Brachial plexus

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Here is a thorough overview of the brachial plexus, drawn from Gray's Anatomy for Students and Bradley & Daroff's Neurology.

Brachial Plexus

Fig. 7.52A - Major components of the brachial plexus in the neck and axilla (Gray's Anatomy for Students)

Formation & Overview

The brachial plexus is a somatic nerve plexus formed by the anterior rami of C5-C8 and most of T1. It originates in the neck (posterior triangle), passes laterally and inferiorly over rib I, and enters the axilla. The plexus runs between the anterior and middle scalene muscles at its root level, then travels with (and surrounds) the axillary artery in the axilla. The roots receive gray rami communicantes from the sympathetic trunk, distributing postganglionic sympathetic fibers to the upper limb.

The classic mnemonic for its organization: R - T - D - C - B (Roots, Trunks, Divisions, Cords, Branches)

Step-by-Step Structure

Complete schematic of brachial plexus showing all branches with axillary artery relationships

Fig. 7.53 - Schematic of all branches, and relationships to the axillary artery (Gray's Anatomy for Students)

1. Roots (5 roots)

The anterior rami of C5, C6, C7, C8, and T1. They emerge between the anterior and middle scalene muscles.

2. Trunks (3 trunks)

Trunk	Formation
Superior trunk	C5 + C6
Middle trunk	C7 alone
Inferior trunk	C8 + T1

The inferior trunk lies on rib I, posterior to the subclavian artery. The middle and superior trunks are more superior.

3. Divisions (6 divisions)

Each trunk splits into an anterior and a posterior division (total 6). No peripheral nerves branch directly from the divisions.

Anterior divisions - supply anterior (flexor) compartments
Posterior divisions - supply posterior (extensor) compartments

4. Cords (3 cords)

Named by their position relative 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 a.
Medial cord	Anterior division of inferior trunk	C8-T1	Medial to axillary a.
Posterior cord	All three posterior divisions	C5-T1	Posterior to axillary a.

5. Terminal Branches (5 major)

Nerve	Origin	Roots	Key Supply
Musculocutaneous	Lateral cord	C5-C7	Anterior arm muscles; lateral forearm skin
Median	Lateral + medial cords (two roots)	C5-T1	Most anterior forearm muscles; thenar muscles; lateral 3.5 fingers
Ulnar	Medial cord	C8-T1	Intrinsic hand muscles; medial 1.5 fingers
Axillary	Posterior cord	C5-C6	Deltoid, teres minor; lateral shoulder skin
Radial	Posterior cord	C5-T1	All posterior arm & forearm muscles; posterior arm/forearm/hand skin

The musculocutaneous nerve, lateral root of median, median nerve, medial root of median, and ulnar nerve form a characteristic "M" shape over the 3rd part of the axillary artery - a key anatomical landmark.

Collateral (Pre-Terminal) Branches

These arise proximal to the cords:

From the roots:

Dorsal scapular nerve (C5) - rhomboids major & minor
Long thoracic nerve (C5-C7) - serratus anterior; injury causes winging of the scapula

From the superior trunk:

Suprascapular nerve (C5-C6) - supraspinatus, infraspinatus
Nerve to subclavius (C5-C6) - subclavius

From the lateral cord:

Lateral pectoral nerve (C5-C7) - pectoralis major (and minor via communication)

From the medial cord:

Medial pectoral nerve (C8-T1) - pectoralis major and minor
Medial cutaneous nerve of arm (C8-T1) - medial distal arm skin
Medial cutaneous nerve of forearm (C8-T1) - medial forearm skin

From the posterior cord:

Superior subscapular nerve - subscapularis
Thoracodorsal nerve - latissimus dorsi
Inferior subscapular nerve - subscapularis, teres major
(plus axillary and radial as terminal branches above)

Clinical: Brachial Plexus Injuries

Injuries are classified by level of damage:

Erb's Palsy (Upper trunk - C5, C6)

Typically from shoulder dystocia at birth, or violent downward traction of the shoulder in adults
"Waiter's tip" posture: arm adducted, medially rotated, forearm pronated
Loss of: shoulder abduction (deltoid), elbow flexion (biceps), forearm supination
Absent biceps reflex

Klumpke's Palsy (Lower trunk / Inferior trunk - C8, T1)

Caused by forced upward traction of the arm (e.g., grabbing a branch to break a fall)
Weakness/paralysis of intrinsic hand muscles; clawing of ring and little fingers
Absent grasp reflex
May be accompanied by Horner's syndrome (ptosis, miosis, anhidrosis) if T1 root avulsed - due to damage to sympathetic fibers

Pancoast Tumor (C8-T1)

Apical lung tumor infiltrating the inferior trunk; classically produces Horner's syndrome + pain down the medial arm/forearm + intrinsic hand weakness

Total plexus palsy (C5-T1)

Flail arm with complete anesthesia of the limb
Usually from high-energy trauma (motorcycle accidents)

Neonatal brachial plexus injury

Erb palsy (C5-C6) is most common; deficits persist in ~25% of cases
Surgical reconstruction should be considered if no spontaneous recovery at 3 months (Bradley & Daroff's Neurology)

Key clinical tools

Electrodiagnostics (NCS/EMG) - assess latency of muscle contraction with nerve stimulation, localize level of injury
MRI or CT myelography - identifies nerve root avulsions (preganglionic vs postganglionic)
Avulsion injuries (preganglionic) cannot regenerate; stretch/rupture injuries (postganglionic) may recover spontaneously or with nerve grafting

Anatomical Relations

Supraclavicular part (roots + trunks): in the posterior triangle of the neck, between anterior and middle scalene muscles, above rib I
Infraclavicular part (cords + branches): in the axilla, arranged around the axillary artery
Compression can occur between rib I and clavicle or below pectoralis minor (thoracic outlet syndrome)
Gray's Anatomy for Students, p. 858-866
Bradley & Daroff's Neurology in Clinical Practice, p. 2870

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