HEAD & NECK

Head & Neck Anatomy

Comprehensive notes · USMLE Step 1 · high‑yield clinical pearls

🦴 Neck & Thoracic Outlet

Thoracic outlet is bounded by the manubrium, first rib, and T1 vertebra. The scalene triangle (between anterior and middle scalene muscles and first rib) transmits the subclavian artery and brachial plexus trunks.

🔬 Thoracic outlet syndrome (TOS)
Compression of brachial plexus trunks (especially C8–T1, lower trunk) and subclavian artery within the scalene triangle. Causes include cervical rib, scalene hypertrophy, or apical lung tumors (Pancoast).
  • Clinical: medial forearm/hand numbness & pain, ulnar-innervated hand weakness (claw hand), diminished radial pulse.
  • May also involve sympathetic trunk (Horner’s) or recurrent laryngeal nerve (hoarseness).
📌 Note: Subclavian vein and phrenic nerve (C3–C5) lie anterior to the anterior scalene, not within the scalene triangle.

🩸 Carotid & Subclavian Arteries

External Carotid (ECA) branches

  • Superior thyroid
  • Ascending pharyngeal
  • Lingual
  • Facial
  • Occipital
  • Posterior auricular
  • Superficial temporal
  • Maxillary → middle meningeal (most significant)

Subclavian artery branches

  • Internal thoracic (cardiac bypass)
  • Vertebral (brain)
  • Costocervical
  • Thyrocervical → inferior thyroid, transverse cervical, suprascapular
⚠️ Middle meningeal artery – branch of maxillary (ECA). Enters skull via foramen spinosum. Laceration → epidural hematoma (lens-shaped, lucid interval).

🧬 Pharyngeal Apparatus

Composed of arches (mesoderm + neural crest), pouches (endoderm), and grooves (ectoderm).

Arch derivatives
  • Arch 1 (CN V): muscles of mastication, mylohyoid, anterior digastric, tensor tympani & veli palatini; maxilla, mandible, incus, malleus.
  • Arch 2 (CN VII): muscles of facial expression, posterior digastric, stylohyoid, stapedius; stapes, styloid process, lesser horn & upper body of hyoid.
  • Arch 3 (CN IX): stylopharyngeus; greater horn & lower body of hyoid; common & internal carotids.
  • Arch 4 (CN X superior laryngeal): cricothyroid, soft palate, pharyngeal muscles; thyroid cartilage; aortic arch (left) / subclavian (right).
  • Arch 6 (CN X recurrent laryngeal): intrinsic laryngeal muscles (except cricothyroid); all other laryngeal cartilages; pulmonary arteries & ductus arteriosus.

Extra-ocular (III, IV, VI) and tongue muscles (XII) derive from occipital somites, not pharyngeal arches.

Pharyngeal pouch derivatives

  • Pouch 1: auditory tube & middle ear cavity
  • Pouch 2: palatine tonsil crypts
  • Pouch 3: inferior parathyroid + thymus
  • Pouch 4: superior parathyroid + C-cells (neural crest)
DiGeorge sequence – failure of pouches 3 & 4 → absent thymus & parathyroids → hypocalcemia, immune deficits, cardiac defects (persistent truncus arteriosus), micrognathia, abnormal ears.

🦋 Thyroid & Tongue

Thyroid: arises from midline endoderm (thyroid diverticulum) at foramen cecum, migrates caudally. Thyroglossal duct normally obliterates; persistent remnants → cysts/fistulas (midline, near hyoid).

Tongue innervation

  • Anterior ⅔: general sensation – lingual (V3); taste – chorda tympani (VII)
  • Posterior ⅓: general & taste – glossopharyngeal (IX)
  • Motor: hypoglossal (XII) except palatoglossus (X)

Clinical correlates

  • Thyroglossal duct cyst – midline, moves with swallowing/tongue protrusion.
  • Ectopic thyroid/parathyroid – midline or lateral neck.

👶 Face & Palate Development

Five primordia from first arch neural crest: frontonasal (single), paired maxillary & mandibular prominences. Intermaxillary segment (fusion of medial nasal prominences) forms philtrum & primary palate. Secondary palate from palatine shelves (maxillary) fuses midline posterior to incisive foramen.

Cleft lip: failure of maxillary prominence to fuse with medial nasal prominence.
Cleft palate: palatine shelves fail to fuse with each other or primary palate.
First arch syndromes (neural crest migration defects): Treacher Collins (mandibular hypoplasia, down-slanted eyes, colobomas, malformed ears) and Pierre Robin (micrognathia, cleft palate, glossoptosis).

💀 Cranium & Foramina

  • Foramen spinosum: middle meningeal artery
  • Jugular foramen: CN IX, X, XI + internal jugular vein
  • Cribriform plate fractures → CSF rhinorrhea & dysosmia.
Jugular foramen syndrome – tumor compression → hoarseness, dysphagia, loss of posterior tongue sensation (IX, X), trapezius/SCM weakness (XI), and tongue deviation to lesioned side (XII).

🧠 Cranial Meninges & Dural Venous Sinuses

Dura: two layers (periosteal + meningeal) fused except where they split to form dural venous sinuses and folds (falx cerebri, tentorium cerebelli). Arachnoid – thin, lines dura; granulations project into sinuses for CSF reabsorption.

Major dural sinuses

  • Superior sagittal → confluence
  • Inferior sagittal → straight sinus (with great cerebral vein)
  • Straight → confluence
  • Transverse (2) → sigmoid → internal jugular
  • Cavernous (2) – clinical significance

Cavernous sinus

  • Lateral wall: CN III, IV, V1, V2
  • Central: CN VI + internal carotid (sympathetic plexus)
  • Drains face via ophthalmic veins & pterygoid plexus.
Cavernous sinus thrombosis – infection from face spreads via emissary veins. CN VI affected earliest (internal strabismus), then III, IV, V1/V2 involved → ophthalmoplegia, upper face sensory loss.

🩸 Intracranial Hemorrhage

Epidural hematoma
Artery (middle meningeal) laceration. Lens-shaped, biconvex. Lucid interval then rapid decline (headache, hemiparesis, herniation). Surgical emergency.
Subdural hematoma
Tearing of bridging veins → crescent-shaped. Often elderly/alcoholics. Chronic onset; signs of raised ICP.
Subarachnoid hemorrhage – berry aneurysm rupture (circle of Willis, AComA or MCA bifurcation). Sudden severe headache (“thunderclap”).

👁️ Orbital Muscles & Innervation

  • CN III (oculomotor): superior, inferior, medial rectus; inferior oblique; levator palpebrae superioris (skeletal part).
  • CN IV (trochlear): superior oblique (pulls eye down & laterally).
  • CN VI (abducens): lateral rectus.
  • Sympathetic: superior tarsal (smooth) from carotid plexus → elevates eyelid.
Clinical pearl: CN VI palsy → medial strabismus (eye adducted). CN III palsy → “down and out” eye with ptosis and dilated pupil.
CN III→ SR, IR, MR, IO, LPS CN IV→ SO CN VI→ LR
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