Larynx Anatomy
The larynx acts as a sphincter guarding
the lower respiratory tract and is responsible for
phonation. It lies in the neck and its inlet is continuous with the
laryngopharynx (Figs 7.61 & 7.62). Inferiorly,
the larynx is continuous with the trachea. It consists of a framework
of cartilages and bone, which supports the vocal and vestibular folds and the
muscles that move them. Anteriorly lie the infrahyoid strap muscles.
Skeleton
The skeleton of the larynx comprises the
thyroid, cricoid and arytenoid cartilages, the epiglottis and the hyoid bone.
The thyroid cartilage (Figs 7.67 &
7.68) has two flat laminae joined anteriorly to form the midline laryngeal
prominence (Adam’s apple). The posterior margin of each lamina is free and
bears a superior and an inferior horn. The tips of the inferior horns
articulate by tiny synovial joints with the lateral surfaces of the cricoid
cartilage.
The cricoid cartilage (Figs 7.67 &
7.71) lies below the thyroid cartilage and is signet ring-shaped, with a narrow
anterior arch and a wide posterior lamina. Its inferior border lies
horizontally, while the superior border slopes upwards posteriorly. Movement at
the cricothyroid joints allows the arch of the cricoid to tip upwards, while
the lamina tips backwards.
In addition to its articulations with the
thyroid cartilage, the upper border of the cricoid lamina bears articular
surfaces, one on each side of the midline, for synovial joints with the two
arytenoid cartilages. Each arytenoid (Figs 7.64, 7.67 &
7.71) is pyramidal in
shape, with its base on the cricoid lamina and its apex superiorly. It has four
surfaces (medial, posterior, anterolateral and inferior) and two processes. Anteriorly lies the
vocal process, to which the vocal fold is attached, while its muscular process
projects laterally. Each arytenoid is able to rotate around its own vertical
axis. In addition, the arytenoids can slide laterally and downwards away from
the midline on the slope of the articular surface on the cricoid.
The epiglottis (Figs 7.64 & 7.67) is
attached in the midline to the inner surface of the thyroid cartilage, just
below the laryngeal prominence. It extends upwards and backwards behind the
tongue to which it is attached in the midline by the median glossoepiglottic
fold. From each side of the epiglottis a lateral glossoepiglottic fold (Fig.
7.61) extends to the side of the tongue. These folds bound a recess on each
side of the midline, the vallecula (Fig. 7.61), which is a site where swallowed
foreign bodies may lodge. The epiglottis overlies the laryngeal inlet when the
larynx is raised against the tongue when swallowing.
The hyoid bone (Figs 7.62 & 7.67) is
the uppermost skeletal element of the larynx. It consists of a midline body
bearing greater horns projecting posteriorly. On each side a lesser horn is
located on the upper surface where the body and greater horn unite.
The inelastic quadrangular (aryepiglottic)
membrane (Fig. 7.66) attaches posteriorly to the body and apex of the arytenoid
and anteriorly to the lower part of the lateral edge of the epiglottis. It has
a lower free border which forms the vestibular fold (false vocal cord), while
the upper free edge forms the aryepiglottic fold (part of the laryngeal inlet).
The thyrohyoid membrane (Fig. 7.68) fills
the gap between the thyroid cartilage and the hyoid bone. Its posterior free
edges are thickened to form the lateral thyrohyoid ligaments, while the midline
thickening is the median thyrohyoid ligament.
Mucous membrane (Fig. 7.65) lines the interior of the larynx. The epithelium is
of the respiratory type, except over the vocal and aryepiglottic folds where it
is stratified and squamous. A pouch of mucous membrane protrudes laterally
between the vocal and vestibular folds, forming the saccule. It contains numerous
mucous glands, the secretions of which moisten the vocal folds.
Intrinsic muscles
The intrinsic laryngeal muscles control
the position and tension of the vocal and aryepiglottic folds, therefore
modifying the shape of the airway through the larynx, acting both at the
glottis (rima glottidis), the gap between the vocal folds, and at the inlet
(Fig. 7.62). During swallowing, both openings narrow, but in coughing and
phonation only the glottis narrows. The glottis widens during inspiration,
especially when deep. The muscles are symmetrical in their arrangement.
The cricothyroid muscle (Fig. 7.68)
attaches to the anterolateral surface of the cricoid cartilage and to the
inferior edge of the thyroid cartilage and adjacent part of the inferior horn.
It raises the arch of the cricoid cartilage, tipping the lamina backwards and
thereby increasing the tension and length of the vocal folds. This has the
effect of raising the pitch of the voice and narrowing the glottis.
The thyroarytenoid muscle (Fig. 7.70)
attaches to the posterior surface of the thyroid cartilage adjacent to the
conus elasticus. Its fibres pass posteriorly to gain the vocal process and
adjacent body of the arytenoid cartilage.
The muscle opposes the action of cricothyroid, drawing the arytenoids
forwards and relaxing the vocal folds. Part of the muscle (vocalis) lies in the
free edge of the vocal fold. The vocalis gives rigidity to the edge of the fold
and also modifies tension differentially along its length.
Two muscles attach to the muscular process
of each arytenoid. The lateral cricoarytenoid (Fig. 7.69) attaches to the lateral surface of the cricoid, its
fibres passing posterosuperiorly to reach the muscular process of the
arytenoid. The muscle rotates the arytenoid around its vertical axis, adducting
the vocal folds and thereby closing the glottis. The posterior cricoarytenoid
(Figs 7.63 & 7.69) inclines anterosuperiorly from the lamina of the cricoid
to the muscular process of the arytenoid. Its action opposes that of the
lateral cricoarytenoid, rotating the arytenoid so that the folds are abducted
and the glottis opened. Simultaneous contraction of the lateral and posterior
cricoarytenoid muscles separates the arytenoids by sliding them down the
cricoid lamina.
The arytenoid muscle (Figs 7.63 &
7.65) spans the gap between the bodies of the arytenoid cartilages and has
transverse and oblique parts. The muscle approximates the arytenoids, closing
the glottis. The oblique arytenoids continue anterosuperiorly in the
aryepiglottic fold as the aryepiglottic muscles (Fig. 7.69). These assist in closing the laryngeal inlet and are
important during swallowing.
Nerve supply
The larynx is supplied by the right and
left superior and recurrent laryngeal branches of the vagus (X) nerves. Each
recurrent laryngeal nerve (Figs 7.68 & 7.69) ascends from the root of the neck (p. 331) in the
groove between the trachea and oesophagus. Passing beneath the lower border of
the inferior constrictor, the nerve enters the larynx behind the cricothyroid
joint.
It supplies motor fibres to all of the
intrinsic muscles (except cricothyroid) and carries sensory fibres from all of
the structures within the larynx and laryngopharynx below the level of the vocal folds.
The superior laryngeal nerve (Figs 7.58
& 7.68) arises from the vagus nerve just below the skull and descends to
the thyrohyoid membrane where it divides, forming a motor external laryngeal
branch and a sensory internal laryngeal branch. The external laryngeal nerve
(Fig. 7.68) descends on the outer surface of the larynx to supply the cricothyroid muscle.
The internal laryngeal branch (Figs 7.62 & 7.68) pierces the thyrohyoid
membrane and provides sensory fibres to the larynx and laryngopharynx above the
vocal folds. These sensory nerves contribute to the cough reflex.
Blood supply
The laryngeal branches of the superior and
inferior thyroid arteries (Figs 7.13 & 7.15) supply the larynx, while
venous blood drains via superior and middle thyroid vessels into the internal
jugular veins.
