Brachial Plexus Anatomy

The brachial plexus is the intricate neural wiring that empowers every deliberate gesture and nuanced sensation in your arm, hand, and shoulder. For any aspiring clinician, a deep understanding of this structure is non-negotiable, as it is frequently implicated in trauma, childbirth injuries, and compressive syndromes, forming the basis for accurate diagnosis and effective treatment planning.

Foundations: Spinal Roots and Initial Formation

The brachial plexus originates from the ventral rami—the anterior branches—of spinal nerves C5, C6, C7, C8, and T1. Occasionally, contributions from C4 (pre-fixed) or T2 (post-fixed) may occur, but the classic model centers on these five roots. These ventral rami carry motor, sensory, and autonomic fibers destined for the upper limb. They emerge between the scalene muscles in the neck, where they converge to begin their organized rearrangement. Think of this stage as gathering the raw electrical cables before they are bundled and routed through the complex conduit of the shoulder region. A key clinical point is that injuries at this root level, such as from a cervical disc herniation, can mimic more distal plexus problems, making a systematic assessment critical.

Architectural Divisions: From Trunks to Cords

The reorganization of the ventral rami follows a consistent, mnemonic-friendly sequence: Roots, Trunks, Divisions, Cords, and Branches. The five roots (C5-T1) first combine to form three trunks. The C5 and C6 roots unite to become the upper trunk, C7 continues alone as the middle trunk, and C8 and T1 merge into the lower trunk. These trunks pass behind the clavicle, where each splits into an anterior division and a posterior division. The divisions then recombine to form the three cords, named for their spatial relationship to the axillary artery: the lateral cord (from anterior divisions of upper and middle trunks), the medial cord (from the anterior division of the lower trunk), and the posterior cord (from all three posterior divisions). This staged architecture ensures that fibers destined for anterior (flexor) and posterior (extensor) compartments of the limb are systematically sorted.

The Final Pathways: Major Terminal Nerves

The cords give rise to the five major terminal nerves that you will encounter constantly in clinical practice. Each has a distinct origin, course, and functional domain.

• Musculocutaneous Nerve: Arising from the lateral cord, it innervates the coracobrachialis, biceps brachii, and brachialis muscles—the primary flexors of the elbow. It then continues as the lateral cutaneous nerve of the forearm.
• Median Nerve: Formed by contributions from both the lateral and medial cords, it is the "eye of the hand," supplying most flexor muscles in the forearm and the thenar eminence (responsible for thumb opposition). It provides sensation to the palmar aspect of the thumb, index, middle, and lateral half of the ring finger.
• Ulnar Nerve: Derives from the medial cord. It innervates intrinsic hand muscles (except some thenar muscles), the flexor carpi ulnaris, and the medial half of the flexor digitorum profundus. Its sensory territory includes the little finger and the medial half of the ring finger.
• Radial Nerve: The continuation of the posterior cord, it is the great extensor nerve. It supplies the triceps brachii, all muscles in the posterior forearm (extensors), and provides sensation to the posterior arm, forearm, and the dorsal aspect of the lateral hand.
• Axillary Nerve: Also from the posterior cord, it curls around the surgical neck of the humerus to innervate the deltoid and teres minor muscles, enabling shoulder abduction and external rotation.

Consider a patient attempting to lift a coffee cup: the musculocutaneous nerve initiates elbow flexion, the median nerve stabilizes the wrist and allows the thumb to grasp, and the radial nerve subsequently extends the fingers to release it—a symphony conducted by the brachial plexus.

When Things Go Wrong: Upper and Lower Plexus Injuries

Injuries to specific segments of the plexus produce classic, predictable syndromes. Understanding the involved anatomy allows you to localize the lesion precisely.

Erb-Duchenne Palsy results from excessive traction on the upper trunk (C5-C6 roots), commonly seen in motorcycle accidents (forceful separation of head and shoulder) or during difficult childbirth. This injury leads to a characteristic "waiter's tip" posture: the arm hangs adducted and internally rotated at the shoulder, the elbow is extended, and the forearm is pronated. This occurs due to paralysis of the deltoid (axillary nerve), supraspinatus/infraspinatus (suprascapular nerve), biceps (musculocutaneous nerve), and brachioradialis (radial nerve). The patient cannot abduct the shoulder, externally rotate the arm, or flex the elbow.

In contrast, Klumpke Palsy involves the lower trunk (C8-T1 roots), often from a sudden upward pull on the arm, as in grabbing a ledge during a fall. This damages nerves derived from the medial cord, primarily affecting the ulnar nerve and the median nerve's contributions to the hand. The result is a "claw hand" deformity: hyperextension at the metacarpophalangeal joints and flexion at the interphalangeal joints (especially in the 4th and 5th digits), due to loss of intrinsic hand muscle function. There is also sensory loss along the ulnar border of the hand and forearm, and, if T1 sympathetic fibers are involved, an ipsilateral Horner's syndrome (ptosis, miosis, anhidrosis) may be present.

Common Pitfalls

1. Confusing Median and Ulnar Nerve Clawing: A true "ulnar claw" hand is most apparent when the patient attempts to extend their fingers, as the unopposed pull of the long extensors causes hyperextension at the MCP joints. A "median nerve" palsy primarily affects thumb function (opposition), not causing the classic claw. Remember, the lumbricals to the index and middle fingers are median-innervated and help prevent clawing in those digits.
2. Overlooking the Thoracic Outlet: When assessing lower trunk (C8-T1) symptoms like hand weakness and ulnar sensory loss, clinicians may jump to an ulnar neuropathy at the elbow. Always consider thoracic outlet syndrome, where the lower trunk can be compressed by a cervical rib or fibrous band between the scalene muscles, producing similar symptoms.
3. Misattributing Shoulder Pain: An upper trunk lesion (Erb's palsy) causes profound shoulder weakness. However, persistent pain after the acute phase should prompt evaluation for a concomitant rotator cuff tear or glenohumeral joint injury, which are common in the same traumatic mechanisms and require different treatment.
4. Forgetting Autonomic Fibers: The lower trunk carries sympathetic fibers from T1. In a lower plexus injury, the absence of an ipsilateral Horner's syndrome might suggest a more distal lesion, while its presence confirms a very proximal injury at the root level, impacting prognosis.

Summary

• The brachial plexus is formed from the ventral rami of spinal nerves C5 through T1, organized sequentially into Roots, Trunks, Divisions, Cords, and terminal Branches.
• Its five major nerves—musculocutaneous, median, ulnar, radial, and axillary—each have specific motor and sensory territories that dictate the functional loss when injured.
• Erb-Duchenne palsy (upper trunk, C5-C6) results in a "waiter's tip" posture with loss of shoulder abduction and elbow flexion.
• Klumpke palsy (lower trunk, C8-T1) leads to a "claw hand" and may include Horner's syndrome due to involvement of sympathetic fibers.
• Clinical assessment must proceed anatomically, from history and physical exam to electrodiagnostics, to localize the lesion accurately and guide appropriate management.
• Always consider anatomical variations and rule out mimicking conditions, such as thoracic outlet syndrome for lower trunk symptoms or peripheral nerve entrapments.