What is the structure and function of the brachial plexus and how can it be remembered?

Brachial Plexus: Detailed Structure, Function, and Memory Techniques

Anatomical Organization

The brachial plexus is best understood as a sequential transformation from 5 nerve roots into 5 terminal nerves, passing through intermediate stages organized in sets of threes. 1

The "5-3-3-3-5" Pattern

The plexus follows this memorable numerical sequence:

• 5 Roots → C5, C6, C7, C8, and T1 ventral rami (occasionally with contributions from C4 or T2) 2, 3
• 3 Trunks → Superior, middle, and inferior
• 3 Divisions → Each trunk splits into anterior and posterior divisions (6 total divisions, but 3 pairs) 2
• 3 Cords → Lateral, posterior, and medial 2, 1
• 5 Terminal Nerves → Musculocutaneous, median, ulnar, axillary, and radial 1

Anatomical Pathway Through the Neck

The nerve roots emerge and pass between the anterior and middle scalene muscles alongside the subclavian artery to form the trunks 2. This is a critical anatomical relationship because variations can occur where roots emerge anterior to the anterior scalene muscle, which has implications for nerve blocks and compression syndromes 4.

The cords travel with the subclavian artery and vein in the infraclavicular region, then form terminal branches at the lateral margin of the pectoralis minor muscle, continuing through the axilla 2.

Functional Organization

Trunk Formation and Division Pattern

• Superior trunk (C5-C6): Divides into anterior and posterior divisions
• Middle trunk (C7): Divides into anterior and posterior divisions
• Inferior trunk (C8-T1): Divides into anterior and posterior divisions 1

Cord Formation Rule

All posterior divisions unite to form the posterior cord, while anterior divisions split to form lateral and medial cords 1. This is functionally significant because:

• Posterior cord structures innervate extensor compartments
• Lateral and medial cord structures innervate flexor compartments

Terminal Branch Distribution

Each cord gives rise to exactly two branches 1:

• Lateral cord → Musculocutaneous nerve + lateral contribution to median nerve 1
• Medial cord → Ulnar nerve + medial contribution to median nerve 1
• Posterior cord → Axillary nerve + radial nerve 1

Memory Techniques

The "Randy Travis Drinks Cold Beer" Mnemonic

For the sequential organization:

• Roots
• Trunks
• Divisions
• Cords
• Branches 5

The "M-M-U-A-R" Terminal Nerve Mnemonic

From lateral to medial, the five terminal nerves spell out a pattern:

• Musculocutaneous (lateral cord)
• Median (lateral + medial cords)
• Ulnar (medial cord)
• Axillary (posterior cord)
• Radial (posterior cord) 1

Numerical Pattern Memory Aid

Remember "5 becomes 3 becomes 3 becomes 5" - this captures the entire transformation from roots through terminal branches 1.

Functional Division Memory

"Posterior = Extensors, Anterior = Flexors" - All posterior divisions merge into the posterior cord, which supplies extensor muscles via axillary and radial nerves, while anterior divisions supply flexor compartments 1.

Clinical Significance

Preganglionic vs Postganglionic Lesions

Distinguishing between preganglionic (nerve root) and postganglionic (plexus) injuries is essential because treatment approaches differ fundamentally 2. Preganglionic injuries occur proximal to the dorsal root ganglion and have worse prognosis, while postganglionic injuries may be amenable to surgical repair 5.

Common Pathological Patterns

The inferior trunk (C8-T1) is particularly vulnerable to invasion by superior sulcus lung tumors (Pancoast tumors), which characteristically present with Horner syndrome and pain in the ulnar nerve distribution 6.

Primary tumors affecting the plexus are most commonly benign schwannomas and neurofibromas, with malignant peripheral nerve sheath tumors being rare and associated with neurofibromatosis type 1 6.

Anatomical Variations

Prefixed plexuses (with significant C4 contribution) and postfixed plexuses (with significant T2 contribution) occur with enough frequency that clinicians must consider them when interpreting imaging or planning interventions 3, 4. These variations can affect the success of interscalene nerve blocks and surgical approaches 4.

Diagnostic Approach

MRI of the brachial plexus is the primary imaging modality for evaluating plexopathies, with sensitivity of 81%, specificity of 91%, and overall accuracy of 88% 6. The superior soft-tissue contrast and spatial resolution allow detailed visualization of intraneural anatomy and localization of pathologic lesions 6.

Clinical diagnosis is confirmed through electrodiagnostic studies, which help differentiate between various etiologies when imaging findings are nonspecific 2.