The brachial plexus: roots, trunks, cords and the nerves of the arm

1. Roots between the scalenes

The plexus is formed by the anterior rami of C5 to T1. The roots leave the intervertebral foramina and pass between scalenus anterior and scalenus medius, together with the subclavian artery; the subclavian vein stays in front of scalenus anterior. From the roots: the dorsal scapular nerve (C5, rhomboids and levator scapulae) and the long thoracic nerve (C5–C7, serratus anterior).

Surgeon’s corner: The interscalene block puts local anaesthetic around the roots between the two scalenes: ideal for shoulder surgery, but it almost always blocks the phrenic nerve (C3–C5, on the front of scalenus anterior) and paralyses half the diaphragm. Avoid it in patients with poor lung function.

2. Three trunks above the clavicle

C5 and C6 join to form the superior trunk, C7 continues alone as the middle trunk, and C8 and T1 form the inferior trunk, which lies on the first rib behind the subclavian artery. The trunks cross the posterior triangle of the neck. The superior trunk gives the suprascapular nerve and the nerve to subclavius.

Surgeon’s corner: Upper plexus injury (C5–C6, Erb): the head and shoulder are forced apart (motorbike fall, shoulder dystocia). The arm hangs adducted and internally rotated with the elbow extended and the forearm pronated: the "waiter’s tip". Lower plexus injury (C8–T1, Klumpke): the arm is pulled up; the intrinsic muscles of the hand are weak (claw hand) and T1 avulsion can cause Horner’s syndrome.

3. Divisions and cords

Behind the clavicle each trunk splits into an anterior division (flexor compartments) and a posterior division (extensor compartments). The anterior divisions of the superior and middle trunks form the lateral cord; the anterior division of the inferior trunk forms the medial cord; all three posterior divisions form the posterior cord. The cords are named by their position around the axillary artery behind pectoralis minor.

Surgeon’s corner: Memory aid, roots to branches: Real Teenagers Drink Cold Beer (roots, trunks, divisions, cords, branches). The infraclavicular block is placed around the cords, next to the axillary artery under pectoralis minor.

4. The axillary artery and its branches

The subclavian artery becomes the axillary artery at the outer border of the first rib and the brachial artery at the lower border of teres major. Pectoralis minor divides it into three parts with one, two and three branches: superior thoracic; thoraco-acromial and lateral thoracic; subscapular, anterior and posterior circumflex humeral.

Surgeon’s corner: The scapular anastomosis (suprascapular and dorsal scapular arteries with the subscapular artery) keeps the arm alive if the artery is tied between the first rib and the subscapular origin. In elderly patients an anterior dislocation or its reduction can tear the axillary artery: always check the radial pulse.

5. The axillary nerve and the quadrangular space

The axillary nerve (C5–C6) leaves the posterior cord, runs below the shoulder joint capsule and passes backwards through the quadrangular space (teres minor above, teres major below, long head of triceps medially, surgical neck of the humerus laterally) with the posterior circumflex humeral artery. It supplies the deltoid and teres minor and the skin over the lower deltoid through the superior lateral cutaneous nerve of the arm.

Surgeon’s corner: Injured in anterior shoulder dislocation (up to a third of dislocations in older patients), surgical neck fractures and deltoid-splitting approaches (the nerve lies about 5–7 cm below the acromion). Signs: weak abduction beyond the first 15°, a flattened shoulder with time, and numbness over the "regimental badge" area. Test sensation before and after every reduction.

6. The radial nerve: the end of the posterior cord

The radial nerve (C5–T1) is the continuation of the posterior cord. It leaves the axilla below teres major through the triangular interval, then winds round the back of the humerus in the radial groove with the deep brachial artery, between the lateral and medial heads of triceps. It supplies triceps and, in the forearm, all the extensors; its sensory branch supplies the back of the hand over the first web space.

Surgeon’s corner: Humeral shaft fractures (especially spiral distal-third fractures), crutches pressing in the axilla and sleeping with the arm over a chair back ("Saturday night palsy") injure it. Sign: wrist and finger drop with numbness over the first dorsal web space; elbow extension is spared when the lesion is below the branches to triceps.

7. Lateral cord: the musculocutaneous nerve

The lateral cord gives the lateral pectoral nerve, the musculocutaneous nerve and the lateral root of the median nerve. The musculocutaneous nerve (C5–C7) pierces coracobrachialis, runs between biceps and brachialis and supplies the three flexors of the arm, then becomes the lateral cutaneous nerve of the forearm.

Surgeon’s corner: It enters coracobrachialis only 5–8 cm below the coracoid: strong medial retraction of the conjoint tendon in a deltopectoral approach (Latarjet, shoulder arthroplasty) stretches it. Isolated injury: weak elbow flexion and supination with numbness on the lateral forearm.

8. Medial cord: ulnar and median nerves

The medial cord gives the medial pectoral nerve, the medial cutaneous nerves of the arm and forearm, the ulnar nerve (C8–T1) and the medial root of the median nerve. The two roots of the median nerve join in front of the axillary artery (the "M" of the plexus). In the arm the median nerve runs with the brachial artery, crossing it from lateral to medial; the ulnar nerve runs down the medial side to pass behind the medial epicondyle.

Surgeon’s corner: Ulnar nerve: compressed at the elbow (cubital tunnel) and at the wrist (Guyon’s canal); weak intrinsic muscles, clawing of the ring and little fingers, positive Froment’s sign. Median nerve: carpal tunnel syndrome at the wrist; a high lesion (supracondylar fracture in children, stab wounds) loses thumb, index and middle finger flexion ("benediction sign" when making a fist).

9. Nerves to the shoulder girdle

Long thoracic nerve (C5–C7, from the roots): runs down the outer surface of serratus anterior. Suprascapular nerve (superior trunk): through the suprascapular notch to supraspinatus and infraspinatus. Upper and lower subscapular nerves and the thoracodorsal nerve (posterior cord): subscapularis, teres major and latissimus dorsi.

Surgeon’s corner: Long thoracic nerve injury (axillary node dissection, first rib resection, carrying heavy bags) causes medial winging of the scapula when the patient pushes against a wall. The thoracodorsal nerve is kept in axillary dissection and is the pedicle of the latissimus dorsi flap. A ganglion at the spinoglenoid notch compresses the suprascapular nerve: isolated infraspinatus wasting.