Larynx — Cartilages, Muscles & Nerve Supply

The larynx is the organ of phonation and the guardian of the lower airway, spanning vertebrae C3–C6 in the adult. For NEET PG it is a dense integration zone where Anatomy meets ENT: cartilage and joint mechanics, the precise innervation rule, and the predictable cord positions after nerve injury are repeatedly tested.

Overview & boundaries

The larynx lies in the anterior midline of the neck, continuous above with the laryngopharynx and below with the trachea. In adults it occupies the C3–C6 level; in infants and children it sits two to three vertebrae higher (C2–C3), which explains the more vertical, anterior, cephalad paediatric airway and the difficulty of intubation in children. Its skeleton is a framework of nine cartilages connected by joints, membranes and ligaments, moved by intrinsic muscles, and slung from the hyoid by extrinsic muscles.

High-yield: Adult larynx = C3–C6. Infant larynx is higher (≈ C2–C3) and the narrowest part of the paediatric airway is the subglottis (cricoid ring), whereas in adults the narrowest part is the rima glottidis (vocal cords).

The nine cartilages

There are three unpaired and three paired cartilages — six named cartilages, nine pieces in total.

Cartilage	Number	Type	Key fact
Thyroid	1 (unpaired)	Hyaline	Largest; forms laryngeal prominence (Adam's apple); superior + inferior horns
Cricoid	1 (unpaired)	Hyaline	Only complete ring; signet-ring shape (lamina posterior); landmark for cricoid pressure
Epiglottis	1 (unpaired)	Elastic	Leaf-shaped; attached to thyroid by thyroepiglottic ligament
Arytenoid	2 (paired)	Hyaline (apex elastic)	Pyramidal; vocal process + muscular process; sit on cricoid lamina
Corniculate (of Santorini)	2 (paired)	Elastic	Apex of arytenoid; in aryepiglottic fold
Cuneiform (of Wrisberg)	2 (paired)	Elastic	In aryepiglottic fold; no cartilaginous attachment

High-yield: Elastic cartilages of the larynx = epiglottis, corniculate, cuneiform, and the apex (vocal process) of the arytenoid. The rest (thyroid, cricoid, body of arytenoid) are hyaline — and so they ossify with age (begin ~20–25 years), a fact exploited in forensic age estimation and seen on radiographs.

Mnemonic for elastic cartilages — "Every Cool Cat Climbs": Epiglottis, Corniculate, Cuneiform, Cartilage apex of arytenoid.

The cricoid is clinically the most important because it is the only complete cartilaginous ring; backward pressure on it (Sellick manoeuvre) compresses the oesophagus against C6 to prevent regurgitation during rapid-sequence induction.

Laryngeal joints

Two synovial joints govern cord movement, and both are vulnerable in disease.

1. Cricothyroid joint — between inferior horn of thyroid and the side of the cricoid. Allows the thyroid cartilage to rotate/tilt forward about a transverse axis → lengthens and tenses the vocal cords. Moved by cricothyroid muscle.
2. Cricoarytenoid joint — between base of arytenoid and the upper border of the cricoid lamina. Allows rotation (abduction/adduction of the vocal process) and gliding (approximation/separation of arytenoids). This is the joint affected in rheumatoid arthritis, causing fixation and hoarseness.

Functional flow of cord movement: Posterior cricoarytenoid contracts → rotates muscular process backward → vocal process swings laterally → abduction (cords open, airway widens). The lateral cricoarytenoid does the reverse for adduction.

Membranes & ligaments

• Thyrohyoid membrane — connects thyroid cartilage to hyoid; pierced by the internal laryngeal nerve and superior laryngeal vessels.
• Cricothyroid membrane (median + lateral) — the avascular midline structure incised in an emergency cricothyroidotomy.
• Quadrangular membrane — upper; its lower free margin = vestibular ligament (false cord).
• Conus elasticus (cricovocal membrane) — lower; its upper free margin = vocal ligament (true cord), running from thyroid angle to vocal process of arytenoid.

High-yield: The vocal ligament is the thickened upper free margin of the conus elasticus. True cords are pearly white and avascular (poor lymphatics → glottic cancers metastasise late and present early with hoarseness — good prognosis). Supraglottis and subglottis are richly lymphatic → early nodal spread.

Cavity of the larynx — the three compartments

The cavity is divided by the vestibular folds (false cords) and vocal folds (true cords):

Compartment	Boundaries	Lymphatics	Clinical note
Supraglottis (vestibule)	Inlet → vestibular folds	Rich, bilateral	Cancer spreads early to nodes
Glottis	Between true cords (ventricle laterally)	Sparse	Hoarseness early, spreads late, best prognosis
Subglottis	Below true cords → lower border of cricoid	Moderate	Paediatric narrowest point; croup affects this

The rima glottidis is the fissure between the two true cords + arytenoids — the narrowest part of the adult larynx.

Intrinsic muscles — actions on the cords

All intrinsic muscles are paired except the transverse arytenoid (unpaired). They are classified by function: openers, closers, tensors and relaxers.

Muscle	Action on cords	Notes
Posterior cricoarytenoid (PCA)	ABDUCTS (opens)	Only abductor; the "safety muscle" of the larynx
Lateral cricoarytenoid (LCA)	Adducts	Closes rima
Transverse + oblique arytenoid (interarytenoid)	Adducts	Closes posterior rima; transverse is unpaired
Cricothyroid (CT)	Tenses / lengthens	Only tensor; supplied by external laryngeal nerve
Thyroarytenoid (incl. vocalis)	Relaxes / shortens	Fine pitch control; vocalis = medial part
Aryepiglottic + thyroepiglottic	Close/open inlet	Sphincter of vestibule

High-yield: The only abductor of the vocal cords is the posterior cricoarytenoid (PCA) — "PCA = Please Come Apart." The only tensor is the cricothyroid, and it is the only intrinsic muscle NOT supplied by the recurrent laryngeal nerve.

Mnemonic for adductors — "LIT": Lateral cricoarytenoid, Interarytenoid, Thyroarytenoid all adduct.

Nerve supply — the single most tested concept

Both laryngeal nerves are branches of the vagus (CN X).

The rule (learn it verbatim):

• All intrinsic muscles are supplied by the recurrent laryngeal nerve (RLN) EXCEPT the cricothyroid, which is supplied by the external laryngeal branch of the superior laryngeal nerve (SLN).

Superior laryngeal nerve divides into:

• Internal laryngeal nerve — pierces thyrohyoid membrane → sensory to the laryngeal mucosa above the vocal cords (and supraglottis); afferent limb of the cough reflex.
• External laryngeal nerve — motor to cricothyroid (and inferior constrictor); runs with the superior thyroid artery — at risk in thyroidectomy.

Recurrent laryngeal nerve — motor to all intrinsic muscles except cricothyroid, and sensory below the vocal cords (subglottis).

High-yield: Sensory watershed = the vocal cords. Above cords → internal laryngeal nerve (SLN). Below cords → RLN. Motor: everything → RLN except cricothyroid → external laryngeal nerve.

Asymmetry of the RLN — a classic MCQ:

• Right RLN hooks around the right subclavian artery.
• Left RLN hooks around the arch of aorta (ligamentum arteriosum) → much longer intrathoracic course → more often injured by aortic aneurysm, left atrial enlargement (mitral stenosis → Ortner's syndrome / cardiovocal syndrome), mediastinal nodes and bronchogenic carcinoma.

A non-recurrent laryngeal nerve occurs almost always on the right side, associated with an aberrant right subclavian artery (arteria lusoria) and absent brachiocephalic trunk — a surgical trap.

Blood supply

• Above the cords: superior laryngeal artery (branch of superior thyroid artery ← external carotid), accompanies internal laryngeal nerve.
• Below the cords: inferior laryngeal artery (branch of inferior thyroid artery ← thyrocervical trunk ← subclavian), accompanies the RLN.
• Venous drainage parallels arteries → superior thyroid → IJV; inferior thyroid → brachiocephalic vein.

Nerve injury patterns & cord positions

This is the highest-yield clinical-anatomy crossover. The position a paralysed cord adopts depends on which nerve is cut and whether it is unilateral or bilateral.

Injury	Voice	Cord position	Airway
Unilateral RLN	Hoarse / breathy	Cord in paramedian position	Usually adequate
Bilateral RLN (sudden, complete)	Near-normal/weak	Both paramedian (adducted unopposed by lost PCA)	Stridor, emergency → tracheostomy
Unilateral SLN (external br.)	Weak, easy fatigue, loss of high notes	Cord wavy, slightly lower	Adequate
Combined / complete vagus (above SLN)	Hoarse	Cord in cadaveric (intermediate) position	Often aspiration

Why paramedian in RLN palsy? The PCA (the abductor) is paralysed, so the unopposed (still partly functioning early) adductors and the intact cricothyroid (external laryngeal nerve, tensor) keep the cord near the midline → paramedian.

Why cadaveric in complete vagal/combined palsy? Both RLN and the external laryngeal nerve (cricothyroid) are lost → no tensor, no abductor, no adductor → the cord lies midway between abduction and adduction = cadaveric / intermediate position.

High-yield: Semon's law — in a progressive lesion of the RLN, the abductors (PCA) are paralysed before the adductors. So a slowly growing lesion first fixes the cord in adduction (median/paramedian) before later moving it to the cadaveric position.

High-yield: Bilateral abductor (RLN) paralysis is the dangerous one — both cords sit paramedian, the airway is a slit → inspiratory stridor and respiratory obstruction needing urgent airway management. Bilateral adductor paralysis (rare, often functional) → aphonia but a safe airway.

Memory hook for cord positions (medial → lateral): Median → paramedian → cadaveric/intermediate → gentle abduction → full abduction.

Cricothyroidotomy & surgical anatomy

In a "can't intubate, can't oxygenate" emergency, the cricothyroid membrane is the access point.

Surface anatomy / steps: Palpate the laryngeal prominence (thyroid) → slide finger inferiorly into the soft depression → that gap is the cricothyroid membrane, between the lower thyroid and upper cricoid → stabilise larynx, make a horizontal incision → insert tube.

• The membrane is subcutaneous, midline and relatively avascular (the small cricothyroid branch of the superior thyroid artery anastomoses across the top — incise lower/midline to avoid it).
• Cricothyroidotomy is preferred over tracheostomy in an emergency because it is faster and the landmark is superficial; tracheostomy (between 2nd–4th tracheal rings, ideally) is the elective/definitive procedure.
• Cricothyroidotomy is contraindicated in children < 12 years (small, soft cricoid; risk of subglottic stenosis) — needle cricothyroidotomy is used instead.

High-yield: Structures pierced in cricothyroidotomy (superficial → deep): skin → superficial fascia → investing deep fascia (pretracheal continuation) → cricothyroid membrane. The isthmus of the thyroid (over rings 2–4) is avoided because the incision is above it.

Key differentials / clinical correlates

• Hoarseness of voice: unilateral RLN palsy (commonest cause iatrogenic = thyroidectomy; malignant = bronchogenic Ca / lung apex), laryngeal carcinoma (glottic), laryngitis, vocal nodules.
• Stridor: bilateral abductor palsy, laryngomalacia (commonest congenital stridor — omega-shaped epiglottis), croup (subglottis), epiglottitis (supraglottis).
• Ortner's syndrome: left RLN palsy from a dilated left atrium (mitral stenosis) or pulmonary artery.
• Rheumatoid arthritis → cricoarytenoid joint fixation → hoarseness, sense of fullness.

Recently asked / exam angle

NEET PG and INI-CET have repeatedly framed the larynx through these stems:

• "All intrinsic muscles of the larynx are supplied by the RLN except…" → Cricothyroid (answer is constant).
• "Only abductor of vocal cord" → Posterior cricoarytenoid.
• "Sensory supply above the vocal cords" → Internal laryngeal nerve.
• "Cord position in complete unilateral vagal palsy" → Cadaveric / intermediate, contrasted with paramedian in isolated RLN palsy.
• "Semon's law" stem → abductors paralysed first.
• "Cricothyroidotomy — structure incised / not at risk" → cricothyroid membrane; thyroid isthmus avoided.
• "Left RLN relation" → arch of aorta / ligamentum arteriosum; explains Ortner's syndrome in mitral stenosis.
• "Non-recurrent laryngeal nerve" → right side, aberrant right subclavian artery.
• "Narrowest part of airway" → adult = rima glottidis; child = subglottis (cricoid).
• Image-based: identifying the signet-ring cricoid or the vocal process of the arytenoid on a diagram.

High-yield: A favourite distractor: students confuse the internal laryngeal nerve (sensory, above cords, branch of SLN) with the recurrent laryngeal nerve (motor + sensory below cords). Lock in: SLN-internal = sensation above; SLN-external = cricothyroid motor; RLN = the rest.

Rapid revision

1. Nine cartilages: 3 unpaired (thyroid, cricoid, epiglottis) + 3 paired (arytenoid, corniculate, cuneiform).
2. Cricoid = only complete ring; signet-ring shape; site of cricoid pressure (Sellick).
3. Elastic cartilages = epiglottis, corniculate, cuneiform + apex/vocal process of arytenoid; the rest are hyaline and ossify with age.
4. Only abductor = posterior cricoarytenoid (PCA); paralysis → airway danger.
5. Only tensor = cricothyroid; the only muscle not supplied by RLN (supplied by external laryngeal nerve).
6. RLN = motor to all intrinsics except cricothyroid + sensory below cords; internal laryngeal nerve = sensory above cords.
7. Left RLN loops around arch of aorta (ligamentum arteriosum) → Ortner's syndrome in mitral stenosis; right RLN loops around right subclavian artery.
8. Semon's law: abductors paralysed before adductors in progressive lesions.
9. Unilateral RLN palsy → paramedian cord; complete vagal/combined palsy → cadaveric (intermediate) cord; bilateral RLN → both paramedian = stridor, emergency.
10. Cricothyroidotomy through the cricothyroid membrane; emergency airway of choice in adults; contraindicated < 12 years.
11. Vocal ligament = upper free margin of conus elasticus; true cords avascular with sparse lymphatics → glottic cancer presents early, spreads late, best prognosis.
12. Narrowest airway: adult = rima glottidis (cords); child = subglottis (cricoid ring).