Brachial Plexus

Root Origin

Spinal nerve roots: C5, C6, C7, C8, T1
Prefixed plexus: C4-C8 (the plexus shifts one level higher — uncommon variant)
Postfixed plexus: C6-T2 (the plexus shifts one level lower — uncommon variant)
Location of formation: Posterior triangle of the neck, between the anterior and middle scalene muscles

Plexus Architecture

The brachial plexus has five levels of organization. Each level is a clinical landmark for lesion localization.

Roots (C5-T1)

The five ventral rami exit the intervertebral foramina and pass between the anterior and middle scalene muscles (the interscalene gap). At this level, two important pre-trunk branches exit directly from the roots:

Branch	Root	Muscle(s) Supplied	Page
Dorsal scapular nerve	C5	Rhomboids, levator scapulae	anatomy/nerves/dorsal-scapular-nerve
Long thoracic nerve	C5, C6, C7	Serratus anterior	anatomy/nerves/long-thoracic-nerve

Both nerves pierce the middle scalene — scalene pathology can compress them at their origin.

Trunks

The five roots merge into three trunks in the posterior triangle of the neck:

Trunk	Roots	Clinical Significance
Upper trunk	C5 + C6	Lesion = Erb's palsy ("waiter's tip" — loss of shoulder abduction, external rotation, elbow flexion, forearm supination)
Middle trunk	C7 alone	Lesion = isolated C7 deficits (triceps weakness, wrist drop, finger extension weakness) — rare in isolation
Lower trunk	C8 + T1	Lesion = Klumpke's palsy (loss of intrinsic hand muscles, finger flexors, "claw hand" — historically from birth traction injury, now more commonly from lower plexus traction in adults)

Erb's point — the junction where C5 and C6 merge to form the upper trunk — is a critical anatomical landmark. The suprascapular nerve branches from the upper trunk at this point:

Branch	Trunk	Muscle(s) Supplied	Page
Suprascapular nerve	Upper trunk (C5, C6)	Supraspinatus, infraspinatus	anatomy/nerves/suprascapular-nerve

Divisions

Each trunk splits into an anterior and a posterior division behind the clavicle. This is the only level with no named branches — divisions are a routing system, not a clinical landmark.

Division	Destination	Functional Significance
Anterior divisions	Lateral and medial cords	Supply flexor compartments of the arm and forearm
Posterior divisions	Posterior cord	Supply extensor compartments of the arm and forearm

Cords

The six divisions recombine into three cords, named for their relationship to the axillary artery:

Cord	Formed From	Relationship to Axillary Artery
Lateral cord	Anterior divisions of upper and middle trunks (C5, C6, C7)	Lateral to the artery
Posterior cord	All three posterior divisions (C5-T1)	Posterior to the artery
Medial cord	Anterior division of lower trunk (C8, T1)	Medial to the artery

Terminal Branches

Each cord gives rise to terminal nerves — these are the individual nerve pages in this wiki: Lateral Cord (C5-C7):

Terminal Branch	Root Levels	Primary Function	Page
Musculocutaneous nerve	C5, C6, C7	Elbow flexion (biceps, brachialis), lateral forearm sensation	anatomy/nerves/musculocutaneous-nerve
Lateral contribution to median nerve	C6, C7	Joins with medial contribution to form the median nerve	anatomy/nerves/median-nerve

Posterior Cord (C5-T1):

Terminal Branch	Root Levels	Primary Function	Page
Axillary nerve	C5, C6	Shoulder abduction (deltoid), external rotation (teres minor)	anatomy/nerves/axillary-nerve
Radial nerve	C5-T1	Elbow, wrist, and finger extension; posterior arm and dorsal hand sensation	anatomy/nerves/radial-nerve

Medial Cord (C8-T1):

Terminal Branch	Root Levels	Primary Function	Page
Ulnar nerve	C8, T1	Intrinsic hand muscles, wrist flexion (FCU), medial hand sensation	anatomy/nerves/ulnar-nerve
Medial contribution to median nerve	C8, T1	Joins with lateral contribution to form the median nerve	anatomy/nerves/median-nerve
Medial cutaneous nerve of arm	C8, T1	Medial arm sensation	—
Medial cutaneous nerve of forearm	C8, T1	Medial forearm sensation	—

The Median Nerve is unique — it receives contributions from BOTH the lateral cord (C6-C7) and the medial cord (C8-T1), making it the only terminal nerve with dual-cord origin. This is why the median nerve carries fibers from C6-T1, the broadest root representation of any terminal branch.

Clinical Patterns by Lesion Level

Lesion Level	Pattern Name	Presentation	Common Cause
C5-C6 (upper trunk)	Erb's palsy	Shoulder adducted, internally rotated; elbow extended, forearm pronated ("waiter's tip"). Loss of deltoid, supraspinatus, infraspinatus, biceps, brachioradialis.	Birth injury (shoulder dystocia), motorcycle accidents, stingers in contact sports
C8-T1 (lower trunk)	Klumpke's palsy	Claw hand — intrinsic hand muscle paralysis, finger flexor weakness. Sensory loss in medial arm and hand. May include Horner syndrome (ptosis, miosis, anhidrosis) if T1 sympathetic fibers are damaged.	Birth injury (arm traction), Pancoast tumor (lung apex), lower plexus traction
All roots/trunks	Complete plexus palsy	Flail arm — complete motor and sensory loss in the entire upper extremity.	High-energy trauma (motorcycle accident, industrial), severe birth injury
Lateral cord	—	Weak biceps and impaired median nerve function (pronation, wrist flexion — but hand intrinsics preserved because medial cord contribution to median is intact). Loss of lateral forearm sensation.	Anterior shoulder dislocation, axillary compression
Posterior cord	—	Weak deltoid (axillary) and wrist drop (radial). Loss of posterior arm/forearm and dorsal hand sensation.	Anterior shoulder dislocation, axillary compression
Medial cord	—	Weak intrinsic hand muscles (ulnar) and partial median hand weakness. Loss of medial arm, forearm, and ulnar hand sensation.	Hyperabduction injury, sternal retraction during cardiac surgery

Thoracic Outlet Syndrome

The brachial plexus passes through the thoracic outlet — the space between the anterior scalene, middle scalene, first rib, and clavicle. Compression here produces conditions/thoracic-outlet-syndrome, which can be neurogenic (plexus compression — 95% of cases), arterial (subclavian artery), or venous (subclavian vein).

Neurogenic TOS typically compresses the lower trunk (C8-T1), producing intrinsic hand weakness and medial arm/forearm numbness. This mimics cubital tunnel syndrome or C8-T1 radiculopathy.
Provocative tests: Adson's test (scalene compression), costoclavicular test (clavicle-rib compression), Roos test (sustained provocative position), ULTT (neural tension).
MT relevance: Scalene release, first rib mobilization, pectoralis minor release, and postural correction are first-line conservative approaches for neurogenic TOS. The interscalene gap is the most common compression site.

Clinical Notes

Stingers are the most common brachial plexus injury MTs will see. In contact sports (football, hockey, rugby), brachial plexus traction injuries ("stingers" or "burners") occur when the head is forcefully laterally flexed away from the affected shoulder, stretching the upper trunk. The athlete reports burning pain shooting down the arm, followed by transient weakness. Most stingers resolve in seconds to minutes. Recurrent stingers require cervical spine assessment and return-to-play evaluation.

The brachial plexus passes between the scalenes. The interscalene gap (between the anterior and middle scalene muscles) is the primary passageway for the plexus. Scalene hypertonicity, anatomical variations (cervical ribs, fibrous bands), and postural dysfunction (forward head posture increasing scalene load) all narrow this gap. When treating the scalenes, you are working directly around the plexus.

The "hand of the cord" mnemonic. A cord lesion produces a predictable pattern based on which terminal nerves are affected. Lateral cord: musculocutaneous + lateral median = weak elbow flexion and partial median loss. Posterior cord: axillary + radial = weak shoulder abduction and wrist drop. Medial cord: ulnar + medial median = weak hand intrinsics and medial sensory loss. Recognizing these patterns localizes the lesion more precisely than testing individual muscles alone.

Upper trunk injuries spare the hand; lower trunk injuries spare the shoulder. This is the most practical summary for clinical reasoning. If the hand works but the shoulder is weak, the lesion is at the upper trunk. If the shoulder works but the hand is weak, the lesion is at the lower trunk.

Key Takeaways

Five roots (C5-T1) organize into three trunks, six divisions, three cords, and five terminal nerves — the architecture determines lesion patterns.
Upper trunk (C5-C6) lesion = Erb's palsy (shoulder/elbow dysfunction, hand spared); lower trunk (C8-T1) lesion = Klumpke's palsy (hand dysfunction, shoulder spared).
The plexus passes between the anterior and middle scalenes — scalene pathology is the most common cause of neurogenic thoracic outlet syndrome.
Stingers (upper trunk traction) are the most common brachial plexus injury — transient burning and arm weakness in contact athletes.