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.

Structure

• Suspended from the hyoid apparatus

• Moves position when the animal swallows due to its attachments to the tongue and the hyoid apparatus

• 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 throid cartilage

• 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

• Vocal folds and vestibular folds
  • Made of (slightly stiffer) elastic ligaments
  • Pass between the arytenoid cartilages and the laryngeal floor

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

• Intrinsic musculature connects the laryngeal cartilages (see here)

• 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

• Rostral part of laryngeal floor to arytenoid cartilages

• Run caudodorsally

• Ligament medially

• Vocalis muscle laterally

• Fat surrounds vocalis muscle

• Form part of the glottis

Vestibular folds

• 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

Innervation

• Branches of the vagus nerve (CN X)
  • Cranial larygeal nerve
  • Caudal (recurrent) laryngeal nerve

Lymphatics

• Lymphoid tissue present

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 least extensive 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

Canine

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

Links

Pathology of the Larynx

Cartilage - Anatomy & Physiology