Anatomy and Physiology of the Larynx

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🎤 The larynx (“voice box”) is a highly specialised organ in the anterior neck, spanning roughly C3–C6. It sits between the pharynx and trachea and functions as:

• ➡️ An airway conduit (controls airflow and resistance)
• 🎶 A sound generator for speech (phonation)
• 🛡️ An airway guardian during swallowing (prevents aspiration)

📍 Position, Relations & Surface Landmarks

• Location: anterior neck, suspended from the hyoid, continuous inferiorly with the trachea.
• Anterior relations: strap muscles (infrahyoids) and thyroid gland (especially anterior/lateral to lower larynx).
• Posterior relations: laryngopharynx; oesophagus begins posteriorly at cricopharyngeus level.
• Key palpable landmarks:
  • Thyroid cartilage (“Adam’s apple”).
  • Cricothyroid membrane between thyroid and cricoid cartilages (front-of-neck access).
  • Cricoid cartilage (firm complete ring).

🧱 Cartilaginous Framework

The laryngeal cartilages form a protective “box” with mobile joints that allow fine adjustment of vocal fold position and tension. The cricoid is the only complete ring; the arytenoids are the key moving parts that abduct/adduct the vocal folds.

• Unpaired cartilages
  • Thyroid cartilage: largest; forms the anterior wall and protects the vocal folds.
    📌 Clinical: landmark for laryngeal level and emergency airway orientation.
  • Cricoid cartilage: complete ring; forms the base of the larynx; articulates with thyroid (cricothyroid joints) and arytenoids (cricoarytenoid joints).
    📌 Exam tip: functionally crucial in airway patency; paediatric airway is narrow and easily compromised.
  • Epiglottis: leaf-shaped elastic cartilage; folds posteriorly during swallowing to help protect the laryngeal inlet.
• Paired cartilages
  • Arytenoids: sit on cricoid lamina; rotate/slide to abduct/adduct vocal folds and adjust tension.
  • Corniculates: small “horns” on arytenoids; support posterior laryngeal inlet.
  • Cuneiforms: support aryepiglottic folds and maintain inlet patency.

🪢 Ligaments, Membranes & Joints

• Extrinsic supports
  • Thyrohyoid membrane: suspends larynx from hyoid; transmits the internal branch of SLN and superior laryngeal vessels.
  • Cricotracheal ligament: connects cricoid to trachea.
• Intrinsic fibroelastic membranes
  • Quadrangular membrane: forms the vestibular (false) fold margin superiorly; contributes to supraglottic structure.
  • Cricovocal membrane (conus elasticus): forms the vocal ligament margin inferiorly; essential for true vocal folds.
  • Cricothyroid membrane (median part): high-yield emergency access site (front-of-neck airway).
• Key synovial joints
  • Cricothyroid joints: tilt thyroid cartilage forward/back → changes vocal fold tension (pitch control).
  • Cricoarytenoid joints: arytenoid rotation/glide → vocal fold abduction/adduction (airway vs phonation).

🏛️ Internal Divisions of the Laryngeal Cavity

• Supraglottis: epiglottis, aryepiglottic folds, vestibule, and false (vestibular) folds.
  🛡️ Main role: airway protection and shaping airflow.
• Glottis: true vocal folds + rima glottidis (the opening between them).
  🎶 Main role: phonation; also critical for airway resistance and cough.
• Subglottis: from below the vocal folds to inferior cricoid border → trachea.
  📌 Important in paediatric airway obstruction and subglottic stenosis.

🎶 True vs False Cords (What’s the difference?)

• False (vestibular) folds: protective, contribute to resonance and closure during swallowing/straining; not the main sound source.
• True vocal folds: contain the vocal ligament + vocalis muscle; vibrate to create sound.
  • Covered by stratified squamous epithelium to withstand friction.
  • The superficial lamina propria (“Reinke’s space”) is important for normal vibration and is affected in Reinke’s oedema.

💪 Musculature

Intrinsic laryngeal muscles provide fine control of vocal fold position and tension (airway vs voice). Extrinsic muscles move the whole larynx up/down, crucial during swallowing and speech.

• Intrinsic muscles (key actions)
  • Posterior cricoarytenoid: only abductor → opens rima glottidis (breathing).
    📌 Bilateral dysfunction → potentially life-threatening airway compromise (stridor).
  • Lateral cricoarytenoid: adducts vocal folds (phonation, airway protection).
  • Interarytenoids (transverse/oblique): adduct arytenoids → closes posterior glottis.
  • Cricothyroid: tenses/elongates vocal folds → ↑ pitch (🎵).
    Nerve: external branch of superior laryngeal nerve.
  • Thyroarytenoid + vocalis: fine-tune tension and mass → controls timbre, pitch modulation, and subtle voice quality.
• Extrinsic muscles (move larynx as a unit)
  • Suprahyoids elevate larynx during swallowing.
  • Infrahyoids (strap muscles) depress/stabilise larynx during speech and after swallowing.

🩸 Blood Supply & Lymphatics

• Arteries:
  • Superior laryngeal artery (from superior thyroid artery).
  • Inferior laryngeal artery (from inferior thyroid artery).
• Veins: drain via superior/inferior thyroid veins.
• Lymphatics: classically divided by the vocal folds:
  • Supraglottis: rich lymphatics → upper deep cervical nodes.
  • Glottis: relatively sparse lymphatics → early cancers may present with hoarseness before nodal spread.
  • Subglottis: drains to pretracheal/paratracheal and lower deep cervical nodes.

🧠 Innervation (High-yield)

The larynx is innervated by the vagus nerve (CN X) via the superior laryngeal nerve and recurrent laryngeal nerve. A simple rule: RLN does almost all motor, except cricothyroid (external SLN). Sensation is split by vocal fold level.

• Superior laryngeal nerve (SLN)
  • Internal branch: sensation to supraglottis (above vocal folds); triggers protective cough reflex when stimulated.
  • External branch: motor to cricothyroid (pitch control).
• Recurrent laryngeal nerve (RLN)
  • Motor to all other intrinsic laryngeal muscles (abduction/adduction/closure).
  • Sensory to subglottis (below vocal folds).
  • 📌 Clinical: unilateral palsy → hoarseness/weak voice; bilateral palsy → airway obstruction/stridor.

⚙️ Physiology

🌬️ 1) Respiration and Airway Resistance

• The larynx regulates airflow by altering rima glottidis size.
• Abduction of vocal folds during inspiration reduces resistance; slight narrowing can increase airflow velocity (useful in certain speech sounds).
• Protective reflexes (cough, laryngospasm) prevent aspiration but can acutely obstruct airflow.

🎶 2) Phonation (Sound Production)

Voice is produced when air from the lungs passes through adducted vocal folds, causing them to vibrate. Pitch depends largely on vocal fold tension/length (cricothyroid + vocalis control), while loudness depends on subglottic pressure and degree of closure. The supraglottic tract (pharynx, oral/nasal cavities) shapes resonance to create intelligible speech.

• Adduction (lateral cricoarytenoid + interarytenoids) approximates cords for vibration.
• Tension control:
  • Cricothyroid: increases tension/length → ↑ pitch.
  • Vocalis: fine-tunes stiffness and segmental vibration → nuanced tone.
• Resonance/articulation: shaped by pharynx, tongue, palate, lips (larynx is the sound source; upper airway is the “instrument body”).

🛡️ 3) Swallowing & Airway Protection

• Laryngeal elevation (suprahyoids) moves larynx upward/anteriorly, helping open the upper oesophageal sphincter and protecting the airway.
• Epiglottic deflection plus aryepiglottic fold closure helps shield the inlet.
• Vocal fold closure (true cords) and vestibular fold approximation provide a layered seal.
• Reflexes: supraglottic sensory input (internal SLN) triggers cough/laryngospasm if material approaches the inlet.

💥 4) Pressure Functions (Valsalva, Cough, Lifting)

• Glottic closure enables generation of high intrathoracic pressure for:
  • Cough (airway clearance)
  • Valsalva manoeuvre (straining)
  • Stabilisation during lifting

⚕️ Clinical Relevance

• 🗣️ Vocal cord paralysis: RLN injury (thyroid/parathyroid surgery, mediastinal/lung pathology) → hoarseness, aspiration, weak cough; bilateral palsy can cause stridor.
• 🎵 External SLN injury: weak/monotone voice, impaired high pitch/projection (cricothyroid dysfunction) — important in professional voice users.
• 🛠️ Airway procedures: cricothyrotomy uses the cricothyroid membrane; intubation requires knowledge of glottic anatomy to avoid trauma.
• 🤧 Laryngitis: inflammation → hoarseness; persistent hoarseness needs assessment.
• 🧪 Laryngeal cancer: persistent hoarseness (especially >3 weeks), dysphagia/odynophagia, neck lumps → urgent ENT pathway.
• 🧒 Paediatric airway: small calibre means oedema causes large resistance increase → croup/subglottic narrowing can be dramatic.

📌 Key Exam Pearls

• Posterior cricoarytenoid = only vocal fold abductor.
• RLN supplies all intrinsic muscles except cricothyroid (external SLN).
• Sensation: above cords = internal SLN; below cords = RLN.
• Glottis is the primary sound source; supraglottis is crucial for protection and resonance shaping.

✅ Conclusion

The larynx is a finely balanced organ integrating respiration, phonation, and airway protection. Its cartilages provide a stable yet mobile framework; intrinsic muscles precisely position and tense the vocal folds; and vagal innervation partitions sensation and motor control in a clinically predictable pattern. Understanding these anatomy–physiology links underpins safe airway management, thyroid/neck surgery, and early recognition of laryngeal pathology.