What is the anatomy and physiology of the brachial plexus?

Anatomy and Physiology of the Brachial Plexus

The brachial plexus is a complex neural network formed from the ventral rami of spinal nerves C5-T1 (occasionally including C4 and/or T2), which supplies motor, sensory, and sympathetic innervation to the upper extremity through its terminal branches. 1

Anatomical Organization

The brachial plexus follows a systematic organization that can be remembered as a progression from 5 roots to 5 terminal nerves:

Components

1. Roots: Ventral rami of C5-T1 spinal nerves
2. Trunks: Three trunks form from the roots
   • Upper trunk (C5-C6)
   • Middle trunk (C7)
   • Lower trunk (C8-T1)
3. Divisions: Each trunk divides into anterior and posterior divisions (6 total)
4. Cords: Three cords form from the divisions
   • Lateral cord (anterior divisions of upper and middle trunks)
   • Medial cord (anterior division of lower trunk)
   • Posterior cord (posterior divisions of all three trunks)
5. Terminal branches: Five major nerves emerge from the cords
   • Musculocutaneous nerve (from lateral cord)
   • Median nerve (from lateral and medial cords)
   • Ulnar nerve (from medial cord)
   • Axillary nerve (from posterior cord)
   • Radial nerve (from posterior cord) 2

Anatomical Course

The brachial plexus emerges between the anterior and middle scalene muscles alongside the subclavian artery. The nerve roots form trunks, which then split into anterior and posterior divisions. These divisions reorganize to form cords named for their relationship to the axillary artery (lateral, medial, and posterior). The cords form terminal branches at the lateral margin of the pectoralis minor muscle and continue through the axilla. 1

Functional Physiology

Innervation Patterns

• Upper trunk (C5-C6): Primarily innervates proximal muscles of the shoulder and upper arm
• Middle trunk (C7): Contributes to innervation of the arm and forearm
• Lower trunk (C8-T1): Primarily supplies hand and wrist function 3

Functional Distribution

1. Motor Function: The brachial plexus provides motor innervation to:

   • All muscles of the upper limb
   • Some muscles of the neck and shoulder girdle
   • Specific distribution by terminal nerves:
     • Musculocutaneous nerve: Anterior arm muscles (biceps, brachialis, coracobrachialis)
     • Median nerve: Most anterior forearm muscles and thenar muscles
     • Ulnar nerve: Most intrinsic hand muscles and medial forearm muscles
     • Axillary nerve: Deltoid and teres minor
     • Radial nerve: Posterior compartment muscles of arm and forearm 4

2. Sensory Function: Provides sensory innervation to:

   • Entire upper limb except for the medial aspect of the arm (intercostobrachial nerve, T2)
   • Specific cutaneous distributions corresponding to each terminal nerve 4

3. Sympathetic Function: Carries sympathetic fibers that:

   • Regulate vasomotor activity
   • Control sweat glands
   • Influence temperature regulation in the upper limb 1

Clinical Significance

Plexopathy Manifestations

Brachial plexus injuries (plexopathies) can manifest as:

• Neuropathic pain in multiple peripheral nerve distributions
• Dysesthesia or burning/electric sensations
• Weakness or paralysis in affected muscle groups
• Sensory loss in corresponding dermatomes
• Flaccid loss of tendon reflexes
• Potential temperature and color changes due to autonomic dysfunction 1, 5

Localization of Injuries

• Upper trunk injuries (C5-C6): Result in shoulder and elbow dysfunction (Erb's palsy)
• Lower trunk injuries (C8-T1): Cause hand and wrist dysfunction (Klumpke's palsy)
• Complete plexus injury: Results in total limb paralysis 5

Common Variations

Anatomical variations in the brachial plexus are common and clinically significant:

• Prefixed plexus: Greater contribution from C4, lesser from T1
• Postfixed plexus: Lesser contribution from C5, greater from T2
• Variations in cord formation and branching patterns 6, 7

Diagnostic Evaluation

MRI of the brachial plexus is the gold standard imaging modality (sensitivity 84%, specificity 91%) and should include:

• T1-weighted sequences
• T2-weighted sequences
• Fat-saturated T2-weighted or STIR sequences 5

For optimal visualization after trauma, MRI evaluation should be delayed approximately one month to allow resolution of hemorrhage and edema. 5

Clinical Pearls

• The brachial plexus can be affected by various pathologies including trauma, neoplasms, radiation therapy, inflammatory processes, and entrapment syndromes 5
• Understanding the complex anatomy of the brachial plexus is essential for accurate diagnosis and management of upper extremity neurological deficits
• Anatomical variations are common and must be considered during surgical procedures in the neck, axilla, and upper limb 7
• The optimal window for surgical intervention in traumatic injuries is within 1-3 months post-injury 5

Understanding the detailed anatomy and physiology of the brachial plexus is crucial for clinicians to accurately diagnose and effectively manage conditions affecting the upper extremity.