The Radial, Axillary and Musculocutaneous Nerves
Three nerves open the hand, lift the arm, and bend the elbow — and each of them is famous for the moment it fails. One spirals around the back of the humerus, pressed against naked bone, so a night asleep in the wrong chair can leave you unable to lift your own wrist. One hugs the surgical neck of the humerus so closely that a dislocated shoulder can rob you of the power to raise your arm at all. And one pierces straight through a muscle belly to reach the biceps. Learn where they run, and every one of their injuries becomes predictable — you can name the level of the damage from the pattern of the weakness alone.
A man falls asleep in an armchair after a long night, one arm slung over the wooden back, his head resting on it. He wakes six hours later and the hand will not obey. He can still feel his fingers, mostly. He can still bend the elbow. But when he tries to lift the wrist, nothing happens — the hand hangs from the forearm like a dropped glove, limp and useless, and the back of his thumb feels as though it belongs to someone else. Nothing was cut. No bone was broken. For six hours the hard edge of a chair pressed a single nerve against the back of his own humerus, and that was enough. Doctors have a name for it, half joke and half diagnosis: Saturday-night palsy. It is the most vivid lesson in anatomy the upper limb ever gives — that a nerve's fate is written by the path it takes.
Three nerves, two cords: where they come from
Two of these nerves are the entire output of the posterior cord; the third is the first branch of the lateral cord. In the axilla, the cords of the brachial plexus are named for their position around the axillary artery. The posterior cord gathers fibres from all five roots (C5–T1) and ends by dividing into its two terminal branches: the radial nerve and the axillary nerve. That single fact carries a functional truth — the posterior cord is the extensor cord. Everything on the back of the limb, from deltoid at the shoulder to the last extensor tendon in the hand, is posterior-cord territory. The lateral cord (C5–C7) gives the musculocutaneous nerve to the front of the arm before contributing its lateral root to the median nerve. The medial cord and the median and ulnar nerves — the flexor half of the story — are covered in the median and ulnar nerves. Together these five terminal branches account for every muscle and every patch of skin below the shoulder.
The radial nerve: the great extensor nerve (C5–T1)
The radial nerve is the largest branch of the brachial plexus and the only nerve that supplies extensors. It begins in the axilla behind the axillary artery, gives its first branches to the long and medial heads of triceps, then leaves the axilla through the triangular interval — the gap bounded above by teres major, laterally by the humerus, and medially by the long head of triceps. From there it enters the posterior compartment of the arm and does the thing that defines it: it winds obliquely down and around the back of the humerus, lying directly on bone in the radial (spiral) groove, accompanied by the profunda brachii artery. Halfway down it pierces the lateral intermuscular septum to re-enter the anterior compartment, and runs down in the groove between brachialis and brachioradialis. Anterior to the lateral epicondyle, at the level of the elbow, it splits into its two terminal branches: a superficial branch that is purely sensory, and a deep branch that is purely motor.
Picture the radial nerve as a cable strapped to the outside of a pillar rather than run safely inside the wall. The median and ulnar nerves travel down the soft, protected front and inside of the arm, cushioned by muscle. The radial nerve does the opposite: it deliberately leaves cover, spirals across the naked back of the humerus in a shallow groove, with only the triceps between it and the world, and only then dives back into muscle. Any cable taped to the outside of a pillar will be the first thing damaged when the pillar cracks — and that is precisely why a mid-shaft humeral fracture is a radial nerve story every single time.
What the radial nerve moves, and what it feels
Read its motor list from proximal to distal and the lesion levels fall out by themselves. In the arm the radial nerve supplies the three heads of triceps brachii and the small anconeus — the elbow extensors described in the muscles of the arm. Just above and below the elbow, still as the main trunk, it supplies brachioradialis, extensor carpi radialis longus (ECRL), and often a slip of brachialis. Its superficial branch then runs under brachioradialis, has no motor fibres at all, and emerges near the wrist to supply the skin of the dorsum of the lateral hand — the lateral three and a half digits over their proximal segments, and above all the first web space and the floor of the anatomical snuffbox. Its deep branch pierces the supinator muscle, winds round the neck of the radius, and re-emerges on the back of the forearm renamed the posterior interosseous nerve (PIN). From there the PIN supplies everything left in the posterior compartment covered in the muscles of the forearm: supinator itself, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and the deep group — abductor pollicis longus, extensor pollicis longus and brevis, and extensor indicis. Sensation for the back of the limb comes from its cutaneous branches: the posterior and inferior lateral cutaneous nerves of the arm, and the posterior cutaneous nerve of the forearm. One rule captures it: the radial nerve opens the hand and feels the back of it.
Reading the level of a radial nerve lesion
Because the branches leave in a strict order, the pattern of loss tells you exactly where the injury is. A lesion in the axilla — from crutches jammed into the armpit, or from an arm hooked over a chair back overnight — is the highest level: triceps is weak (so elbow extension fails), there is wrist drop, finger and thumb extension are lost, and sensation is gone over the posterior arm, posterior forearm and the dorsum of the lateral hand. A lesion in the radial groove, classically from a mid-shaft humeral fracture, spares triceps, because its branches were given off before the nerve entered the groove; elbow extension is therefore normal, but the hallmark wrist drop appears, with loss of finger and thumb extension and numbness over the first web space and snuffbox. A lesion of the posterior interosseous nerve — a fracture or dislocation at the elbow, a compressing lipoma, or entrapment at the arcade of Fröhse in supinator — produces finger drop with the wrist still extending (because ECRL was supplied by the main trunk before the split), and, crucially, no sensory loss at all, because the PIN is purely motor. Finally, a lesion of the superficial branch alone — a tight watch strap, handcuffs, a laceration at the wrist — is the mirror image: numbness over the snuffbox with completely normal power.
💡 The single most useful question in a wrist drop is: can the patient still straighten the elbow? Triceps branches leave the radial nerve before it enters the spiral groove, so a preserved triceps places the lesion at or below mid-shaft humerus, while a weak triceps drags it up into the axilla. Then ask the second question: is the snuffbox numb? Numb means the superficial branch is involved, so the lesion is at or above the elbow; normal sensation with finger drop means the pure motor PIN — and PIN lesions never cause numbness. Two questions, three levels, no imaging required. This is why anatomists say the radial nerve is examined, not guessed at.
The axillary nerve: the deltoid's lifeline (C5–C6)
The shortest course of the three — and the most exposed. The axillary nerve is the other terminal branch of the posterior cord. It leaves the axilla by passing backwards through the quadrangular space — bounded above by teres minor and the shoulder capsule, below by teres major, medially by the long head of triceps and laterally by the surgical neck of the humerus — travelling with the posterior circumflex humeral artery. It then wraps around the surgical neck itself, deep to deltoid, and divides. The posterior division supplies teres minor (giving it a small ganglion-bearing branch) and the posterior part of deltoid, then continues as the superior lateral cutaneous nerve of the arm, which curls around the posterior border of deltoid to supply the skin over the lower deltoid — the famous "regimental badge" area, so called because it is exactly where a soldier's shoulder patch sits. The anterior division runs deep to deltoid around the humerus and supplies its anterior and middle parts. Motor loss therefore means loss of the main abductor of the arm beyond the first 15 degrees, and loss of one of the rotator cuff's external rotators — muscles set out in the shoulder and scapular muscles.
A rugby player lands on an outstretched arm and the head of the humerus is driven forwards and downwards out of the glenoid — an anterior dislocation, the commonest of all major joint dislocations, described in the shoulder joint. As the head slips inferiorly it can stretch or tear the axillary nerve looping under it. In the emergency department the examination is fixed ritual: before and after any reduction, test light touch over the regimental badge, because deltoid power cannot be tested reliably in a painful, freshly dislocated shoulder. If that patch of skin is numb, the axillary nerve is injured, and it must be documented then — not after the joint has been relocated, when everyone will argue about who caused it. The same nerve is threatened by a fracture of the surgical neck of the humerus in an elderly patient who falls, and by an intramuscular injection placed too low into deltoid instead of into its upper third. The result in every case is the same picture: the arm cannot be abducted, the deltoid contour flattens over weeks as the muscle wastes, and the badge is numb.
The musculocutaneous nerve: the nerve that bends the elbow (C5–C7)
Its name is its summary: it is muscular first and cutaneous afterwards. Arising from the lateral cord, it does something no other major limb nerve does — it pierces straight through the belly of coracobrachialis, then runs down the arm sandwiched in the plane between biceps brachii in front and brachialis behind. Along the way it supplies all three muscles of the anterior compartment of the arm: coracobrachialis (flexes and adducts the arm at the shoulder), biceps brachii (the powerful supinator of the forearm and a flexor of the elbow), and brachialis (the workhorse pure flexor of the elbow). Just lateral to the biceps tendon at the elbow it pierces the deep fascia and changes identity, continuing as the lateral cutaneous nerve of the forearm, which supplies the skin of the lateral forearm from the elbow down to the wrist. Note the elegance of the arrangement: one nerve, one compartment, one action. Compartment and nerve are the same thought.
Isolated injury is rare — and that is itself diagnostically useful. Because the musculocutaneous nerve is buried between muscle bellies for its whole course, it is well protected; isolated lesions come mainly from penetrating trauma to the axilla or upper arm, from violent shoulder hyperextension, from anterior shoulder surgery, or from strenuous repetitive elbow extension that compresses it against the biceps aponeurosis. When it does fail, the picture is clean: markedly weakened elbow flexion (though brachioradialis, a radial-supplied muscle, still gives some flexion in the mid-prone position), weakened supination (though the radial-supplied supinator survives), an absent biceps reflex, and numbness down the lateral forearm. If instead the weakness of elbow flexion comes with a weak deltoid and a numb badge patch, think higher — an upper trunk C5–C6 injury (Erb's palsy) knocks out the musculocutaneous and axillary nerves together, producing the classic "waiter's tip" arm.
Think of the arm as a building with two rooms and a service corridor running round the back. The front room holds the flexors and is wired by a single dedicated cable, the musculocutaneous nerve, which enters through the wall of coracobrachialis and never leaves the room. The back room holds the extensors and is wired by the radial nerve, which reaches it by running along the outside of the building in the spiral groove. Damage the front-room cable and only elbow flexion goes dark. Damage the outside cable and the whole back of the limb, all the way to the fingertips' extensor tendons, loses power at once. The wiring diagram predicts the blackout.
💡 Every one of these nerves can be switched off deliberately, and surgeons do it every day. An interscalene or axillary block floods the plexus with local anaesthetic, which shuts sodium channels and silences conduction, so a shoulder can be reconstructed on a patient who is awake and chatting. The anatomy is the whole technique: the anaesthetist finds the axillary artery because the cords are named for their position around it, and knows that the musculocutaneous nerve has already left the sheath inside coracobrachialis — which is exactly why a standard axillary block so often misses the lateral forearm and must be topped up separately. A nerve's course does not just predict its injuries; it predicts how to switch it off on purpose.
Reach up to take a book from a high shelf: the axillary nerve fires deltoid to abduct the arm, and if that nerve were cut you could not get the hand above shoulder height at all. Curl the book toward your chest: the musculocutaneous nerve drives biceps and brachialis, and rotates your palm upward to read the spine. Now open your hand to set the book down flat: that is the radial nerve, extending the wrist to stabilise it and opening every finger and the thumb. Three ordinary seconds; three separate nerves, in order. And the reverse also holds — a man who leans on crutches all day with the padding pressing high into his armpit will notice, weeks later, that he cannot straighten his elbow well and his wrist has begun to drop. Crutch palsy is the same anatomy, read backwards.
- Radial nerve (C5–T1) is the terminal branch of the posterior cord and the ONLY nerve supplying extensors of the limb.
- Course: axilla → triangular interval → radial (spiral) groove on the humerus with profunda brachii → anterior to the lateral epicondyle → superficial (sensory) + deep (motor) branches.
- Radial motor: triceps + anconeus in the arm; brachioradialis + ECRL at the elbow; then all remaining extensors via the posterior interosseous nerve after it pierces supinator.
- Radial sensory: posterior cutaneous nerves of arm and forearm, plus the dorsum of the lateral hand — above all the first web space / anatomical snuffbox.
- Axillary nerve (C5–C6) passes through the quadrangular space with the posterior circumflex humeral vessels and wraps the surgical neck of the humerus.
- Axillary supply: deltoid + teres minor, and the superior lateral cutaneous nerve of the arm over the "regimental badge" patch.
- Musculocutaneous nerve (C5–C7) from the lateral cord pierces coracobrachialis and supplies the whole anterior arm compartment.
- Its three muscles: coracobrachialis, biceps brachii, brachialis; it ends as the lateral cutaneous nerve of the forearm.
- Axillary radial lesion (crutch / Saturday-night palsy): triceps ALSO weak, plus wrist drop and wide posterior sensory loss.
- Mid-shaft humeral fracture in the radial groove: triceps SPARED (branches given off proximally); hallmark = wrist drop + loss of finger/thumb extension + snuffbox numbness.
- PIN lesion: finger drop with wrist extension PRESERVED (via ECRL) and NO sensory loss — the PIN is purely motor.
- Axillary nerve injury (shoulder dislocation, surgical-neck fracture, bad injection) = loss of abduction + badge-area numbness; musculocutaneous injury = weak elbow flexion/supination, absent biceps reflex, lateral forearm numbness.
- Assuming any wrist drop means a humeral shaft fracture. Check triceps first: if elbow extension is weak too, the lesion is higher, in the axilla.
- Expecting numbness in a PIN lesion. The posterior interosseous nerve carries no cutaneous fibres — finger drop with normal sensation is its signature.
- Confusing the surgical neck (axillary nerve) with the mid-shaft (radial nerve) of the humerus. Neck = cannot abduct; shaft = cannot extend the wrist.
A patient with a mid-shaft humeral fracture has wrist drop, cannot extend the fingers or thumb, and is numb over the anatomical snuffbox — but elbow extension is normal. Why is triceps spared?
- The posterior cord (C5–T1) ends as the radial and axillary nerves — the extensor cord; the lateral cord gives the musculocutaneous nerve to the anterior arm.
- Radial nerve: triangular interval → radial groove on the humerus → divides at the lateral epicondyle into a sensory superficial branch and a motor deep branch that becomes the posterior interosseous nerve through supinator; it supplies triceps, anconeus, brachioradialis, ECRL and every forearm extensor, and feels the back of the arm, forearm and lateral hand.
- Lesion level is read from the pattern: axilla = triceps weak too; radial groove = triceps spared with wrist drop and snuffbox numbness; PIN = finger drop with preserved wrist extension and no sensory loss.
- Axillary nerve (quadrangular space, surgical neck) supplies deltoid + teres minor and the regimental-badge skin — dislocation or a surgical-neck fracture costs abduction; the musculocutaneous nerve pierces coracobrachialis to supply biceps, brachialis and coracobrachialis, ending as the lateral cutaneous nerve of the forearm.
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- Moore KL, Dalley AF, Agur AMR. Clinically Oriented Anatomy — Brachial plexus and the terminal branches.
- Netter FH. Atlas of Human Anatomy — Plates: radial, axillary and musculocutaneous nerves.
- Sinnatamby CS. Last's Anatomy: Regional and Applied — The upper limb.
- Snell RS. Clinical Anatomy by Regions — Nerve injuries of the upper limb.
- TeachMeAnatomy — The Radial Nerve; The Axillary Nerve; The Musculocutaneous Nerve.
