3.1 Myology of the Appendicular Skeleton

Myology of the Appendicular Skeleton

Myology of the appendicular skeleton is 12% of General Anatomy on the NBCE Part I test plan. General Anatomy itself is 20% of the 255-item basic-sciences exam. Items reward a lesion-to-deficit map: if you know which named nerve supplies which named muscle, winged scapula, Trendelenburg gait, wrist drop, and foot drop become one-step questions. Origins and insertions matter when they explain a test (supraspinatus on the superior facet of the greater tubercle; iliopsoas on the lesser trochanter), but innervation and compartment membership are the scoring core.

This section stays on the limbs and pelvic girdle. Axial myology (erector spinae, suboccipitals, abdominal wall) belongs to Spinal Anatomy.

Structure and Histology

Skeletal muscle is voluntary, striated, and multinucleated. Embryonic myoblasts fuse, so a mature fiber carries many peripheral nuclei. Repair after injury depends on satellite cells that sit between the sarcolemma and the basal lamina; they are the resident stem-cell pool and are not a feature of cardiac myocytes.

Connective-tissue sheaths organize the mechanical chain from fiber to bone:

• Endomysium surrounds each fiber (and carries capillaries and terminal axons).
• Perimysium wraps a fascicle.
• Epimysium wraps the whole muscle and is continuous with tendon. Collagen of tendon continues as Sharpey fibers into bone.

The contractile unit is the sarcomere, Z disc to Z disc. The dark A band contains thick myosin filaments (with overlapping actin). The light I band is thin actin only. The H zone is myosin without overlap; the M line cross-links myosin. Thin filaments also carry tropomyosin and the troponin complex (TnC binds Ca2+, TnT binds tropomyosin, TnI inhibits actin–myosin until Ca2+ arrives).

In skeletal muscle, T-tubules invaginate at the A–I junction. Each T-tubule plus two sarcoplasmic-reticulum terminal cisternae is a triad. Voltage-gated dihydropyridine receptors couple mechanically to ryanodine receptors and dump stored Ca2+ onto troponin C. (Cardiac muscle uses a diad at the Z line and calcium-induced calcium release; that comparison is high-yield when Part I mixes histology across tissues.)

A motor unit is one lower motor neuron plus every fiber it innervates. Fine muscles (lumbricals, extraocular homologues are axial) have small motor units; gluteus maximus has large ones. The neuromuscular junction releases acetylcholine onto nicotinic receptors; acetylcholinesterase clears the cleft.

Fiber types (all can exist in one muscle, but postural muscles are Type I–rich):

Sensory organs in the muscle–tendon unit: muscle spindles (intrafusal fibers; group Ia and II afferents) monitor length and velocity; Golgi tendon organs (Ib) monitor tension. These receptors are the anatomic substrate of stretch and autogenic-inhibition reflexes tested in physiology, but Part I anatomy expects you to place them in perimysium/tendon and name the afferent classes.

Development

Limb buds appear in week 4. Hypaxial cells from the dermomyotome of somites migrate into the limb and split into dorsal and ventral muscle masses. Epaxial myotomes stay in the true back and take dorsal rami—they are not appendicular.

• The upper limb rotates laterally about 90°, so the dorsal (extensor/supinator) mass finishes posterior (triceps, posterior forearm).
• The lower limb rotates medially about 90°, so the dorsal mass finishes anterior. That is why quadriceps and tibialis anterior are developmental extensors sitting on the front of the limb, and why the dermatome map looks “twisted” relative to the upper limb.

Each spinal segment keeps its myotome identity after rotation. C5–C6 still flex the elbow; L3–L4 still extend the knee. Limb muscles are innervated by ventral rami collected into the brachial plexus (C5–T1) and the lumbar (L1–L4) and sacral (L4–S4) plexuses. Prefixed or postfixed plexuses shift one segment but do not change compartment logic.

Upper Limb and Shoulder (Including Innervation)

Scapular and pectoral muscles

Serratus anterior failure produces medial winged scapula (long thoracic nerve: superficial on the muscle, vulnerable in axillary dissection or traction). Dorsal scapular loss (rhomboids) can also wing, with the vertebral border drifting laterally. Trapezius palsy wings with difficulty abducting above the horizontal because upward rotation is lost.

Rotator cuff (SITS)

The cuff tendons blend with the glenohumeral capsule and dynamically center the humeral head on the glenoid. They are not the prime movers of large arcs; they set the fulcrum so deltoid can abduct.

Suprascapular nerve path: upper trunk → suprascapular notch (nerve under the superior transverse scapular ligament; the artery usually passes over—Army over Navy) → supraspinatus → spinoglenoid notch → infraspinatus. A spinoglenoid cyst can spare abduction and still weaken external rotation. Supraspinatus is the cuff tendon most often torn and the one that occupies the subacromial space.

The quadrangular space (teres minor, teres major, long head of triceps, surgical neck of the humerus) transmits the axillary nerve and posterior circumflex humeral artery. Surgical-neck fracture or inferior GH dislocation can take deltoid and teres minor together and numb the superior lateral cutaneous patch over deltoid.

Arm compartments

Anterior compartment — musculocutaneous nerve (C5–C7): coracobrachialis (the nerve pierces it), biceps brachii (long head: supraglenoid tubercle, through the intertubercular groove; short head: coracoid), brachialis (workhorse flexor; a small lateral part may take radial innervation). Distal to biceps, musculocutaneous continues as the lateral cutaneous nerve of the forearm—a sensory-only remnant useful for localizing a lesion.

Posterior compartment — radial nerve: triceps brachii (long head: infraglenoid tubercle; lateral and medial heads from the humerus) and anconeus. The radial nerve and profunda brachii travel in the spiral groove. A mid-shaft humeral fracture can produce wrist drop while sparing some elbow extension if the lesion is distal to branches to the long and lateral heads; the medial head lies deep to the groove and is more often affected.

Forearm compartments

Anterior (flexor–pronator) rule: median nerve except flexor carpi ulnaris and the medial half of flexor digitorum profundus (ulnar nerve). Superficial muscles arise from the medial epicondyle (common flexor origin).

The median nerve lies medial to the biceps tendon in the cubital fossa, usually passes between the two heads of pronator teres, then travels between FDS and FDP. It gives the anterior interosseous nerve (pure motor: FPL, PQ, lateral FDP—the OK-pinch sign fails when the distal thumb and index IP joints cannot flex). At the wrist it enters the carpal tunnel with nine tendons (four FDS, four FDP, FPL). The palmar cutaneous branch arises proximal to the flexor retinaculum, so classic carpal-tunnel sensory loss spares the palm.

The ulnar nerve passes behind the medial epicondyle (cubital tunnel), between the two heads of FCU, and through Guyon canal (not the carpal tunnel) with the ulnar artery.

Posterior (extensor–supinator) compartment is radial/posterior-interosseous. Brachioradialis and extensor carpi radialis longus take the radial nerve before it divides. In the cubital fossa the radial nerve splits into the superficial radial (sensory, under brachioradialis) and the deep radial / posterior interosseous nerve, which pierces supinator (arcade of Frohse). A PIN lesion spares brachioradialis and ECRL: the wrist can still extend, but it radially deviates, and finger MCP extension is lost.

Deep posterior muscles: supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, extensor indicis. The anatomical snuffbox is bounded by APL/EPB (anterior) and EPL (posterior); the scaphoid is the floor and the radial artery crosses it.

Hand intrinsics

Median recurrent motor (thenar LOAF): lumbricals 1–2, opponens pollicis, abductor pollicis brevis, flexor pollicis brevis (superficial head). The deep head of FPB is often ulnar. Adductor pollicis is ulnar, not thenar-median.

Ulnar deep branch: all dorsal and palmar interossei, adductor pollicis, hypothenar muscles (abductor, flexor, and opponens digiti minimi), lumbricals 3–4, and palmaris brevis (superficial ulnar). Dorsal interossei abduct (DAB); palmar interossei adduct (PAD). Lumbricals flex the MCP and extend the IP joints via the extensor expansion.

Ulnar claw is most marked in digits 4–5. The ulnar paradox: a proximal ulnar lesion also knocks out medial FDP, so IP clawing is less severe than a wrist-level lesion that leaves FDP intact and unopposed at the DIP joints. Froment sign (paper between thumb and index) fails when adductor pollicis is lost and FPL substitutes with IP flexion.

Lower Limb and Pelvic Girdle (Including Innervation)

Gluteal region

Trendelenburg sign: during single-leg stance, a weak gluteus medius/minimus lets the contralateral pelvis drop. The superior gluteal nerve leaves the sciatic foramen above piriformis. Inferior gluteal vessels and nerve, pudendal nerve, and usually the sciatic nerve leave below piriformis (a piriformis-piercing sciatic variant exists). Obturator externus is an external rotator of the thigh but belongs to the medial compartment (obturator nerve).

Intramuscular injection in the superolateral gluteal quadrant is aimed to miss the sciatic nerve; a misplaced needle can still injure the superior gluteal nerve and produce Trendelenburg gait.

Thigh compartments

Anterior — femoral nerve (L2–L4): iliacus, sartorius, quadriceps femoris. Rectus femoris is the only two-joint quadriceps (AIIS → tibial tuberosity via the patellar tendon). Psoas major is lumbar plexus (L1–L3), not femoral; together iliacus and psoas are iliopsoas, the prime hip flexor, inserting on the lesser trochanter. Pectineus is usually femoral (occasionally dual femoral/obturator).

Femoral triangle (inguinal ligament, sartorius, adductor longus): lateral to medial NAVEL—femoral Nerve (outside the femoral sheath), Artery, Vein, Empty femoral canal, Lymph (node of Cloquet). A femoral hernia enters the canal medial to the vein. Distally, the femoral artery, vein, saphenous nerve, and nerve to vastus medialis travel in the adductor (Hunter) canal under sartorius, then the vessels pass the adductor hiatus to become popliteal.

Medial — obturator nerve (L2–L4): adductor longus, adductor brevis, adductor part of adductor magnus, gracilis, obturator externus. Adductor magnus hamstring part is tibial division of sciatic, from ischial tuberosity to the adductor tubercle. Obturator nerve splits around adductor brevis into anterior and posterior branches; a small medial-thigh sensory strip is the cutaneous flag.

Posterior — sciatic nerve: tibial division to the long head of biceps femoris, semitendinosus, and semimembranosus; common fibular division to the short head of biceps femoris only. Pes anserinus (sartorius, gracilis, semitendinosus) is three muscles, three compartments, three nerves (femoral, obturator, tibial).

Leg compartments

Common fibular nerve winds around the neck of the fibula—the most commonly injured lower-limb nerve. Combined anterior and lateral compartment loss produces foot drop, steppage gait, and a sensory map over the dorsum of the foot. Isolated deep fibular loss drops the foot but preserves eversion. Isolated tibial loss drops plantarflexion and the sole’s sensation; inversion via tibialis posterior is weak but tibialis anterior can still invert in dorsiflexion.

Tarsal tunnel (behind the medial malleolus): Tom, Dick, And Very Nervous Harry—tibialis posterior, flexor digitorum longus, posterior tibial artery, vein, tibial nerve, flexor hallucis longus.

Foot muscles

Dorsal: extensor digitorum brevis and extensor hallucis brevis (deep fibular).

Plantar layers (superficial to deep):

1. Abductor hallucis, flexor digitorum brevis, abductor digiti minimi
2. Quadratus plantae and lumbricals (with FDL tendons)
3. Flexor hallucis brevis, adductor hallucis (oblique and transverse heads), flexor digiti minimi brevis
4. Plantar and dorsal interossei

Innervation copies the hand: medial plantar nerve (median analogue) supplies abductor hallucis, FDB, FHB, and the first lumbrical. Lateral plantar nerve (ulnar analogue) supplies the rest, including adductor hallucis and all interossei.

Clinical Clustering for Part I

Recite the nerve and the named loss before you look at options:

• Long thoracic → serratus anterior → medial winged scapula
• Axillary (surgical neck or quadrangular space) → deltoid + teres minor → lost abduction and external rotation
• Radial in the spiral groove → wrist drop; triceps often partly spared
• PIN → finger extension lost; ECRL spared so wrist extends with radial deviation
• Median at the carpal tunnel → thenar LOAF; palm sensation spared
• Ulnar at Guyon canal → interossei, adductor pollicis, hypothenar; FCU and FDP spared
• Superior gluteal → Trendelenburg (contralateral drop)
• Inferior gluteal → weak hip extension
• Obturator → weak adduction
• Common fibular at the fibular neck → foot drop plus lost eversion
• Tibial → lost plantarflexion and sole sensation

After the maps are automatic, work mixed items on /practice/nbce-part1. Part I will pair a named muscle with a named nerve far more often than it will ask an isolated origin.