Larynx - Anatomy & Physiology

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