Larynx

The larynx is a cartilaginous tube that connects the pharynx and the trachea. It is involved in producing sound, preventing aspiration, swallowing, breathing, and the cough reflex. It is composed of several cartilages (e.g., cricoid, thyroid, arytenoid) connected by ligaments, muscles (intrinsic and extrinsic), and membranes. The intrinsic muscles produce the fine movements necessary for sound production and breathing. The laryngeal cavity is divided into three parts: the supraglottis (contains the false vocal cords), glottis (contains the true vocal cord), and subglottis. The laryngeal cavity is lined by pseudostratified ciliated columnar epithelium, except for the true vocal cords (area of physiological stress), which are lined by stratified squamous epithelium. Smoking increases the risk of metaplasia in the transition zone between these two epithelial linings, which can progress to squamous cell carcinoma of the larynx. The larynx is supplied by the superior and inferior laryngeal arteries and drains into the superior and inferior laryngeal veins. It is innervated by the superior laryngeal nerve and recurrent laryngeal nerve. The recurrent laryngeal nerve is at particular risk of injury during thyroidectomy.

Overview

• The larynx is a hollow, tube-shaped organ continuous with the trachea below and the pharynx above in the anterior compartment of the neck.
• Most superior part of the lower respiratory tract
• Extends from C3–C6
• Muscles and ligaments support the cartilaginous skeleton of the larynx

Function

• Prevents aspiration
• Produces sound (phonation)
• Controls ventilation
• Cough reflex
• Effort closure in Valsalva maneuver

Regions of the laryngeal cavity

• Supraglottis: from the inferior surface of the epiglottis to the false vocal cords (or vestibular folds)
• Glottis: contains true vocal cords (or vocal folds)
• Subglottis: from inferior border of the glottis to the inferior border of the cricoid cartilage

Cartilages

• Unpaired:
  • Cricoid: : most inferior of the laryngeal cartilages, ring-shaped
  • Thyroid: largest of the laryngeal cartilages, contains the laryngeal prominence (Adam's apple)
  • Epiglottis: petal-shaped, attached to the inside of the thyroid cartilage on the posterior wall
• Paired:
  • Arytenoid (regulate the width of the glottis)
  • Corniculate
  • Cuneiform

Ligaments and joints

• Extrinsic ligaments
  • Thyrohyoid membrane: fibroelastic, connects thyroid cartilage and hyoid bone
  • Hyoepiglottic ligament: connects epiglottis and hyoid bone
  • Cricotracheal ligament: connects first tracheal cartilage and cricoid cartilage
• Intrinsic ligaments
  • Cricothyroid ligament: connects cricoid cartilage to a free upper edge called the vocal ligament, which runs between the thyroid cartilage anteriorly and the arytenoid cartilages posteriorly (location of incision for cricothyrotomy)
  • Quadrangular membrane: connects epiglottis and arytenoid and corniculate cartilages; has free lower margin called the vestibular ligament
• Joints
  • Cricothyroid: allows movement between the inferior horns of the thyroid cartilage and the cricoid cartilage
  • Cricoarytenoid: allows movement between the cricoid cartilage and the arytenoid cartilages (abduction and adduction of the vocal ligaments)

The cricothyroid ligament is where the incision is made for a cricothyrotomy.

The cricoarytenoid joint allows the vocal ligaments to be abducted and adducted.

Musculature

Extrinsic muscles

The extrinsic muscles connect the thyroid, cricoid, and arytenoid cartilage to other structures of the head and neck.

Intrinsic muscles

The intrinsic muscles open and close the glottis and control the amount of tension applied to the vocal folds and ligaments.

The paired posterior cricoarytenoid (PCA) muscles are the only muscles responsible for opening (abducting) the vocal cords. Paralysis of the PCA causes the vocal folds to close, potentially leading to asphyxiation.

Vasculature

Innervation

• Recurrent laryngeal nerve
  • Motor innervation: all intrinsic muscles except for cricothyroid muscle
  • Sensory innervation: below the vocal cords
• Superior laryngeal nerve
  • Motor innervation (external branch): cricothyroid muscle
  • Sensory innervation (Internal laryngeal nerve): above the vocal cords

Except for the cricothyroid, all intrinsic muscles of the larynx are innervated by the recurrent laryngeal nerve. The cricothyroid is innervated by the superior laryngeal nerve. Denervation of the cricothyroid occurs in up to 30% of thyroid dissections and causes monotone voice and impaired ability to produce pitched sounds.
The recurrent laryngeal nerve is at risk of injury during thyroid surgery because it passes close to the thyroid gland on its ascent to the larynx. While unilateral vocal cord paresis leads to paramedian positioning of the vocal cords, which may cause hoarseness, acute bilateral paralysis is a medical emergency that may require a tracheotomy to secure the airway.

• The larynx is lined by a mucous membrane which contains ciliated columnar epithelium (respiratory epithelium).
• Vocal cords: stratified squamous nonkeratinized epithelium (an area of physiological stress)

Smoking increases the risk of squamous metaplasia at the transitional zone between respiratory and stratified squamous nonkeratinized epithelium, which may progress to squamous cell carcinoma of the larynx.

• The structures of the larynx are derived from the 4^th–6^th pharyngeal arches.
  • 4^th pharyngeal arch: cricothyroid muscle, superior laryngeal nerve, epiglottic cartilage
  • 6^th pharyngeal arch: all other intrinsic muscles of the larynx, recurrent laryngeal nerve, all other laryngeal cartilage

• Physiologic respiration
• Forced respiration
• Phonation, articulation: arytenoid cartilages and vocal folds abducted → air forced through the closed rima glottidis → vocal folds vibrate against each other → production of sound → modification by upper parts of airway and oral cavity
• Effort closure (the retention of air in the thoracic cavity, e.g., during heavy lifting)
• Swallowing: rima glottidis, rima vestibuli, and vestibule are closed and larynx moves up and forward → epiglottis swings downward to close the laryngeal inlet → opening of the esophagus posterior to the cricoid cartilage → prevention of liquids or food being aspirated

• Vocal cord abnormalities
  • Laryngeal papillomatosis
  • Vocal cord dysfunction
• Airway conditions
  • Foreign body aspiration
  • Stridor
  • Laryngomalacia
  • Laryngocele
  • Airway management and ventilation
  • Cricothyroidotomy
• Inflammatory/infectious lesions
  • Epiglottitis
  • Laryngitis
  • Diphtheria
  • Croup
  • Laryngeal tuberculosis
• Tumors
  • Benign tumors and precanceroses of the larynx
  • Laryngeal carcinoma (squamous cell carcinoma)