Larynx anatomy: cartilages, vocal folds and laryngeal muscles

The larynx is a cartilaginous structure below the laryngopharynx that connects the pharynx to the trachea. It is part of the conducting airway, and the inner edges of its true vocal cords are free, so they can vibrate to produce sound. (OpenStax Anatomy and Physiology 2e, 22.1: Organs and Structures of the Respiratory System.)

The larynx sits in the front of the neck in the midline, in front of the oesophagus, normally at the level of vertebrae C3 to C6. It is continuous with the oropharynx above and the trachea below, and is built from nine cartilages joined by membranes, ligaments and muscles. (Kenhub: Larynx.)

Radiologists define the larynx as running from the epiglottis down to the lower edge of the cricoid cartilage, below which it continues as the cervical trachea. It develops from the laryngotracheal groove in the floor of the primitive pharynx, and its cartilages come from the 4th and 6th pharyngeal arches. (Radiopaedia: Larynx.)

In infants the larynx sits higher in the neck, which lets the epiglottis lock into the nasopharynx so a baby can breathe through the nose while feeding. This arrangement is lost between about 2 and 6 years of age. (Kenhub: Larynx.)

The larynx has nine cartilages: three unpaired (thyroid, cricoid and epiglottis) and three pairs (arytenoid, corniculate and cuneiform). Together they form the laryngeal skeleton. The thyroid and cricoid cartilages are hyaline cartilage, while the epiglottis is elastic cartilage. (Kenhub: Larynx.)

The thyroid cartilage is the largest. Its two flat laminae fuse in the front midline to form the laryngeal prominence (Adam's apple), while their back borders are free and extend up as the superior horns and down as the inferior horns. The superior horns are tied to the hyoid bone by the lateral thyrohyoid ligaments; the inferior horns articulate with the cricoid at the cricothyroid joints. An oblique line on each lamina gives attachment to the inferior pharyngeal constrictor, thyrohyoid and sternothyroid muscles. Its upper border lies at about C4. (Radiopaedia: Thyroid cartilage.)

The cricoid cartilage is a complete ring of hyaline cartilage, with a broad lamina at the back and a narrow arch at the front. It marks the lower border of the larynx at C6 and is the only complete ring of cartilage in the larynx and trachea. Pressing on it (cricoid pressure, or Sellick's manoeuvre) squeezes the oesophagus behind it, which is used during emergency intubation to reduce regurgitation of stomach contents. (TeachMeAnatomy: Laryngeal Cartilages.)

The epiglottis is a leaf-shaped flap of elastic cartilage. Its narrow stalk (petiolus) is attached to the inside of the thyroid cartilage, behind the laryngeal prominence, by the thyroepiglottic ligament, and the hyoepiglottic ligament ties its front surface to the hyoid bone. Its free upper end projects up and back over the laryngeal inlet. On the tongue side the mucosa forms glossoepiglottic folds with a shallow pit, the vallecula, on each side. (Kenhub: Epiglottis.)

The arytenoid cartilages are paired, pyramid-shaped hyaline cartilages sitting on the sloping upper border of the cricoid lamina, where they form synovial cricoarytenoid joints. Each has a vocal process pointing forwards, to which the vocal fold attaches, a muscular process pointing laterally for the posterior and lateral cricoarytenoid muscles, and an apex that articulates with the corniculate cartilage. (Radiopaedia: Arytenoid cartilages.)

The corniculate cartilages sit on the apices of the arytenoids. The cuneiform cartilages lie within the aryepiglottic folds, have no direct attachment to the other cartilages, and simply stiffen the folds. (TeachMeAnatomy: Laryngeal Cartilages.)

The larynx has two pairs of joints. At the cricothyroid joints the inferior horns of the thyroid cartilage articulate with the side of the cricoid. At the cricoarytenoid joints the arytenoids rotate on the upper cricoid lamina, which opens (abducts) the vocal folds during breathing and closes (adducts) them during swallowing. (Kenhub: Larynx.)

Extrinsic membranes and ligaments tie the larynx to its neighbours. The thyrohyoid membrane joins the thyroid cartilage to the hyoid bone, with median and lateral thickenings; the superior laryngeal vessels and the internal laryngeal nerve pierce it on each side. The hyoepiglottic ligament joins the hyoid to the epiglottis, and the cricotracheal ligament joins the cricoid to the trachea. (TeachMeAnatomy: Laryngeal Ligaments and Folds.)

Inside, the cricothyroid ligament (conus elasticus) rises from the cricoid cartilage. Its free upper edge, stretched between the thyroid cartilage in front and the arytenoid behind, is the vocal ligament. Higher up, the quadrangular membrane runs from the arytenoid to the side of the epiglottis, and its lower free edge is the vestibular ligament. The median cricothyroid ligament is the thick midline part of the cricothyroid membrane between the thyroid and cricoid cartilages. (TeachMeAnatomy: Laryngeal Ligaments and Folds.)

The vocal folds (true vocal cords) are controlled by the muscles of phonation. From the surface inwards each is made of non-keratinised stratified squamous epithelium, Reinke's space, the vocal ligament and the vocalis muscle. They have little blood supply and look white. The gap between them is the rima glottidis. (TeachMeAnatomy: Laryngeal Ligaments and Folds.)

The vestibular folds (false vocal cords) lie just above the vocal folds. Each is the vestibular ligament covered by mucosa; they are pink, do not move, and mainly protect the larynx. (TeachMeAnatomy: Laryngeal Ligaments and Folds.)

The two pairs of folds divide the laryngeal cavity into three parts: the vestibule, from the laryngeal inlet down to the vestibular folds; a middle part between the vestibular and vocal folds, which includes the laryngeal ventricles and saccules; and the infraglottic cavity, from the vocal folds down to the trachea. The glottis is the vocal folds together with the rima glottidis. (Kenhub: Larynx.)

For cancer staging, radiologists use three subsites: the supraglottis, from the tip of the epiglottis to the laryngeal ventricle (including the aryepiglottic folds and false cords); the glottis (the true vocal cords and the anterior and posterior commissures); and the subglottis, from the underside of the true cords to the lower edge of the cricoid. (Radiopaedia: Larynx.)

The intrinsic muscles lie within the larynx and control the shape of the rima glottidis and the tension of the vocal folds. All are supplied by the inferior laryngeal nerve, the terminal branch of the recurrent laryngeal nerve, except the cricothyroid, which is supplied by the external laryngeal nerve. (Radiopaedia: Intrinsic muscles of the larynx.)

The posterior cricoarytenoid runs from the back of the cricoid lamina to the muscular process of the arytenoid. It is the only muscle that abducts the vocal folds, so it is the only muscle that opens the airway at the glottis. (TeachMeAnatomy: Posterior Cricoarytenoid.)

It works by pulling the muscular processes backwards, which rotates the vocal processes outwards. The lateral cricoarytenoid, from the upper border of the cricoid arch to the same muscular process, does the opposite: it pulls the muscular process forwards, swinging the vocal processes towards the midline. It is the principal adductor. (Radiopaedia: Intrinsic muscles of the larynx.)

The transverse arytenoid joins the back surfaces of the two arytenoids and pulls them together. The oblique arytenoid runs from the muscular process of one arytenoid to the apex of the opposite one, so the two oblique muscles cross in an X. Some of its fibres continue around the arytenoid apex into the aryepiglottic fold as the aryepiglottic muscle (aryepiglotticus). Together they act as a sphincter of the laryngeal inlet that helps prevent aspiration during swallowing and coughing. (Kenhub: Oblique arytenoid muscle.)

The cricothyroid runs from the front of the cricoid arch to the thyroid cartilage in two parts: a straight part to the lower margin of the thyroid lamina and an oblique part to the inferior horn. It rotates the thyroid cartilage forwards and down at the cricothyroid joint, moving it away from the arytenoids, so the vocal folds are stretched, tightened and thinned and the pitch rises. (Kenhub: Cricothyroid muscle.)

The thyroarytenoid (often spelled thyro-arytenoid) runs from the inner surface of the thyroid cartilage near the midline, and from the cricothyroid ligament, back to the arytenoid. It draws the arytenoids forwards, which shortens, thickens and relaxes the vocal folds. Some anatomists describe the part along the vocal ligament as a separate vocalis muscle, and a few fibres continue into the aryepiglottic fold as the thyroepiglotticus. (Kenhub: Thyroarytenoid muscle.)

Radiopaedia sums up the groups: the posterior cricoarytenoids are the only abductors; the lateral cricoarytenoids are the principal adductors, helped by the transverse and oblique arytenoids; the cricothyroids are the tensors; the thyroarytenoids are the relaxers; and the vocalis makes fine adjustments of tension when the folds are together for speech or singing. (Radiopaedia: Intrinsic muscles of the larynx.)

To make a sound, the vocal folds are brought close together without being pressed tight, and air breathed out makes their edges vibrate. Pitch rises when the cricothyroid tenses the folds and falls when the thyroarytenoid relaxes them; loudness depends on fold tension and how hard the air is pushed out. (Kenhub: Vocal cords.)

The folds take different positions for different jobs: relaxed and open in quiet breathing, fully abducted in forced inspiration, adducted for speech, adducted with a small gap at the back for a whisper, and tightly closed during effort closure, as when coughing or lifting. (Kenhub: Vocal cords.)

The extrinsic muscles move the larynx as a whole. They attach to the hyoid bone, which is tied to the larynx by strong ligaments. The suprahyoid muscles and stylopharyngeus raise the larynx; the infrahyoid muscles lower it. (TeachMeAnatomy: Muscles of the Larynx.)

During the pharyngeal phase of swallowing, breathing stops briefly, the larynx is pulled upward and the epiglottis folds down over the glottis, blocking the way into the trachea. If food does enter the trachea, coughing usually brings it back up into the pharynx. (OpenStax Anatomy and Physiology 2e, 23.3: The Mouth, Pharynx, and Esophagus.)

Kenhub describes the same closure in more detail: the base of the tongue presses the epiglottis backwards, the hyoid moves up and forwards, and the aryepiglottic and arytenoid muscles pull the epiglottis over the laryngeal inlet. (Kenhub: Epiglottis.)

The vocal folds are the dividing line for the larynx's supply. Above them, blood comes from the superior laryngeal artery (from the superior thyroid artery) and drains through the superior laryngeal vein to the internal jugular vein. Below them, the inferior laryngeal artery (from the inferior thyroid artery) supplies it and the inferior laryngeal vein drains to the brachiocephalic vein. (Radiopaedia: Larynx.)

Lymph from above the vocal folds pierces the thyrohyoid membrane with the superior laryngeal artery and drains to the superior deep cervical and pre-epiglottic nodes. Lymph from below the folds drains to the inferior deep cervical nodes and the prelaryngeal (Delphian) nodes. (Radiopaedia: Larynx.)

All laryngeal nerves come from the vagus nerve (CN X). The superior laryngeal nerve splits into an internal branch, sensory to the cavity above the vocal folds, which enters through the thyrohyoid membrane with the superior laryngeal artery, and an external branch, motor to the cricothyroid. The recurrent (inferior) laryngeal nerve is motor to all the other intrinsic muscles and sensory below the vocal folds. (Kenhub: Larynx.)

The two recurrent laryngeal nerves take different routes. The right hooks under the right subclavian artery; the left hooks under the aortic arch, next to the ligamentum arteriosum. Both then climb in the groove between the trachea and oesophagus, beside the thyroid gland, and enter the larynx behind the cricothyroid joint. (Radiopaedia: Recurrent laryngeal nerve.)

Vocal fold paralysis is usually caused by damage to the recurrent laryngeal nerve. It is far more often one-sided, and the left is affected more often, probably because the left nerve has a longer course through the chest. Causes include thyroid and parathyroid surgery, lung cancer and aortic aneurysm. A one-sided palsy gives a hoarse, breathy, weak voice; a two-sided palsy can narrow the airway and cause breathlessness. (Radiopaedia: Vocal cord paralysis.)

Kenhub notes that unilateral injury can also cause aspiration, especially of liquids, while bilateral injury causes stridor and breathing difficulty but rarely hoarseness. The cricothyroid is spared in recurrent laryngeal nerve injury because the superior laryngeal nerve supplies it. (Kenhub: Muscles of the larynx.)

Because the posterior cricoarytenoid is the only abductor, losing its function affects both breathing and voice. (TeachMeAnatomy: Posterior Cricoarytenoid.)

Epiglottitis is inflammation of the epiglottis, classically caused by Haemophilus influenzae type b. It causes fever, difficulty breathing and swallowing, a tripod posture and inspiratory stridor, and can obstruct the airway, so securing the airway comes first. (Kenhub: Epiglottis.)

When the airway is blocked at or above the larynx and intubation has failed, an emergency cricothyroidotomy is made through the cricothyroid ligament, which is found by feeling for the thyroid cartilage and moving just below it. (TeachMeAnatomy: Laryngeal Ligaments and Folds.)

Aryepiglotticus

The aryepiglottic muscle is a thin muscle in the aryepiglottic fold, continuing from the oblique arytenoid muscle.

Arytenoid cartilage

The arytenoid cartilages are a pair of pyramid-shaped cartilages at the back of the larynx.

Conus elasticus

An elastic membrane of the larynx.

Corniculate cartilage

The corniculate cartilages are two small conical cartilages on the apex of each arytenoid cartilage.

Cricoid cartilage

The cricoid cartilage is a complete signet-ring-shaped cartilage forming the lower part of the larynx.

Cuneiform cartilage

The cuneiform cartilages are small rod-shaped cartilages in the aryepiglottic folds.

Epiglottis

The epiglottis is a leaf-shaped elastic cartilage that covers the laryngeal inlet during swallowing.

Hyo-epiglottic ligament

The hyo-epiglottic ligament is a band connecting the epiglottis to the hyoid bone.

Hyoid bone

A U-shaped bone in the neck.

Lateral crico-arytenoid

An intrinsic laryngeal muscle that closes the vocal folds.

Lateral thyrohyoid ligament

The thickened posterior edge of the thyrohyoid membrane.

Median cricothyroid ligament

A fibro-elastic ligament in the midline joining the cricoid and thyroid cartilages.

Median thyrohyoid ligament

The thickened midline part of the thyrohyoid membrane.

Oblique arytenoid

An intrinsic laryngeal muscle behind the arytenoid cartilages.

Oblique part of cricothyroid

The oblique fibres of cricothyroid, an intrinsic laryngeal muscle.

Posterior crico-arytenoid

An intrinsic laryngeal muscle on the back of the cricoid cartilage.

Straight part of cricothyroid

The anterior straight fibers of the cricothyroid muscle of the larynx.

Thyro-arytenoid

An intrinsic laryngeal muscle forming the body of the vocal fold.

Thyro-epiglottic ligament

The thyro-epiglottic ligament is a band connecting the epiglottis to the thyroid cartilage.

Thyrohyoid membrane

A fibroelastic sheet connecting the hyoid bone to the thyroid cartilage.

Thyroid cartilage

The thyroid cartilage is the largest cartilage of the larynx, forming the laryngeal prominence.

Transverse arytenoid

An unpaired intrinsic laryngeal muscle crossing between the arytenoid cartilages.

Vocal ligament

The elastic band forming the core of each vocal fold.

Vocalis

The medial part of thyro-arytenoid lying along the vocal ligament.