Upper limb & nerve · Free article
The brachial plexus from C5–T1 roots to terminal branches, with an interactive 3D model: trunks, divisions, cords, every branch with its root values, and the signs that localise an injury.
The brachial plexus is the ventral rami of C5, C6, C7, C8 and T1, which leave the neck between scalenus anterior and scalenus medius and reorganise four times before they become named nerves. The order is roots, trunks, divisions, cords, branches, and each level has a fixed rule: C5 and C6 unite as the upper trunk, C7 continues alone as the middle trunk, C8 and T1 unite as the lower trunk; every trunk splits behind the clavicle into an anterior and a posterior division; the three posterior divisions form the posterior cord, the anterior divisions of the upper and middle trunks form the lateral cord, and the anterior division of the lower trunk continues as the medial cord. The cords are named for their relationship to the second part of the axillary artery, behind pectoralis minor.
The functional logic is simpler than the diagram. Anterior divisions carry fibres to the flexor compartments of the limb and posterior divisions to the extensor compartments, so the posterior cord supplies deltoid, the triceps and every extensor of the forearm through the axillary and radial nerves, while the lateral and medial cords supply the flexors through the musculocutaneous, median and ulnar nerves.
| Level | Branch | Roots | Supplies |
|---|---|---|---|
| Roots | Dorsal scapular nerve | C5 | Rhomboids (and levator scapulae) |
| Roots | Long thoracic nerve | C5, C6, C7 | Serratus anterior |
| Roots | Contributions to phrenic nerve | C5 (with C3, C4) | Diaphragm |
| Upper trunk | Suprascapular nerve | C5, C6 | Supraspinatus, infraspinatus |
| Upper trunk | Nerve to subclavius | C5, C6 | Subclavius |
| Lateral cord | Lateral pectoral nerve | C5, C6, C7 | Pectoralis major |
| Lateral cord | Musculocutaneous nerve | C5, C6, C7 | Coracobrachialis, biceps, brachialis; lateral cutaneous nerve of forearm |
| Lateral cord | Lateral root of median nerve | C5, C6, C7 | Joins the medial root in front of the axillary artery |
| Posterior cord | Upper subscapular nerve | C5, C6 | Subscapularis |
| Posterior cord | Thoracodorsal nerve | C6, C7, C8 | Latissimus dorsi |
| Posterior cord | Lower subscapular nerve | C5, C6 | Subscapularis, teres major |
| Posterior cord | Axillary nerve | C5, C6 | Deltoid, teres minor; regimental badge skin |
| Posterior cord | Radial nerve | C5–T1 | Triceps and all forearm extensors; posterior arm, forearm and dorsal hand skin |
| Medial cord | Medial pectoral nerve | C8, T1 | Pectoralis minor and major |
| Medial cord | Medial cutaneous nerve of arm | C8, T1 | Medial arm skin |
| Medial cord | Medial cutaneous nerve of forearm | C8, T1 | Medial forearm skin |
| Medial cord | Ulnar nerve | C8, T1 (± C7) | FCU, ulnar FDP, most intrinsics; ulnar hand skin |
| Medial cord | Medial root of median nerve | C8, T1 | With the lateral root: forearm flexors, thenar muscles, radial lumbricals |
The written paper tends to describe a traction injury and ask where the lesion is. Work outward from the roots. Rhomboid or serratus anterior weakness, or a Horner’s syndrome, places the injury at root level, because the dorsal scapular and long thoracic nerves and the T1 sympathetic contribution leave before the trunks form. Supraspinatus and infraspinatus weakness places it at or proximal to the upper trunk. A deficit confined to one terminal nerve is a cord or branch lesion.
Then decide whether a root injury is preganglionic (avulsion from the cord) or postganglionic (rupture distal to the dorsal root ganglion). In an avulsion the sensory cell bodies in the ganglion stay connected to their peripheral axons, so nerve conduction studies record a normal sensory nerve action potential from skin the patient cannot feel; Horner’s syndrome, a pseudomeningocele on MRI myelography and paralysis of the rhomboids, serratus anterior or hemidiaphragm point the same way. Avulsed roots cannot be grafted, which is why the finding changes the plan from grafting to nerve transfer.
| Pattern | Roots | Mechanism | Picture |
|---|---|---|---|
| Erb–Duchenne (upper) | C5, C6 (± C7) | Head pushed away from the depressed shoulder — birth, motorcycle | Shoulder adducted and internally rotated, elbow extended, forearm pronated: the waiter’s tip |
| Klumpke (lower) | C8, T1 | Traction on the abducted arm — grabbing a branch, breech delivery | Clawing of the hand, intrinsic wasting; Horner’s if T1 is avulsed |
| Pan-plexus | C5–T1 | High-energy traction | Flail, anaesthetic arm; avulsions common |
Test yourself
Six weeks after a motorcycle crash, a 23-year-old has paralysis of the long finger flexors and hand intrinsics, with anaesthesia of the little finger and medial forearm. Shoulder and elbow function are normal. Nerve conduction studies show a normal ulnar sensory nerve action potential recorded from the anaesthetic little finger.
Where is the lesion most likely to be?
Test yourself
Five months after a closed humeral shaft fracture treated in a functional brace, a 45-year-old still has complete wrist and finger drop. Brachioradialis shows no clinical activity, EMG shows fibrillations without motor unit potentials, and Tinel's sign remains at the fracture site. The fracture has united.
What is the most appropriate management?
Musculocutaneous (lateral cord), median (lateral and medial cords), ulnar (medial cord), axillary and radial (posterior cord).
The dorsal scapular nerve (C5) to the rhomboids and the long thoracic nerve (C5–C7) to serratus anterior; C5 also contributes to the phrenic nerve. Because they leave at root level, their function helps distinguish a root injury from a more distal one.
Sensory cell bodies sit in the dorsal root ganglion. If the root is avulsed proximal to the ganglion, the cell body and its peripheral axon stay connected, so a sensory potential can still be recorded even though the signal never reaches the cord and the skin is anaesthetic.
A prefixed plexus takes a large contribution from C4 and little from T1 (effectively C4–C8); a postfixed plexus takes from T2 and little from C5 (C6–T2). Significant variation is found in roughly half of people.
Written for revision in the FRCS (Plast) style under our editorial standards. It is not clinical advice: always confirm against current guidance. Spotted an error? Tell us.
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