Anatomy chapters
The brachial plexus
The network of nerves from C5 to T1 that supplies the whole upper limb: its parts, its five terminal branches and the injuries examiners ask about.
- Formed by the ventral rami of C5 to T1.
- Five parts in order: roots, trunks, divisions, cords, branches.
- Exams test it through injuries: Erb's (C5–6), Klumpke's (C8–T1) and posterior cord damage.
On this page
Overview
The brachial plexus is the network of nerves that supplies the whole upper limb, both its skin and its muscles.
It is formed by the ventral rami of C5, C6, C7, C8 and T1. It begins in the neck, passes over the first rib and behind the clavicle, and ends in the axilla, where it divides into the nerves of the arm.
It has five parts, always in the same order: roots, trunks, divisions, cords and branches.
"Read That Damn Cadaver Book" gives the order: Roots, Trunks, Divisions, Cords, Branches.
Labelled diagrams and dissection images: TeachMeAnatomy: the brachial plexus, Kenhub: brachial plexus diagrams and quiz and StatPearls: Brachial Plexus.
Roots
The roots are the ventral rami of C5 to T1. They emerge between the anterior and middle scalene muscles.
Two nerves come straight off the roots:
- Dorsal scapular nerve (C5) to the rhomboids and levator scapulae.
- Long thoracic nerve (C5, C6, C7) to serratus anterior.
"C5, 6, 7 raise your wings to heaven": the long thoracic nerve, and a winged scapula when it is damaged.
Trunks
In the posterior triangle of the neck, the roots join to form three trunks.
| Trunk | Formed by |
|---|---|
| Upper | C5 and C6 |
| Middle | C7 alone |
| Lower | C8 and T1 |
The upper trunk gives two small branches: the suprascapular nerve (to supraspinatus and infraspinatus) and the nerve to subclavius.
Divisions
Behind the clavicle, each trunk splits into an anterior and a posterior division.
Anterior divisions go on to supply the front (flexor) compartments. Posterior divisions supply the back (extensor) compartments.
The divisions give no branches. An option naming a "branch of the anterior division" is always wrong.
Cords
In the axilla, the divisions join to form three cords. Each is named by its position around the second part of the axillary artery.
| Cord | Formed by | Roots |
|---|---|---|
| Lateral | Anterior divisions of upper and middle trunks | C5–C7 |
| Medial | Anterior division of lower trunk | C8–T1 |
| Posterior | All three posterior divisions | C5–T1 |
The cords also give smaller branches:
- Lateral cord: lateral pectoral nerve.
- Medial cord: medial pectoral nerve, and the medial cutaneous nerves of the arm and forearm.
- Posterior cord: upper and lower subscapular nerves, and the thoracodorsal nerve (C6–C8) to latissimus dorsi.
The five terminal branches
| Nerve | From | Roots | Main muscles | Skin |
|---|---|---|---|---|
| Musculocutaneous | Lateral cord | C5–C7 | Biceps, brachialis, coracobrachialis | Lateral forearm |
| Axillary | Posterior cord | C5–C6 | Deltoid, teres minor | Over the lower deltoid |
| Radial | Posterior cord | C5–T1 | Triceps and the forearm extensors | Back of arm, forearm and lateral hand |
| Median | Lateral + medial cords | C5–T1 | Most forearm flexors, thenar muscles | Palm side of lateral 3½ fingers |
| Ulnar | Medial cord | C8–T1 | Most small muscles of the hand | Medial 1½ fingers |
The median nerve is the only terminal branch formed from two cords (a lateral and a medial root).
Axillary + radial lost together means the posterior cord is damaged.
Clinical relevance
Erb's palsy (upper trunk, C5–C6)
How it happens: the head and shoulder are forced apart, for example shoulder dystocia at birth or a fall onto the shoulder from a motorbike.
What you see: the "waiter's tip" posture. The arm hangs by the side, turned inwards, with the elbow straight and the forearm pronated. Sensation is lost over the lateral arm.
Klumpke's palsy (lower trunk, C8–T1)
How it happens: the arm is pulled up hard, for example a breech delivery or grabbing a branch while falling. A cervical rib or a Pancoast tumour can press on the lower trunk slowly.
What you see: a claw hand, because the small muscles of the hand are paralysed, and numbness along the medial forearm and hand.
Klumpke's palsy with a droopy eyelid and small pupil means Horner's syndrome: the T1 root, which carries sympathetic fibres to the head, is also injured.
Posterior cord injury
The axillary and radial nerves are lost together. The patient cannot lift the arm sideways and has a wrist drop. Classic causes are badly fitted crutches and anterior dislocation of the shoulder.
Long thoracic nerve injury
This nerve runs down the side of the chest wall, so it can be cut during axillary node clearance or mastectomy. Serratus anterior fails and the scapula wings when the patient pushes against a wall.
Examiners rarely ask you to recite the plexus. They describe an injury and ask where it is. Work backwards: list the nerves that are lost, then find the one part of the plexus they share. Two nerves lost from one cord points to that cord; a whole posture (waiter's tip, claw hand) points to a trunk.
Quick revision
- Roots C5–T1, ventral rami, between the anterior and middle scalenes.
- Trunks: upper C5–6, middle C7, lower C8–T1.
- Divisions lie behind the clavicle and give no branches.
- Cords are named around the second part of the axillary artery.
- Erb's (C5–6): waiter's tip. Klumpke's (C8–T1): claw hand ± Horner's.
- Axillary + radial lost together: posterior cord.
- Long thoracic nerve: serratus anterior, winged scapula.
Written by The House Job from standard anatomy references (Snell's Clinical Anatomy by Regions is the book named in the UHS PGET syllabus). Checked against OpenStax Anatomy and Physiology and StatPearls (Brachial Plexus).
FAQ
What are the roots of the brachial plexus?
The ventral rami of C5, C6, C7, C8 and T1.
Which cord gives the axillary and radial nerves?
The posterior cord. Damage to it causes weak shoulder abduction and wrist drop.
What is the difference between Erb's palsy and Klumpke's palsy?
Erb's palsy is an upper trunk (C5–C6) injury with the waiter's tip posture. Klumpke's palsy is a lower trunk (C8–T1) injury with a claw hand, sometimes with Horner's syndrome.
Which nerve injury causes a winged scapula?
The long thoracic nerve (C5–C7), which supplies serratus anterior.