Larynx - Anatomy & Physiology

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Introduction

The larynx is situated below where the pharynx divides into the trachea and the oesophagus. It is contained partly within the rami of the mandible and extends caudally into the neck. Vocal folds and vestibular folds are present in the larynx and due to this, it is more commonly known as the voice box.

The cartilageous larynx can be manually palpated in the living animal and is commonly implicated in respiratory conditions such as roaring.

Anatomy of the Larynx - Copyright Nottingham

Structure

• Pharynx rostrally

• Trachea caudally

• Bilaterally symmetrical

• Tube-shaped

• Musculocartilagenous organ

• Suspended from the hyoid apparatus

• Moves position when the animal swallows due to its attachments to the tongue and the basihyoid bone of the hyoid apparatus by the thyrohyoid membrane

• Synovial joints
  • Between the thyrohyoid bone and the dorsorostral aspect of the thyroid cartilage
  • Dorsal joint of throid cartilage
  • Lateral aspect of cricoid cartilage and dorsocaudal aspect of thyroid cartilage
  • Between cricoid and arytenoid cartilage allowing abduction and adduction of the arytenoid cartilages

• Movement of the cricoid-arytenoid joint controls the size of the glottic opening, lumen and larynx

• Membranes and elastic ligaments attach the laryngeal cartilages
  • Epiglottis to throid and cricoid cartilage
  • First tracheal ring with cricoid cartilage by the cricotracheal ligament

• Extrinsic musculature connects the larynx to the sternum, tongue, pharynx and hyoid apparatus

• Intrinsic musculature connects the laryngeal cartilages (see here)
  • All skeletal muscles

• The vestibule extends from the entrance of the larynx to the arytenoid cartilages and vocal folds
  • Vestibular folds run parallel, but rostral to, the vocal folds

• The glottic cleft (rima glottidis) is surrounded by the arytenoid cartilages dorsally and vocal folds ventrolaterally
  • Varies in size
  • Diamond shaped
  • Glottic cleft disappears when the glottis is closed
  • Vocal folds run caudodorsally

• The infraglottic cavity extends from the caudal section of the arytenoid cartilages into the lumen of the trachea
  • Fixed size

• Epiglottis is the rostral margin of the larynx

Thyroid Cartilage

• Hyaline cartilage

• Forms most of the floor of the larynx

• Fusion of the two lateral plates varies in different species

• Rostral part forms the 'Adam's apple'

• Articulates with the thyrohyoid bone

• Articulates with the cricoid cartilage

• Becomes brittle as the animal ages

Cricoid Cartilage

• Hyaline cartilage

• Signet ring shape
  • Wider on dorsal surface
  • Narrower on ventral surface

• Crest on midline of dorsal surface

• Facets for arytenoid cartilages on rostral egde

• Articulates with the thyroid cartilage

• Becomes brittle as the animal ages

Arytenoid Cartilage

• Hyaline cartilage

• Paired

• Articulates with the rostral part of the cricoid cartilage

• Vocal process present on caudal surface where the vocal folds attach
  • Muscular process extending laterally
  • Corniculate process extending dorsomedially (elastic cartilage)

Epiglottic Cartilage

• Elastic cartilage

• Flexible

• Most rostral

• Thinner stalk-like part is attached to the root of the tongue, body of the thyroid cartilage and the basihyoid bone

• The larger blade-like part lies behind the soft palate and points dorso-rostrally

• During deglutition the large blade part of the epiglottic cartilage partially covers the entrance to the trachea

Interarytenoid Cartilage

• Nodule of hyaline cartilage

• Between arytenoid cartilages dorsally

Cuneiform Process

• Elastic cartilage

• Supports mucosal folds from epiglottis to arytenoid cartilages

• Not present in all species

• Free or fused with the epiglottic cartilages and arytenoid cartilages depending on species

Vocal Folds

• Made of (slightly stiffer) elastic ligaments

• Pass between the arytenoid cartilages and the laryngeal floor

• Run caudodorsally

• Ligament medially

• Vocalis muscle laterally

• Fat surrounds vocalis muscle

• Form part of the glottis

• Secrete mucous

• Vocalisation

Vestibular folds

• Made of (slightly stiffer) elastic ligaments

• Vestibular ligaments rostral to vocal ligament

• Rostral part of laryngeal floor to arytenoid cartilages

• Run caudodorsally, rostral to vocal folds

• Ligament medially

• Vocalis muscle laterally

• Fat surrounds vocalis muscle

• Do not form the glottis

Intrinsic Musculature

Cricothryroid muscle

• Innervated by the cranial laryngeal nerve a branch of the vagus nerve (CN X)
• Moves cricoid and arytenoid cartilages caudally
• Tenses vocal folds

Dorsal cricoarytenoid muscle

• Innervated by the caudal laryngeal nerve a branch of the vagus nerve (CN X)
• Dorsal surface of cricoid cartilage to arytenoid cartilage
• Abducts vocal process and therefore vocal fold
• Widens glottis

Lateral cricoarytenoid muscle

• Innervated by the caudal laryngeal nerve a branch of the vagus nerve (CN X)
• Adducts vocal processes
• Narrows the glottis

Thyroarytenoid muscle

• Innervated by the caudal laryngeal nerve a branch of the vagus nerve (CN X)
• Laryngeal floor to the thyroid cartilage and arytenoid cartilage
• Alters the tension of the vocal and vestibular folds
• Forms part of the sphincter muscular arrangement

Transverse arytenoid muscle

• Innervated by the caudal laryngeal nerve a branch of the vagus nerve (CN X)
• Completes the muscular sphincter arrangment
• Spans the arytenoid cartilages

Function

• Protects the trachea in Swallowing, preventing aspiration of foreign material
  • During swallowing the larynx is moved rostrally causing the epiglottis to partially cover the laryngeal entrance
  • Solid foods are carried over the laryngeal entrance by the muscles of the pharynx
  • Fluids are deflected by the epiglottis
  • Clsure of the glottis also prevents food passing down the larynx

• Relfex stimulation of the mucosa promotes the coughing reflex

• Communication

• Passage of air to the lungs

• Increases intra-abdominal pressure

• Vocalisation

• The glottis can widen by adduction of the vocal folds when breathing is vigorous

Vasculature

• Laryngeal branch of superior thyroid artery
  • Supplies rostral larynx
  • Branch from carotid artery

• Laryngeal branch of inferior thyroid artery
  • Supplies caudal larynx
  • Branch from subclavian artery from thyrocervical trunk

• Laryngeal branch of cricothyroid artery
  • Branch from superior thyroid artery

Innervation

• Branches of the vagus nerve (CN X)
  • Cranial larygeal nerve
    • Internal branch innervates mucosa
    • External branch innervates cricothyroid muscle and constrictors of pharynx
  • Caudal (recurrent) laryngeal nerve
    • Innervates intrinsic muscles of larynx (except cricothyroid muscle)

Lymphatics

• Lymphoid tissue present

Histology

• See elastic cartilage histology

• See hyaline cartilage histology

• Mucous glands present

• In the ventricles, there is a high density of mucous glands

• Stratified squamous epithelium rostrally around laryngeal entrance

• Ciliated pseudostratified columnar epithelium elsewhere

Species Differences

Equine

• Fusion of the two plates of the thyroid cartilage is incomplete forming a rostral pointing notch which is a good site for surgical entry into the larynx

• Thyroarytenoid muscle is divided into 2 parts- rostral and caudal vocalis which are situated within the vocal folds and vestibular folds

• Cuneiform processes attached to epiglottis

• Prominance (surgical landmark) rostral to opening in thyroid cartilage

Canine

• Thyroarytenoid muscle is divided into 2 parts- rostral and caudal vocalis which are situated within the vocal folds and vestibular folds

• Laryngeal ventricles present

Feline

• Thick vocal folds

• Purring due to vibration of vocal folds (and of the diaphragm) by rapid twitching of laryngeal muscles

• Very sensitive mucosa making intubation tricky

Porcine

• Small laryngeal ventricle present

• Double corniculate process in arytenoid cartilages

• Stimulation of laryngeal mucosa can results in violent laryngeal spasm

Avian

• No vocal folds

• Sound in the bird is produced in the syrinx (specialisation of the trachea bifurcation)

Links

Pathology of the Larynx

Cartilage - Anatomy & Physiology

Aspiratory Pneumonia

Video

Pot 258 Lateral section through the head of a dog

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