, elbow, and wrist, and also help coordinate fine movements of the hand. The posterior divisions supply the muscles that extend the shoulder, elbow, and wrist.

Cords

Now things start to recombine again. The divisions combine to form three cords - the ​lateral cord, medial cord, and posterior cord. 

  1. Lateral Cord: Formed by the upper two anterior divisions​, that is, the anterior divisions of the superior and middle trunks.
  2. Medial Cord: This is a continuation of the anterior division of the inferior trunk.
  3. Posterior Cord: This is formed by the union of all the posterior divisions. An easy one to remember!

Now, the most difficult part about the cords is memorizing the branches that arise from these cords (not the terminal branches/nerves, but smaller nerves that shoot off from the cords). There are 7 in total, which can seem intimidating, but I find it easier to remember them in groups.

  1. Lateral Cord: Gives rise to the lateral pectoral nerve. This can be easy to remember, as they both contain the word "lateral".
  2. Medial Cord: Mirroring the lateral cord, the medial cord gives rise tot he medial pectoral nerve. It also sends off two other branches - the ​medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm. Again, a good way to remember this is by recognizing the word "medial". 
  3. Posterior Cord: The posterior cord gives rise to the upper, middle, and lower subscapular nerves. Where it may confuse some people is that the middle subscapular nerve is often called the thoracodorsal nerve. They're the same thing. 

Branches/Nerves

The cords now transition into the nerves that you may be a little more familiar with, or at least heard of before. These include the 5 nerves I listed previously (MARMU) from superior to inferior: the musculocutaneous nerve, axillary nerve, radial nerve, median nerve, and ulnar nerve.

One good way to identify the median nerve is to look for the telltale 'M' shape in the plexus. The middle of this is the median nerve, and from there, you now have a reference point to identify the other nerves (see figure below).

As a side note, the compression of the median nerve in the carpal tunnel is the course of debilitating pain in carpal tunnel syndrome. The ulnar nerve is also a common source of pain, often best identified when you hit your "funny bone", or inside of the elbow. When you do this, the rapid application of force to the ulnar nerve results in the weird type of pain you feel.

Drawing the Brachial Plexus

The title of this section may have prompted you to ask the legitimate question, "Why would I ever want to draw the brachial plexus?" When I was going through all my anatomy classes, I wondered the exact same thing when looking at additional resources. Turns out, this was probably the most effective tool for me in general, especially when it came to writing exams.

It's also not as difficult as you may think. Simply watch the video first, then watch it again and draw along with it, and then after that try and draw it yourself a few times. Once you feel like you have it locked in, give yourself a break, and try to draw it form memory again the next day. After that, just continue to draw it every now and then until you don't need to remember it anymore. 

One thing I should also mention about this is that it's really quick to draw. I think once you are proficient with it, you can probably draw it in about 30 seconds or less. Here's the video I learned from:

Summary

At this point, we'll call it quits for this article, as we have already thrown a lot of information at you. That said, the information we discussed above cover a lot of the content you may be tested on in an undergraduate-level anatomy course. You may also be asked more advanced questions, and for that reason, we have planned our next article to focus on more specific aspects of the brachial plexus, including how to remember the nerve roots that contribute to the MARMU nerves. We hope you found this helpful!

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