The brachial plexus is a part of anatomy that I am pretty sure everyone (except for me) dreaded learning about. Understanding the various parts, remembering which nerves came from what roots, and were divisions before or after the trunks? I feel like we spend all the time just trying to memorize the drawing for the test but don’t really understand what the drawing means. It took me until I was studying for the certified hand therapist certification exam to really break down each part of the brachial plexus, and it honestly helped me better understand the plexus and its entirety.
They didn’t call me Professor Logan in OT school for nothing! No really…they did.
I want to break down the Brachial Plexus a bit more, to help you better understand its entirety and how crucial it is to the utilization of the upper extremity. In this post, we are going to learn about each section that makes up the Brachial Plexus.
I have another post on the brachial plexus, because I seriously do love studying it. If you don’t believe me, check out this post here. That post is more of the what, and this is more of the why.
Why is the Brachial Plexus so important to hand therapists? Brachial is latin for arm. This plexus supplies all motor and sensory innervation to the upper extremity. So you can see why it is a big deal.
Roots
The roots of the brachial plexus are formed from the ventral rami C5-T1 spinal cord levels. C5, C6, C7, and C8 roots course ABOVE the associated vertebrae. So C5 root is ABOVE C5 vertebrae, C6 root is ABOVE C6 vertebrae, C7 is ABOVE C7 and C8 ABOVE T1, remember there are only seven cervical vertebrae!
T1 (and below) course BELOW the associated vertebrae, or T1 root is BELOW T1 vertebrae.
Roots C5 and C6 converge together to form the Upper Trunk(which we will talk about in the next section). C7 stays in its own lane and forms the Middle Trunk. C8 and T1 converge together to form the Lower Trunk.
Each root has an associated DERMATOME and MYOTOME. Let’s think back to medical terminology and break these down. DERMA means skin, MYO means muscle and TOME means section. Dermatome means an area (or section) of skin innervated by a specific spinal nerve level. Myotome means an a muscle (or muscles) or a movement (caused by the muscles the movement is innervated by) associated with a specific spinal nerve level.
Dermatomes for the BP
- C5: lateral aspect of the arm
- C6: radial aspect of forearm and thumb
- C7: central aspect of posterior arm and index and middle/long finger
- C8: ulnar aspect of the forearm and hand, ring and pinky/small finger
- T1: middle aspect of forearm and distal arm
Myotomes for BP
- C5: shoulder ABDuction
- C6: elbow FLEXION
- C7: elbow EXTENSION
- C8: finger flexion
- T1: finger ABD/ADDuction
This will all come back when we break down the specific branches and their origins.
There are a couple nerves that branch off from the roots. The Dorsal Scapular Nerve branches from the C5 root. This innervates the Levator Scapulae and the Rhomboids. The Long Thoracic Nerve branches from C5, C6, and C7 roots. This innervates the Serratus Anterior. If someone has a winging scapula, this is the nerve typically injured. NO BRANCHES FORM OFF OF ROOTS C8 AND T1.
Trunks
There are 3 trunks that exit between the anterior and middle scalene muscles, along with the subclavian artery. The UPPER trunk is formed from the UPPER roots, C5 and C6. The MIDDLE trunk is formed from the MIDDLE root, C7. The LOWER trunk is formed from the LOWER roots, C8 and T1. The Subclavian Nerve branches from the upper trunk right where C5 and C6 converge, innervating the Subclavius muscle. The Suprascapular Nerve branches off from the upper trunk and innervates the supraspinatus and infraspinatus. To remember this, I think of supra, which means above (or upper) and the upper trunk, or upper and upper.
Divisions
Coming off from the trunks are two divisions: Anterior and Posterior. This can sometimes get a little complex when learning, even though there are only two. The division name or position has to do with it’s relation to the axillary artery. The anterior divisions are anterior to the artery and the Posterior are posterior. NO NERVES BRANCH OFF FROM THE DIVISIONS. The anterior divisions from the Upper and Middle trunks (or roots C5, C6, and C7) form the lateral cord. The anterior division from the lower trunk (or roots C8 and T1) form the medial cord. The posterior divisions from all the trunks (and roots) converge to form the posterior cord. The anterior divisions are what innervate the flexors of the UE. the posterior divisions are what innervate the extensors of the UE.
Cords
Just like the divisions, the cords are named depending on their position to the axillary artery. Each cord gives off terminal and nonterminal branches. The Lateral Cord, formed from the anterior divisions of the upper and middle trunks (roots C5, C6, C7) is lateral to the artery. It gives off nonterminal branches to the lateral pectoral nerve which innervates the pectoralis major muscle. The Medial Cord, formed from the anterior division of the lower trunk (roots C8 and T1) is medial to the artery. It gives off nonterminal branches to the medial pectoral nerve which innervates the pectoralis major and minor, the medial brachial cutaneous which innervates the medial side of the arm, and the medial antebrachial cutaneous nerve which innervates the medial side of the forearm. The Posterior Cord, formed from the posterior divisions of all the trunks (roots C5, C6, C7, C8 and T1) is posterior to the artery. It gives off nonterminal branches to the upper subscapular nerve (formed from C5 and C6 roots) which innervates the subscapularis muscle, the lower subscapular nerve (also formed from C5 and C6 roots) which innervates the subscapularis and teres major muscles, and the middle subscapular/thoracodorsal nerve (formed from C6, C7, and C8 roots) which innervates the latissimus dorsi muscle.
Branches
The branches, or terminal branches are what innervate the rest of the UE. Let’s break each of the 5 terminal branches down. Two branches are formed from the posterior cord; the axillary nerve and the radial nerve. The axillary nerve (roots C5, C6) innervates the deltoid muscles, and the teres minor, as well as the lateral cutaneous nerve of the arm and the lateral brachial cutaneous nerve. The radial nerve (C5-T1) innervates the entire posterior arm, triceps, brachioradialis, anconeus, ECRL, ECRB, supinator, EDC, EDM, ECU, APL, EPL, EPB, and EIP. The radial nerve also gives off the superficial cutaneous branch which gives sensation to the dorsal aspect of the hand up to the IP joint of the thumb and P2 (or middle phalanx) of the index and long fingers. The musculocutaneous nerve branches off from the lateral cord and innervates the biceps, brachialis, and coracobrachialis muscles, as well as the lateral cutaneous nerve of the forearm and the later antebrachial cutaneous nerve, both sensory nerves. The Ulnar nerve branches off from the medial cord. It innervates FCU, the ulnar half of FDP (FDP to ring and small digits), palmaris brevis, ADM, ODM, FDM, lumbricals III and IV, palmar and dorsal interossei, the deep head of FPB, and adductor pollicis. It also gives rise to the superficial cutaneous branch which gives sensation to the volar aspects of 1/2 of the ring finger, all of the small finger, the hypothenar eminence, and the dorsal aspects of 1/2 of the ring finger and all of the small finger. It is the nerve involved with cubital tunnel syndrome. The Median nerve is unique, as it is formed from both the lateral and medial cords. It innervates the pronator teres, FCR, palmaris longus, all of FDS, radial half of FDP (FDP to index and long fingers), FPL, PQ, the superficial head of FPB, APB, OP, and lumbricals I and II. It also gives off the superficial cutaneous branch which gives sensation to the volar aspects of the thumb, index, long and radial half of the ring finger, as well as the dorsal aspects of the thumb, index, long, and radial 1/2 of the ring finger to the PIP joints. It is the nerve that is involved with carpal tunnel syndrome.
Tying It All Together
Okay, now that your brain is mush again, let’s think about all this information. Knowing which muscles come from which root can not only help you when studying and taking the CHT exam, it can help you clinically as well. I had a patient that I was seeing sent to me from their PCP for cubital tunnel. Upon evaluation, during the subjective portion, they stated that they had N/T in their ring and pinky fingers, along with decreased grip strength. They stated they were starting to have similar symptoms on the other side and also complained of some neck and shoulder pain. Knowing that the ulnar nerve, which is the nerve involved in cubital tunnel, comes from the medial cord from roots C8 and T1, I decided to do a quick cervical screen and all of the tests came back positive for possible cervical radiculopathy. I sent him back to the doctor and upon further assessment it was found that he did have cervical radiculopathy at C8 and T1 which were causing his hand symptoms, not actually irritation from the cubital tunnel. This was also confirmed with an EMG. This in depth understanding I have of the brachial plexus helped me find the cause of the issue, not just treat the symptoms.
So why bring in dermatomes and myotomes? Understanding how the brachial plexus redistributes dermatomal and myotomal fibers helps differentiate between radiculopathy (spinal nerve root compression) and peripheral nerve lesions.
If a patient has sensory loss in a dermatomal pattern (e.g., lateral forearm and thumb = C6), it suggests spinal nerve root involvement (C6 radiculopathy). If sensory loss follows a peripheral nerve pattern (e.g., loss over the palmar aspect of the thumb and index finger = median nerve injury), the lesion is likely at the level of the peripheral nerve rather than the spinal nerve root.
If a spinal nerve root is injured, the muscles in the entire myotome will be weak, even though the individual muscles are controlled by different peripheral nerves. However, if a peripheral nerve is injured, only the muscles specifically controlled by that nerve will be affected.
Here are a couple of examples we commonly see in clinic:
C6 Radiculopathy (spinal nerve injury) → Weakness in elbow flexion (musculocutaneous nerve) and wrist extension (radial nerve), plus sensory loss in the C6 dermatome.
Radial Nerve Injury (peripheral nerve injury, e.g., at mid-humerus) → Wrist drop (loss of wrist and finger extensors), but elbow flexion (C6 myotome) remains intact because the musculocutaneous nerve is unaffected.
Below is a summary table bringing all of that together.
| Feature | Radiculopathy (Spinal Nerve Root Injury) | Peripheral Nerve Injury |
|---|---|---|
| Sensory Loss | Follows a dermatomal pattern | Follows a peripheral nerve distribution |
| Motor Weakness | Follows a myotomal pattern (multiple muscles weak) | Limited to muscles supplied by the specific nerve |
| Reflexes | Often diminished if the affected nerve root contributes to a reflex arc | Reflexes may be preserved unless the nerve is involved in the arc |
| Common Causes | Herniated disc, foraminal stenosis | Trauma, compression (e.g., carpal tunnel, radial nerve palsy) |
Tips For Studying
I know that this is a LOT of information. It took me quite some time to fully digest it all. Take your time, watch YouTube videos, draw it out, practice practice practice. The more you study it, the more you will understand it. But make sure to not just know the what, but the why and how! Happy studying!
REFERENCES
Biel, A. (2019). Trail Guide to the body: A hands-on guide to locating muscles, bones, and more. Books of Discovery.
Dermatomes. Kenhub. (n.d.). https://www.kenhub.com/en/library/anatomy/dermatomes
Osterman, A. L., Skirven, T. M., Fedorczyk, J. M., Amadio, P. C., Feldscher, S. B., & Shin, E. K. (2021). Rehabilitation of the hand and upper extremity. Elsevier Mosby.
Weiss, S., & Rogers, L. C. (2019). Hand and upper extremity rehabilitation: A quick reference guide & review. Exploring Hand Therapy, Inc.
