Learning Objective

Understand the anatomy, formation, branches, and clinical relevance of the brachial plexus, including its roots, trunks, divisions, cords, and major nerves, and recognize the features of upper and lower brachial plexus injuries.

Brachial Plexus Overview

The brachial plexus is a network of nerve fibers that supplies the skin and muscles of the upper limb. It originates in the root of the neck, passes through the axilla, and continues along the upper extremity.

Formation: Anterior rami of C5, C6, C7, C8, and T1.
Purpose: Provides motor and sensory innervation to the upper limb.

The brachial plexus is organized into five parts: roots, trunks, divisions, cords, and branches. (Mnemonic: “Read That Damn Cadaver Book”). These divisions are for descriptive purposes; they do not represent functional differences.

Roots

The roots are the anterior rami of spinal nerves C5-T1.
Each spinal nerve exits the spinal cord via the intervertebral foramen and splits into an anterior and posterior ramus.
The anterior rami form the roots of the brachial plexus; the posterior rami supply intrinsic back muscles.
The roots pass between the anterior and middle scalene muscles to enter the base of the neck.

Trunks

Superior trunk: C5 + C6
Middle trunk: C7
Inferior trunk: C8 + T1
Located at the base of the neck, they cross laterally through the posterior triangle.

Divisions

Each trunk splits into anterior and posterior divisions.
Anterior divisions: Move anteriorly; primarily supply flexor compartments.
Posterior divisions: Move posteriorly; primarily supply extensor compartments.
The divisions combine in the axilla to form the cords.

Cords

Named according to their position relative to the axillary artery:

Lateral cord: Anterior divisions of the superior and middle trunks
Posterior cord: Posterior divisions of all three trunks
Medial cord: Anterior division of the inferior trunk

The cords give rise to the major peripheral nerves of the upper limb.

Major Branches

Nerve	Roots	Motor Function	Sensory Function
Musculocutaneous	C5-C7	Biceps brachii, brachialis, coracobrachialis	Lateral forearm (lateral cutaneous branch)
Axillary	C5-C6	Deltoid, teres minor	Superior lateral arm (regimental badge area)
Median	C6-T1	Most forearm flexors, thenar muscles, and lateral lumbricals	Lateral palm and 3½ fingers
Radial	C5-T1	Triceps, posterior forearm muscles	Posterior arm, forearm, and posterolateral hand
Ulnar	C8-T1	Intrinsic hand muscles (except thenar/lateral lumbricals), FCU, medial FDP	Medial 1½ fingers, associated palm

Practical tip: In dissection, the “M” shape formed by the musculocutaneous, median, and ulnar nerves (superficial to the axillary artery) helps identify the cords and branches.

Minor Branches

Roots: Dorsal scapular nerve, long thoracic nerve
Trunks: Suprascapular nerve, nerve to subclavius
Lateral cord: Lateral pectoral nerve
Medial cord: Medial pectoral nerve, medial cutaneous nerves of the arm and forearm
Posterior cord: Superior and inferior subscapular nerves, thoracodorsal nerve

Clinical Relevance

Upper Brachial Plexus Injury (Erb’s Palsy)

Cause: Stretch injury to C5-C6 (common during difficult deliveries)
Nerves affected: Musculocutaneous, axillary, suprascapular, nerve to subclavius
Muscles affected: Deltoid, biceps, brachialis, coracobrachialis, teres minor, supraspinatus, subclavius
Motor deficits: Weak shoulder abduction, lateral rotation, and supination; flexion at the shoulder
Sensory deficits: Lateral arm (C5-C6 dermatomes)
Characteristic posture: “Waiter’s tip” – arm hangs by the side, medially rotated; forearm pronated

Lower Brachial Plexus Injury (Klumpke’s Palsy)

Cause: Injury to C8-T1 (rare, often difficult deliveries)
Nerves affected: Ulnar and median nerves (lower roots)
Muscles affected: Intrinsic hand muscles
Motor deficits: Loss of finger flexion at the MCP joints and extension at the IP joints
Sensory deficits: Medial arm and hand (C8-T1 dermatomes)
Characteristic posture: Clawed hand due to unopposed extensor action