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

Annotated horse larynx- Copyright RVC

Structure

The pharynx is located rostrally to the larynx, whilst the trachea is located caudally. The larynx is suspended from the hyoid apparatus. The larynx is bilaterally symmetrical and 'tube-shaped', it can be described as a musculocartilagenous organ.

The larynx 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

Synovial joints can be found between the thyrohyoid bone and the dorsorostral aspect of the thyroid cartilage. Synovial joints include the dorsal joint of thyroid cartilage; between the lateral aspect of the cricoid cartilage and the dorsocaudal aspect of the thyroid cartilage and between the cricoid and arytenoid cartilage. This allows abduction and adduction of the arytenoid cartilages. Movement of the cricoid-arytenoid joint controls the size of the glottic opening, the lumen and the larynx.

Ligaments

Membranes and elastic ligaments attach the laryngeal cartilages, allowing attatchment of the Epiglottis to the thyroid and cricoid cartilage. The first tracheal ring has attatchment with the cricoid cartilage by the cricotracheal ligament.

Extrinsic musculature

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

Intrinsic musculature

Intrinsic musculature connects the laryngeal cartilages (see here).

Vestibule

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

Glottic cleft

The glottic cleft (rima glottidis) is surrounded by the arytenoid cartilages dorsally and vocal folds ventrolaterally. It varies in size and is diamond shaped. The 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. It is fixed in size.

Epiglottis

The Epiglottis is the rostral margin of the larynx.

The Cartilage of the Larynx

Thyroid Cartilage

Thyroid Cartilage - Copyright nabrown RVC

The thyroid cartilage is hyaline cartilage and forms most of the floor of the larynx. The fusion of the two lateral plates varies in different species. The rostral part forms the 'Adam's apple'. The thyriod cartilage articulates with the thyrohyoid bone and the cricoid cartilage. It becomes brittle as the animal ages.

Cricoid Cartilage

Cricoid Cartilage - Copyright nabrown RVC

The cricoid cartilage is also hyaline cartilage. It is signet ring shaped and is wider on the dorsal surface than the ventral surface. There is a crest on the midline of the dorsal surface and facets for arytenoid cartilages on the rostral egde. The cricoid cartilage articulates with the thyroid cartilage. It also becomes brittle as the animal ages.

Arytenoid Cartilage

Arytenoid Cartilages - Copyright nabrown RVC

The arytenoid cartilage is also hyaline cartilage. It is paired and articulates with the rostral part of the cricoid cartilage. A vocal process is present on the caudal surface where the vocal folds attach; a muscular process extends laterally and a corniculate process extends dorsomedially. (elastic cartilage)

Epiglottic Cartilage

Epiglottic Cartilage - Copyright nabrown RVC

The epiglottic cartilage is elastic cartilage. It is flexible and the most rostral type of cartilage. The 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

The interarytenoid cartilage is a nodule of hyaline cartilage. It is located between the arytenoid cartilages dorsally.

Cuneiform Process

The cuneiform process is elastic cartilage. It supports mucosal folds from the epiglottis to the arytenoid cartilages. It is not present in all species and can be free or fused with the epiglottic cartilages.

The Vocal and Vestibular Folds

Vocal Folds

The vocal folds are made of (slightly stiffer) elastic ligaments and pass between the arytenoid cartilages and the laryngeal floor. They run caudodorsally, with the ligament positioned medially and the vocalis muscle laterally. Fat surrounds the vocalis muscle. The vocal folds form part of the glottis and secrete mucous. They are used for vocalisation.

Vestibular folds

The vestibular folds are made of (slightly stiffer) elastic ligaments. The vestibular ligaments are rostral to the vocal ligament. The vestibular folds run caudodorsally, rostral to the vocal folds with the ligament positioned medially and the vocalis muscle laterally. Fat surrounds vocalis muscle. The vestibular folds differ as they 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

Pharynx Labelled - Copyright C.Clarkson and T.F.Fletcher University of Minnesota

Laryngeal Pharynx

Largest part of the pharynx

Wide rostrally

Narrows caudally

Joins oesophagus at mucosal folds (clearest in the canine, but in other species it is harder to see the demarcation)

Lumen closed at rest by the roof and walls falling towards the floor

The floor of the laryngeal pharynx contains the opening into the larynx- epiglottis, arytenoid cartilages and the aryepiglottic folds

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
- Closure 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)