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Foundational Sciences · Anatomy
Head & Neck: Triangles & Key Structures
A board-focused tour of the neck triangles: how the sternocleidomastoid partitions the neck, the key nerves and vessels each triangle contains, and the classic lesions (CN XI, recurrent vs. external laryngeal, hypoglossal) the USMLE loves to test.
13 min readHigh yield
The SCM: master landmark of the neck
Every question about neck anatomy starts with one muscle. The sternocleidomastoid (SCM) splits each side of the neck into two triangles:
- Anterior triangle — bounded by the midline, the inferior border of the mandible, and the anterior border of the SCM.
- Posterior triangle — bounded by the posterior border of the SCM, the anterior border of trapezius, and the middle third of the clavicle.
The SCM itself is motor-supplied by the spinal accessory nerve (CN XI) with sensory/proprioceptive fibers from C2–C3. Each triangle is further subdivided by strap and digastric muscles, and the boards care about triangles for one reason: they tell you which nerve or vessel is at risk in a given surgical field. A node biopsy in the posterior triangle endangers a different nerve than a thyroidectomy in the muscular/carotid region.
Triangles → boundaries → key contents
| Triangle (subdivision) | Boundaries | High-yield contents |
|---|---|---|
| Submental (ant.) | Both anterior bellies of digastric + hyoid | Submental lymph nodes; origin of anterior jugular vein |
| Submandibular / digastric (ant.) | Two bellies of digastric + lower border of mandible | Submandibular gland, facial artery & vein, hypoglossal nerve (CN XII), mylohyoid nerve |
| Carotid (ant.) | Superior belly of omohyoid + posterior belly of digastric + anterior SCM | Carotid bifurcation, internal jugular vein, CN X, XI, XII, ansa cervicalis, carotid sinus & body, internal/external laryngeal nerves |
| Muscular (ant.) | Superior belly of omohyoid + anterior SCM + midline | Infrahyoid (strap) muscles, thyroid & parathyroid glands |
| Occipital (post.) | Posterior SCM + trapezius + inferior belly of omohyoid | CN XI, cervical plexus (Erb's point), roots/trunks of brachial plexus |
| Subclavian / omoclavicular (post.) | Inferior belly of omohyoid + clavicle + posterior SCM | 3rd part of subclavian artery, brachial plexus trunks, external jugular vein terminus |
Must-know relations
- Common carotid bifurcates at the upper border of the thyroid cartilage (~C4) into internal and external carotid arteries.
- Carotid sheath contents: internal jugular vein (lateral), common/internal carotid artery (medial), and vagus nerve (posterior, in the groove between them) + deep cervical lymph nodes. The sympathetic trunk lies behind the sheath, not inside it.
- CN XI is the only major nerve crossing the roof of the posterior triangle and lies very superficially — the classic casualty of a lymph-node biopsy.
- Erb's point (nerve point of the neck): the cutaneous branches of the cervical plexus emerge at the middle of the posterior SCM border — lesser occipital, great auricular, transverse cervical, supraclavicular nerves.
- Phrenic nerve (C3–C5) descends on the anterior surface of the anterior scalene, deep to prevertebral fascia.
- Brachial plexus roots/trunks and the 3rd part of the subclavian artery pass between anterior and middle scalene; the subclavian vein passes anterior to the anterior scalene.
Nerve → root → motor → sensory → injury
| Nerve | Root / origin | Motor | Sensory | Classic injury & deficit |
|---|---|---|---|---|
| Spinal accessory (CN XI) | Spinal roots C1–C5 (fibers to SCM/trapezius; traditional "cranial root" runs with the vagus) | SCM, trapezius | — | Posterior-triangle injury → shoulder droop, weak abduction >90°, lateral scapular winging |
| Recurrent laryngeal (of CN X) | Vagus | All intrinsic laryngeal muscles except cricothyroid | Mucosa below vocal folds | Hoarseness; left looped under aortic arch (aneurysm/thyroid sx); bilateral → airway obstruction |
| External laryngeal (of superior laryngeal, CN X) | Vagus | Cricothyroid | — | Injured with superior thyroid artery ligation → weak, monotone voice, can't reach high pitch |
| Internal laryngeal (of superior laryngeal, CN X) | Vagus | — | Mucosa above vocal folds | Loss of cough/afferent reflex → aspiration |
| Hypoglossal (CN XII) | CN XII | All intrinsic + extrinsic tongue muscles except palatoglossus | — | Tongue deviates toward the lesion on protrusion |
| Phrenic | C3–C5 | Diaphragm | Pericardium, mediastinal pleura, central diaphragm | Hemidiaphragm paralysis; irritation → referred shoulder-tip pain |
| Ansa cervicalis | C1–C3 | Infrahyoid straps (sterno-/omohyoid, sternothyroid) | — | Rarely symptomatic; landmark on the carotid sheath |
Vignette: the drooping shoulder after a node biopsy
A 40-year-old undergoes excisional biopsy of an enlarged lymph node in the posterior triangle. Weeks later she reports a dull neck ache, cannot shrug that shoulder, and struggles to raise the arm above the horizontal. On exam the shoulder droops and the scapula shows lateral winging that worsens with abduction.
- Lesion: spinal accessory nerve (CN XI) — it runs superficially across the posterior triangle and is the structure most vulnerable there.
- Deficit: trapezius denervation → loss of scapular rotation, so the arm can't be abducted past ~90°.
- Board discriminator: lateral winging (worse on abduction) = trapezius/CN XI. Contrast medial winging (worse on wall-push / forward reach) = serratus anterior / long thoracic nerve (C5–C7).
Vignette: hoarseness — three different nerves
- Post-thyroidectomy hoarseness with a paralyzed vocal cord → recurrent laryngeal nerve (runs near the inferior thyroid artery). Bilateral injury can obstruct the airway.
- A singer who loses her high notes and tires vocally after thyroid surgery, with a near-normal speaking voice → external laryngeal nerve (travels with the superior thyroid artery), denervating the cricothyroid (the tensor that raises pitch).
- New hoarseness with a large aortic-arch aneurysm or a dilated left atrium (mitral stenosis, "Ortner syndrome") → stretched left recurrent laryngeal nerve, which loops under the arch of the aorta at the ligamentum arteriosum (the right nerve loops under the right subclavian artery).
The classics worth memorizing
- Carotid sheath, lateral → medial: "VAN" — Vein (internal jugular), Artery (carotid), Nerve (vagus, posterior in the groove).
- Erb's point cutaneous nerves — "GLTS": Great auricular, Lesser occipital, Transverse cervical, Supraclavicular.
- Hypoglossal lesion: the protruded tongue points toward the side of the lesion ("licks its wounds").
- Recurrent laryngeal: it supplies every intrinsic laryngeal muscle except the cricothyroid — the one muscle spared (and the one the external laryngeal nerve runs to).
- Scalene gap: the subclavian Artery is After (posterior to) the anterior scalene; the subclavian vein runs in front of it.
Carotid reflexes & the danger space
- Carotid sinus — a baroreceptor in the wall of the proximal internal carotid, just above the bifurcation; afferent limb = CN IX (carotid sinus nerve of Hering) → medulla. Hypersensitivity or carotid massage → bradycardia, hypotension, syncope.
- Carotid body — a chemoreceptor at the bifurcation sensing ↓O₂ / ↑CO₂ / ↓pH; afferent mainly CN IX (with CN X).
- Aortic arch baroreceptors and bodies report via CN X (vagus).
- Retropharyngeal / "danger" space — the posterior fascial plane behind the pharynx is a conduit for pharyngeal infection to spread inferiorly into the posterior mediastinum; a retropharyngeal abscess presents with neck stiffness, dysphagia, and widened prevertebral soft tissue.
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