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Roots & Trunks

Roots (C5–T1)

Ventral rami of C5 to T1 emerge between the scalenus anterior and medius. C4 and T2 may contribute, giving prefixed and postfixed plexuses respectively.

Trunks

• Upper — C5 + C6; gives suprascapular nerve and nerve to subclavius.
• Middle — C7 alone; no branches.
• Lower — C8 + T1; lies behind the subclavian artery.

Divisions

Anterior divisions

Supply the flexor compartments. Upper and middle join to form the lateral cord; the lower anterior division continues as the medial cord.

Posterior divisions

All three unite behind the axillary artery to form the posterior cord, which supplies every extensor of the upper limb.

Landmark

Divisions lie behind the clavicle — the retroclavicular part of the plexus, and the reason clavicular fractures can present with neurological signs.

Cords of the Brachial Plexus

Lateral cord

Formed by the anterior divisions of the upper and middle trunks. Gives the lateral pectoral nerve, musculocutaneous nerve and the lateral root of the median nerve.

Medial cord

Continuation of the anterior division of the lower trunk. Gives the medial pectoral nerve, medial cutaneous nerves of arm and forearm, ulnar nerve and the medial root of the median nerve.

Posterior cord

Formed by all three posterior divisions. Gives the upper and lower subscapular nerves, thoracodorsal nerve, axillary nerve and radial nerve.

Terminal Branches

Five terminal nerves

• Musculocutaneous — anterior arm flexors; lateral forearm sensation.
• Axillary — deltoid and teres minor; regimental badge sensation.
• Median — most forearm flexors, thenar muscles, radial 3½ digits.
• Radial — all extensors of arm and forearm; dorsum of hand.
• Ulnar — intrinsic hand muscles, ulnar 1½ digits.

Checkpoint

Which cord supplies the deltoid? The posterior cord, via the axillary nerve.

Applied Anatomy

Brachial plexus block

Interscalene approach targets the roots and upper trunk; supraclavicular the divisions; infraclavicular the cords; axillary the terminal branches.

Thoracic outlet syndrome

A cervical rib or fibrous band elevates the lower trunk, producing C8–T1 sensory loss along the medial forearm with wasting of the intrinsic hand muscles.

Rapid Revision

High-yield associations

• Winged scapula — long thoracic nerve (C5–C7), serratus anterior.
• Claw hand — ulnar nerve / lower trunk.
• Wrist drop — radial nerve at the spiral groove.
• Ape thumb — median nerve, thenar wasting.
• Horner syndrome — T1 root involvement, Pancoast tumour.