The larynx is a hollow musculo-cartilaginous organ which carries out a number of important functions. These include acting as the ‘voice box’ enabling phonation, allowing passage of air into the lower respiratory tract and producing the cough reflex to prevent aspiration. Its many roles are undertaken using a series of intricate cartilages, ligaments, membranes and muscles, which help move certain parts of the larynx at any given time.

Macro-Anatomy

 

Located in the anterior neck, the laryngeal apparatus usually spans from vertebral levels C3-C6 suspended from the hyoid bone (the only disarticulated bone in the body). Superiorly, it is continuous with the laryngopharynx and inferiorly the trachea. As mentioned, the larynx is essentially comprised of a series of cartilages, membranes and folds, ligaments and intrinsic muscles. In this chapter we shall look at each of the components in turn.

 

Laryngeal Cartilages

 

The cartilaginous laryngeal skeleton includes three sets of unpaired and paired structures;

 

Unpaired Cartilages

 

·       The epiglottis is a leaf-shaped cartilage situated superiorly which demarcates the entrance to the larynx. During swallowing it acts to block off the respiratory tract therefore it is vital in preventing aspiration.

·       The thyroid cartilage is a large anterior structure of the larynx comprised of two laminae which join together in the midline to form the laryngeal prominence (known colloquially as the Adam’s apple in men). It has two superior and two inferior horns or ‘cornu’. The posterior aspect The superior horns articulate above with the hyoid bone whereas the inferior articulates with what is known as the cricoid cartilage. The posterior aspect also marks the site of articulation with the epiglottis.

·       The cricoid cartilage is an inferiorly located ‘signet ring’-shaped cartilage made entirely of hyaline (the posterior aspect is the thickest part). It delineates the inferior border of the larynx and lies at C6 level. As mentioned previously, it articulates with the lower part of the thyroid cartilage, as well as the paired arytenoid cartilages.

 

Paired Cartilages

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·       The arytenoid cartilages are small posteriorly situated structures which sit on the cricoid cartilage and are roughly pyramidal in shape. They each have an apex, a base, a vocal process and a muscular process and articulate with two further sets of tiny paired cartilages.

·       The corniculate cartilages are tiny hook-shaped cartilages which articulate with the apices of the arytenoid cartilages.

·       The cuneiform cartilages are found within what is known as the aryepiglottic folds (which will be discussed in more detail in this chapter). They do not articulate with any other laryngeal cartilage directly but help to strengthen the fold.

 

 

Laryngeal membranes, folds and ligaments

 

The key point to remember with laryngeal membranes is that they are the basis of the eventual folds and ligaments. Membranes covered in mucosa are known as folds, and thickenings of the membrane are known as ligaments. There are three main membranes (on each side) to highlight within the larynx that help to form these structures.

Thyrohyoid Membrane

 

As implied by its name, this membrane attaches to the superior aspect of the thyroid cartilage and runs up to the inferior aspect of the hyoid bone. The superior laryngeal vessels and the internal laryngeal nerve (which is a branch of the superior laryngeal nerve) pierce the lateral aspect of this membrane. There are also some important ligaments to mention as part of this membrane:

 

·       The median thyrohyoid ligament is the anteromedial thickening of the membrane

·       The lateral thyrohyoid ligaments are the posterolateral thickenings of the membrane

 

Cricothyroid Membrane

 

This membrane spans from the cricoid cartilage inferiorly and attaches to the arytenoid cartilages posteriorly as well as the posterior surface of the thyroid cartilage anteriorly. This creates a superior ‘free margin’ between its anterior and posterior attachments which is important to note. This free margin is thickened forming the vocal ligament and when covered in mucosa will be known as the vocal fold or true vocal cords. Also encased within this vocal apparatus is the vocalis muscle, which helps with phonation by strengthening the ligament. The left and right vocal folds are dynamic and the space between them is known as the rima golttidis.

 

Aryepiglottic or ‘Quadrangular’ Membrane

 

As with the other membranes, the attachments of this membrane are implied by its name (although it is often referred to as the quadrangular membrane also), spanning between the anterolateral aspect of the arytenoid cartilage and the lateral aspect of the epiglottis. As with the cricothyroid membrane, the aryepiglottic membrane also has a ‘free margin’ on each side, however this time it is the inferior part of the membrane that creates it. These free margins are thickened forming the vestibular ligaments and, once covered in mucosa, are known as the vestibular folds or false vocal cords. Unlike the true vocal cords these are fixed and do not have an equivalent to the vocalis muscles to strengthen the ligaments.

 

 

Extrinsic muscles of the larynx

 

These muscles can be split into two groups; the suprahyoid and infrahyoid muscles. As their names suggest they are above and below the hyoid bone and act to elevate or depress the larynx respectively.

 

Suprahyoids

 

The suprahyoid muscles are paired muscles that attach to the hyoid bone and therefore contraction of these muscles elevates the hyoid bone and helps to initiate swallowing. Below is a table summarising the suprahyoid muscles:

 

	Origin	Insertion	Action	Blood Supply	Innervation
Stylohyoid	Styloid process	Hyoid	Initiates swallowing; Pulls hyoid posteriorly + superiorly	Facial + occipital arteries	Mandibular branch of CN VII
Digastric	Anterior belly: Mandible Posterior belly: Mastoid process	Hyoid	Elevates hyoid + depresses mandible	Anterior belly: Submental branch of facial artery Posterior belly: Posterior auricular + occipital arteries	Anterior belly: inferior alveolar nerve of CN V Posterior belly: VII
Mylohyoid	Mylohyoid line of mandible	Hyoid	Elevates hyoid + is part of the floor of the mouth	Mylohyoid branch of inferior alveolar artery	Inferior alveolar nerve of CN V
Geniohyoid	Inferior mental spine of mandible (genial tubercle)	Hyoid	Elevates hyoid + depresses mandible	Lingual + facial arteries	C1 nerve root (that run alongside CN XII)

NB: Occasionally an extra muscle, the stylopharyngeus muscle, can also be included as an extrinsic elevator of the larynx

 

Infrahyoids

 

The infrahyoids are paired muscles that attach to the hyoid bone, They are located inferior to the hyoid and generally act to depress it and the larynx. There are four muscles included in this group, two of which are in a more superficial plane (sternohyoid and omohyoid) and two are in the deeper plane (sternothyroid and thyrohyoid). Below is a table summarising the infrahyoid muscles:

 

	Origin	Insertion	Action	Blood supply	Innervation
Sternohyoid	Sternum + sterno-clavicular joint	Hyoid	Depresses hyoid	Branches of superior thyroid + lingual arteries	C1-C3 nerve roots (ventral rami)
Omohyoid	Inferior belly: scapula and clavicle	Superior belly: hyoid	Depresses hyoid	Branches of superior thyroid + lingual arteries	C1-C3 nerve roots (ventral rami)
Sternothyroid	Manubrium of sternum	Thyroid cartilage	Depresses thyroid cartilage	Cricothyroid branch of superior thyroid artery	C1-C3 nerve roots  (ventral rami)
Thyrohyoid	Thyroid cartilage	Hyoid	Depresses hyoid	Hyoid branch of superior thyroid artery	C1 nerve root (ventral rami) carried with CN XII

 

 

 

Intrinsic muscles of the larynx

 

This group of muscles are responsible for movement of individual components of the larynx in order to vary phonation and allow passage of air into the lower respiratory tract. The muscles act to tense/relax and lengthen/shorten the true vocal cords, as well as change the shape and size of the rima glottidis. All of the muscles are innervated by the inferior laryngeal nerve (the terminal branch of the recurrent laryngeal nerve – CN X) except for the most inferior intrinsic muscle, the cricothyroid, which is innervated by the external branch of the superior laryngeal nerve.

 

It is easiest to try to think of these muscles in pairs depending on their actions. The muscles below act antagonistically on the true vocal cords to alter pitch and volume:

 

	Origin	Insertion	Action	Blood Supply	Innervation
Cricothyroid	Anterolateral aspect of cricoid cartilage	Inferior horns of thyroid cartilage	Stretches + tenses vocal ligament   Forceful, higher pitched speech	Cricothyroid branch of superior thyroid artery	External laryngeal branch (superior laryngeal – X)
Thyroarytenoid	Postero-inferior angle of thyroid cartilage	Anterolateral aspect of arytenoid cartilage	Relaxes vocal ligament   Softer, lower pitched voice	Laryngeal branch of superior thyroid artery	Inferior laryngeal branch (recurrent laryngeal – X)

 

NB: The vocalis muscle lies in the same plane as the thyroarytenoid muscle but has fibres which directly insert into the vocal ligament, thus helping to tense and strengthen it

 

 

 

 

 

 

 

The following muscles act antagonistically to open (abduct) or close (adduct) the true vocal cords thus widening or narrowing the rima glottidis:

 

	Origin	Insertion	Action	Blood Supply	Innervation
Posterior Cricoarytenoid	Posterior aspect of cricoid cartilage	Muscular process of arytenoid cartilage	Only muscle to abduct true vocal cords; Widens rima glottidis   Modulates tone, louder volume	Superior laryngeal artery + cricothyroid branch of superior thyroid artery	Inferior laryngeal branch (recurrent laryngeal – X)
Lateral cricoarytenoid	Arch of cricoid cartilage	Muscular process of arytenoid cartilage	Major adductors of true vocal cords; Narrows rima glottidis   Modulates tone, quieter volume	Superior laryngeal artery + cricothyroid branch of superior thyroid artery	Inferior laryngeal branch (recurrent laryngeal – X)

 

 

The final pair of muscles act synergistically together and with the lateral cricoarytenoid muscle to help with adduction of the true vocal cords:

 

	Origin	Insertion	Action	Blood Supply	Innervation
Transverse arytenoid	Posterior aspect of one arytenoid cartilage	Posterior aspect of the other arytenoid cartilage   (transversely)	Aids in adduction of true vocal cords; Holds the arytenoid cartilages in place; Narrows posterior part of rima glottidis	Laryngeal branch of superior thyroid artery	Inferior laryngeal branch (recurrent laryngeal – X)
Oblique arytenoid	Postero-superior aspect of one arytenoid cartilage	Postero-inferior aspect of the other arytenoid cartilage   (diagonally)	Aids in adduction of true vocal cords; Holds the arytenoid cartilages in place; Narrows posterior part of rima glottidis	Laryngeal branch of superior thyroid artery	Inferior laryngeal branch (recurrent laryngeal – X)

 

 

Areas of the larynx

 

The larynx itself can be split into three distinct regions:

 

·       SUPRAGLOTTIS – the space between the inferior surface of the epiglottis to the false vocal cords (or vestibular folds).

·       GLOTTIS – the area containing the true vocal cords (or vocal folds) and housing the rima glottidis, the dynamic space between the left and right vocal folds which alters with phonation.

·       SUBGLOTTIS – the space between the inferior border of the glottis to the inferior border of the cricoid cartilage

  

Development

 

The larynx starts to develop around the 4^th week of gestation and occurs alongside the development of the trachea. Embryologically both these structures begin as what is known as the laryngotracheal groove situated in the ventral wall of the pharynx. Over time this groove deepens and its edges fuse forming a septum, which separates the primitive laryngotracheal tube from the pharynx and oesophagus. It is the cranial end of this tube which forms the larynx and trachea. It is for this reason that any congenital abnormality affecting the pharynx or oesophagus is almost always associated with a certain degree of malformation of the larynx.

 

 

Micro-Anatomy

 

The superior surface of the epiglottis is covered by non-keratinised stratified squamous epithelium. All other parts of the larynx (including the inferior surface of the epiglottis) are lined by pseudostratified columnar epithelium except for the true vocal cords, which are lined by stratified squamous epithelium. The large, unpaired cartilages of the larynx are made of hyaline whereas the small, paired cartilages are composed of elastic cartilage.

 

Blood Supply

 

The majority of the larynx is supplied by the superior laryngeal artery (a branch of the superior thyroid) and the inferior laryngeal artery (a branch of the inferior thyroid artery). The superior laryngeal artery courses alongside the internal branch of the superior laryngeal nerve to enter the larynx. In contrast, the inferior laryngeal artery enters the larynx with the recurrent laryngeal nerve. In around 10% of people, the thyroid ima artery exists and this may also have branches that supply the larynx.

 

Venous drainage is generally via the superior and inferior laryngeal veins. The superior laryngeal vein drain into the superior thyroid vein and eventually into the internal jugular, whereas the inferior laryngeal vein drains into the left brachiocephalic vein via the inferior thyroid vein.

 

Innervation

 

As mentioned above, the innervation to the larynx comes from two important branches of the vagus nerve, which are the superior laryngeal and the recurrent laryngeal nerves

 

·       The superior laryngeal nerve gives off an internal and an external branch; the internal branch provides sensation to the supraglottis and the external branch provides the motor innervation to the cricothyroid muscle.

 

·       The recurrent laryngeal nerve accounts for sensory innervation to the infraglottis and motor innervation to all the intrinsic muscles of the larynx except cricothyroid.

 

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