Structure
The brachial plexus is built in five stages, roots, trunks, divisions, cords and branches, and each stage lies in a different region.
Roots, trunks and divisions
The roots are the anterior rami of C5, C6, C7, C8 and T1, which emerge between scalenus anterior and scalenus medius. In the posterior triangle of the neck they join to form the upper trunk (C5–C6), middle trunk (C7) and lower trunk (C8–T1); the lower trunk lies on the first rib behind the subclavian artery. Behind the clavicle each trunk splits into an anterior and a posterior division.
Cords
In the axilla the six divisions form three cords, named by their position round the second part of the axillary artery.
- Lateral cord: anterior divisions of the upper and middle trunks (C5–C7).
- Medial cord: anterior division of the lower trunk (C8–T1).
- Posterior cord: all three posterior divisions (C5–T1).
Anterior divisions supply flexor muscles and posterior divisions supply extensors, so every extensor of the upper limb is on the radial or axillary nerve.
Branches
| Level | Branch | Supplies |
|---|---|---|
| Roots | Dorsal scapular nerve (C5) | Rhomboids, part of levator scapulae |
| Roots | Long thoracic nerve (C5–C7) | Serratus anterior |
| Upper trunk | Suprascapular nerve (C5–C6); nerve to subclavius | Supraspinatus, infraspinatus; subclavius |
| Lateral cord | Lateral pectoral nerve; musculocutaneous nerve; lateral root of median nerve | Pectoralis major; arm flexors |
| Medial cord | Medial pectoral nerve; medial cutaneous nerves of arm and forearm; ulnar nerve; medial root of median nerve | Pectoralis minor and major; medial arm and forearm skin; hand |
| Posterior cord | Upper and lower subscapular nerves; thoracodorsal nerve; axillary nerve; radial nerve | Subscapularis, teres major; latissimus dorsi; deltoid, teres minor; extensors |
Relations in the axilla
In the axilla the cords and their branches are wrapped round the axillary artery inside the axillary sheath, bounded by four walls of muscle.
- Anterior wall: pectoralis major and pectoralis minor, with the clavipectoral fascia between them.
- Posterior wall: subscapularis, teres major and latissimus dorsi.
- Medial wall: serratus anterior over the upper ribs.
- Lateral wall: the intertubercular sulcus of the humerus, with coracobrachialis and the short head of biceps.
- Apex: the cervico-axillary canal between the clavicle, first rib and upper border of the scapula.
Pectoralis minor divides the axillary artery into three parts. The first part, above the muscle, gives the superior thoracic artery; the second, behind it, gives the thoraco-acromial trunk and lateral thoracic artery; the third, below it, gives the subscapular artery and the anterior and posterior circumflex humeral arteries. The subscapular artery, the largest branch, divides into the circumflex scapular artery, which passes through the triangular space, and the thoracodorsal artery, which runs with the thoracodorsal nerve to latissimus dorsi. The axillary vein lies medial to the artery throughout.
Clinical relevance
Brachial plexus injuries are grouped by which roots are damaged and by whether the root is torn from the spinal cord.
| Injury | Mechanism | Picture |
|---|---|---|
| Erb's palsy (C5–C6, upper trunk) | Head forced away from the shoulder: shoulder dystocia, motorcycle fall | Arm adducted and internally rotated, elbow extended, forearm pronated (waiter's tip); numb lateral arm |
| Klumpke's palsy (C8–T1, lower trunk) | Arm forced above the head: breech delivery, grabbing a support in a fall | Clawing and wasting of the small hand muscles; numb medial forearm; Horner's syndrome if T1 is avulsed |
| Thoracic outlet syndrome | Cervical rib or band pressing on the lower trunk | Pain and paraesthesia in the medial forearm and hand; wasting of the small hand muscles, often most marked in the thenar eminence |
Preganglionic or postganglionic
A root torn from the cord (preganglionic avulsion) cannot be repaired directly. Clues are Horner's syndrome (T1), a winged scapula (long thoracic nerve) or weak rhomboids (dorsal scapular nerve), because these branches leave the roots close to the cord. Sensory nerve action potentials are preserved in numb skin, because the dorsal root ganglion and its peripheral axons survive.
Shoulder dislocation
An anterior dislocation of the shoulder can stretch the cords and terminal nerves as well as the axillary nerve. Nerve function and distal pulses are checked and recorded before and after reduction.
On the specimen
On an axilla prosection the plexus is identified by the letter M lying in front of the axillary artery, with the radial nerve hidden behind it.
- The M: the lateral limb is the musculocutaneous nerve running into coracobrachialis; the lateral cord and the lateral root of the median nerve form the next stroke; the median nerve is the centre; the medial root and medial cord form the fourth stroke; the ulnar nerve is the medial limb.
- Median nerve: its two roots straddle the front of the artery before fusing.
- Ulnar nerve: lies between the axillary artery and the axillary vein.
- Radial nerve: the thickest nerve, directly behind the artery on the posterior wall; lift the artery to show it.
- Axillary vein: thin-walled and medial, often collapsed or partly removed on a prosection.
- Arteries: the thoraco-acromial trunk pierces the clavipectoral fascia at the upper border of pectoralis minor; the subscapular artery runs down the lower border of subscapularis and splits into the circumflex scapular artery, turning backwards, and the thoracodorsal artery, continuing on to latissimus dorsi.
The muscles pinned are those of the walls: pectoralis major and minor in front; subscapularis, teres major and latissimus dorsi behind; serratus anterior medially; coracobrachialis and the two heads of biceps laterally, with deltoid and triceps at the edges of the dissection.