Brachial Plexus: Roots, Trunks, Cords and the Injuries That Matter

Almost nobody fails a brachial plexus question because they do not know the anatomy. They fail because they cannot draw it fast enough under pressure. Learn it as five stages in a fixed order and it becomes a two-minute diagram.

At a glance

Building the diagram

Draw five roots. Join C5 and C6 into the upper trunk, leave C7 alone as the middle trunk, join C8 and T1 into the lower trunk. Split every trunk into an anterior and a posterior division. Then combine: all three posterior divisions form the posterior cord; the anterior divisions of upper and middle trunks form the lateral cord; the anterior division of the lower trunk alone continues as the medial cord.

The logic is worth holding on to: anterior divisions supply flexor compartments, posterior divisions supply extensor compartments. That single rule tells you that the radial and axillary nerves must come off the posterior cord, and that the musculocutaneous and ulnar nerves cannot.

Branches

From the roots

• Dorsal scapular nerve (C5) — rhomboids and levator scapulae.
• Long thoracic nerve (C5, C6, C7) — serratus anterior. Remember “C5, 6, 7 — bells of heaven”.
• Contributions to the phrenic nerve (C5) and branches to scalenes and longus colli.

From the upper trunk

• Suprascapular nerve (C5, C6) — supraspinatus and infraspinatus.
• Nerve to subclavius.

Nothing arises from the middle or lower trunk, and nothing from the divisions.

From the cords

The two roots of the median nerve, one from each of the lateral and medial cords, clasp the axillary artery anteriorly — the classical M shape that makes the plexus recognisable in a dissection.

Clinical syndromes

Erb’s palsy (upper trunk, C5–C6)

Caused by forcible widening of the angle between the neck and shoulder — shoulder dystocia at birth, or a fall onto the shoulder. Deltoid, supraspinatus, infraspinatus, biceps and brachialis are paralysed. The arm hangs adducted and medially rotated, the elbow extended, the forearm pronated: the waiter’s tip position. Sensory loss runs down the lateral arm and forearm. The biceps reflex is lost.

Klumpke’s palsy (lower trunk, C8–T1)

Caused by forcible abduction of the arm above the head — grabbing at something while falling, or a breech delivery. The small muscles of the hand are paralysed, producing a claw hand, with sensory loss along the medial forearm and hand. If the T1 root is avulsed proximal to the sympathetic contribution, Horner’s syndrome appears on the same side — ptosis, miosis, anhidrosis, apparent enophthalmos — and its presence indicates a severe, proximal, avulsion-type injury with a poor prognosis.

Long thoracic nerve injury

Serratus anterior is paralysed, so the scapula loses its anchor against the chest wall. Winging appears, made obvious by asking the patient to push against a wall, and the arm cannot be abducted beyond about 90 degrees because the scapula cannot rotate. The nerve is vulnerable during axillary surgery and in radical mastectomy.

Axillary nerve injury

Typically from surgical neck fracture of the humerus or anterior shoulder dislocation. Deltoid wastes, abduction is weak, and there is a patch of sensory loss over the regimental badge area on the lateral shoulder — which must be tested before and after reducing any dislocated shoulder.

Last-minute checklist

• Randy Travis Drinks Cold Beer, with divisions behind the clavicle.
• Cords are named for their relation to the second part of the axillary artery.
• ULTRA for the posterior cord.
• Erb’s: waiter’s tip. Klumpke’s: claw hand, plus Horner’s if T1 is avulsed.
• Winged scapula equals long thoracic nerve.

Frequently asked questions

Why are the cords named lateral, medial and posterior?

Because of their positions around the second part of the axillary artery, not their positions in the neck. Anatomical names here describe a relationship, not a location.

Why does Horner’s syndrome accompany some Klumpke’s palsies?

Preganglionic sympathetic fibres for the head and neck leave the cord in the T1 ventral ramus. An avulsion proximal to where they branch off takes the sympathetic supply with it.

Why can the arm not be raised above the shoulder after long thoracic nerve injury?

Full abduction requires upward rotation of the scapula, which serratus anterior performs together with trapezius. Without it the glenoid cannot be tilted upwards.

References

• Gray’s Anatomy for Students — the brachial plexus.
• B. D. Chaurasia’s Human Anatomy, Volume 1 — upper limb.
• Snell’s Clinical Anatomy by Regions.