Last's Anatomy: Regional and Applied

Part thirteen. Larynx

The larynx is a respiratory organ, set in the respiratory tract between the pharynx and trachea. Although phonation is important in man, the main function of the larynx is to provide a protective sphincter for the air passages. The larynx lies below the hyoid bone in the midline of the neck at the level of C4–6 vertebrae.

Skeleton of the larynx

The framework of the larynx consists of cartilages, ligaments and membranes. There are three single cartilages (thyroid, cricoid and epiglottic) and three pairs of cartilages (arytenoid, corniculate and cuneiform). The ligaments and membranes are extrinsic (thyrohyoid membrane and cricotracheal, hyoepiglottic and thyroepiglottic ligaments) and intrinsic (quadrangular membrane and cricothyroid ligament). The vocal cords are the upper part of the cricothyroid ligament (cricovocal membrane).

Cartilages

The thyroid, cricoid and arytenoid cartilages are composed of hyaline cartilage and with age parts of them may calcify or ossify; the epiglottic, corniculate and cuneiform cartilages are elastic fibrocartilage.

The thyroid cartilage consists of two laminae whose anterior borders are fused at a median angle, or laryngeal prominence. A thyroid notch marks the upper end of the prominence. The posterior borders are free and projected upwards and downwards as the superior and inferior horns (Fig. 6.43). Each inferior horn articulates with the cricoid cartilage to form the cricothyroid joint. The outer surface of each lamina possesses an oblique ridge running downwards and forwards, and bounded above and below by a tubercle.

The cricoid cartilage is the foundation of the larynx; to this signet-ring structure (Fig. 6.44) the thyroid and arytenoid cartilages are articulated by synovial joints. It is the only complete cartilaginous ring in the whole of the air passages. The anterior part of the ring is the arch; posteriorly it is projected upwards as a quadrangular flat lamina. Near the junction of the arch and lamina is an articular facet for the inferior horn of the thyroid cartilage. The upper part of the lamina has sloping shoulders, which carry articular facets for the arytenoids. A vertical ridge in the midline of the lamina produces a shallow concavity on each side for the attachment of the posterior cricoarytenoid muscle; the ridge gives attachment to longitudinal muscle fibres of the oesophagus (see p. 209).

The epiglottic cartilage is a slightly curled, leaf-shaped structure, prolonged below into a slender process (the stalk of the leaf) attached in the midline to the back of the laryngeal prominence, below the thyroid notch. The epiglottic cartilage leans back from its attached stalk to overhang the vestibule of the larynx. Fibrous tissue attaching the front and sides of the epiglottis to the body and greater horns of the hyoid bone (hyoepiglottic ligaments) form the framework of the glossoepiglottic folds that bound the valleculae (see p. 387). The posterior surface below the apex is pitted by mucous glands. A bulge on the lower part of this surface is the tubercle of the epiglottis.

Each of the pair of arytenoid cartilages is a three-sided pyramid with anterolateral, medial and posterior surfaces. The inferior base has a forward projection, the vocal process, and a lateral projection, the muscular process. The base articulates with the sloping shoulder on the upper border of the cricoid lamina. A very small corniculate cartilage articulates with the apex of each arytenoid cartilage and a tiny cuneiform cartilage lies nearby in the aryepiglottic fold; they are unimportant.

Joints

The cricothyroid joint, between the inferior horn of the thyroid cartilage and the facet on the side of the arch of the cricoid, is synovial. Movement between the cricoid and thyroid occurs round an axis that passes transversely between the two joints, so that one cartilage can rock backwards and forwards on the other. The recurrent laryngeal nerve lies immediately behind this joint.

The cricoarytenoid joint is also synovial. The capsule here is lax, allowing both rotary and lateral gliding movements. When the arytenoids are pulled laterally and downwards they slide apart from one another along the sloping shoulders of the cricoid lamina. This gliding of the arytenoids opens the gap between the vocal folds (the rima of the glottis) in the shape of a V; rotation opens the glottis in the shape of a diamond. In man there is a greater range of gliding than of rotary movement, and the open human glottis resembles a V and not a diamond (Fig. 6.45).

Ligaments and membranes

Of the extrinsic membranes, the thyrohyoid membrane connects the whole length of the upper border of the thyroid laminae and the superior horns to the body and greater horns of the hyoid bone (Fig. 6.37). The thyrohyoid membrane passes up behind the body of the hyoid bone to be attached to its upper border; a bursa lies between the membrane and the back of the bone. It is here that remnants of the thyroglossal duct (see p. 26) may persist, necessitating resection of the central part of the bone to give adequate removal.

The thyrohyoid membrane forms the lateral wall of the piriform recess and is perforated by the internal laryngeal nerve and the superior laryngeal vessels. It is not part of the larynx, but anchors the skeleton of the larynx to the hyoid bone.

The epiglottis is attached to the hyoid bone and thyroid cartilage by the hyoepiglottic and thyroepiglottic ligaments. The former are described above; the latter is a strong band attaching the stalk of the cartilage to the angle between the thyroid laminae just below the thyroid notch.

The cricotracheal membrane connects the lower border of the cricoid cartilage to the first cartilaginous ring of the trachea.

Of the intrinsic membranes, the quadrangular membrane is a thin fibroelastic membrane that extends between the arytenoid cartilage and the epiglottis (Fig. 6.46). Its anterior border is attached to the side of the lower half of the epiglottis. Its posterior border, much shorter, is attached to the anterolateral surface of the arytenoid. Its lower border is free, constituting the vestibular ligament (‘false vocal cord’). The mucous membrane covering its much longer upper border constitutes the aryepiglottic fold. The two aryepiglottic folds form the margins of the oval inlet of the larynx.

The other intrinsic ligament is the cricothyroid ligament. This is composed of mainly elastic tissue and has distinct anterior and lateral parts, continuous with each other. The anterior part is a thick band in the midline connecting the upper border of the cricoid to the lower border of the thyroid, the anterior (median) cricothyroid ligament. The paired, thinner lateral cricothyroid ligaments (also termed cricothyroid or cricovocal membranes) are attached below to the upper border of the cricoid, but as they ascend they converge and pass up deep to the lamina of the thyroid on each side (Fig. 6.46). Each lateral ligament has a free, thickened superior edge, attached in front to the back of the angle of the thyroid cartilage, midway between the notch and the lower border, and at the back to the vocal process of the arytenoid cartilage. This free edge constitutes the vocal ligament or vocal cord (hence the term cricovocal membrane). The free edge of the quadrangular membrane, the vestibular ligament, lies above the vocal ligament and there is a gap between the two ligaments. The quadrangular and cricovocal membranes are lined on their inner aspects by mucous membrane, and that part of it which covers the vestibular and vocal ligaments forms the vestibular and vocal folds respectively.

Cavity of the larynx

The inlet (aditus) of the larynx, through which it communicates with the pharynx, faces backwards and upwards and is bounded in front by the upper edge of the epiglottis, at the sides and back by the aryepiglottic folds, and in the posterior midline by the transverse mucosal fold between the arytenoids (Figs 6.40 and 6.47). The space below the level of the inlet down as far as the vestibular folds is the vestibule. In the gap between the vestibular and vocal ligaments, the mucous membrane of the larynx bulges outwards, forming a deep horizontal groove, the ventricle or laryngeal sinus (Fig. 6.48). Opening from its anterior end is a small pouch of mucous membrane, the laryngeal saccule, which extends upwards between the vestibular fold and the thyroid lamina.

The gap between the vocal folds is the rima of the glottis (or simply ‘the glottis’), the anteroposterior slit through which air passes (Fig. 6.45). The anterior 60% of the glottis (intermembranous part) is bounded on each side by the vocal fold itself. The posterior 40% (intercartilaginous part) lies between the vocal processes of the arytenoid cartilages and the medial margins of their bases (covered of course by mucous membrane). In the resting state during quiet respiration, the glottis is triangular and about 8mm wide at the back; the sagittal length is about 23mm in the male and 17mm in the female. On looking down into the larynx from above, as with a laryngoscope, the vestibular folds appear as bulges of mucosa above and lateral to the vocal folds, which are lower and closer together and move with respiration and phonation.

Below the glottis the infraglottic part of the larynx extends down to the level of the lower border of the cricoid cartilage where it becomes continuous with the trachea.

Mucous membrane

As part of the respiratory tract, the larynx in general is lined by pseudostratified columnar ciliated epithelium. The anterior surface of the epiglottis (not in the larynx) faces the tongue. Its mucosa is covered by stratified squamous epithelium, which ‘climbs over’ from the front of the epiglottis on to the aryepiglottic folds and the upper part of the posterior epiglottic surface, before being replaced by the ciliated variety. However, over the vocal folds the epithelium is always stratified squamous. The folds are a whitish colour since blood vessels do not show through here, due to the firm attachment of the mucosa to the vocal ligaments. The saccules contain many mucous glands whose secretion flows down to lubricate the vocal folds, where mucous glands are absent. Taste buds are present on the posterior epiglottic surface and the aryepiglottic folds.

The lamina propria is loose in all parts except over the vocal folds where the mucous membrane is very firmly attached. It therefore allows great swelling except at the glottis; in ‘oedema of the glottis’ the swelling accumulates above the rima, but may still cause dangerous obstruction to the airflow.

Muscles

The muscles of the larynx alter the size and shape of the inlet, or affect the vocal ligaments causing their movements or changing their tension. The muscles that act on the inlet are the aryepiglottic and oblique arytenoid muscles, assisted by the transverse arytenoid and thyroepiglottic muscles. Those that affect the vocal ligaments are the posterior and lateral cricoarytenoids, oblique and transverse arytenoids, thyroarytenoids and vocalis, and the cricothyroids. Apart from transverse arytenoid, all the muscles are paired.

The transverse arytenoid is attached to the posterior surfaces of the arytenoid cartilages (Fig. 6.47). It draws the arytenoids (and their vocal processes) nearer to each other and adducts the vocal folds, helping to close the glottis. The oblique arytenoids pass from the back of the muscular process of one arytenoid to the apex of the opposite one, crossing each other on the posterior surface of the transverse arytenoid, and have the same action. Some fibres continue from the arytenoid apex into the aryepiglottic fold and reach the edge of the epiglottis, so forming the aryepiglottic muscle (Fig. 6.40); they approximate the aryepiglottic folds and close the laryngeal inlet.

The posterior cricoarytenoid arises from the concavity on the back of the lamina of the cricoid whence its fibres converge on the back of the muscular process of the ipsilateral arytenoids (Fig. 6.47). Its upper fibres are almost horizontal, its lower lateral fibres almost vertical. Their combined action is to move the arytenoid laterally and rotate its vocal process outwards. It is the most important muscle of the larynx as it is the only muscle that abducts the vocal folds and opens the glottis.

The lateral cricoarytenoid arises from the upper border of the cricoid arch and passes upwards and backwards to be attached to the front of the muscular process of the arytenoid (Fig. 6.46B). By drawing the muscular process forwards it rotates the vocal process inwards and closes the glottis.

The thyroarytenoid muscle extends backwards and laterally from the angle of the thyroid to the anterolateral surface of the arytenoid and the lateral surface of its vocal process (Fig. 6.46B). It shortens and relaxes the vocal ligament, thereby altering the pitch of the voice. A part of this muscle runs parallel and lateral to the vocal ligament, and some of its fibres arise from the ligament rather than the thyroid; these form the vocalis muscle and act on the posterior part of the vocal ligament.

Many thyroarytenoid fibres ascend up to the aryepiglottic fold and some even reach the side of the epiglottis. They constitute the thyroepiglottic muscle and they open the laryngeal inlet by abducting the aryepiglottic fold. Some of these fibres pass lateral to the saccule, which they compress.

The cricothyroid muscle is a fan-shaped muscle on the outer surface of the larynx; it arises from the lateral aspect of the cricoid arch and is attached to the inferior horn and adjacent lower border of the thyroid lamina (Fig. 6.49). Its contraction makes the thyroid tilt slightly downwards and forwards, thereby lengthening and tensing the vocal ligament.

Swallowing. Protection of the inlet during swallowing is provided by the sphincteric action of the aryepiglottic muscles. A second sphincter is provided by closure of the glottis, but swallowed material very rarely enters the vestibule. Elevation of the larynx beneath the posteriorly bulging tongue displaces the epiglottis backwards, assisting closure of the larynx. A large passing bolus may fold the epiglottis over the closed inlet, but the epiglottis is not essential for the protection of the airway. Indeed the epiglottis often stays upright during swallowing, food passing beside it on either side into the piriform fossa (the lateral food channel; see Fig. 6.38).

Phonation. Phonation or voice production involves the making of sounds that can be varied in pitch, intensity and quality (timbre). The stream of air emitted during phonation emerges as a series of discrete jets, as from a siren. This is not only a more effective means of sound production, but is very economical of expired air. At rest the vocal folds are separated. During phonation they are held together. The apposed vocal folds are blown apart by the pressure of the air below them, and elastic recoil returns them to their original position; the rapid repetition of these movements results in vibration of the folds, so giving rise to sound waves with a certain pitch. The frequency of emission of the jets depends on the length and tension of the folds, and it is these features that are adjusted by the intrinsic muscles to vary the pitch. The intensity of the sound varies with the pressure of the air forced through the glottis. The qualityor timbre of the voice depends on the resonating chambers above the glottis; these include the vestibule of the larynx, pharynx, mouth, nose and paranasal sinuses, and their overall shape and volume can be altered by the soft palate, tongue and other muscles. Depression of the larynx (see p. 337) increases the volume of the resonating chambers. Articulation depends on breaking up the sound into recognizable consonants and vowels by the use of tongue, teeth and lips.

In whispering the vocal folds are separated, and vibrations are imparted to a constant stream of expired air. This is inefficient as a means of sound production, and is very wasteful of air.

Various muscular efforts such as heavy lifting, coughing and abdominal straining are accompanied by closure of the glottis, and also by some medial movement of the vestibular folds and compression of the laryngeal ventricle.

A cough or sneeze is an explosion of compressed air. The vocal folds are powerfully adducted, a strong expira-tory contraction is made to build up the intrathoracic pressure (see p. 188), the folds are then suddenly abducted and the blast of compressed air explodes through the larynx (its expulsive force increased by the simultaneous ‘choke-barrel’ narrowing of the trachea). In the cough reflex, afferents from the mucous membrane supplied by the glossopharyngeal and vagus nerves pass to the nucleus of the tractus solitarius (see p. 480). There are widespread connections in the brainstem and spinal cord for the efferent side of the reflex, which involves muscles of the larynx, pharynx, palate, tongue, diaphragm and other thoracic and abdominal muscles.

Abdominal straining is made more effective by adduc-tion of the vocal folds. The diaphragm is weaker than the muscles of the anterior abdominal wall. To prevent loss of intra-abdominal pressure by upward displacement of the diaphragm the folds are closed after a deep breath and the diaphragm is held down by a cushion of compressed air. This manoeuvre is used for evacuation of pelvic effluents and also for the straining of heavy lifting. Escape of a jet of compressed air causes the characteristic accompanying grunt.

Blood supply

Above the vocal folds blood is brought to the larynx by the superior laryngeal branch of the superior thyroid artery. This enters the piriform recess below the internal laryngeal nerve by piercing the thyrohyoid membrane (Fig. 6.40). The superior laryngeal veins accompany the artery and empty into the superior thyroid veins.

The lower half of the larynx is supplied from the inferior laryngeal branch of the inferior thyroid artery; it accompanies the recurrent laryngeal nerve beneath the inferior constrictor of the pharynx. Venous return is by the inferior laryngeal veins to the inferior thyroid veins.

Lymph drainage

From the supra- and infraglottic parts of the larynx, lymphatics accompany the superior or inferior thyroid vessels and drain to the upper or lower groups of deep cervical nodes respectively. A few infraglottic lymphatics pass through the cricothyroid membrane and drain initially to prelaryngeal and to pretracheal nodes.

Nerve supply

All the muscles of the larynx are supplied by the recurrent larnygeal nerve except cricothyroid which is innervated by the external laryngeal nerve. All the motor fibres in both nerves are from cell bodies in the nucleus ambiguus derived mainly via the cranial part of the accessory nerve.

The recurrent laryngeal nerve enters the pharynx by passing upwards under the lower border of the inferior constrictor behind the cricothyroid joint. By this stage it has often divided into an anterior (motor) and a posterior (sensory) branch, at the level of the upper border of the isthmus of the thyroid gland. The nerve reaches the lower part of the piriform recess and then penetrates the laryngeal wall.

With complete recurrent laryngeal nerve paralysis the vocal fold takes up a variable position. Respiratory problems are rare but there may be stridor (noisy respira-tion) if the airflow is substantially increased for any reason. The voice is initially hoarse but with compensatory movement of the other vocal fold disability is reduced. With acute bilateral complete palsies there is significant inspiratory stridor and an immediate tracheostomy may be needed. The position of the vocal folds following partial lesions of the recurrent laryngeal nerves is contentious. The traditional view, that the only abductor muscle (posterior cricoarytenoid) is more vulnerable, is disputed and the vocal folds may assume a paramedian or intermediate (half abducted) position with differing effects on phonation and respiration.

Paralysis of the external laryngeal nerve affecting cricothyroid may pass unnoticed, or perhaps cause some hoarseness of the voice which appears to recover (due to hypertrophy of the opposite cricothyroid) but with a residual inability to produce higher frequencies, as in the high notes in singing. Examination reveals that the vocal fold on the damaged side is slightly bowed and at a lower level than the normal, due to loss of the tension normally provided by cricothyroid.

The mucous membrane of the larynx above the level of the vocal folds is supplied by the internal laryngeal nerve; that of the folds and the larynx below them is supplied by the recurrent laryngeal nerve.

The sympathetic supply (vasoconstrictor) comes in with the superior and inferior laryngeal arteries from the middle and inferior cervical sympathetic ganglia.

Development

The larynx develops from the laryngotracheal groove at the caudal end of the floor of the primitive pharynx, with the laryngeal cartilages being derived from the fourth and sixth arches (see pp. 25 and 26).

Laryngotomy

In very acute airway obstruction at the level of the glottis or above, when an endotracheal tube cannot be successfully introduced through the oral or nasal cavities, an emergency laryngotomy is preferred to tracheostomy (see p. 341). The laryngeal prominence and cricoid cartilage are palpated and entry is made through the cricothyroid ligament between the cricoid and the lower border of the thyroid cartilage. There are no large midline vessels here; an anastomosis between the small cricothyroid branches of the superior thyroid arteries, high up on the cricothyroid ligament, does not usually cause problems. The proximity of the vocal cords, at the level of the middle of the thyroid angle, must be borne in mind both during the procedure and thereafter; an airway introduced through a laryngotomy is usually not maintained for more than 48 hours, lest it leads to subglottic stenosis. This site is also used for the insertion of a minitracheal tube (for suction rather than as an airway).