brachial plexus
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Here is a complete overview of the Brachial Plexus, sourced from Gray's Anatomy for Students:
Brachial Plexus
The brachial plexus is a somatic nerve plexus formed by the anterior rami of C5 to C8 and most of the anterior ramus of T1. It originates in the neck, passes laterally and inferiorly over rib I between the anterior and middle scalene muscles, and enters the axilla. All major nerves supplying the upper limb originate from it.
The mnemonic for its parts is: R-T-D-C-B (Roots - Trunks - Divisions - Cords - Branches).
Overview Diagram
Fig. 7.52A: Major components in the neck and axilla. Roots pass between the anterior and middle scalene muscles.
Fig. 7.52B: Schematic showing parts of the brachial plexus. Cords are arranged around the 2nd part of the axillary artery.
1. Roots (anterior rami C5-T1)
The roots pass between the anterior scalene and middle scalene muscles. They receive gray rami communicantes from the sympathetic trunk, carrying postganglionic sympathetic fibers distally.
Branches from roots:
- Dorsal scapular nerve (C5): innervates rhomboid major and minor (and levator scapulae)
- Long thoracic nerve (C5, C6, C7): runs down medial wall of axilla on serratus anterior; innervates serratus anterior
- Small contribution from C5 to the phrenic nerve
2. Trunks
| Trunk | Formed by |
|---|---|
| Superior (upper) | C5 + C6 |
| Middle | C7 only |
| Inferior (lower) | C8 + T1 |
The inferior trunk lies on rib I posterior to the subclavian artery.
Branches from trunks (from the superior trunk only):
- Suprascapular nerve (C5, C6): passes through suprascapular foramen, innervates supraspinatus and infraspinatus
- Nerve to subclavius (C5, C6): innervates subclavius muscle
3. Divisions
Each trunk divides into anterior and posterior divisions (6 divisions total). No peripheral nerves arise directly from the divisions.
- Anterior divisions supply the anterior (flexor) compartments
- Posterior divisions supply the posterior (extensor) compartments
4. Cords
The cords are named by their position relative to the 2nd part of the axillary artery:
| Cord | Formed from | Roots |
|---|---|---|
| Lateral | Anterior divisions of superior + middle trunks | C5, C6, C7 |
| Medial | Anterior division of inferior trunk | C8, T1 |
| Posterior | All three posterior divisions | C5 - T1 |
5. Branches (Terminal and Pre-terminal)
Fig. 7.53: Full schematic (A) and relationships to the axillary artery (B).
From the Lateral Cord:
| Branch | Roots | Function |
|---|---|---|
| Lateral pectoral nerve | C5-C7 | Motor: pectoralis major (clavicular head) |
| Musculocutaneous nerve | C5-C7 | Motor: coracobrachialis, biceps, brachialis; Sensory: lateral forearm skin |
| Lateral root of median nerve | C5-C7 | Contributes to median nerve |
From the Medial Cord:
| Branch | Roots | Function |
|---|---|---|
| Medial pectoral nerve | C8, T1 | Motor: pectoralis minor and major |
| Medial cutaneous nerve of arm | C8, T1 | Sensory: medial distal third of arm |
| Medial cutaneous nerve of forearm | C8, T1 | Sensory: medial forearm to wrist |
| Medial root of median nerve | C8, T1 | Contributes to median nerve |
| Ulnar nerve | C8, T1 | Motor: most intrinsic hand muscles, FCU, medial FDP; Sensory: medial hand and little/ring fingers |
From the Posterior Cord:
| Branch | Roots | Function |
|---|---|---|
| Upper subscapular nerve | C5, C6 | Motor: upper subscapularis |
| Thoracodorsal nerve | C6, C7, C8 | Motor: latissimus dorsi |
| Lower subscapular nerve | C5, C6 | Motor: lower subscapularis and teres major |
| Axillary nerve | C5, C6 | Motor: deltoid, teres minor; Sensory: skin over deltoid (regimental badge area) |
| Radial nerve | C5-C8, T1 | Motor: all posterior compartment muscles of arm and forearm; Sensory: posterior arm, forearm, and hand |
Joined from lateral + medial cords:
- Median nerve (C5-T1): forms anterior to the 3rd part of the axillary artery; innervates most anterior forearm muscles and thenar muscles + lateral two lumbricals; sensory: lateral 3.5 fingers and palm
Key Clinical Points
- Erb's palsy (C5-C6 injury / upper trunk): "waiter's tip" posture - arm adducted, internally rotated, elbow extended, forearm pronated. Affects deltoid, supraspinatus, infraspinatus, biceps, brachioradialis.
- Klumpke's palsy (C8-T1 injury / lower trunk): "claw hand" - intrinsic hand muscles paralyzed; may be associated with Horner syndrome (T1 disruption involving sympathetic chain).
- Long thoracic nerve injury (C5-C7): medial winging of the scapula - serratus anterior paralysis, scapula swings away from thorax when pushing forward.
- Axillary nerve injury (surgical neck of humerus fracture or shoulder dislocation): loss of deltoid function and sensory loss over the "regimental badge" area.
- Radial nerve injury (midshaft humerus fracture): wrist drop - loss of wrist and finger extension.
- Median nerve injury at wrist (carpal tunnel or laceration): "ape hand" - loss of thenar eminence, sensory loss over lateral 3.5 fingers.
- Ulnar nerve injury at medial epicondyle: "claw hand" (ring and little fingers) - loss of hypothenar, interossei, and medial two lumbricals; sensory loss over medial 1.5 fingers.
Source: Gray's Anatomy for Students, pp. 858-864
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