Brachial Plexus 1 — MRCS Part B Anatomy

What this station tests

The axilla opened from the front: the muscles that form its walls, the axillary vessels, and the cords of the brachial plexus wrapped around the artery. It then turns clinical — the directions the shoulder dislocates, how to reduce it, and what an axillary nerve injury looks like.

Walls of the axilla

• Anterior: pectoralis major and pectoralis minor, with the clavipectoral fascia between them.
• Posterior: subscapularis, teres major and latissimus dorsi.
• Medial: serratus anterior over the upper ribs.
• Lateral: the intertubercular groove of the humerus, with coracobrachialis and the short head of biceps.
• Apex: between the clavicle, the first rib and the upper border of the scapula — the route by which vessels and nerves enter from the neck.

How the plexus is built

The anterior rami of C5 to T1 are the roots (a prefixed plexus also takes C4, a postfixed one T2). They join into upper (C5–C6), middle (C7) and lower (C8–T1) trunks in the posterior triangle. Behind the clavicle each trunk splits into an anterior and a posterior division. In the axilla the divisions form three cords, named by where they lie around the second part of the axillary artery:

• Lateral cord — anterior divisions of the upper and middle trunks. Gives the musculocutaneous nerve and the lateral root of the median nerve.
• Medial cord — anterior division of the lower trunk. Gives the ulnar nerve, the medial root of the median nerve, and the medial cutaneous nerves of the arm and forearm.
• Posterior cord — all three posterior divisions. Gives the axillary and radial nerves, the subscapular nerves and the thoracodorsal nerve to latissimus dorsi.

Anterior divisions supply flexors and posterior divisions extensors, which is why every extensor of the upper limb is on the radial or axillary nerve.

Shoulder dislocation

Anterior dislocation is by far the commonest, from a fall on the abducted, externally rotated arm. Posterior dislocation follows seizures and electric shock and is easily missed on an AP film (the “light bulb” humeral head). Inferior dislocation (luxatio erecta) presents with the arm held above the head. Check and document axillary nerve function and distal pulses before and after any reduction, and confirm reduction on a radiograph.

Axillary nerve

From the posterior cord (C5–C6), it passes through the quadrangular space with the posterior circumflex humeral artery and winds around the surgical neck of the humerus. Injury in dislocation or surgical neck fracture weakens abduction beyond the first few degrees (deltoid) and numbs the skin over the lower deltoid, the “regimental badge” area.