Next Lesson - Head and Neck Cancers
Core
The larynx (‘voice box’) is a component of the respiratory tract found in the anterior neck. It lies at the level of the C3-6 vertebral bodies (the bodies of the 3rd to 6th cervical vertebrae, in the neck) and connects the laryngopharynx (the lowest part of the pharynx, or throat) to the trachea.
The larynx has several functions:
- Airway Protection - this is arguably the most important role of the larynx, preventing food and fluid from entering the airways. When food/fluid enters the airway, this is known as aspiration.
- Ventilation - as part of the respiratory tract, the larynx provides a channel for the movement of air in and out of the lungs.
- Coughing - the larynx plays an important role in the cough reflex, which rapidly expels unwanted material from the respiratory tract.
- Phonation - the production of sound.
The larynx lies anterior to the laryngopharynx. Food should not enter the larynx. During swallowing it is normally diverted around the laryngeal inlet (the entrance to the larynx) through recesses on either side of it, called the piriform fossae, and then passes into the oesophagus.
The larynx is divided into 3 sections: the supraglottis, glottis and infraglottis (also called the subglottis). The glottis is the narrow area at the level of the vocal cords. It includes the vocal cords and the gap between them (the rima glottidis), and it divides the supraglottis from the infraglottis.
Diagram - Anatomy of the larynx
Public Domain Source by Alan Hoofring (Illustrator), edited by Dr. Joshua Bray [Public domain]
The larynx is formed by a framework of cartilages, membranes and ligaments which is predominantly lined by respiratory mucosa, with the true vocal cords covered by stratified squamous epithelium (a tough lining of flat cells arranged in layers).
The laryngeal skeleton consists of 9 cartilages: 3 are single and 6 are paired (3 pairs).
The 3 unpaired cartilages are:
- Epiglottis - the most superior of the laryngeal cartilages, the epiglottis is a leaf-shaped cartilage which plays an important role in airway protection. Its elastic cartilage allows it to fold over the laryngeal inlet while swallowing to prevent material from entering the larynx. It lies just posterior to the root of the tongue and the space between the two is known as the vallecula.
- Thyroid Cartilage - the largest of the cartilages, the thyroid cartilage consists of two laminae (flat plates) which extend anteriorly and join to form the laryngeal prominence (Adam’s apple).
- Cricoid Cartilage - the most inferior cartilage, the cricoid cartilage is a complete ring of hyaline cartilage (the smooth, glassy type of cartilage) which encircles the airway and forms the inferior border of the larynx. It is shaped like a signet ring with its band facing anteriorly.
The 6 paired cartilages (3 pairs) are:
- Arytenoid Cartilages - these pyramid-shaped cartilages articulate with the superior aspect of the cricoid cartilage. These cartilages are important as they provide attachment for the aryepiglottic folds (the folds of tissue that run between the arytenoid cartilages and the epiglottis) and are also visible on laryngoscopy (looking at the larynx with a camera or scope).
- Corniculate Cartilages - these smaller cartilages articulate with the apex of the arytenoid cartilages.
- Cuneiform Cartilages - these cartilages sit within the aryepiglottic folds to provide reinforcement to the folds.
All nine cartilages are connected by numerous membranes and ligaments to unite the larynx as a single structure.
Three membranes connect the unpaired cartilages of the larynx to each other and to other nearby structures:
- Thyrohyoid Membrane - connecting the hyoid bone (the small bone in the neck above the larynx) to the thyroid cartilage.
- Cricothyroid Membrane - connecting the thyroid cartilage to the cricoid cartilage. A cricothyroidotomy is an emergency procedure in which a cut is made through this membrane, usually with a scalpel, to create an airway when the airway cannot be secured in any other way.
- Cricotracheal Membrane - connecting the cricoid cartilage to the 1st tracheal cartilage.
Diagram - External laryngeal cartilages and membranes
Creative commons source by Olek Remesz [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The quadrangular membrane spans between the epiglottis and arytenoid cartilages. The free edges of the membrane form important structures:
- Aryepiglottic Folds - formed by the superior free edge of the quadrangular membrane, connecting the epiglottis to the arytenoid cartilages. These form an oval entrance to the larynx called the laryngeal inlet.
- Vestibular Ligaments - formed by the inferior free edge of the quadrangular membrane, connecting the angle of the thyroid cartilage to the arytenoid cartilage. These are otherwise known as the false vocal cords.
The true vocal cord, otherwise known as the vocal ligament, is formed by the thickened superior free border of the conus elasticus (also called the cricovocal membrane), an elastic membrane that rises from the cricoid cartilage beneath the vocal cords.
Diagram - View of the larynx and vocal cords on nasendoscopy (a camera passed through the nose)
Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
The larynx is predominantly lined with pseudostratified ciliated epithelium (a single layer of cells that looks layered, with tiny hair-like cilia on the surface). The exceptions are the true vocal cord and the lingual surface (the surface facing the tongue) of the epiglottis. These are lined by stratified squamous non-keratinised epithelium (flat cells in layers, without a tough outer layer of keratin).
The larynx has many muscles. This article will focus on some of the more important muscles.
In broad terms, there are 2 groups of laryngeal muscles: extrinsic and intrinsic. The extrinsic muscles attach the larynx (directly or indirectly) to other structures and act to move the entire larynx. The intrinsic muscles are found within the larynx itself and act primarily on the vocal cords.
Extrinsic Muscles:
- Stylopharyngeus - this muscle of the pharynx acts to elevate the larynx.
- Suprahyoids - this group of muscles act to elevate the hyoid bone and therefore elevate the larynx.
- Infrahyoids - this group of muscles act to depress the hyoid bone and therefore depress the larynx.
Intrinsic Muscles:
Most of the intrinsic laryngeal muscles (e.g. lateral cricoarytenoid, transverse arytenoid) act to adduct the vocal cords. Other important muscles of note are:
- Posterior Cricoarytenoids - this pair of muscles are the only muscles to abduct the vocal cords.
- Cricothyroid - this pair of muscles increase tension in the vocal cords in order to produce higher pitched sounds.
The larynx executes different movements depending on the action which is being performed:
- Ventilation - when taking a deep breath in or out, the vocal cords are fully abducted (via posterior cricoarytenoids) in order to create a wide airway.
- Phonation - in order to create sounds, the vocal cords adduct to vibrate a thin column of air. The pitch of the sound created is altered by varying the length and tension of the vocal cords (via cricothyroid).
- Coughing - in order to produce a cough, the vocal cords initially adduct in order to build up intrathoracic pressure. The cords then suddenly snap open which causes the air to be expelled at a high velocity.
During swallowing, it is imperative that food/drink does not enter the airway (aspiration). To avoid this, the larynx has several protective mechanisms:
- The vocal cords adduct in order to close the glottis.
- Contraction of the aryepiglottic muscles acts to narrow the laryngeal inlet and fold the epiglottis over the inlet.
- The larynx is pulled upwards by contraction of stylopharyngeus and the suprahyoid muscles. This helps to close the laryngeal inlet.
The larynx is innervated by branches of the vagus nerve (CN X, short for cranial nerve 10). These branches are the superior laryngeal nerve and the recurrent laryngeal nerve.
The recurrent laryngeal nerve has an important path, branching distally from the vagus nerve and then travelling back on itself towards the larynx:
- Right Recurrent Laryngeal Nerve - loops under the right subclavian artery
- Left Recurrent Laryngeal Nerve - loops under the aortic arch
Diagram - Course of the left recurrent laryngeal nerve
Creative commons source by Jkwchui [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
In terms of sensory innervation, the supraglottis is supplied by the internal branch of the superior laryngeal nerve, while the infraglottis is supplied by the recurrent laryngeal nerve. Sensation at the level of the glottis (the vocal cords) is generally attributed to the internal branch of the superior laryngeal nerve, also called the internal laryngeal nerve.
As for motor supply, all intrinsic muscles of the larynx are innervated by the recurrent laryngeal nerve, except for cricothyroid which is innervated by the external branch of the superior laryngeal nerve.
Because of its long course, the recurrent laryngeal nerve is vulnerable to damage, for example during thyroid surgery. Injury to the recurrent laryngeal nerve leads to paralysis of the ipsilateral vocal cord.
- If the damage (lesion) is unilateral (on one side only) - this will lead to a hoarse voice and/or an ineffective cough reflex. These lesions are often well tolerated as the contralateral vocal cord compensates over time.
- If the lesion is bilateral (on both sides) - this can be a surgical emergency. Both vocal cords are paralysed and often lie in a paramedian position (close to the midline), which compromises the airway and can cause stridor (a harsh, high-pitched noise on breathing in). The voice may sound fairly normal. An emergency airway may be needed.
Conditions Affecting the Larynx
Acute laryngitis is inflammation of the larynx. It is associated with upper respiratory tract infections and is most commonly viral, for example parainfluenza or influenza viruses. Symptoms include dry cough, hoarse voice and sore throat. If the inflammation extends to the trachea and bronchi, this is known as laryngotracheobronchitis, or croup. Croup is common in children, typically aged between 6 months and 3 years. It is characterised by a barking cough (which sounds like a seal) and may also cause a hoarse voice and stridor (a harsh, high-pitched noise on breathing in).
Epiglottitis is inflammation of the epiglottis. It is a medical emergency as the swollen epiglottis can compromise the airway. It presents with dyspnoea (difficulty breathing), stridor (a harsh, high-pitched noise on breathing in), a very sore throat with painful swallowing, hoarseness of voice and often drooling. All patients require urgent senior assessment. If there are signs of airway compromise, experienced specialists should manage the airway in a controlled setting. In children, epiglottitis is classically caused by Haemophilus influenzae type b (Hib) infection. Children in the UK are routinely vaccinated against Hib (as part of the 6-in-1 vaccine), so epiglottitis is now much less common in children and most cases occur in adults.
The airway can also become compromised during anaphylaxis, a severe allergic reaction. Mast cells (immune cells) release histamine (a chemical that causes swelling) and other chemicals, which can cause swelling of the throat and tongue, including laryngeal oedema. The most important drug is adrenaline, given early by intramuscular injection (into a muscle). If tracheal intubation (placing a breathing tube in the windpipe) is needed, it should be done by the most experienced clinician available.
Laryngeal nodules (vocal nodules) are benign growths on the vocal cords. They are generally due to voice overuse which causes chronic irritation of the vocal cords. Therefore, they are common in singers, cheerleaders, teachers, etc. Patients generally present with voice changes (e.g. hoarse, raspy, low-pitched voice) and/or pain in the throat. Initial treatment is voice therapy, usually led by a speech and language therapist. If this does not work, the nodules can be removed surgically.
Laryngeal cancer often presents with voice changes. Other symptoms include dysphagia (difficulty swallowing), pain on swallowing, globus (a feeling of a lump in the throat), shortness of breath, earache and weight loss. A lump or swelling in the neck is also a common presenting feature. NICE advises considering a suspected cancer pathway referral (an urgent referral to a cancer specialist team) for laryngeal cancer in people aged 45 and over with persistent unexplained hoarseness or an unexplained lump in the neck. Treatment depends on the size, site and spread of the cancer. Surgery (including laser surgery) is often the main treatment, especially for early cancers, but radiotherapy, or chemotherapy given with radiotherapy, is an alternative for some people. For more information, see our article on Head and Neck Cancers.
Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge
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