Unit 16 — Spinal Nerves · Question Bank

Formation

The brachial plexus is formed from the anterior rami of C5, C6, C7, C8, and T1 (pre-fixed plexus includes C4; post-fixed includes T2). It lies in the posterior triangle of the neck, passes posterior to the clavicle, and enters the axilla.

Roots → Trunks (3)

• Superior trunk: C5 + C6 unite at Erb's point (posterior triangle, 2–3 cm above clavicle behind SCM)
• Middle trunk: C7 alone
• Inferior trunk: C8 + T1 unite (lies posterior to subclavian artery)

Trunks → Divisions (6)

Each trunk divides into an anterior and posterior division behind the clavicle (6 divisions total).

Divisions → Cords (3)

• Lateral cord: anterior divisions of superior + middle trunks
• Medial cord: anterior division of inferior trunk
• Posterior cord: all three posterior divisions
• Named by their relationship to the axillary artery (second part)

Cords → Terminal Branches

• Lateral cord: musculocutaneous nerve + lateral root of median nerve
• Medial cord: ulnar nerve + medial root of median nerve + medial cutaneous nerves of arm and forearm
• Posterior cord: radial nerve + axillary nerve (+ thoracodorsal + subscapular nerves)
• Median nerve = lateral root (lateral cord) + medial root (medial cord)

Erb's Palsy (Upper Trunk C5–C6)

Caused by forcible lateral neck flexion (birth injury: shoulder dystocia; adults: motorcycle fall). Muscles paralysed: deltoid, supraspinatus, infraspinatus, biceps brachii, brachialis, brachioradialis. Deformity: arm hangs in adduction, medial rotation at shoulder, elbow extended, forearm pronated, wrist flexed = "waiter's tip". Reflexes lost: biceps (C5–C6) and brachioradialis (C6). Sensory loss: lateral shoulder and forearm (C5–C6).

Klumpke's Palsy (Lower Trunk C8–T1)

Caused by upward arm traction (grabbing a branch during a fall) or cervical rib compression. Muscles paralysed: all intrinsic hand muscles (interossei, hypothenar, thenar adductor pollicis, medial 2 lumbricals). Deformity: claw hand. Sensory loss: medial forearm and hand (C8–T1). Horner's syndrome (ptosis, miosis, anhidrosis, enophthalmos) if T1 root is avulsed, interrupting preganglionic sympathetic fibres to the superior cervical ganglion.

Formation and Course

The median nerve is formed in the axilla by the union of its lateral root (from the lateral cord, C5–C7) and medial root (from the medial cord, C8–T1). It descends in the medial compartment of the arm, initially lateral to the brachial artery, then crosses medially at the mid-arm. In the cubital fossa it lies medial to the brachial artery. It passes between the two heads of pronator teres, then deep to the flexor digitorum superficialis (FDS) in the forearm, running on flexor digitorum profundus (FDP). At the wrist it enters the carpal tunnel (beneath the flexor retinaculum) with the nine flexor tendons, and divides into medial and lateral branches in the palm.

Motor Supply

• Forearm: pronator teres, flexor carpi radialis, palmaris longus, FDS; anterior interosseous branch → pronator quadratus, flexor pollicis longus, lateral half of FDP (index/middle)
• Hand (thenar): abductor pollicis brevis, opponens pollicis, superficial head of flexor pollicis brevis (LOAF: Lumbricals 1+2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis)
• 1st and 2nd lumbricals

Sensory Supply

Lateral 3½ fingers (palmar surface) and corresponding palm; dorsal tips of lateral 3½ fingers.

High Lesion (above elbow)

All forearm muscles are paralysed in addition to thenar muscles. When the patient is asked to make a fist, index and middle fingers cannot flex (FDP lateral half + FDS to these digits lost) while ring/little can (FDP medial half is ulnar nerve). The index finger remains extended = "benediction sign" (or pointing hand / hand of blessing). Pronation is lost. Thenar wasting + ape hand also present. Sensory loss as above.

Low Lesion (carpal tunnel / wrist)

Forearm muscles are intact. Only thenar muscles (abductor pollicis brevis, opponens pollicis, flexor pollicis brevis superficial head) and 1st/2nd lumbricals are affected. Result: loss of thumb opposition, thenar wasting = "ape hand". Sensory loss over lateral 3½ fingers and lateral palm. No benediction sign because FDS/FDP to index/middle are working.

Origin

The radial nerve is the largest branch of the brachial plexus, arising from the posterior cord (C5–C8, T1).

Course

• Axilla: arises from posterior cord, gives branches to triceps long and medial heads
• Arm: winds around the posterior humerus in the radial (spiral) groove between the medial and lateral heads of triceps, accompanied by the profunda brachii artery; supplies triceps lateral head and anconeus, and gives the posterior cutaneous nerve of forearm
• Lateral epicondyle region: pierces lateral intermuscular septum, lies in front of the lateral epicondyle between brachialis and brachioradialis; divides into superficial and deep branches
• Superficial radial nerve: purely sensory; descends in forearm under brachioradialis; emerges at the wrist to supply the dorsum of lateral hand and lateral 3½ fingers (proximal phalanges)
• Deep radial nerve (PIN — posterior interosseous nerve): motor; winds around the radial neck between the two heads of supinator (through the arcade of Frohse), enters the posterior compartment; supplies all extensor muscles of the forearm

Wrist Drop

Injury of the radial nerve in the radial groove (mid-shaft humeral fracture, Saturday night palsy) paralyses extensor carpi radialis longus + brevis, extensor carpi ulnaris + all finger extensors → wrist drop. Triceps function is usually preserved (its branches arise above the groove). Sensory loss: small area over dorsum of first web space (only consistent area due to overlap).

PIN Syndrome

The posterior interosseous nerve (PIN) is the deep branch of the radial nerve. Compression at the radial neck (arcade of Frohse, radial head fracture, rheumatoid synovitis) causes PIN syndrome: weakness of finger and thumb extension, radial deviation of wrist (extensor carpi radialis longus is spared because it branches proximal to the compression). No sensory loss because the PIN is purely motor.

Origin

The ulnar nerve arises from the medial cord of the brachial plexus (C7, C8, T1; mainly C8–T1).

Course

• Arm: descends medial to brachial artery in the medial compartment; pierces medial intermuscular septum in the distal arm
• Medial epicondyle: passes posterior to the medial epicondyle in the cubital tunnel (ulnar groove / cubital fossa) — most vulnerable site, palpable here
• Forearm: passes between the two heads of flexor carpi ulnaris; runs on flexor digitorum profundus; supplies FCU and medial half of FDP (ring and little fingers)
• Wrist: passes superficial to the flexor retinaculum through Guyon's canal (between the pisiform and hook of hamate), lateral to the ulnar artery
• Hand: divides into superficial (sensory) and deep (motor) branches in the palm; deep branch hooks around the hook of hamate to supply all interossei, hypothenar muscles, medial two lumbricals, and adductor pollicis

Claw Hand

Ulnar nerve palsy (medial epicondyle injury) paralyses interossei and medial two lumbricals (ring, little fingers). These muscles normally flex MCP and extend IP joints. Without them: hyperextension at MCP + flexion at PIP/DIP of ring and little fingers = claw hand. Ulnar paradox: low lesion (at wrist) → more severe claw than high lesion (elbow), because FDP (ring/little) is intact in a low lesion, allowing greater IP flexion.

Guyon's Canal Syndrome

Compression of the ulnar nerve in Guyon's canal (between the pisiform and hamate at the wrist) by ganglion, lipoma, ulnar artery aneurysm, or prolonged cycling (handlebar palsy). Results in intrinsic hand muscle weakness + sensory loss over medial 1½ fingers. Forearm muscles (FCU, medial FDP) are spared because they are supplied by branches proximal to Guyon's canal.

Origin

The sciatic nerve is the largest nerve in the body, formed from the sacral plexus (L4, L5, S1, S2, S3). It is actually two nerves (tibial + common fibular) bound together in a common sheath.

Exit from Pelvis

The sciatic nerve exits the pelvis through the greater sciatic foramen, passing below piriformis (infrapiriform foramen). It emerges between the greater trochanter of the femur and the ischial tuberosity, deep to gluteus maximus. Note: in 10–12% of individuals, the common fibular component passes above or through piriformis (piriformis syndrome).

Course in Thigh

Descends vertically in the posterior compartment of the thigh, lying on the posterior surface of the adductor magnus. Supplies the posterior thigh muscles (semimembranosus, semitendinosus, biceps femoris long head — via tibial component; biceps femoris short head — via common fibular component) and the adductor magnus (hamstring part). It is accompanied by the perforating branches of the profunda femoris artery.

Terminal Branches

The sciatic nerve divides into the tibial nerve and common fibular (peroneal) nerve, usually at the apex of the popliteal fossa (may divide more proximally in the thigh).

• Tibial nerve: descends through the popliteal fossa, posterior leg (supplies posterior compartment — gastrocnemius, soleus, popliteus, tibialis posterior, FDL, FHL = plantarflexors + invertors), and enters the foot through the tarsal tunnel; divides into medial and lateral plantar nerves. Injury → hook-like foot.
• Common fibular nerve: winds around the neck of the fibula; divides into superficial peroneal nerve (lateral compartment = evertors) and deep peroneal nerve (anterior compartment = dorsiflexors). Injury → foot drop.

Foot Drop

Injury to the common fibular nerve at the fibular neck (fracture, tight plaster cast, leg-crossing) denervates the anterior compartment (tibialis anterior, EDL, EHL) → inability to dorsiflex the foot. Also loss of eversion (superficial peroneal). Patient walks with high-stepping gait to prevent toe drag. Resting posture: talipes equinovarus (foot plantarflexed + inverted). Sensory loss: dorsum of foot and lateral leg.