Larynx - Anatomy & Physiology
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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 cartilagenous larynx can be manually palpated in the living animal and is commonly implicated in respiratory conditions such as roaring.
The pharynx is located rostrally to the larynx, whilst the trachea is located caudally. The larynx is suspended from the hyoid apparatus. It is bilaterally symmetrical and 'tube-shaped' and can be described as a musculocartilagenous organ.
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.
Membranes and elastic ligaments attach the laryngeal cartilages, allowing attachment of the epiglottis to the thyroid and cricoid cartilage. The first tracheal ring has attachment with the cricoid cartilage by the cricotracheal ligament.
Intrinsic musculature connects the laryngeal cartilages (see below).
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.
The glottic cleft (rima glottidis) is surrounded by the arytenoid cartilages dorsally and vocal cords 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.
The epiglottis is the rostral margin of the larynx. It is a flap of elastic cartilage covered by mucous membrane. It forms the rostral boundary of the larynx and prevents food particles from entering the trachea. The epiglottis can return to its normal size and shape after distortion due to the vast amount of elastic fibres present within.
Avian species do not have epiglottis.
The Cartilage of the Larynx
The thyroid cartilage is a 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 thyroid cartilage articulates with the thyrohyoid bone and the cricoid cartilage. It becomes brittle as the animal ages.
The cricoid cartilage is also a 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 edge. The cricoid cartilage articulates with the thyroid cartilage. It also becomes brittle as the animal ages.
The arytenoid cartilage is also a 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.
The epiglottic cartilage is an elastic cartilage, which is the most flexible and most rostral type of cartilage. The thinner stalk-like part, is attached to the root of the tongue, the 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.
The interarytenoid cartilage is a nodule of hyaline cartilage. It is located between the arytenoid cartilages dorsally.
The cuneiform process is formed by 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
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.
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.
The cricothyroid muscle is innervated by the cranial laryngeal nerve, a branch of the vagus nerve (CN X). It moves the cricoid and arytenoid cartilages caudally to tense the vocal folds.
Dorsal cricoarytenoid muscle
The dorsal cricoarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It runs from the dorsal surface of the cricoid cartilage to the arytenoid cartilage. It abducts the vocal process and therefore the vocal fold to widen the glottis.
Lateral cricoarytenoid muscle
The lateral cricoarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It adducts the vocal processes and narrows the glottis.
The thyroarytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It runs from the laryngeal floor to the thyroid cartilage and arytenoid cartilage and alters the tension of the vocal and vestibular folds. It forms part of the sphincter muscular arrangement.
Transverse arytenoid muscle
The transverse arytenoid muscle is innervated by the caudal laryngeal nerve, a branch of the vagus nerve (CN X). It completes the muscular sphincter arrangment and spans the arytenoid cartilages.
The laryngeal pharynx is the largest part of the pharynx. It is wide rostrally and narrows caudally. The laryngeal pharynx joins the oesophagus at the mucosal folds (clearest in the canine, in other species it is harder to see the demarcation). The lumen is 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.
The larynx 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. The reflex stimulation of the mucosa promotes the coughing reflex.
The larynx also allows the passage of air to the lungs and increases the intra-abdominal pressure. The glottis can widen by adduction of the vocal folds when breathing is vigorous. In addition to this, the larynx is used for vocalisation.
Laryngeal branch of the superior thyroid artery supplies the rostral larynx and is a branch of the carotid artery.
Laryngeal branch of inferior thyroid artery supplies the caudal larynx and is branching from the subclavian artery from the thyrocervical trunk.
Laryngeal branch of cricothyroid artery branches from the superior thyroid artery.
The larynx is innervated by branches of the vagus nerve (CN X).
Cranial laryngeal nerve has two branches. The internal branch innervates the mucosa and the external branch innervates the cricothyroid muscle and constricts the pharynx.
Caudal (recurrent) laryngeal nerve innervates the intrinsic muscles of the larynx (except the cricothyroid muscle).
Lymphoid tissue is present.
The variation of the larynx between species is significant. More information can be found by clicking here.
Mucous glands are present in the larynx with a particularly high density in the ventricles. Stratified squamous epithelium is located rostrally around the laryngeal entrance, whilst ciliated pseudostratified columnar epithelium is elsewhere.
The epiglottis is covered by mucous membrane and contains irregular elastic fibres. They form a dense network of branching fibres around the chondrocytes and less dense branching fibres towards the perichondrium.
See elastic cartilage histology for more information.
See hyaline cartilage histology for more information.
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