brachial plexus
Summary
The brachial plexus is a network of nerves formed by the ventral rami of C5-T1 that innervates the upper limb, providing motor and sensory function to the shoulder, arm, forearm, and hand. It is organized into Roots, Trunks, Divisions, Cords, and Branches (mnemonic: Randy Travis Drinks Cold Beer). Injuries to specific parts produce characteristic clinical syndromes.
Detail
The brachial plexus arises from the anterior rami of spinal nerves C5-T1 and is organized proximal to distal as: Roots (C5-T1) → Trunks (upper, middle, lower) → Divisions (anterior/posterior) → Cords (lateral, posterior, medial, named by relation to axillary artery) → Branches (terminal nerves: musculocutaneous, axillary, radial, median, ulnar). Key clinical correlations: Erb-Duchenne palsy (upper trunk, C5-C6) from traction injury during birth or falls causes 'waiter's tip' deformity (arm adducted, internally rotated, forearm pronated) due to loss of shoulder abduction (deltoid, supraspinatus) and elbow flexion (biceps). Klumpke palsy (lower trunk, C8-T1) results from upward traction injury (e.g., grabbing a branch while falling) causing claw hand due to loss of intrinsic hand muscles, often with Horner syndrome if T1 sympathetic fibers are affected. Posterior cord injury affects axillary and radial nerves causing wrist drop and deltoid/triceps weakness. Long thoracic nerve injury (from roots, not part of terminal branches) causes winged scapula due to serratus anterior paralysis. Understanding the plexus is essential for localizing lesions based on the pattern of motor/sensory loss, critical for both anatomy and clinical neurology boards.
Sources
- Moore's Clinically Oriented Anatomy
- First Aid for the USMLE Step 1
- Kaplan USMLE Step 1 Anatomy Lecture Notes
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