Conquer the Brachial Plexus — The Mnemonic-Friendly Visual Guide

Why the Brachial Plexus Matters

The brachial plexus is the network of nerves supplying the entire upper limb — motor and sensory. Injury to it from trauma (motorcycle accidents, traction injuries during difficult labour, shoulder dislocations) produces predictable patterns of muscle paralysis and sensory loss that can only be correctly diagnosed — and correctly assessed for prognosis — if you understand the anatomy. The brachial plexus features on virtually every medical licensing exam in the English-speaking world, because getting the anatomy right is the difference between diagnosing a C5–C6 root avulsion and missing it.

The Five Levels — Roots, Trunks, Divisions, Cords & Branches

The brachial plexus has five sequential anatomical levels, remembered by the mnemonic: "Robert Taylor Drinks Cold Beer"

• Roots
• Trunks
• Divisions
• Cords
• Branches

1. Roots (C5, C6, C7, C8, T1)

The brachial plexus arises from the anterior rami (ventral rami) of spinal nerves C5, C6, C7, C8, and T1. These emerge between the anterior and middle scalene muscles in the posterior triangle of the neck.

Root-level branches (leave before forming trunks):

• Long thoracic nerve (C5, C6, C7) — innervates serratus anterior. Damage → winging of scapula (scapula rises off ribcage on push against resistance).
• Dorsal scapular nerve (C5) — innervates rhomboids and levator scapulae.
• Nerve to subclavius (C5, C6).
• Suprascapular nerve (C5, C6) — leaves the upper trunk → supraspinatus and infraspinatus. "Suprascapular notch syndrome" causes posterior shoulder pain and rotator cuff weakness.

2. Trunks (3)

• Upper Trunk (C5 + C6)
• Middle Trunk (C7 alone)
• Lower Trunk (C8 + T1)

Formed between the anterior and middle scalene muscles in the posterior triangle of the neck, anterior to the subclavian artery.

3. Divisions (6)

Each of the three trunks divides into an anterior and a posterior division — giving six divisions total. These pass posterior to the clavicle. No named branches arise at this level.

• Anterior divisions → eventually supply flexor (anterior) compartments
• Posterior divisions → eventually supply extensor (posterior) compartments

4. Cords (3) — Named by Relation to Axillary Artery

• Lateral Cord — from anterior divisions of upper + middle trunks (C5–C7)
• Medial Cord — from anterior division of lower trunk (C8–T1)
• Posterior Cord — from posterior divisions of all three trunks (C5–T1)

5. Terminal Branches (5 main + multiple smaller)

Mnemonic — "My Aunt Rides A Mule"

Nerve	Origin	Roots	Primary Supply
Musculocutaneous	Lateral cord	C5–C7	Biceps, brachialis, coracobrachialis; lateral forearm sensation
Axillary	Posterior cord	C5–C6	Deltoid, teres minor; skin over deltoid (regimental badge)
Radial	Posterior cord	C5–T1	All posterior compartment muscles of arm and forearm; dorsum of hand
Median	Lateral + medial cords	C6–T1	Anterior forearm (most), thenar muscles, lateral 3½ digits
Ulnar	Medial cord	C8–T1	Hypothenar muscles, intrinsics, medial 1½ digits

Brachial Plexus Injuries — Clinical Patterns

Erb's Palsy — Upper Trunk Injury (C5–C6)

Mechanism: Forceful separation of head and shoulder (traction) → stretching or avulsing the upper trunk. Classic obstetric injury during difficult delivery, or motorcycle accidents. Classic posture: "Waiter's tip" — arm hangs at side, internally rotated and extended (forearm pronated, wrist flexed) — because shoulder abductors (supraspinatus, deltoid), external rotators, elbow flexors (biceps, brachialis), and supinators are all paralysed. Sensation lost over the lateral arm and forearm.

Klumpke's Palsy — Lower Trunk Injury (C8–T1)

Mechanism: Forceful abduction of arm upwards (grabbing a rail during a fall, pulling on an outstretched arm during delivery). Classic pattern: "Claw hand" — loss of intrinsic hand muscles (interossei, lumbricals, hypothenar and thenar) → MCP joint hyperextension + PIP/DIP flexion. Medial forearm and ulnar border of hand anaesthesia. If T1 root involved → Horner's syndrome (ptosis, miosis, anhidrosis) from interruption of preganglionic sympathetic fibres.

Crutch Palsy / Saturday Night Palsy — Radial Nerve at Spiral Groove

The radial nerve winds around the posterior aspect of the humerus in the spiral (radial) groove — rendering it vulnerable to compression between the humerus and an external surface (arm over back of chair, crutch in axilla, humeral shaft fracture). Result: Wrist drop (paralysis of wrist and finger extensors), spared triceps (branch leaves before spiral groove), impaired sensation over anatomical snuffbox and first dorsal web space.

Carpal Tunnel Syndrome — Median Nerve Compression

The most common peripheral nerve entrapment — median nerve compressed beneath the flexor retinaculum at the wrist. Symptoms: Tingling and numbness in the lateral 3½ digits (thumb, index, middle, and lateral half of ring finger), wasting of thenar eminence, positive Tinel's sign (tapping over carpal tunnel → paraesthesia) and Phalen's test (sustained wrist flexion → symptoms). Treatment: wrist splinting, corticosteroid injection, surgical decompression.

Ulnar Nerve Lesion — "Cubital Tunnel Syndrome"

Ulnar nerve most vulnerable posterior to medial epicondyle (cubital tunnel) or at Guyon's canal (wrist). Pattern: Loss of intrinsic hand muscles → "ulnar claw" — hyperextension at MCPs and flexion at PIPs/DIPs of ring and little fingers (worse with low lesions where long flexors are preserved). Sensation loss over medial 1½ fingers and medial palm. Note: ulnar claw is LESS severe with high lesions (at elbow) because finger flexors are also lost, reducing the clawing.

For educational purposes only. Medical disclaimer →