2.1 Musculoskeletal Anatomy of Upper and Lower Extremities
Key Takeaways
- Brachial plexus ventral rami C5-T1 organize sequentially into roots, trunks, divisions, cords, and terminal nerves.
- Upper limb nerve lesions produce characteristic deformities: C5-C6 Erb-Duchenne ('waiter's tip'), C8-T1 Klumpke ('claw hand'), radial ('wrist drop'), and median ('ape hand' / 'hand of benediction').
- Lower limb innervation relies on lumbar (L1-L4) and sacral (L4-S4) plexuses; deep peroneal nerve damage around the fibular neck leads to foot drop.
- Arterial vascular trees transition smoothly from subclavian to palmar arches in the arm, and external iliac to plantar arteries in the leg.
Last updated: July 2026
2.1 Musculoskeletal Anatomy of Upper and Lower Extremities
Mastering the peripheral nervous system and musculoskeletal architecture of the extremities is essential for the NPLEX Part I. This section details the plexus origins, nerve pathways, peripheral nerve lesions, muscle compartments, and arterial supply of the upper and lower limbs.
Brachial Plexus Origins, Structure, and Branching
The brachial plexus provides motor and sensory innervation to the entire upper extremity. It is formed by the anterior (ventral) rami of spinal nerves C5 through T1. Anatomically, the plexus is divided sequentially into five roots, three trunks, six divisions, three cords, and five terminal branches.
- Roots: Ventral rami of C5, C6, C7, C8, and T1 emerge between the anterior and middle scalene muscles.
- Trunks:
- Superior Trunk: Union of C5 and C6 rami.
- Middle Trunk: Continuation of C7 ramus.
- Inferior Trunk: Union of C8 and T1 rami.
- Divisions: Each trunk splits into an anterior division (supplying flexor compartments) and a posterior division (supplying extensor compartments).
- Cords: Named relative to their anatomical relationship to the axillary artery deep to the pectoralis minor muscle:
- Lateral Cord: Formed by anterior divisions of superior and middle trunks (C5, C6, C7).
- Posterior Cord: Formed by posterior divisions of all three trunks (C5, C6, C7, C8, T1).
- Medial Cord: Formed by anterior division of inferior trunk (C8, T1).
Terminal Nerves and Functional Innervation
- Musculocutaneous Nerve (C5\u2013C7): Originates from the lateral cord, pierces coracobrachialis, and innervates anterior arm flexors (biceps brachii, brachialis, coracobrachialis). Provides cutaneous sensation to the lateral forearm (lateral antebrachial cutaneous nerve).
- Axillary Nerve (C5\u2013C6): Branches from posterior cord, passes through the quadrangular space with the posterior circumflex humeral artery. Innervates deltoid and teres minor. Sensory over lateral shoulder ("badge area").
- Radial Nerve (C5\u2013T1): Main continuation of posterior cord. Traverses radial groove of humerus. Innervates all posterior compartment extensor muscles of arm (triceps brachii) and forearm (brachioradialis, extensor carpi radialis longus/brevis, extensor digitorum, extensor carpi ulnaris). Sensory to posterior arm, posterior forearm, and dorsal aspect of lateral 3.5 digits.
- Median Nerve (C5\u2013T1): Formed by contributions from lateral and medial cords. Runs in anterior forearm between flexor digitorum superficialis and profundus, passing under flexor retinaculum through carpal tunnel. Innervates anterior forearm flexors (except flexor carpi ulnaris and medial half of flexor digitorum profundus) and intrinsic thenar muscles (LOAF: 1st/2nd Lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis superficial head). Sensory to palmar aspect of lateral 3.5 digits.
- Ulnar Nerve (C8\u2013T1): Terminal branch of medial cord. Passes posterior to medial epicondyle of humerus ("funny bone") and enters forearm under flexor carpi ulnaris. Innervates flexor carpi ulnaris, medial half of flexor digitorum profundus, hypothenar muscles, all interossei, adductor pollicis, and 3rd/4th lumbricals. Sensory to medial 1.5 digits (palmar and dorsal).
Clinical Upper Limb Nerve Lesions
Nerve injuries produce classic clinical manifestations frequently tested on board examinations:
| Pathology / Nerve | Level & Etiology | Characteristic Physical Presentation |
|---|---|---|
| Erb-Duchenne Paralysis | Upper trunk (C5\u2013C6) traction; birth trauma or shoulder depression | "Waiter's tip" hand: Arm adducted, medially rotated, elbow extended, forearm pronated. Loss of abductors, lateral rotators, and biceps. |
| Klumpke Paralysis | Lower trunk (C8\u2013T1) traction; upward pull on arm or thoracic outlet syndrome | "Total claw hand": Flexion of PIP/DIP and extension of MCP joints due to loss of intrinsic hand muscles. May present with ipsilateral Horner syndrome (T1 sympathetics). |
| Radial Nerve Injury | Mid-shaft humeral fracture or compression in axilla ("Saturday night palsy") | Wrist drop: Inability to extend wrist and MCP joints. Loss of sensation over dorsal webspace between thumb and index finger. |
| Median Nerve (Proximal) | Supracondylar humeral fracture | "Hand of benediction": Inability to flex digits 1\u20133 when attempting to make a fist due to loss of FDS, lateral FDP, and FPL. |
| Median Nerve (Distal) | Carpal tunnel compression or wrist laceration | "Ape hand": Loss of thumb opposition and thenar atrophy. Dysesthesia over lateral 3.5 palmar digits. |
| Ulnar Nerve Injury | Cubital tunnel syndrome (elbow) or Guyon canal compression (wrist) | "Ulnar claw": Hyperextension of 4th/5th MCP joints with PIP/DIP flexion when extending fingers. Weakened wrist adduction and thumb adduction (positive Froment sign). |
Lower Limb Plexus & Peripheral Nerves
The lower extremity is innervated by the lumbar plexus (L1\u2013L4) and sacral plexus (L4\u2013S4).
Lumbar Plexus Key Branches
- Femoral Nerve (L2\u2013L4): Emerges lateral to psoas major, passes under inguinal ligament into femoral triangle. Innervates anterior thigh muscles (rectus femoris, vastus lateralis/medialis/intermedius, pectineus, sartorius). Sensory to anterior thigh and medial leg (saphenous nerve). Injury causes loss of knee extension, absent patellar reflex, and sensory loss over anterior thigh/medial leg.
- Obturator Nerve (L2\u2013L4): Emerges medial to psoas major, passes through obturator foramen. Innervates medial thigh adductors (adductor longus, brevis, magnus, gracilis). Injury causes impaired thigh adduction.
Sacral Plexus Key Branches
- Sciatic Nerve (L4\u2013S3): Emerges through greater sciatic foramen inferior to piriformis muscle. Divides into tibial and common peroneal (fibular) nerves in lower thigh. Innervates posterior hamstrings (biceps femoris, semitendinosus, semimembranosus) and adductor magnus posterior portion.
- Tibial Nerve (L4\u2013S3): Descends through popliteal fossa and posterior compartment of leg. Innervates plantar flexors (gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, flexor digitorum longus, flexor hallucis longus) and intrinsic sole muscles. Injury impairs plantar flexion and inversion; tarsal tunnel compression causes sole pain.
- Common Peroneal (Fibular) Nerve (L4\u2013S2): Winds around neck of fibula (vulnerable to trauma/casts). Branches into:
- Deep Peroneal Nerve: Innervates anterior compartment (tibialis anterior, EHL, EDL). Deficit results in Foot Drop (loss of dorsiflexion, steppage gait) and sensory loss in 1st webspace.
- Superficial Peroneal Nerve: Innervates lateral compartment (fibularis longus/brevis). Deficit results in loss of eversion and sensory loss over dorsum of foot.
Compartmental Organization & Arterial Blood Supply
Understanding limb compartments aids in diagnosing compartment syndrome and arterial occlusive disease.
Extremity Muscle Compartments
- Upper Arm: Anterior (musculocutaneous n., brachial a.); Posterior (radial n., profunda brachii a.).
- Forearm: Anterior flexor (median and ulnar n., radial and ulnar a.); Posterior extensor (deep radial n., posterior interosseous a.).
- Thigh: Anterior (femoral n., femoral a.); Medial (obturator n., deep femoral a. branches); Posterior (sciatic n., perforating branches of deep femoral a.).
- Leg: Anterior (deep peroneal n., anterior tibial a.); Lateral (superficial peroneal n., fibular a. branches); Posterior (tibial n., posterior tibial a.).
Arterial Tree of Extremities
- Upper Limb: Subclavian artery becomes axillary artery at outer border of 1st rib. Axillary artery (divided into 3 parts by pectoralis minor) becomes brachial artery at inferior border of teres major. Brachial artery bifurcates in cubital fossa into radial artery and ulnar artery, which form the superficial and deep palmar arches.
- Lower Limb: External iliac artery passes under inguinal ligament to become femoral artery. In femoral triangle (NAVEL: Nerve, Artery, Vein, Empty space, Lymphatics from lateral to medial), it gives off deep artery of thigh. Passes through adductor hiatus to become popliteal artery. Popliteal artery divides into anterior tibial artery (continues as dorsalis pedis) and posterior tibial artery (gives off fibular artery and divides into medial/lateral plantar arteries).
Test Your Knowledge
A 24-year-old motorcyclist suffers a high-speed trauma resulting in severe depression of the right shoulder. Physical examination reveals an arm that hangs limply at his side, rotated medially, with the forearm pronated and elbow extended ('waiter's tip' position). Which root origins of the brachial plexus were most likely injured?
A
B
C
D
Test Your Knowledge
A 45-year-old construction worker presents with a fibular neck fracture following a crushing injury. Physical examination reveals an inability to dorsiflex the foot, resulting in a pronounced steppage gait, alongside sensory loss in the webspace between the first and second toes. Which nerve has been damaged?
A
B
C
D
Test Your Knowledge
A surgeon performing a procedure in the femoral triangle needs to identify key neurovascular structures. Ordering structures from lateral to medial, which sequence correctly identifies the contents of the femoral triangle?
A
B
C
D