AssessmentsAnatomy of the brachial plexus
USMLE® Step 1 style questions USMLE
Content Reviewers:Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.), Viviana Popa, MD
Contributors:Ursula Florjanczyk, MScBMC, Elizabeth Nixon-Shapiro, MSMI, CMI, Sam Gillespie, BSc, Maddison Caterine, MSc, MB, BCh, BAO
Have you ever fallen asleep with your shoulder in a weird position only to wake up to your arm completely asleep? Or hit the inside of your elbow, or the famous ‘funny bone’, and cause part of your hand to go numb?
This happens when we compress or bang the nerves found in our arms, all of which originate in the brachial plexus. The brachial plexus is a vast network of nerves originating from the anterior rami of C5 to T1, which extends through the axilla into the shoulder, arm, and hand, providing afferent, or sensory, nerve fibers from the skin, as well as efferent, or motor, nerve fibers to the muscles.
Alright, so, the brachial plexus is divided into five roots, three trunks, six divisions, three cords, and five terminal branches. The order can be remembered using the mnemonic. “Remember To Drink Cold Beer.”
Additionally, there are branches that leave the brachial plexus at various points along its length. Since the branches that come off of the roots and trunks are located above the clavicle, they are sometimes called the supraclavicular branches of the brachial plexus.
And since the branches that come off of the cords as well as the terminal branches are located below the clavicle, they are sometimes called the infraclavicular branches of the brachial plexus.
Okay, so starting with the roots, the five roots come from the anterior rami of the last four cervical nerves, C5-C8, as well as the anterior ramus of the first thoracic nerve or T1. These roots usually travel between the anterior and middle scalene muscles along with the subclavian artery.
These five roots give off some branches: the long thoracic nerve, which arises from C5-C7, and gives motor innervation to the serratus anterior muscle, the dorsal scapular nerve from C5, which gives motor innervation to the rhomboid and levator scapulae muscles, and a contribution to the phrenic nerve from C5, which gives motor and sensory innervation to the diaphragm.
At the inferior part of the neck, the five roots combine to form three trunks: C5 and C6 merge to form the superior or upper trunk, C7 remains as the middle trunk, and C8 and T1 merge to form the inferior or lower trunk.
The superior trunk gives off another branch, the suprascapular nerve, which travels across the posterior triangle of the neck, which we can see here highlighted in blue, and through the scapular notch to give motor innervation to the supraspinatus and infraspinatus muscles.
The superior trunk also gives off the nerve to subclavius, which gives motor innervation to the subclavius muscle. OK, now that we’ve gone over the roots, trunks, and the supraclavicular branches, let’s pause and do a quick quiz! Pause the video and see if you can identify them!
Moving on, each trunk splits into an anterior and a posterior division as they travel through the cervicoaxillary canal or the apex of the axilla, giving rise to a total of six divisions.
Think of it as travelling through a wormhole - and on the other side, the six divisions regroup with each other to form three cords.
The cords are named for their relationship to the second part of the axillary artery; so lateral to the axillary artery, there’s the lateral cord, which is formed by the anterior divisions of the superior and middle trunks; posterior to the axillary artery, there’s, you guessed it, posterior cord, formed by the posterior divisions of the superior, middle, and inferior trunks.
And medial to the axillary artery, there’s the medial cord, formed by the anterior division of the inferior trunk - no surprises there. Now, these cords sprout a number of branches.
The lateral cord gives rise to the lateral pectoral nerve, which gives motor innervation to the pectoralis major muscle and also to the pectoralis minor muscle via a communicating branch to the medial pectoral nerve.
Next, the medial cord gives rise to the medial cutaneous nerve of the arm and the medial cutaneous nerve of the forearm, which give sensory innervation to the medial skin of the arm and forearm.
And more proximally, there’s the medial pectoral nerve, which gives motor innervation to the pectoralis minor and major muscles. And finally, three cords give rise to the five terminal branches.
The median nerve is formed by contributions from both the lateral and medial cords and is made up of fibers from C5 to T1. The other terminal branch of the lateral cord is the musculocutaneous nerve, which receives contributions from C5 to C7.