Brachial Plexus and Lumbosacral Plexus
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Posterior cord – dotted line
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Muscles innervated by musculocutaneous nerve are biceps, brachialis, and coracobrachialis
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Cervical plexus from CN XII-C5; only know phrenic nerve (C3-C5)
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Finger flexion best tested at DIPs where flexor digitorum profundus (median nerve for digits 2-3; ulnar nerve for digits 4-5) acts without significant contributions from other muscles
Simplification: Three Nerves Acting on Thumb
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RUM – Radial nerve (abduction in plane of palm done by abductor pollicis longus), Ulnar nerve (adduction by adductor pollicis), and Median nerve (flexion by flexor pollicis longus and superficial head of flexor pollicis brevis)
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Abduction perpendicular to palm mediated by abductor pollicis brevis, which is innervated by median nerve after it passes through carpal tunnel

Intrinsic and Extrinsic Hand Muscles
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Muscles of thenar eminence (opponens pollicis, abductor policis brevis, flexor pollicis brevis, and adductor pollicis)
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Muscles of hypothenar eminence (opponens digiti minimi, abductor digiti minimi, flexor digiti minimi)
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Intrinsic hand muscles innervated by ulnar nerve except LOAF (Lumbricals I and II, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis (superficial head)) which are innervated by median nerve after it passes
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 through carpal tunnel
All intrinsic hand muscles supplied by C8 and T1
Extrinsic muscles in forearm important for finger movements too
Extensor digitorum (radial nerve, C7) best tested at MCP joints; lumbricals predominantly responsible for finger extension at PIPs and DIPs (median nerve digits 2-3, ulnar nerve digits 4-5)
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Palmar interossei adduct fingers; dorsal interossei abduct them
Simplification: Five Nerves to Remember in Leg
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Tibial and common peroneal nerves most important branches of sciatic nerve
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Hamstring muscles (semitendinosus, semimembranosus, and biceps femoris) innervated by sciatic nerve before it divides
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Common peroneal nerve divides into superficial peroneal and deep peroneal nerves

Common Plexus and Nerve Syndromes
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Caused mainly by mechanical factors or diabetes
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Erb-Duchenne palsy – caused by traction on infant’s shoulder during difficult delivery and motorcycle accidents o Damage to upper trunk of brachial plexus causes loss of function in C5-C6 innervated muscles, resulting in prominent weakness in deltoid, biceps, infraspinatus, and wrist extensors o Bellman’s position (waiter’s tip position) o Finger and hand movements relatively spared o Most infants recover fully o Surgical repair of plexus occasionally pursued
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Klumpke’s palsy – caused by upward traction produced by grabbing branch during fall from tree, thoracic outlet syndrome, and Pancoast’s syndrome o Damage to lower trunk of brachial plexus causes weakness of C8-T1 innervated muscles o Hand and finger weakness, atrophy of hypothenar muscles, sensory loss on ulnar aspect o If T1 nerve root damaged proximal to SNS trunk, may be Horner’s syndrome o Thoracic outlet syndrome – lower brachial plexus compressed as it passes between clavicle and first rib
 Symptoms increased by raising and external rotation of arm
 Look for cervical rib
 Treatment includes strengthening shoulder muscles and surgical decompression o Pancoast’s syndrome – apical lung tumor extends into lower brachial plexus; sometimes includes
Horner’s syndrome, recurrent laryngeal nerve impingement (hoarseness)
 Ultimately, entire brachial plexus may be invaded
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Axillary neuropathy – dislocation or fracture of proximal humerus compressing axillary nerve; doesn’t involve biceps (C5 radiculopathy does)
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Parsonage-Turner syndrome – onset in adulthood; burning shoulder or lateral neck pain followed by weakness of muscles innervated by brachial plexus; most patients fully recover in 6-12 weeks of onset
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Radial neuropathy – sleeping with arm slung over park bench, compression in axilla by improper crutch use, or fracture of humerus damaging nerve as it travels in spiral groove o Weakness of extensors, supination, loss of triceps reflex, and sensory loss of radial distribution o Posterior interosseous nerve purely motor branch of radial nerve; damage to it results in weakness of radial nerve-innervated muscles without triceps involvement or sensory loss o Cheiralgia paresthetica – caused by tight wrist bands or handcuffs compressing superficial branch of radial nerve, causing isolated sensory loss in dorsal lateral hand
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Median neuropathy – honeymooner’s palsy (caused by sleeping with lover’s head on upper arm); can be caused by fractures of humerus or distal radius; entrapment can occur as nerve passes through pronator teres o Weakness of wrist flexion and abduction, opposition of thumb, and flexion of digits 2-3 o Attempt to make fist produces hand of benediction
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Carpal tunnel syndrome – caused by compression of median nerve as it passes with tendons of hand under flexor retinaculum on flexor surface of wrist o Test abductor pollicis brevis (abducts thumb perpendicular to plane of palm) o Flick sign – patient shakes hand to try to relieve paresthesias o Treatment includes immobilization with wrist splint, steroid injections, and surgical decompression
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Ulnar neuropathy – entrapment of ulnar nerve at elbow in cubital canal (region of ulnar groove) o Sometimes called tardy ulnar palsy (delayed result of posttraumatic, degenerative, or congenital increased carrying angle of elbow) o Ulnar nerve damaged by fractures of medial epicondyle or compressed by resting elbows on table o Weakness of wrist flexion and adduction, finger adduction and abduction, and flexion of digits 4-5 o Sensory paresthesias in ulnar distribution o Ulnar claw when asked to straighten fingers o Entrapment in cubital canal can be treated by surgically translocating ulnar nerve to flexor side of elbow
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Compression of ulnar nerve in hand as it passes over hamate bone in Guyon’s canal can occur from prolonged leaning forward while cycling o Weakness of finger adduction and abduction without sensory loss
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Combination of chronic median and ulnar nerve lesions leads to simian hand (lack of thumb opposition)

Lower-Extremity Nerve Injuries
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Femoral neuropathy – femoral nerve injured during pelvic surgery or retroperitoneal hematoma or pelvic mass; weakness of thigh flexion and knee extension, loss of patellar reflex, and sensory loss in anterior thigh
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Sciatic neuropathy – caused by posterior hip dislocation, acetabular fracture, and IM injection placed too medially and inferiorly in buttocks o Weakness of all foot and ankle muscles and knee flexion, loss of Achilles tendon reflex, and sensory loss in foot and lateral leg below knee o Sciatica – all disorders causing painful paresthesias in sciatic distribution; most common cause is compression of lumbosacral roots by disc material and osteophytes o Rarely may be compressed more distally by muscular or skeletal elements
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Peroneal nerve palsy – common peroneal nerve passes around fibular head near skin surface; vulnerable to laceration, stretch injury by foot inversion, or compression by tight stockings, cast, crossed legs, or trauma o Foot drop, weakness of dorsiflexion and eversion of foot, sensory loss over dorsolateral foot and shin o Most recover spontaneously when mechanical cause removed
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Obturator nerve palsy – obturator nerve originates from L2-L4; can be compressed during complicated delivery, pelvic trauma, or surgery o Gait instability due to weakness of leg adductor muscles and pain and numbness in medial thigh
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Meralgia paresthetica – lateral femoral cutaneous nerve (L2-L3) entrapped as it passes under inguinal ligament
 or fascia lata, producing paresthesias and loss of sensation in lateral thigh o No motor involvement or reflex changes o Caused by obesity, pregnancy, weight loss, or heavy equipment belts o Symptoms may be worse after prolonged walking, standing, or sitting o Symptoms often resolve spontaneously or by avoidance of mechanical precipitants
Morton’s metatarsalgia – tight-fitting shoes can compress digital nerves, especially of 3 rd and 4 th toes, producing patches of numbness and paresthesias
Electromyography (EMG) and Nerve Conduction Studies
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Nerve conduction studies – stimulating electrodes placed on skin overlying nerve, and recording electrodes placed at different point along same nerve or overlying muscle innervated by nerve o When stimulus given to nerve, compound motor action potential (CMAP) recorded over belly of muscle innervated by that nerve o If distal nerve branch with purely sensory function is used for recording or stimulation and second set of stimulating or recording electrodes placed somewhere along nerve, compound sensory nerve action potential (SNAP) recorded over nerve o Lesions proximal to DRG leave SNAPs intact o Proximal lesions of motor nerve roots reduce CMAPs o Latency or velocity measured to determine if demyelination exists o Decreased SNAP amplitudes suggest conduction in some axons interrupted (axonal damage) o CMAP studies evaluate neuromuscular junction by use of repetitive stimulation; slow repetitive stimulation depletes presynaptic stores of ACh, and faster repetitive stimulation increases presynaptic
Ca 2+ , facilitating neurotransmitter release
 Under normal conditions, repetitive stimulation doesn’t affect amplitude (safety factor)
 In myasthenia gravis (decrease in postsynaptic ACh receptors on muscle cells) – slow repetitive stimulation results in gradual decrement in CMAP amplitude
 Lambert-Eaton myasthenic syndrome and botulism have decreased presynaptic neurotransmitter release; fast repetitive stimulation (or active volitional muscle contraction) causes CMAPs to increment in amplitude from abnormally low starting point
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EMG – electrode inserted directly into muscle, and motor unit action potentials (MUPs) recorded from muscle o EMG pattern provides info useful in distinguishing weakness caused by neuropathic disorders (nerve or motor disease) from that caused by myopathic disorders (muscle disease) o Neuropathic disorders – increased spontaneous activity (fibrillation potentials and positive sharp waves) often recorded on EMG and sometimes visible as fasciculations
 Occurs due to chronic denervation of muscle cells; denervation causes adjacent motor axons to sprout and re-innervate larger region, resulting in abnormally large motor units

 MUPs usually abnormally large amplitude and duration o Myopathic disorder – shown by reduced MUP amplitude and duration o When muscle voluntarily contracted, EMG shows pattern of continuous firing of motor units (normal recruitment pattern)
 In neuropathic disorders, recruitment pattern has normal amplitude but shows interrupted firing, since some motor units not successfully (decreased, reduced, or incomplete recruitment)
 In myopathic disorders, recruitment pattern continuous or increased (since more motor units need to be activated for given force) but amplitude often decreased