Guide · 8 min read
How to learn the brachial plexus
The brachial plexus is the diagram everyone redraws and nobody remembers. Copying it is not the problem - the problem is that a copied diagram is recognition, and an exam asks for recall, usually in the form of a single nerve, its root values, one muscle it supplies, and what happens when it is cut.
This guide reorganises the plexus around that question. Instead of learning the network and hoping the branches fall out of it, learn each branch by the level it leaves the plexus at, then hang the root values, one muscle and one lesion on it. The seven nerves below all have their own cards in the nervous system section, and every root value quoted here is the string on that card.
Three levels of exit, not one diagram
Nerves do not all leave the plexus at the same depth, and the level a nerve comes off is the single most useful thing to know about it - it predicts the root values, the muscles, and the injury.
The long thoracic nerve leaves earliest, directly from the roots of the plexus, carrying C5, C6 and C7. Because it comes off before anything else, it runs its own course on the external surface of serratus anterior along the mid-axillary line, exposed and superficial - which is why it is the nerve injured in axillary surgery.
The suprascapular nerve leaves next, from the upper trunk, with C5 and C6. It is the only branch in this set to come off a trunk, and it heads backwards through the suprascapular notch to the scapula rather than down into the arm.
The remaining five come off the cords, and it is worth stating the split as a fact you can recall rather than a shape you can see: the lateral cord gives the musculocutaneous nerve, the medial cord gives the ulnar nerve, both cords together give the median nerve, and the posterior cord gives the radial nerve and the axillary nerve.
| Nerve | Root values | Comes off | One muscle |
|---|---|---|---|
| Long thoracic | C5, C6, C7 | The roots | Serratus anterior |
| Suprascapular | C5, C6 | Upper trunk | Supraspinatus |
| Musculocutaneous | C5, C6, C7 | Lateral cord | Biceps brachii |
| Median | C6, C7, C8, T1 | Lateral and medial cords | Pronator teres |
| Ulnar | C8, T1 (variable C7) | Medial cord | Flexor carpi ulnaris |
| Axillary | C5, C6 | Posterior cord | Deltoid |
| Radial | C5, C6, C7, C8, T1 | Posterior cord | Triceps brachii |
The root values are the spine of the whole thing
Root values look like the most arbitrary part of the plexus and are in fact the most orderly. Read the table top to bottom and the pattern is that the nerves supplying the shoulder are the highest, the nerves supplying the hand are the lowest, and the one nerve that supplies everything from shoulder to fingertips carries the whole range.
The radial nerve is the largest branch of the plexus and the continuation of the posterior cord, and its card gives it C5 through T1 - every root. Its job explains the width: triceps brachii, anconeus, brachioradialis, supinator, and all the posterior forearm extensors. At the other end, the ulnar nerve carries C8 and T1 with a variable C7 contribution, which is what you would expect of a nerve whose motor territory is dominated by the intrinsic muscles of the hand.
One value is worth memorising as a sentence rather than a list. The long thoracic nerve is C5, C6, C7, and its card carries the line students have used for a century: C5, 6, 7 raise your arms to heaven. Serratus anterior is the muscle that holds the scapula against the chest wall and rotates it for overhead abduction, so a nerve that fails leaves the arm unable to rise above the horizontal.
One muscle per nerve, then widen
Trying to hold a full motor territory for each branch is what makes the plexus feel unlearnable. Anchor each nerve to a single muscle first, and only widen once the anchors are secure.
The musculocutaneous nerve supplies coracobrachialis, biceps brachii and brachialis - the whole anterior compartment of the arm, and nothing else motor. Its card compresses this to BBC, which is a small enough set that the initialism is genuinely worth it. It pierces coracobrachialis on the way through and emerges as the lateral cutaneous nerve of the forearm, so a lesion costs both elbow flexion and supination.
The radial nerve gets the mirror-image territory: everything on the back. Its card offers BEST - brachioradialis, extensors, supinator, triceps. Between them, those two mnemonics account for the arm and forearm in two lines.
The hand is split between median and ulnar on a rule that is easier to state as an exception. The ulnar nerve supplies flexor carpi ulnaris and the medial half of flexor digitorum profundus in the forearm, plus most of the intrinsic hand muscles; the median nerve supplies the rest of the anterior forearm flexors, the thenar muscles, and the lateral two lumbricals. Learn the ulnar list, and the median list is what is left over.
Every lesion names its level
The clinical signs are not extra material on top of the plexus - they are the fastest way to check that you have the anatomy right, because each one follows from where the nerve runs.
The axillary nerve passes through the quadrangular space and winds around the surgical neck of the humerus, so a surgical neck fracture or an anterior shoulder dislocation costs the deltoid and teres minor, giving weak abduction and a patch of numbness over the lateral shoulder. The radial nerve runs in the radial groove of the humerus, so a midshaft fracture of that same bone gives wrist drop instead. Two nerves, one bone, two completely different pictures, and the difference is only where along the shaft each one lies.
- Long thoracic - winged scapula, the medial border lifting away from the chest wall.
- Suprascapular - weak initiation of abduction and weak lateral rotation, from entrapment at the suprascapular notch.
- Musculocutaneous - weak elbow flexion and supination.
- Axillary - deltoid wasting, weak abduction, 'regimental badge' sensory loss over the lateral shoulder.
- Radial - wrist drop after a midshaft humeral fracture, or 'Saturday night palsy' from prolonged compression.
- Median - thenar wasting and numbness of the lateral three and a half digits in carpal tunnel syndrome.
- Ulnar - claw hand, more pronounced with a wrist-level lesion than a proximal one, and a positive Froment sign.
Sensory territory settles the ambiguous cases
Motor findings often overlap; cutaneous territory usually does not, which makes it the tiebreaker in a question that describes a patient rather than naming a nerve.
The hand divides cleanly. The median nerve takes the palmar surface of the lateral three and a half digits and their nail beds, and the lateral palm. The ulnar nerve takes the medial one and a half digits and the medial palm and dorsum. The radial nerve takes the dorsolateral hand and the dorsum of the lateral three and a half digits, but only as far as the nail beds - which is why numbness over the back of the thumb web and numbness over the pad of the thumb point at different nerves.
Higher up, the musculocutaneous nerve becomes the lateral cutaneous nerve of the forearm and takes the lateral surface of the forearm, while the axillary nerve takes only the small patch over the inferior deltoid. Two of these seven have no cutaneous territory at all: the long thoracic nerve is purely motor, and the suprascapular nerve is articular, supplying the glenohumeral and acromioclavicular joints.
A practical drill order
The plexus rewards being tested in more than one direction. Being able to go from nerve to muscle is not the same skill as going from muscle to nerve, and exams ask for both.
Every one of these seven cards lists the other six under Related, so the group is navigable without a list page, and all of them sit in the upper limb region with the muscles they supply.
- Write the seven nerves grouped by exit level - roots, upper trunk, cords - before writing any root values.
- Add root values from memory, then check each against its card.
- Name one muscle per nerve, then one lesion per nerve, in separate passes rather than all at once.
- Use the nerve and innervation modes in the quiz, which ask the question in both directions.
- Run the upper limb flashcards until the sensory territories are as automatic as the motor ones.
Frequently asked questions
What are the terminal branches of the brachial plexus?
The musculocutaneous, median, ulnar, radial and axillary nerves. The musculocutaneous comes off the lateral cord, the ulnar off the medial cord, the median from both, and the radial and axillary from the posterior cord. The long thoracic and suprascapular nerves leave earlier, from the roots and the upper trunk.
Which nerve of the brachial plexus has the widest root values?
The radial nerve, recorded in this library as C5, C6, C7, C8 and T1. It is the largest branch of the plexus and the continuation of the posterior cord, supplying triceps brachii, anconeus, brachioradialis, supinator and all the posterior forearm extensors.
Why does a winged scapula point to the long thoracic nerve?
Because that nerve's only motor target is serratus anterior, the muscle that holds the medial border of the scapula against the thoracic wall and rotates it for overhead abduction. When it fails, the border lifts away and the arm cannot be raised above the horizontal.
How do I tell a median from an ulnar nerve lesion in the hand?
Use the cutaneous territory. The median nerve covers the palmar surface of the lateral three and a half digits and the lateral palm; the ulnar nerve covers the medial one and a half digits and the medial palm and dorsum. Thenar wasting points at the median nerve, clawing and a positive Froment sign at the ulnar.
Do I need to memorise the whole plexus diagram?
For most courses you need to reproduce the level at which each named branch leaves, its root values, and its motor and sensory territory. Learning the branches in that order gives you the diagram as a by-product, and it is the form questions are actually asked in.