Must Know

Plexus forms from anterior rami C5 to T1.
Roots lie between scalenus anterior and medius. Trunks cross the posterior triangle.
Divisions lie behind the clavicle. Cords lie in the axilla.
Cords are named by their relation to the second part of the axillary artery.
C5 and C6 form the upper trunk, C7 the middle, C8 and T1 the lower.
Anterior divisions of upper and middle trunks form the lateral cord.
Anterior division of the lower trunk forms the medial cord.
All three posterior divisions form the posterior cord.
Divisions give no branches.
Axillary sheath, from prevertebral fascia, encloses the artery and cords and is the target of plexus blocks.

Branches and Root Values

Origin	Branch	Roots	Notes
Roots	Dorsal scapular	C5	Pierces scalenus medius, supplies rhomboids
Roots	Nerve to subclavius	C5, 6	May give an accessory phrenic nerve
Roots	Long thoracic	C5, 6, 7	C5, 6 pierce scalenus medius. C7 joins on serratus anterior
Upper trunk	Suprascapular	C5, 6	Under transverse scapular ligament. Supplies supraspinatus, infraspinatus, shoulder and AC joints
Lateral cord	Lateral pectoral	C5, 6, 7	Pierces clavipectoral fascia. No skin branch
Lateral cord	Musculocutaneous	C5, 6, 7	Leaves high, pierces coracobrachialis
Lateral cord	Lateral root of median	C5, 6, 7	Joins medial root in front of the artery
Medial cord	Medial pectoral	C8, T1	Pierces pectoralis minor to reach pectoralis major
Medial cord	Medial root of median	C8, T1	Crosses in front of the artery
Medial cord	Medial cutaneous nerve of arm	C8, T1	Smallest and most medial branch
Medial cord	Medial cutaneous nerve of forearm	C8, T1	Between artery and vein
Medial cord	Ulnar	C7, 8, T1	C7 fibres may come from the lateral cord
Posterior cord	Upper subscapular	C5, 6	Upper subscapularis
Posterior cord	Thoracodorsal	C6, 7, 8	Latissimus dorsi. At risk in lower axillary surgery
Posterior cord	Lower subscapular	C5, 6	Lower subscapularis and teres major
Posterior cord	Axillary	C5, 6	Quadrangular space
Posterior cord	Radial	C5 to T1	Largest branch of the plexus

Branch count is roots 3, upper trunk 1, lateral cord 3, medial cord 5, posterior cord 5.
Musculocutaneous nerve can escape an axillary block because it leaves the lateral cord high.
Medial and lateral pectoral nerves take their names from their cords, not their position.

More on plexus variants and sympathetic supply

About 10 percent of plexuses are prefixed (C4 to C8).
About 10 percent are postfixed (C6 to T2).
Formation can vary but each branch keeps its segmental content.
Sympathetic outflow for the limb comes mainly from T2 to T5 and joins the roots as grey rami.

Plexus Injury Patterns

Lesion	Roots	Picture
Erb	C5, 6	Arm adducted, medially rotated, elbow extended, forearm pronated. Lateral arm and forearm numb
Klumpke	C8, T1	Claw hand from intrinsic loss. Medial forearm numb
Complete	C5 to T1	Flail anaesthetic limb, Horner syndrome with root avulsion

Motorcycle falls on the shoulder with the neck forced away avulse roots.
Working serratus anterior and rhomboids place the lesion distal to the roots.
Working supraspinatus and infraspinatus place the lesion distal to the upper trunk.
Pectoralis major is the only muscle supplied by all five segments.

Axilla and Axillary Artery

Axillary artery runs from the outer border of the first rib to the lower border of teres major.
Pectoralis minor divides it into three parts with one, two and three branches.
Surface marking with the arm abducted runs from mid clavicle to the groove behind coracobrachialis.
Axillary vein lies anteromedial to the artery and begins at teres major from the basilic vein.
Medial cord, ulnar nerve and medial cutaneous nerve of forearm lie between artery and vein.

Part	Branches
First (above pectoralis minor)	Superior thoracic
Second (behind pectoralis minor)	Thoracoacromial, lateral thoracic
Third (below pectoralis minor)	Subscapular, anterior and posterior circumflex humeral

Subscapular artery is the largest branch. It gives the circumflex scapular, then continues as thoracodorsal.
Anterior circumflex humeral gives an ascending branch up the intertubercular sulcus to the humeral head.
Posterior circumflex humeral is larger and travels with the axillary nerve.

Space	Boundaries	Contents
Quadrangular	Subscapularis, teres major, long head of triceps medially, surgical neck laterally	Axillary nerve, posterior circumflex humeral vessels
Triangular	Subscapularis, teres major, long head of triceps laterally	Circumflex scapular artery
Triangular interval	Teres major, long head of triceps, humerus	Radial nerve, profunda brachii

Root of the Neck and Scalenus Anterior

Thoracic outlet is bounded by T1 vertebra, first ribs with their cartilages and the manubrium.
Scalenus anterior arises from anterior tubercles of C3 to C6 and inserts on the scalene tubercle of rib 1.
Scalenus anterior is supplied by C4 to 6.
Phrenic nerve descends on scalenus anterior from its lateral to its medial border under prevertebral fascia.
Subclavian vein lies in front of scalenus anterior and below its insertion, separated from the artery.
Vein joins the internal jugular at the medial border of scalenus anterior to form the brachiocephalic vein.
Subclavian artery and lower roots of the plexus pass between scalenus anterior and medius.
Scalenus medius arises from atlas, axis and posterior tubercles and inserts behind the subclavian groove of rib 1.

Subclavian part	Position	Branches
First	Medial to scalenus anterior	Vertebral, thyrocervical trunk, internal thoracic
Second	Behind scalenus anterior	Costocervical trunk only
Third	Lateral to scalenus anterior	Dorsal scapular usually

Pyramidal space lies between scalenus anterior and longus colli, with no fascial roof.
Apex of the pyramidal space is the carotid (Chassaignac) tubercle of C6.
Base of the space is the subclavian artery on the suprapleural membrane.
Stellate ganglion lies in the space with the vertebral vessels in front of it.
Vertebral artery usually enters the C6 transverse foramen, occasionally a higher one.
Subclavian artery is marked by a curve from sternoclavicular joint to mid clavicle, peaking about 2 cm above it.
In thoracic outlet surgery scalenus anterior is divided close to its first rib insertion.

Source

Sinnatamby CS. Last’s Anatomy. Regional and Applied. 12th ed. Edinburgh, Churchill Livingstone Elsevier, 2011. Chapter 6, Head and neck and spine. Root of the neck.

Numbers to Remember

Branches. Roots 3, trunk 1, cords 3, 5 and 5, divisions 0.
Artery branches. Parts one, two, three give 1, 2 and 3 branches.
Variants. About 10 percent prefixed, 10 percent postfixed.

Memory Aids

Roots, trunks, divisions, cords. Scalenes, posterior triangle, clavicle, axilla.
Long thoracic. C5, 6, 7 raise the wing of the scapula to heaven.

Exam Traps

Suprascapular nerve comes from the upper trunk, not a cord.
Klumpke numbness is on the medial forearm.
Prefixed means a C4 contribution. Postfixed means a T2 contribution.

Source

Sinnatamby CS. Last’s Anatomy. Regional and Applied. 12th ed. Edinburgh, Churchill Livingstone Elsevier, 2011. Chapter 2, Upper limb.

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 17 October 2026.