Larynx and Cervical Airway

Thyroid cartilage:

• The largest laryngeal cartilage, formed by two laminae fusing anteriorly at the laryngeal prominence (Adam's apple), more prominent in males (reflecting the larger, more acutely angled fusion typical of the male larynx); superior and inferior horns project from the posterior laminae, the inferior horns articulating with the cricoid cartilage (cricothyroid joint).

Cricoid cartilage:

• A complete ring (the only complete cartilaginous ring in the airway, all tracheal rings are C-shaped/incomplete posteriorly, see Trachea below), signet-ring shaped (narrow anteriorly, broad posteriorly), located below the thyroid cartilage, marking the C6 vertebral level and the transition point to the trachea/oesophagus.

Epiglottis:

• A leaf-shaped elastic cartilage, attached inferiorly to the thyroid cartilage (via the thyroepiglottic ligament) and to the hyoid bone (via the hyoepiglottic ligament), projecting upward behind the tongue base and hyoid, covered by mucosa; folds downward during swallowing to help direct food/liquid away from the laryngeal inlet and into the piriform recesses/oesophagus.

Arytenoid cartilages:

• Paired, pyramidal cartilages sitting atop the posterior cricoid lamina, articulating with it (cricoarytenoid joint); each has a vocal process (anterior projection, giving attachment to the vocal ligament) and a muscular process (posterolateral projection, giving attachment to the intrinsic laryngeal muscles that move the cartilage), the key mobile cartilages controlling vocal fold position/tension.

Corniculate cartilages:

• Small, paired cartilages sitting atop the arytenoid cartilages' apices, contributing to the aryepiglottic fold's structure.

Cuneiform cartilages:

• Small, paired cartilages within the aryepiglottic folds (anterior to the corniculate cartilages), providing additional structural support to the fold without a direct articulation to other cartilages.

Cricothyroid joint:

• Synovial joint between the inferior horn of the thyroid cartilage and the cricoid cartilage, permitting a rocking/gliding movement that tilts the thyroid cartilage relative to the cricoid, increasing the distance between the thyroid prominence and the arytenoid cartilages, this movement tenses the vocal folds, the mechanical action underlying the cricothyroid muscle's pitch-raising function.

Cricoarytenoid joint:

• Synovial joint between each arytenoid cartilage and the cricoid cartilage's superior lamina, permitting both a rotational movement (swinging the vocal process medially/laterally, abducting or adducting the vocal folds) and a gliding movement (altering the arytenoid's position slightly), the essential joint enabling vocal fold abduction/adduction (opening/closing the rima glottidis).

Thyrohyoid membrane:

• Connects the superior border of the thyroid cartilage to the hyoid bone, pierced laterally by the internal laryngeal nerve and superior laryngeal vessels (a key anatomical relationship, relevant to laryngeal nerve blocks and surgical approaches to this region).

Cricothyroid membrane:

• Connects the cricoid cartilage to the thyroid cartilage anteriorly (its midline portion is the palpable, superficial structure targeted in emergency airway access; see Applied Anatomy); its lateral, deeper parts thicken to form the conus elasticus, whose upper free margin forms the vocal ligament.

Quadrangular membrane:

• A paired, sheet-like membrane extending from the lateral epiglottis to the arytenoid/corniculate cartilages, whose lower free margin forms the vestibular ligament (the fibrous core of the vestibular/false vocal fold).

Conus elasticus (cricovocal membrane):

• The paired, deeper, lateral thickening of the cricothyroid membrane, extending from the cricoid cartilage upward, whose upper free margin forms the vocal ligament (the fibrous core of the true vocal fold), attaching anteriorly to the thyroid cartilage (near the midline) and posteriorly to the arytenoid cartilage's vocal process, directly linking the mobile arytenoid cartilage's position to vocal fold tension/position.

True vocal folds:

• Formed by the vocal ligament (free edge of the conus elasticus) covered by mucosa, plus the underlying vocalis muscle (the medial part of thyroarytenoid), the primary sound-producing structures, vibrating in the airflow to generate the fundamental voice frequency.

Vestibular folds (false vocal folds):

• Formed by the vestibular ligament (free edge of the quadrangular membrane) covered by mucosa, positioned superior to the true vocal folds, do not normally contact/vibrate during phonation (their apposition instead occurs during breath-holding/Valsalva manoeuvre, or pathologically in certain voice disorders where they compensate for true vocal fold dysfunction).

Rima glottidis:

• The opening between the two true vocal folds (and, more posteriorly, between the two arytenoid cartilages), its width varies with vocal fold position, from fully open (quiet breathing) to a narrow slit (phonation) to fully closed (breath-holding/protective airway closure), the narrowest part of the adult airway at the level of the vocal folds themselves (though in infants/young children, the subglottic region, at the cricoid cartilage level, is the narrowest airway point instead, a clinically important paediatric anatomical distinction relevant to appropriately sized endotracheal tube selection).

• Innervation summary: the single most important organising fact in this section: All intrinsic laryngeal muscles are supplied by the recurrent laryngeal nerve EXCEPT cricothyroid, which is supplied by the external laryngeal nerve: this single exception underlies the differing clinical presentations of recurrent vs external laryngeal nerve injury (see Applied Anatomy).

Superior laryngeal nerve:

• A branch of the vagus nerve, arising high in the neck (near the nodose ganglion), descending alongside the pharynx before dividing into internal and external branches near the greater horn of the hyoid bone.

Internal laryngeal nerve:

• The larger, sensory branch, piercing the thyrohyoid membrane (alongside the superior laryngeal artery) to provide sensation to the laryngeal mucosa above the vocal folds (supraglottic region, including the epiglottis's laryngeal surface and the piriform recess mucosa).

External laryngeal nerve:

• The smaller, motor branch, descending on the external surface of the inferior constrictor muscle, closely related to the superior thyroid artery/vessels (running alongside or crossing them near the superior thyroid pole, a critical relationship for thyroid surgery), to supply the cricothyroid muscle (the sole exception) and a small motor contribution to the inferior pharyngeal constrictor.

Recurrent laryngeal nerve:

• A branch of the vagus nerve, with markedly asymmetric courses (right: loops under the right subclavian artery; left, loops under the aortic arch, a much longer intrathoracic course), ascending in the tracheo-oesophageal groove, closely related to the inferior thyroid artery (a critical relationship for thyroid surgery), entering the larynx (typically deep to the inferior constrictor's lower border, near the cricothyroid joint) to supply all intrinsic laryngeal muscles except cricothyroid, and providing sensation to the laryngeal mucosa below the vocal folds (subglottic region).

• Sensory summary: Above the vocal folds → internal laryngeal nerve; below the vocal folds → recurrent laryngeal nerve: the vocal folds themselves represent the sensory dividing line, directly paralleling (though anatomically distinct from) the motor "all except cricothyroid" division.

Cervical extent:

• Begins at the lower border of the cricoid cartilage (C6), descending through the neck to enter the thorax at the level of the suprasternal notch/thoracic inlet, continuing to the tracheal bifurcation at the sternal angle (T4/T5).

Relations (cervical part):

• Anterior: Isthmus of the thyroid gland (crossing anterior to the 2nd-4th tracheal rings), pretracheal fascia, strap muscles, and (in the lower neck) the inferior thyroid veins and, occasionally, a thyroid ima artery when present.
• Posterior: Oesophagus (immediately posterior throughout).
• Lateral: Lobes of the thyroid gland, carotid sheath contents, and the recurrent laryngeal nerve (ascending in the tracheo-oesophageal groove on each side).

Blood supply:

• Inferior thyroid artery branches (predominant supply to the cervical trachea, reflecting its close relationship to the thyroid gland).

Innervation:

• Parasympathetic and sensory: recurrent laryngeal nerve branches; sympathetic, from the cervical sympathetic trunk.

Recurrent laryngeal nerve palsy:

• Unilateral palsy: the affected vocal fold typically comes to lie in a paramedian position (given the balance of remaining intact/compensatory muscle tone), causing a weak, breathy voice (hoarseness) but usually adequate airway patency, since the unaffected side can still abduct.
• Bilateral palsy: both vocal folds lie paramedian, critically narrowing the rima glottidis and causing significant airway obstruction/stridor, a genuine emergency, since neither side retains functional abduction, paradoxically, bilateral RLN palsy can produce a relatively preserved (though weak) voice, since the paramedian folds can still approximate for phonation, even as the airway itself becomes critically compromised, a counterintuitive but classically tested clinical pattern (voice may sound reasonably normal while the airway is in genuine danger).

External laryngeal nerve injury:

• Isolated injury (e.g., during thyroid surgery, given its close relationship to the superior thyroid vessels) selectively weakens cricothyroid, causing an inability to tense the vocal folds, resulting in a weakened, lower-pitched voice with reduced range (particularly loss of the ability to hit higher notes/project volume), rather than the hoarseness/breathiness typical of recurrent laryngeal nerve injury, a more subtle deficit, often most noticeable to professional voice users (singers, public speakers).

Vocal cord paralysis:

• See recurrent laryngeal nerve palsy above; the distinction between unilateral (hoarse voice, adequate airway) and bilateral (weak-but-present voice, critical airway compromise) is the single most clinically important pattern to recognise.

Cricothyrotomy:

• An emergency airway procedure, accessing the airway through the cricothyroid membrane (palpable in the midline, between the thyroid and cricoid cartilages), chosen specifically because this membrane is superficial, relatively avascular (compared to the surrounding structures), and provides direct, rapid access to the subglottic airway, the technique of choice when oral/nasal intubation is impossible and immediate airway access is required ("can't intubate, can't oxygenate" scenarios).

Tracheostomy:

• A surgical airway procedure creating an opening directly into the trachea (typically at the level of the 2nd-4th tracheal rings), requiring careful identification and retraction/division of the thyroid isthmus (which crosses anterior to this level) and awareness of the inferior thyroid veins and (rarely) a thyroid ima artery, which may cross this region, a planned, typically non-emergency procedure (in contrast to the emergency cricothyrotomy), used for longer-term airway management.