Axilla & Brachial Plexus Formation

The axilla is the pyramidal gateway between neck and arm transmitting the great neurovascular bundle of the upper limb, while the brachial plexus (C5–T1) is the most heavily examined neural network in NEET PG anatomy. Master the "Real Texans Drink Cold Beer" sequence and the four classic palsies and you will reliably bag 1–3 marks every cycle.

The Brachial Plexus: Overall Architecture

The brachial plexus is formed by the ventral (anterior) rami of C5, C6, C7, C8 and T1. It is conventionally divided into five components arranged proximal-to-distal:

High-yield: The classic order is Roots → Trunks → Divisions → Cords → Branches. Mnemonic: "Read That Damn Cadaver Book" or "Real Texans Drink Cold Beer."

Part	Components	Key location
Roots	C5, C6, C7, C8, T1 (ventral rami)	Between scalenus anterior & medius
Trunks	Upper (C5+C6), Middle (C7), Lower (C8+T1)	Posterior triangle of neck / supraclavicular
Divisions	Each trunk → anterior + posterior	Behind clavicle
Cords	Lateral, Medial, Posterior	Related to 2nd part of axillary artery
Branches	Terminal nerves	Axilla & beyond

Stepwise formation flow:

C5+C6 → Upper trunk → anterior & posterior divisions → (anterior joins C7 anterior to form lateral cord; posterior divisions of all three trunks → posterior cord) → terminal branches.

The cords are named by their relationship to the second part of the axillary artery: lateral cord is lateral, medial cord medial, posterior cord posterior.

Cord composition (must memorise)

• Lateral cord = anterior divisions of upper + middle trunks (C5, C6, C7)
• Medial cord = anterior division of lower trunk (C8, T1) — direct continuation
• Posterior cord = posterior divisions of all three trunks (C5–T1)

High-yield: The posterior cord carries fibres from all five roots (C5–T1) and gives the axillary and radial nerves — both extensors/abductors. Mnemonic for posterior cord branches: "ULTRA" = Upper subscapular, Lower subscapular, Thoracodorsal, Radial, Axillary.

Branches of the Brachial Plexus by Level

Branches from ROOTS

Nerve	Root value	Function/Note
Dorsal scapular nerve	C5	Rhomboids + levator scapulae
Long thoracic nerve (of Bell)	C5, C6, C7	Serratus anterior → winged scapula if injured
Branch to phrenic nerve	C5	Contributes to diaphragm

High-yield: Long thoracic nerve = "C5, 6, 7 raise your arm to heaven." Damage (e.g., axillary node dissection in mastectomy, or stab wound) → serratus anterior palsy → winging of scapula + inability to abduct arm above 90°.

Branch from UPPER TRUNK

• Suprascapular nerve (C5, C6): supplies supraspinatus & infraspinatus. Entrapment at the suprascapular notch is a cause of shoulder pain and weak abduction/external rotation.
• Nerve to subclavius (C5, C6).

Branches from CORDS

Lateral cord (3 branches):

• Lateral pectoral nerve
• Musculocutaneous nerve (C5–C7) — pierces coracobrachialis; supplies the anterior arm flexors (biceps, brachialis, coracobrachialis); continues as lateral cutaneous nerve of forearm.
• Lateral root of median nerve.

Medial cord (5 branches):

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

Posterior cord (5 branches — ULTRA):

• Upper subscapular nerve → subscapularis
• Thoracodorsal nerve (middle subscapular, C6–C8) → latissimus dorsi
• Lower subscapular nerve → subscapularis + teres major
• Axillary nerve (C5, C6) → deltoid + teres minor; regimental badge sensory area
• Radial nerve (C5–T1) → all extensors of arm & forearm

High-yield: The median nerve is formed by two roots — lateral root from lateral cord + medial root from medial cord — clasping the third part of the axillary artery (forms an "M" over the artery with musculocutaneous laterally and ulnar medially).

The Axilla: Boundaries & Contents

The axilla is a truncated, four-sided pyramid between the upper thoracic wall and the arm.

Wall	Formed by
Apex (cervico-axillary canal)	Outer border of 1st rib, posterior border of clavicle, superior border of scapula
Base (floor)	Skin, axillary fascia, hollow of armpit
Anterior wall	Pectoralis major, pectoralis minor, subclavius, clavipectoral fascia
Posterior wall	Subscapularis, latissimus dorsi, teres major
Medial wall	Serratus anterior + upper 4–5 ribs/intercostals
Lateral wall	Intertubercular (bicipital) groove of humerus (narrowest wall)

Contents of the axilla

• Axillary artery (continuation of subclavian, from outer border of 1st rib to lower border of teres major where it becomes brachial artery) — divided into 3 parts by pectoralis minor.
• Axillary vein (medial to artery).
• Cords & branches of the brachial plexus.
• Axillary lymph nodes (5 groups: anterior/pectoral, posterior/subscapular, lateral, central, apical) — critical in breast cancer staging.
• Axillary tail of Spence (breast tissue), fat, and the long thoracic & intercostobrachial nerves.

High-yield: Axillary artery parts relative to pectoralis minor: 1st part (proximal, 1 branch — superior thoracic), 2nd part (deep to muscle, 2 branches — thoraco-acromial & lateral thoracic), 3rd part (distal, 3 branches — subscapular, anterior & posterior circumflex humeral). Mnemonic for branches: "Screw The Lawyer, Save A Patient."

The Four Classic Palsies (Exam Goldmine)

1. Erb–Duchenne Palsy (Upper Trunk, C5–C6)

Mechanism: Excessive increase in angle between neck and shoulder — birth trauma (shoulder dystocia), fall on shoulder, or a blow forcing head away from shoulder.

Muscles paralysed: Deltoid, supraspinatus, infraspinatus, biceps, brachialis, brachioradialis.

Deformity — "Waiter's tip / Policeman receiving a tip":

• Arm adducted (deltoid out)
• Medially rotated (infraspinatus/teres minor out)
• Forearm pronated (biceps out)
• Forearm extended at elbow

High-yield: Erb's point = junction where C5 + C6 unite to form the upper trunk; six nerves meet here. Position = arm hangs by side, medially rotated, forearm pronated — "waiter's tip."

2. Klumpke's Palsy (Lower Trunk, C8–T1)

Mechanism: Excessive abduction of arm — sudden upward pull (catching a branch while falling, breech delivery with arm extended above head).

Result: Paralysis of intrinsic hand muscles → claw hand (true claw hand, all fingers). Because T1 contributes sympathetic fibres to the head via the stellate ganglion, Horner's syndrome (ptosis, miosis, anhidrosis, enophthalmos) may accompany if T1 root avulsed.

Feature	Erb's palsy	Klumpke's palsy
Roots	C5–C6 (upper trunk)	C8–T1 (lower trunk)
Mechanism	Shoulder pushed down	Arm pulled up/abducted
Deformity	Waiter's tip	Claw hand
Muscles	Shoulder/proximal	Small muscles of hand
Horner's	Absent	May be present

3. Long Thoracic Nerve Injury → Winged Scapula

Serratus anterior palsy. Medial border of scapula protrudes ("winging"), worsened by pushing against a wall. Cannot abduct arm beyond horizontal. Common after mastectomy/axillary clearance.

4. Crutch Palsy → Radial Nerve

Pressure of crutch in the axilla compresses the radial nerve → wrist drop, loss of extension of wrist/fingers/thumb, sensory loss over dorsum of 1st web space. (Radial nerve injury at mid-shaft humerus = "Saturday night palsy.")

High-yield: Wrist drop = radial nerve. Claw hand (ulnar/intrinsics) = Klumpke or ulnar. Ape thumb / pointing index = median nerve. Waiter's tip = Erb's.

Individual Terminal Nerve Lesions (Quick Compare)

Nerve	Roots	Classic lesion site	Deformity/Sign
Axillary	C5,C6	Surgical neck of humerus, shoulder dislocation	Deltoid wasting, loss of "regimental badge" sensation, weak abduction
Radial	C5–T1	Mid-shaft humerus (spiral groove), axilla	Wrist drop, ↓ triceps if proximal
Median	C6–T1	Wrist (carpal tunnel), elbow	Ape thumb, "hand of benediction," pointing index, loss of thumb opposition
Ulnar	C8,T1	Medial epicondyle, Guyon's canal	Claw hand (4th/5th), Froment's sign, hypothenar wasting
Musculocutaneous	C5–C7	Rare; coracobrachialis	Weak elbow flexion/supination, lateral forearm sensory loss

High-yield: Ulnar paradox — a higher (proximal, at elbow) ulnar lesion produces less clawing than a distal (wrist) lesion, because high lesions also paralyse the medial half of flexor digitorum profundus, reducing flexion at the DIP joints.

Pathophysiology of Plexus Injuries

• Pre-ganglionic (root avulsion): proximal to dorsal root ganglion; poor prognosis, no surgical re-implantation; associated with Horner's (if T1), winged scapula, and intact sensory nerve action potentials with absent motor response (because cell body in DRG is intact).
• Post-ganglionic (rupture): distal to DRG; better surgical prospects (nerve grafting).
• Neurapraxia → axonotmesis → neurotmesis (Seddon classification) determines recovery: neurapraxia recovers fully; neurotmesis needs surgery.

Diagnosis & Investigation of Choice

Clinical examination localises the lesion (motor + dermatomal sensory mapping). Then:

Clinical exam → Nerve conduction studies (NCS) + Electromyography (EMG) → MRI brachial plexus / CT myelography (for root avulsion / pseudomeningocele).

• EMG/NCS: investigation of choice for localisation and prognosis; best done after 3 weeks (allows Wallerian degeneration to manifest).
• MRI is the imaging modality of choice for soft-tissue plexus visualisation and tumours (e.g., Pancoast tumour affecting lower trunk → arm pain + Horner's).
• CT myelography: best to demonstrate pseudomeningocele of root avulsion.

Management / Drug of Choice

• Obstetric (Erb's) palsy: initial physiotherapy & splinting to prevent contractures; most C5–C6 cases recover spontaneously by 3 months. If no biceps recovery by 3 months → microsurgical nerve grafting/repair.
• Traumatic plexus injury: early exploration for sharp lacerations; nerve grafting/neurotisation (nerve transfer, e.g., Oberlin transfer of ulnar fascicle to musculocutaneous) for avulsions.
• Crutch palsy / Saturday night palsy: remove offending pressure, splint wrist, physiotherapy — usually recovers (neurapraxia).
• Brachial plexus block (regional anaesthesia): interscalene, supraclavicular, infraclavicular, axillary approaches for upper limb surgery — anatomy directly tested.
• Neuropathic pain: gabapentin/pregabalin; amitriptyline as adjuncts.

Complications

• Permanent muscle wasting & joint contractures (especially internal rotation contracture of shoulder in Erb's).
• Trophic ulcers and disuse atrophy in chronic lesions.
• Complex regional pain syndrome.
• Glenohumeral dysplasia/subluxation in untreated obstetric palsy.
• Phrenic nerve palsy if interscalene block too high (C3–C5 contribution).

Key Differentials

• Cervical radiculopathy / disc prolapse — single dermatomal/myotomal pattern, neck pain, positive Spurling's test.
• Thoracic outlet syndrome — lower trunk (C8–T1) compression by cervical rib/scalene; ulnar-side symptoms + vascular signs.
• Pancoast (superior sulcus) tumour — lower plexus involvement + Horner's; weight loss, smoker.
• Parsonage-Turner syndrome (neuralgic amyotrophy) — acute idiopathic/post-viral plexitis, severe pain then weakness.
• Peripheral neuropathy / mononeuritis multiplex — diabetes, vasculitis.

Recently asked / exam angle

• "Waiter's tip" deformity → Erb's palsy, C5–C6, upper trunk. Repeatedly asked.
• Nerve at risk in fracture surgical neck of humerus → axillary nerve (vs radial at mid-shaft, vs ulnar at medial epicondyle, vs median at supracondylar fracture).
• Posterior cord branches (ULTRA) and which nerves arise from which cord — favourite single-best-answer.
• Long thoracic nerve → winged scapula; injured during modified radical mastectomy / axillary dissection.
• Median nerve formation by two roots clasping 3rd part of axillary artery.
• Klumpke's palsy + Horner's syndrome (T1 root avulsion) — assertion-reason style.
• Boundaries of quadrangular & triangular spaces (axillary nerve + posterior circumflex humeral artery pass through the quadrangular space) — increasingly asked image-based.
• Axillary artery branches & parts relative to pectoralis minor.
• Erb's point = upper trunk; six nerves converge.

High-yield: Quadrangular space (bounded by teres minor above, teres major below, long head of triceps medially, surgical neck of humerus laterally) transmits the axillary nerve + posterior circumflex humeral artery — hence axillary nerve injury in shoulder dislocation.

Rapid revision

1. Brachial plexus = ventral rami C5–T1; order = Roots, Trunks, Divisions, Cords, Branches ("Real Texans Drink Cold Beer").
2. Trunks: Upper (C5+C6), Middle (C7), Lower (C8+T1).
3. Cords named by relation to 2nd part of axillary artery.
4. Posterior cord branches = ULTRA (Upper subscap, Lower subscap, Thoracodorsal, Radial, Axillary).
5. Erb's palsy = upper trunk C5–C6 = waiter's tip; arm adducted, medially rotated, forearm pronated.
6. Klumpke's palsy = lower trunk C8–T1 = claw hand ± Horner's.
7. Long thoracic nerve (C5,6,7) → serratus anterior → winged scapula.
8. Crutch palsy = radial nerve = wrist drop; surgical neck fracture = axillary nerve.
9. Median nerve = two roots (lateral + medial cord) clasping 3rd part of axillary artery forming "M".
10. Axillary artery branches: 1st part 1, 2nd part 2, 3rd part 3 ("Screw The Lawyer, Save A Patient").
11. Quadrangular space transmits axillary nerve + posterior circumflex humeral artery.
12. EMG/NCS = investigation of choice for localisation/prognosis; MRI for soft-tissue/tumour; CT myelography for root avulsion (pseudomeningocele).