Brachial plexus
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brachial plexus anatomy overview roots trunks divisions cords branches
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Brachial Plexus
The brachial plexus is a somatic nerve network formed by the anterior rami of spinal nerves C5 to T1. It originates in the neck between the anterior and middle scalene muscles, crosses rib I, and enters the axilla to supply motor and sensory innervation to the entire upper limb (except the trapezius and the skin of the axilla and dorsal scapular area).
Structure: Roots - Trunks - Divisions - Cords - Branches
The classic mnemonic: "Really Tired? Drink Cold Beer" (Roots, Trunks, Divisions, Cords, Branches).
Fig. 7.52B - Brachial Plexus schematic (Gray's Anatomy for Students)
1. Roots
The roots are the anterior rami of C5, C6, C7, C8, and T1. They receive gray rami communicantes from the sympathetic trunk (postganglionic sympathetic fibers) close to their origin. The roots pass between the anterior and middle scalene muscles and lie posterior to the subclavian artery.
| Root | Contribution |
|---|---|
| C5 | Upper trunk |
| C6 | Upper trunk |
| C7 | Middle trunk (alone) |
| C8 | Lower trunk |
| T1 | Lower trunk |
2. Trunks
Three trunks form from the roots, cross the posterior triangle of the neck, and pass over rib I:
- Superior trunk - union of C5 + C6
- Middle trunk - continuation of C7
- Inferior trunk - union of C8 + T1 (lies on rib I, posterior to subclavian artery)
Branches from trunks (from the superior trunk only):
- Suprascapular nerve (C5, C6) - innervates supraspinatus and infraspinatus
- Nerve to subclavius (C5, C6) - innervates subclavius muscle
3. Divisions
Each trunk splits into anterior and posterior divisions - 6 divisions total. No peripheral nerves arise directly from divisions.
- Anterior divisions - supply anterior (flexor) compartments
- Posterior divisions - supply posterior (extensor) compartments
4. Cords
Named by their relationship to the 2nd part of the axillary artery:
| Cord | Formation | Roots | Position |
|---|---|---|---|
| Lateral cord | Anterior divisions of superior + middle trunks | C5-C7 | Lateral to axillary artery |
| Medial cord | Anterior division of inferior trunk | C8-T1 | Medial to axillary artery |
| Posterior cord | All three posterior divisions | C5-T1 | Posterior to axillary artery |
Fig. 7.53 - Brachial Plexus branches and relationship to axillary artery (Gray's Anatomy for Students)
5. Branches
From the Roots
| Nerve | Roots | Muscles Innervated |
|---|---|---|
| Dorsal scapular nerve | C5 | Rhomboid major and minor |
| Long thoracic nerve | C5, C6, C7 | Serratus anterior |
| Contribution to phrenic nerve | C5 | Diaphragm (partial) |
From the Lateral Cord
| Nerve | Function |
|---|---|
| Lateral pectoral nerve | Pectoralis major (and minor via communication) |
| Musculocutaneous nerve | Coracobrachialis, biceps brachii, brachialis; sensory as lateral cutaneous nerve of forearm |
| Lateral root of median nerve | Contributes to median nerve (C5-C7 fibers) |
From the Medial Cord
| Nerve | Function |
|---|---|
| Medial pectoral nerve | Pectoralis minor and major |
| Medial cutaneous nerve of arm | Skin of medial arm |
| Medial cutaneous nerve of forearm | Skin of medial forearm |
| Medial root of median nerve | Contributes to median nerve (C8-T1 fibers) |
| Ulnar nerve | Intrinsic hand muscles (except thenar + 2 lateral lumbricals); flexor carpi ulnaris, medial FDP; sensory - medial 1.5 fingers |
From the Posterior Cord
| Nerve | Function |
|---|---|
| Upper subscapular nerve | Subscapularis (upper) |
| Thoracodorsal nerve | Latissimus dorsi |
| Lower subscapular nerve | Subscapularis (lower) + teres major |
| Axillary nerve | Deltoid, teres minor; sensory - "regimental badge" area (lateral arm) |
| Radial nerve | All posterior compartment muscles of arm and forearm; sensory - posterior arm/forearm, anatomical snuffbox, dorsum of hand |
Terminal Nerves (5 major)
- Musculocutaneous (lateral cord)
- Axillary (posterior cord)
- Radial (posterior cord)
- Median (lateral + medial cords - forms the characteristic "M" shape in the axilla)
- Ulnar (medial cord)
Anatomical Relations
- In the neck: roots lie between anterior and middle scalene muscles, posterior to the subclavian artery
- At the clavicle: trunks cross over rib I
- In the axilla: cords surround the axillary artery (named by position relative to 2nd part)
Fig. 7.52A - Brachial plexus components in the neck and axilla (Gray's Anatomy for Students)
Clinical Correlates: Brachial Plexus Injuries
Erb-Duchenne Palsy (Upper Plexus Injury - C5, C6)
Mechanism: Excessive widening of the angle between the shoulder and neck - birth injury (during difficult labor), motorcycle accident, fall on shoulder.
Muscles affected: Deltoid, biceps, brachialis, brachioradialis, supraspinatus, infraspinatus, supinator.
Classic deformity: Arm adducted, internally rotated, elbow extended and pronated - "Policeman's tip" or "Waiter's tip" posture.
Sensory loss: Lateral aspect of arm and upper forearm (C5-C6 dermatomes).
Klumpke's Palsy (Lower Plexus Injury - C8, T1)
Mechanism: Forced hyperabduction of the shoulder (breech delivery with arms raised, person falling and grasping an object).
Muscles affected: Intrinsic muscles of the hand (all interossei, hypothenar, thenar) - presents as claw hand with features of combined median and ulnar palsy.
Sensory loss: Medial forearm, medial hand, medial 1.5 fingers.
Associated finding: Horner syndrome (ptosis, miosis, enophthalmos, anhidrosis) - due to involvement of T1 sympathetic fibers.
Total Plexus Injury (C5-T1)
Severe trauma (high-speed RTA). Complete anaesthesia of the limb (except upper medial arm supplied by intercostobrachial nerve T2) and flaccid paralysis of the entire limb.
Quick Summary Table
| Component | Level | Key Points |
|---|---|---|
| Roots | C5-T1 | Between scalene muscles; receive sympathetic gray rami |
| Superior trunk | C5+C6 | Gives suprascapular nerve + nerve to subclavius |
| Middle trunk | C7 | No named branches |
| Inferior trunk | C8+T1 | Behind subclavian artery on rib I |
| Lateral cord | C5-C7 | Musculocutaneous nerve, lateral root of median nerve |
| Medial cord | C8-T1 | Ulnar nerve, medial root of median nerve, cutaneous nerves |
| Posterior cord | C5-T1 | Radial and axillary nerves, thoracodorsal, subscapular nerves |
Sources: Gray's Anatomy for Students, p. 858-862 | S. Das Manual on Clinical Surgery, 13th Ed., p. 140 | Miller's Review of Orthopaedics, 9th Ed.
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