Brachial Plexus

The brachial plexus is formed by the ventral rami of C5–T1, organised sequentially into roots → trunks → divisions → cords → terminal branches as it passes from the neck, over the 1st rib, through the axilla.

Mnemonic for the sequence: Real Texans Drink Cold Beer (Roots, Trunks, Divisions, Cords, Branches).

• Ventral rami of C5, C6, C7, C8, T1.
• Emerge between anterior and middle scalene muscles (interscalene groove).
• C5 and C6 unite to form the upper trunk.
• C7 continues alone as the middle trunk.
• C8 and T1 unite to form the lower trunk (lower trunk lies on the 1st rib, closely related to the subclavian artery, and can be compressed in thoracic outlet syndrome or stretched by a cervical rib).

Branches directly from roots:

• Dorsal scapular nerve (C5): pierces scalenus medius, runs deep to levator scapulae, supplies levator scapulae, rhomboid major, rhomboid minor.
• Long thoracic nerve (C5, C6, C7): runs on the surface of serratus anterior; supplies serratus anterior; injury causes winged scapula.

Trunks lie in the posterior triangle of the neck, crossing the 1st rib to enter the axilla.

Branches from trunks (both from the upper trunk):

• Suprascapular nerve (C5, C6): passes through the suprascapular notch (below the superior transverse scapular ligament); supplies supraspinatus and infraspinatus; also sensory to the glenohumeral and acromioclavicular joints.
• Nerve to subclavius (C5, C6): passes anterior to the plexus and subclavian vessels; supplies subclavius.

Each trunk splits into an anterior and a posterior division at (or just proximal to) the clavicle:

• Anterior divisions: supply the future flexor (anterior) compartments of the limb.
• Posterior divisions: supply the future extensor (posterior) compartments of the limb.

No named branches arise at the divisional level; divisions simply regroup to form the cords.

Named by their relationship to the 2nd part of the axillary artery (posterior to pectoralis minor):

From the lateral cord:

• Lateral pectoral nerve (C5–C7)
• Musculocutaneous nerve (C5–C7)
• Lateral root of the median nerve (C5–C7)

From the medial cord:

• Medial pectoral nerve (C8, T1)
• Medial cutaneous nerve of the arm (C8, T1)
• Medial cutaneous nerve of the forearm (C8, T1)
• Ulnar nerve (C8, T1)
• Medial root of the median nerve (C8, T1)

From the posterior cord:

• Upper subscapular nerve (C5, C6): supplies upper subscapularis.
• Thoracodorsal nerve (C6–C8): supplies latissimus dorsi.
• Lower subscapular nerve (C5, C6): supplies lower subscapularis and teres major.
• Axillary nerve (C5, C6)
• Radial nerve (C5–T1): largest branch of the entire plexus.

Median nerve is formed by the union of its lateral root (from lateral cord) and medial root (from medial cord), typically anterior or lateral to the 3rd part of the axillary artery.

• All three cords surround the 2nd part of the axillary artery (posterior to pectoralis minor), which is the anatomical basis for their names.
• Lateral cord lies lateral, medial cord lies medial, posterior cord lies posterior to this part of the artery.
• This relationship is surgically important during axillary artery exposure or brachial plexus block at the infraclavicular level, and explains why axillary artery aneurysms or trauma can produce combined nerve deficits.

Erb palsy (upper brachial plexus injury, C5–C6):

• Caused by excessive increase in the angle between neck and shoulder: birth trauma (shoulder dystocia, traction on the head during delivery) or a fall onto the shoulder with the head forced away.
• Affects axillary, suprascapular, musculocutaneous nerves, and nerve to subclavius (all C5–C6 derived).
• Results in "waiter's tip" position: arm adducted, medially rotated, elbow extended, forearm pronated, wrist flexed, due to unopposed action of intact antagonist muscles.
• Sensory loss over the regimental badge area and lateral forearm.

Klumpke palsy (lower brachial plexus injury, C8–T1):

• Caused by excessive upward pulling of the arm: birth trauma (breech delivery with arm traction) or a fall while grasping something overhead.
• Affects intrinsic hand muscles (median and ulnar-derived, C8–T1).
• Results in "claw hand": hyperextension at MCP joints, flexion at IP joints, due to loss of lumbrical function.
• May involve T1 sympathetic fibres, producing Horner syndrome (ptosis, miosis, anhidrosis) if the injury is proximal enough to affect the sympathetic chain.

Long thoracic nerve injury:

• Vulnerable due to its long, superficial course on serratus anterior; injured in axillary surgery, mastectomy, or blunt trauma.
• Causes winged scapula: medial border of scapula lifts off the thoracic wall, worsened by pushing against resistance.

Upper trunk lesion:

• Equivalent clinically to Erb palsy; also seen in thoracic outlet compression from a cervical rib affecting the lower trunk preferentially, or in traction injuries at birth.

Lower trunk lesion:

• Equivalent clinically to Klumpke palsy; classically associated with a cervical rib compressing the lower trunk and subclavian artery, or traction injury from grasping something while falling.

Posterior cord lesion:

• Affects axillary and radial nerves together (plus subscapular and thoracodorsal).
• Produces combined deltoid and triceps/wrist extensor weakness: weak abduction and extension, wrist drop, sensory loss over the posterior arm/forearm and regimental badge area.
• Can occur with improper use of axillary crutches ("crutch palsy") or prolonged pressure on the axilla ("Saturday night palsy" more classically affects the radial nerve alone in the radial groove, but posterior cord compression produces a broader pattern).

Important brachial plexus trauma patterns:

• Traction injuries (motorcycle accidents, birth trauma), most common overall mechanism; severity ranges from neurapraxia to root avulsion.
• Root avulsion: proximal to the dorsal root ganglion, carries the worst prognosis (no surgical repair possible for avulsed roots; nerve transfers may be used).
• Penetrating trauma (stab, gunshot), can produce isolated cord or terminal branch lesions depending on the level.
• Total brachial plexus palsy (C5–T1): flail, anaesthetic limb; from severe traction (e.g., high-speed motorcycle accidents).