Explain me the brachial plexus clearly so that I'll never confuse or forget it again
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The Brachial Plexus - A Complete, Unforgettable Guide
The Big Picture First
The brachial plexus is the nerve network supplying the entire upper limb. Five spinal nerve roots enter, get reorganized through a series of junctions, and exit as five major terminal nerves. Think of it like a train-line interchange where passengers (nerve fibers) change platforms before reaching their final destination.
The 5-level hierarchy (mnemonic: "Read That Damn Cold Beer"):
| Level | Name | Notes |
|---|---|---|
| R - Roots | C5, C6, C7, C8, T1 | Anterior rami between scalene muscles |
| T - Trunks | Superior, Middle, Inferior | 3 trunks over rib I |
| D - Divisions | Anterior + Posterior | 6 divisions - NO nerves branch here |
| C - Cords | Lateral, Posterior, Medial | Named by position relative to axillary artery |
| B - Branches | 5 terminal + collateral nerves | The final nerves to the arm |
LEVEL 1: Roots (C5-T1)
The anterior rami of five spinal nerves emerge between the anterior and middle scalene muscles.
- Receives gray rami communicantes from the sympathetic trunk (postganglionic sympathetic fibers)
- Roots already give off two important nerves directly:
- Dorsal scapular nerve (C5) - rhomboids
- Long thoracic nerve (C5, C6, C7) - serratus anterior
LEVEL 2: Trunks (3 trunks)
The roots merge to form three trunks that cross the base of the posterior triangle over rib I.
| Trunk | Formed by | Memory hook |
|---|---|---|
| Superior (upper) | C5 + C6 | "5 and 6 go up together" |
| Middle | C7 alone | "C7 stands alone in the middle" |
| Inferior (lower) | C8 + T1 | "8 and T1 go down together" |
Branches from trunks:
- Suprascapular nerve (superior trunk, C5-C6) - supraspinatus + infraspinatus
- Nerve to subclavius (superior trunk, C5-C6) - subclavius muscle
LEVEL 3: Divisions (6 divisions - the "silent" level)
Each trunk splits into an anterior and a posterior division = 6 divisions total.
- Anterior divisions = future supply for flexor/anterior compartments
- Posterior divisions = future supply for extensor/posterior compartments
- No peripheral nerves originate directly from the divisions (this is the one "silent" level)
LEVEL 4: Cords (3 cords)
Three cords form from the divisions and are positioned around the 2nd part of the axillary artery (named by their position relative to it).
| Cord | Formed from | Roots | Position |
|---|---|---|---|
| Lateral | Anterior divisions of Superior + Middle trunks | C5, C6, C7 | Lateral to axillary artery |
| Posterior | ALL THREE posterior divisions | C5-T1 | Posterior to axillary artery |
| Medial | Anterior division of Inferior trunk | C8, T1 | Medial to axillary artery |
Memory trick: "Lateral and Medial cords carry ANTERIOR (flexor) fibers. Posterior cord carries ALL POSTERIOR (extensor) fibers."
LEVEL 5: Branches (Terminal Nerves)
The cords give rise to the major terminal nerves:
From the Lateral Cord (C5-C7):
- Lateral pectoral nerve - pectoralis major (and minor via communication)
- Musculocutaneous nerve - anterior arm muscles (biceps, brachialis, coracobrachialis) + sensory to lateral forearm
- Lateral root of median nerve (joins medial root to form the median nerve)
From the Medial Cord (C8-T1):
- Medial pectoral nerve - pectoralis major + minor
- Medial cutaneous nerve of arm - sensory medial arm
- Medial cutaneous nerve of forearm - sensory medial forearm
- Ulnar nerve - intrinsic hand muscles, medial 1.5 fingers
- Medial root of median nerve (joins lateral root)
From the Posterior Cord (C5-T1):
- Upper subscapular nerve - subscapularis
- Thoracodorsal nerve (C6-C8) - latissimus dorsi
- Lower subscapular nerve - subscapularis + teres major
- Axillary nerve (C5-C6) - deltoid + teres minor, sensory lateral arm
- Radial nerve (C5-T1) - all posterior compartment muscles of arm + forearm
The Median Nerve (special case):
Formed by contributions from BOTH the lateral cord (C5-C7) and medial cord (C8-T1). The two roots unite in a distinctive "M" or "V" shape on the front of the axillary artery. Mnemonic: the Median nerve takes an M-shaped path.
Full Branch Table (Grouped by Origin)
| Origin | Nerve | Spinal Level | Target |
|---|---|---|---|
| C5 root | Dorsal scapular | C5 | Rhomboids |
| C5-C7 roots | Long thoracic | C5-C7 | Serratus anterior |
| Superior trunk | Suprascapular | C5-C6 | Supraspinatus, infraspinatus |
| Superior trunk | Nerve to subclavius | C5-C6 | Subclavius |
| Lateral cord | Lateral pectoral | C5-C7 | Pec major/minor |
| Lateral cord | Musculocutaneous | C5-C7 | Anterior arm + lateral forearm skin |
| Lateral + Medial | Median nerve | C5-T1 | Anterior forearm, lateral palm, radial 3.5 fingers |
| Medial cord | Medial pectoral | C8-T1 | Pec major/minor |
| Medial cord | Med. cutaneous (arm) | C8-T1 | Medial arm skin |
| Medial cord | Med. cutaneous (forearm) | C8-T1 | Medial forearm skin |
| Medial cord | Ulnar nerve | C8-T1 | Intrinsic hand, medial 1.5 fingers |
| Posterior cord | Upper subscapular | C5-C6 | Subscapularis |
| Posterior cord | Thoracodorsal | C6-C8 | Latissimus dorsi |
| Posterior cord | Lower subscapular | C5-C6 | Subscapularis + teres major |
| Posterior cord | Axillary nerve | C5-C6 | Deltoid, teres minor |
| Posterior cord | Radial nerve | C5-T1 | All posterior compartments |
Clinical Lesions - The "Exam Favorites"
Erb-Duchenne Palsy (Upper Plexus, C5-C6)
Mechanism: Forceful widening of the angle between head and shoulder - motorcycle accident, shoulder dystocia during birth delivery.
Muscles lost: Deltoid, biceps, brachialis, brachioradialis, supraspinatus, infraspinatus
Classic posture: "Waiter's tip" or "Policeman taking a tip" - arm hangs at side, internally rotated, elbow extended, forearm pronated, wrist flexed.
Sensory loss: Lateral arm + upper lateral forearm
Klumpke's Palsy (Lower Plexus, C8-T1)
Mechanism: Forceful hyperabduction of the arm - catching a branch when falling, difficult breech delivery with arms above head.
Muscles lost: Intrinsic hand muscles (all those small thenar, hypothenar, interossei muscles).
Classic posture: "Claw hand" - hyperextended MCP joints, flexed IP joints (combined median + ulnar intrinsic hand paralysis).
Sensory loss: Medial forearm, hand, medial 1.5 fingers (C8-T1 distribution).
Important: Often associated with Horner's syndrome (ptosis, miosis, anhidrosis, enophthalmos) because T1 fibers also carry sympathetic fibers to the face. Horner's = avulsion injury (worst prognosis).
Other Clinically Important Lesions
| Nerve | Common injury | Key deficit |
|---|---|---|
| Long thoracic (C5-C7) | Stab to neck, radical mastectomy | Winged scapula (serratus anterior loss) |
| Axillary (C5-C6) | Anterior shoulder dislocation, surgical neck of humerus fracture | Deltoid paralysis (can't abduct >15°), patch of sensory loss over "regimental badge" area |
| Radial (C5-T1) | Midshaft humerus fracture ("spiral groove") | Wrist drop |
| Musculocutaneous | Rare; coracobrachialis entrapment | Weak flexion at elbow, lateral forearm sensory loss |
The One Memory Framework That Ties It All Together
ROOTS: C5 + C6 → SUPERIOR trunk
C7 → MIDDLE trunk
C8 + T1 → INFERIOR trunk
↓ (each splits anterior + posterior)
CORDS: Ant(Superior) + Ant(Middle) = LATERAL cord (C5-C7)
Post(all three) = POSTERIOR cord (C5-T1)
Ant(Inferior) = MEDIAL cord (C8-T1)
↓
TERMINAL: Lateral → Musculocutaneous + (half of) Median
Posterior → Axillary + Radial
Medial → Ulnar + (half of) Median
The Median nerve is the "bridge" child - it has parents from both the Lateral AND Medial cords.
The Posterior cord is the largest - it drains ALL three posterior divisions from all 5 roots, which is why the radial nerve (its main branch) has the widest distribution of any upper limb nerve.
Sources: Gray's Anatomy for Students (Elsevier), S. Das Manual on Clinical Surgery 13th Ed., Bradley and Daroff's Neurology in Clinical Practice
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