A fracture, a crossed leg, a botched injection. Each one hits a named nerve, and the deficit is the receipt. Trace the wire, predict the loss. Then nail the dorsal versus ventral rami question that sinks half the room.
Before any list, a patient. Read the injury, predict the loss, then pick the nerve. The trap here is the one almost everyone reaches for first.
A 26-year-old man is brought to the emergency department after a fall from a ladder. Radiographs show a transverse fracture through the middle third of the humeral shaft. On exam his elbow flexes and extends fine and his grip strength at rest is preserved, but he cannot lift his wrist or straighten his fingers, so the hand hangs limp. He has numbness over the back of the thumb and the web between the thumb and index finger.
Which nerve did that fracture catch?
Ulnar nerve
Radial nerve
Median nerve
Axillary nerve
🧠The deficit IS the map. A limp wrist with the back of the hand gone means the wire that runs the extensors got cut. That wire spirals right along the back of the humerus. Break the bone there, you snap the wire.
Brachial Plexus
The Five Terminal Nerves
Five wires leave the plexus for the arm. Tap each one. Same five rows every time: roots, what it moves, what it feels, the classic break that gets it, and the sign on exam.
The brachial plexus: roots (C5 to T1) braid into trunks, divisions, and cords, then split into the five terminal nerves below.
Axillary
Musculocutaneous
Radial
Median
Ulnar
AxillaryC5 to C6
MovesDeltoid and teres minor. Abducts the arm from 15 to 90 degrees (the supraspinatus starts the first 15).
FeelsSkin over the deltoid, the regimental badgeThe patch of skin where a soldier's shoulder badge sits. That oval over the deltoid is the axillary nerve's sensory zone, so numbness there points right at it. patch.
BreakSurgical neck of the humerus fracture, or an anterior shoulder dislocation, wraps around right there.
On exam: can't start abduction off the side, and the badge over the shoulder goes numb.
MusculocutaneousC5 to C7
MovesBiceps, brachialis, coracobrachialis. Flexes the elbow and supinates the forearm.
FeelsA strip down the lateral forearm.
BreakIsolated injury is rare. Think weak curl and weak palm-up turn.
On exam: weak elbow flexion and supination, lateral forearm numb. The bicep is the headline.
RadialC5 to T1
📷 Radial nerve, posterior arm · tap to expand
MovesBEST🔑Brachioradialis, Extensors (wrist + fingers), Supinator, Triceps. The radial is the great extender. BEST runs the back of the arm.: Brachioradialis, Extensors of wrist and fingers, Supinator, Triceps. It is the great extender.
FeelsBack of the hand and the anatomic snuffboxThe little hollow at the base of the thumb. Its skin is radial nerve territory, so dorsal hand numbness there fits a radial lesion..
BreakMidshaft humerus in the spiral groove gives wrist drop with the triceps spared. Axilla compression (a crutch, or sleeping over a chair) takes the triceps too.
On exam: wrist drop. The wrist and fingers hang because nothing extends them.
MedianC5 to T1
MovesLOAF🔑Lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis. Plus most forearm flexors and the pronators. The median is the great flexor. hand muscles plus most forearm flexors and the pronators. The thumb's worker.
FeelsLateral palm and the palm side of digits 1 to 3.5.
BreakCarpal tunnel at the wrist gives thenar wasting and an ape handThe thumb can no longer swing out in front of the palm, so it sits flat in the same plane as the fingers, like a primate hand. That is lost opposition and abduction from median wrist injury.. Supracondylar fracture higher up gives the hand of benedictionTry to make a fist: digits 2 and 3 stay straight because their deep flexors are median-run, while 4 and 5 curl. The result looks like a blessing hand. Proximal median lesion. on attempted fist.
On exam: thenar wasting, lost thumb opposition. Make a fist and the index and middle won't curl if it's proximal.
UlnarC8 to T1
📷 Ulnar claw hand · tap to expand
MovesMedial 2 lumbricals, all interossei, hypothenar, adductor pollicis, flexor carpi ulnaris. The hand's fine-tuner.
FeelsMedial 1.5 digits (little finger and half the ring).
BreakMedial epicondyle or cubital tunnel at the elbow, or Guyon canalA tunnel at the wrist on the pinky side where the ulnar nerve passes. Hook-of-hamate fractures and handlebar pressure crush it here. A distal lesion paradoxically claws worse. / hook of hamate at the wrist gives the ulnar claw.
On exam: ulnar claw, positive Froment signAsk the patient to pinch paper between thumb and index. With a weak adductor pollicis (ulnar), they cheat by flexing the thumb tip using the median-run flexor pollicis longus. The bent thumb is a positive sign., can't cross the fingers.
⚠ The ulnar paradox
You'd think a lesion higher up the nerve would look worse. It doesn't. A distal ulnar lesion at the wrist makes a nastier claw than a proximal one at the elbow. Why? The proximal lesion also kills the ulnar half of the deep finger flexor, so the ring and little fingers can't curl as hard, and a finger that can't flex can't claw. Spare that flexor (distal lesion) and the claw deepens. More distal ulnar lesion equals a worse-looking claw.
Whole-Trunk Failures
Erb, Klumpke, and Two Scapular Wires
Sometimes the injury is bigger than one terminal nerve. A whole trunk goes, and the posture tells you which.
Erb palsyC5 to C6 · upper trunk
📷 Erb palsy, the waiter's-tip arm · tap to expand
The waiter's tip: arm adducted, internally rotated, elbow extended, forearm pronated, palm turned back like begging for a tip. Cause: shoulder dystocia at birth, or a fall that yanks the head away from the shoulder and widens that angle.
Waiter's tip equals Erb, C5 to C6. Upper trunk, upper roots, arm hangs and rotates in.
Klumpke palsyC8 to T1 · lower trunk
A total claw hand means lower trunk weakness of the small hand muscles.
Horner syndrome can join when T1 sympathetics are dragged down.
Classic causes: upward arm traction at birth or an apical lung tumor invading from below.
Total claw equals Klumpke, C8 to T1. Lower trunk. Add Horner and think apical lung tumor in an adult.
Long thoracicC5 to C7
📷 Winged scapula, long thoracic nerve · tap to expand
Runs the serratus anterior, the muscle that pins the scapula flat against the ribs. Cut it and you get a winged scapula: push on a wall and the medial border lifts off the back like a wing. Classic cause: axillary node dissection or mastectomy.
📷 Medial winging on a wall push · tap to expand
Wall push, wing pops out equals long thoracic. Serratus anterior, C5 to C7.
Dorsal scapularC5
Runs the rhomboids and levator scapulae, the retractors that pull the shoulder blades together. Lose it and the scapula drifts laterally. Hold this name. It comes back to bite in the back-muscle trap, because despite the word dorsal, this nerve is a branch of the ventral rami.
Lumbosacral Plexus
The Lower-Limb Wires
One giant nerve, the sciatic, drops down the back of the leg and splits at the knee into two branches that divide the foot between them. Learn the split and the foot makes sense.
📷 Lumbar plexus plan: L1 to L4 feed femoral, obturator, and the lateral femoral cutaneous. Tap to expand.
The sciatic splits at the knee. Tibial dives straight down the back (sole, tiptoe). Common fibular wraps the fibular neck, where it is easy to crush.
🥋TIP and PED.Tibial Inverts and Plantarflexes (toe-down, like pressing a gas pedal). Peroneal (fibular) Everts and Dorsiflexes (toe-up). Lose the fibular and the toe-up fails: that's foot drop.🔑TIP: Tibial Inverts and Plantarflexes. PED: Peroneal Everts and Dorsiflexes. The fibular nerve is the foot's gas-pedal lifter. Crush it, the foot drops.
Femoral
Obturator
Tibial
Common fibular
Sup. gluteal
Inf. gluteal
FemoralL2 to L4
MovesQuadriceps (extends the knee), iliopsoas (flexes the hip), sartorius.
FeelsAnterior thigh and the medial leg (via the saphenous nerveThe femoral nerve's pure sensory end branch. It runs down the medial leg, so medial-leg numbness with weak knee extension localizes to femoral.).
BreakPelvic fracture, retroperitoneal hematoma, or prolonged lithotomyThe legs-up-in-stirrups position for pelvic surgery and delivery. It stretches and compresses the femoral nerve under the inguinal ligament. positioning.
On exam: weak knee extension, the leg buckles, and the patellar reflex is gone.
ObturatorL2 to L4
MovesThe thigh adductors. Squeezes the legs together.
FeelsA patch of medial thigh.
BreakAn obturator herniaA loop of bowel pushes through the obturator canal and presses the nerve, classically in a thin older woman. Medial-thigh pain that worsens on hip extension is the Howship-Romberg sign. or pelvic surgery.
On exam: weak adduction, medial thigh numb. The legs drift apart.
TibialL4 to S3
MovesTIP: Tibial Inverts and Plantarflexes. Posterior leg, toe flexors. Stand on tiptoe.
FeelsThe sole of the foot.
BreakKnee dislocation, Baker cystA fluid sac behind the knee that can compress the tibial nerve in the popliteal fossa., or tarsal tunnelA tunnel behind the medial ankle where the tibial nerve passes. Compression here gives sole pain and numbness, the foot's version of carpal tunnel. at the ankle.
On exam: can't stand on tiptoes, ankle jerk gone, sole numb. At rest the foot sits dorsiflexed and everted.
Common fibularL4 to S2
📷 Foot drop, right foot · tap to expand
MovesPED: Peroneal Everts and Dorsiflexes. Lifts the toes and turns the sole out.
FeelsDorsum of the foot, and the first web space (deep branch).
BreakFibular neck fracture, crossing the legs, or a tight cast. It sits right on the bone with no padding. Most common lower-limb nerve injury.
Foot drop and steppage gaitTo clear a dropped foot the patient lifts the whole leg high and slaps it down, like marching, so the toes don't catch.. Lost eversion and dorsiflexion, numb dorsum.
Superior glutealL4 to S1
📷 Trendelenburg drop · tap to expand
MovesGluteus medius and minimus plus tensor fasciae latae. Abducts the hip and stabilizes the pelvis when you stand on one leg.
BreakA misplaced intramuscular injection in the buttock. This is exactly why gluteal shots go in the upper outer quadrant.
Trendelenburg signStand the patient on the affected leg. Normally the hip abductors hold the opposite pelvis level. When they fail, the unsupported side sags down.: stand on the bad leg and the opposite hip drops. The sign points away from the lesion.
Inferior glutealL5 to S2
MovesGluteus maximus. Extends the hip: rising from a chair, climbing stairs, standing from a squat.
BreakDeep buttock injury or surgery.
On exam: trouble standing up from a seat and climbing stairs. No gait drop, that's the superior gluteal.
Two more to know
Pudendal (S2 to S4): the perineum, the external anal and urethral sphincters, genital sensation. Blocked at the ischial spineA bony landmark on the pelvis. A pudendal nerve block is placed here for pain relief during childbirth. for childbirth pain relief.
Lateral femoral cutaneous (L2 to L3): pure sensory. Trapped under the inguinal ligament it gives meralgia parestheticaBurning, tingling numbness over the anterolateral thigh with no weakness, from a tight belt, obesity, or pregnancy squeezing the nerve at the inguinal ligament.: a burning anterolateral thigh from a tight belt, obesity, or pregnancy. No weakness, just the burn.
The Question That Sinks Half The Room
Dorsal Rami vs Ventral Rami
The board shows you a muscle on the back and asks which spinal ramus runs it. Sort each muscle into the right bin. The rule clicks the moment you see why the answer is what it is.
🧑🎓Here is the whole trick before you start. Dorsal rami run only the deep muscles that move the spine itself. Every muscle that sits on the back but moves the arm or shoulder blade is a migrant: it crawled there in development but kept its old wiring from the ventral rami.
Drag each muscle into its bin (or tap a muscle, then tap a bin).
Dorsal rami
Deep, intrinsic, moves the spine. The true back muscles.
Ventral rami / migrants
Sits on the back but moves the arm or scapula, or breathes.
0 of 7 sorted.
The punch. If a muscle on the back moves the arm or the scapula, it is a ventral-rami migrant: trapezius runs off the spinal accessory nerve (CN XI), latissimus dorsi off the thoracodorsal nerve, the rhomboids and levator scapulae off the dorsal scapular nerve, and serratus posterior off the intercostal nerves. Only the deep muscles that bend and twist the spine itself, the erector spinae and the transversospinalis group and the splenius, run off dorsal rami. Naming trap: the dorsal scapular nerve has dorsal in its name but it is a branch of the ventral rami (brachial plexus). The word lies.
⚠ Why the names fight you
It feels backwards that a muscle literally on your back is run by a ventral ramus. The fix is to stop trusting position and trust the job. Position is where the muscle ended up. The ramus follows the job it does. Spine-mover equals dorsal. Arm or scapula mover equals ventral. A muscle on the back that moves the arm or scapula is always ventral.
The Signature Game
The Lesion Localizer
A patient arrives with one specific injury. Watch the nerve light up along the limb, predict the deficit, and the pattern card pays it off. Three patients, three traps.
Patient I
Marcus
Age 24
Where does the wire run?
Which deficit shows up?
Pattern Locked
Route
Pattern
Pearl
Rounds complete
3 / 3
Every wire traced. You read the deficit off the injury.
Build The Algorithm
The Localizer Tree
Guess the branch before it opens. Each step quizzes you first, then shows where the path forks.
1.The deficit is in the hand or the foot. First fork: where does this even live?
A hand problem means an upper-limb nerve
A foot problem means a lower-limb nerve
Hand / armBrachial plexus. Sort by what's lost: extend (radial), oppose the thumb (median), claw (ulnar).
Foot / legLumbosacral plexus. Sort by the move: toe-up (fibular), tiptoe (tibial), pelvis (gluteal).
2.Upper limb: the wrist hangs limp and the back of the hand is numb. Which wire?
Claw + FromentUlnar. Interossei gone. Medial epicondyle or Guyon canal.
Ape hand / benedictionMedian. Thumb opposition gone. Carpal tunnel or supracondylar.
3.Lower limb: the patient sat with crossed legs and now the foot drops and the toes catch. Which wire?
Tibial nerve
Common fibular nerve
Femoral nerve
Foot dropCommon fibular at the fibular neck. PED gone (no toe-up). Crossed legs, tight cast.
Can't tiptoeTibial. TIP gone (no toe-down). Ankle jerk lost.
Hip dropsSuperior gluteal. Trendelenburg toward the opposite side. Bad gluteal injection.
Board-Style Walkthrough
Read The Injury, Pick The Nerve
Full clinical vignettes, one at a time, shuffled and never-repeating. Answer first, then the clues light up and every choice gets explained.
Six hooks to carry in · tap to reveal
🥋 TIP / PEDTibial Inverts and Plantarflexes. Peroneal Everts and Dorsiflexes. The fibular nerve lifts the toes; crush it and the foot drops.🦷 Spiral grooveRadial nerve in the spiral groove equals wrist drop. Midshaft humerus fracture snaps the wire that runs the extensors.✋ LOAFLOAF muscles live in the median nerve's lateral palm: Lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis.🧕 TrendelenburgSuperior gluteal lesion drops the OTHER hip. Trendelenburg points away from the bad side. Bad gluteal injection is the classic cause.👶 Waiter / clawWaiter's tip equals Erb (C5 to C6, upper trunk). Total claw equals Klumpke (C8 to T1, lower trunk). Add Horner, think apical lung tumor in an adult.🦴 Dorsal = deepDorsal rami run the deep back only. Trapezius, latissimus, and rhomboids are ventral migrants. A back muscle that moves the arm or scapula is ventral.
Vignette 1 of 28
Exam tools: right-click (or long-press) a choice to cross it out. Double-click (or double-tap) to highlight it.
Sources: Gray's Anatomy (plexus and limb-nerve plates), Moore Clinically Oriented Anatomy, Netter Atlas of Human Anatomy. Clinical photographs via Wikimedia Commons under their respective Creative Commons and public-domain licenses.
