The Larynx: The Valve That Guards the Airway and Makes the Voice

First a valve, and only then a voice

Understand the larynx as a sphincter that learned to sing, and every muscle and nerve falls into place. The larynx sits at the crossroads where the airway and the food-way part company — at the level of the C3 to C6 vertebrae in the adult, suspended from the hyoid bone above and continuous with the trachea below. Its oldest and most vital function is protective: three tiers of sphincter — the aryepiglottic folds, the false (vestibular) folds, and the true vocal folds — snap shut in sequence during swallowing, so that the bolus streams over a sealed inlet and into the oesophagus behind, a choreography set out in the pharynx and tonsils. Only in the human line did this guard acquire a second job. By stretching a slit of mucosa across a rushing column of air and vibrating its edges, the larynx became a reed — the source of the human voice, shaped into words by the pharynx, tongue and lips downstream. Every fact that follows serves one of these two masters: the valve that keeps you alive, and the instrument that lets you speak.

The cartilages — the skeleton of the box

Three single cartilages down the midline, three paired ones sitting on top of the ring. Nine cartilages give the larynx its shape. The three UNPAIRED ones form the frame. The THYROID cartilage is the largest — two flat laminae meeting in front at an angle whose upper edge is the LARYNGEAL PROMINENCE, the "Adam's apple", with a superior and an inferior horn at its posterior borders (the inferior horn articulating with the cricoid). Below it the CRICOID cartilage is the only COMPLETE ring in the entire airway — signet-shaped, narrow in front and tall behind, lying at the C6 level, and structurally the keystone: the one place the airway cannot be crushed circumferentially, and the landmark used both for a cricothyroidotomy and for cricoid pressure. The EPIGLOTTIS is a leaf of elastic cartilage attached behind the thyroid angle that folds down over the inlet in swallowing. The three PAIRED cartilages sit on the cricoid's back plate: the ARYTENOIDS, small pyramids with a forward-pointing VOCAL PROCESS (to which the vocal ligament attaches) and a lateral MUSCULAR PROCESS (to which the moving muscles attach) — these are the pivots on which the cords swing; and perched on their apices the tiny CORNICULATE and, in the aryepiglottic folds, the CUNEIFORM cartilages, which stiffen the inlet.

The membranes, and the ligament that is the cord

The cartilages are strung together by joints and membranes that turn a pile of gristle into a tunable instrument. Two synovial joints do the work: the CRICOTHYROID joints, between the inferior horns of the thyroid and the cricoid, which let the thyroid tilt forward and back — lengthening and tensing the cords; and the CRICOARYTENOID joints, on the cricoid's back plate, which let the arytenoids rotate and glide — abducting and adducting the cords. Slung between the frame and the hyoid is the THYROHYOID MEMBRANE, and its lateral part is pierced by the INTERNAL LARYNGEAL NERVE and the SUPERIOR LARYNGEAL VESSELS — a fact that matters the moment you meet the nerves. Inside, two fibroelastic sheets line the cavity. The QUADRANGULAR MEMBRANE runs from the epiglottis to the arytenoids, its lower free edge thickening into the VESTIBULAR (false) LIGAMENT. Below it the CONUS ELASTICUS (the cricovocal membrane) rises from the whole upper border of the cricoid ring and converges on the midline, and — this is the sentence to memorise — its upper free edge, thickened and taut, IS the VOCAL LIGAMENT: the core of the true vocal fold, running from the vocal process of each arytenoid behind to the back of the thyroid angle in front.

Three storeys: supraglottis, glottis, subglottis

The cavity is divided by the two pairs of folds into an upstairs, a landing and a downstairs — and the divisions decide how a cancer behaves. From the inlet down, the laryngeal cavity has three parts. The SUPRAGLOTTIS runs from the inlet to just above the vocal folds and includes the VESTIBULE and the false (vestibular) folds. The GLOTTIS is the level of the true VOCAL FOLDS themselves and the RIMA GLOTTIDIS, the slit between them — which is the NARROWEST part of the adult airway, so anything that swells or is inhaled tends to lodge here. The SUBGLOTTIS descends from below the cords to the lower border of the cricoid, where the larynx becomes the trachea; encircled by the rigid cricoid ring, it is the NARROWEST part of the airway in a small CHILD, which is exactly why a viral swelling here — croup — produces the barking cough and stridor of a paediatric emergency while barely troubling an adult. Between the false and true folds on each side is a slit-like recess, the laryngeal VENTRICLE, whose mucous glands lubricate the cords. These divisions are not academic: they follow the embryology of the larynx, and with it the lymphatic drainage, and with THAT the fate of a tumour, as the clinical section will show.

The intrinsic muscles — one job each

This is the cleanest muscle group in the whole body to learn: name the muscle, name its single action. The intrinsic muscles move the cartilages relative to one another and so move the cords. Learn them by their single verbs. The POSTERIOR CRICOARYTENOID is the ONLY ABDUCTOR — it rotates the arytenoids so the vocal processes swing apart and the rima opens; it is the one muscle that keeps the airway open for breathing, which is why it is remembered as "the only one that keeps you alive." Opposing it, the LATERAL CRICOARYTENOID and the transverse and oblique ARYTENOID muscles ADDUCT — closing the cords for phonation and for the protective sphincter. Tension is set by two more: the CRICOTHYROID tilts the thyroid forward on the cricothyroid joint, LENGTHENING and TENSING the cords to RAISE PITCH; the THYROARYTENOID, whose deep fibres form the VOCALIS, relaxes and shortens the cord and fine-tunes the vibrating edge. The ARYEPIGLOTTIC muscles (with the oblique arytenoids) close the laryngeal INLET, adding the topmost tier of the sphincter. That is the entire group: one abductor, the adductors, one tensor, one fine-tuner, and the inlet closers — and the whole of it, bar a single muscle, answers to a single nerve.

The EXTRINSIC muscles play a different game: instead of moving the cartilages on each other, they move the whole larynx up and down. The INFRAHYOID (strap) muscles depress and steady it; the SUPRAHYOID muscles, acting through the hyoid, haul it UPWARD and forward. You feel this every time you swallow — the larynx leaps up under the chin, tucking itself under the tongue base and behind the descending epiglottis so the bolus is deflected past a sealed, elevated inlet into the pharynx and oesophagus. That upward lurch is not decoration; it is a core part of the protective mechanism, and its failure — a larynx that no longer rises, in stroke or in the frail elderly — is one of the commonest routes to aspiration pneumonia.

The nerve supply — the spine of the whole story

Two nerves, both branches of the vagus, split the larynx between them along one horizontal line: the vocal cords. Every muscle and every mucosal surface of the larynx is supplied by the VAGUS NERVE (CN X, mapped among its fellows in the cranial nerves overview) through two branches. The SUPERIOR LARYNGEAL NERVE arises high in the neck and divides into two. Its INTERNAL branch is SENSORY: it pierces the thyrohyoid membrane with the superior laryngeal vessels and supplies mucosal sensation ABOVE the vocal folds — and it is the afferent limb of the COUGH REFLEX, so that a crumb touching the supraglottis triggers an explosive protective cough. Its EXTERNAL branch is MOTOR, and to ONE muscle only: the CRICOTHYROID, running down alongside the superior thyroid artery to reach it. The second branch, the RECURRENT LARYNGEAL NERVE, supplies ALL the other intrinsic muscles — every adductor, the sole abductor, the tensor's antagonist — and carries SENSATION BELOW the vocal folds. The rule to carry out of this article, verbatim, is: all intrinsic muscles by the recurrent laryngeal EXCEPT cricothyroid (external laryngeal); all sensation below the cords by the recurrent laryngeal, above by the internal laryngeal. Two nerves, one dividing line, the cords themselves.

The recurrent laryngeal nerve earns its name from a detour that belongs to the thorax, not the neck, and is drawn in full in the nerves of the thorax: on the LEFT it loops under the ARCH OF THE AORTA, on the RIGHT under the RIGHT SUBCLAVIAN ARTERY, before climbing back up into the neck. What concerns the larynx is where it finishes that climb — in the TRACHEO-OESOPHAGEAL GROOVE, running upward immediately BEHIND the thyroid gland and crossing the inferior thyroid artery near the lower pole. That intimate relationship is the whole reason a thyroidectomy threatens the voice, and why a surgeon dissects that groove as if defusing a bomb.

The blood supply mirrors the nerves, above and below the same line. The arteries follow the same over-and-under logic. The SUPERIOR LARYNGEAL ARTERY, a branch of the SUPERIOR THYROID artery (itself off the external carotid), accompanies the internal laryngeal nerve through the thyrohyoid membrane to supply the upper larynx; the INFERIOR LARYNGEAL ARTERY, a branch of the INFERIOR THYROID artery (from the thyrocervical trunk), ascends with the recurrent laryngeal nerve to supply the lower larynx. Because the arteries travel with their matching nerves, the surgeon who controls the vessels at the poles of the thyroid is working exactly where the nerves lie — which is precisely why the vessels must be tied close to the gland, taking the artery without taking the nerve beside it.

When the recurrent laryngeal fails

A UNILATERAL recurrent laryngeal palsy denervates every intrinsic muscle on that side but one, so the affected cord loses both its abductor and its adductors and comes to rest in the PARAMEDIAN position — not fully closed, not fully open. The healthy cord can often swing across the midline to meet it, so the voice recovers partially but stays weak, breathy and easily tired: HOARSENESS. Because the nerve's course is so long and so exposed, the causes read like a tour of the neck and chest — THYROID SURGERY above all, but also an AORTIC ANEURYSM stretching the left nerve at the arch, and a LEFT HILAR LUNG CANCER or mediastinal nodes invading it there. That is why a hoarse voice that does not settle is never dismissed: it can be the first sign of a tumour nowhere near the throat. Far more dangerous is a BILATERAL recurrent laryngeal palsy — both abductors gone, both cords sitting near the midline with no muscle able to pull them apart. The voice may be almost normal, but the airway is a slit: the patient develops STRIDOR and can obstruct, sometimes needing an emergency tracheostomy. It is the cruel paradox of this nerve — lose one side and you keep your airway but lose your voice; lose both and you may keep your voice but lose your airway.

Emergencies at the front of the neck

When the airway above is lost, one soft membrane at the front of the neck is the way in. The same anatomy that makes the voice becomes, in a crisis, a set of landmarks felt through the skin. Run a finger down the front of the neck and the first firm shelf is the thyroid prominence; drop into the soft dip just below it and you are on the CRICOTHYROID MEMBRANE, the gap between the thyroid and cricoid cartilages. This is where an EMERGENCY CRICOTHYROIDOTOMY is performed — a horizontal stab through skin and that membrane into the subglottis — when the upper airway cannot be secured any other way, a procedure catalogued with the region's landmarks in head and neck surface anatomy and procedures. It is chosen because the membrane is superficial, relatively avascular, and framed by two cartilages a finger can find in the dark. A little lower and less dramatically, CRICOID PRESSURE — firm backward pressure on the complete cricoid ring during rapid-sequence intubation — was long taught to compress the oesophagus behind and guard against aspiration, and it works precisely because the cricoid is the one airway cartilage that forms an unyielding circle to push against.

Cancer, nodules, and testing the reflexes

The three storeys explain why laryngeal cancer behaves so differently depending on where it starts. A GLOTTIC tumour, arising on the vocal cord itself, disturbs the vibrating edge from the very first millimetre, so it presents EARLY with persistent HOARSENESS — and, by luck of embryology, the true cords have a SPARSE lymphatic supply, so a glottic cancer spreads to nodes late and is often curable. A SUPRAGLOTTIC tumour is the opposite: it grows in a region rich in lymphatics, causes only vague throat discomfort or a change in voice quality at first, and so presents LATE, frequently with cervical NODAL metastasis already present. This is why any adult — especially a smoker — with hoarseness lasting more than three weeks must have the cords visualised. Less sinister are VOCAL CORD NODULES ("singer's nodules"): benign, bilateral, symmetrical thickenings at the junction of the anterior and middle thirds of the cords, the point of maximal vibratory contact, caused by voice overuse and abuse and treated largely with speech therapy. And at the bedside the larynx lets you test two protective reflexes directly: the COUGH REFLEX (afferent: internal laryngeal nerve) and the GAG REFLEX (afferent glossopharyngeal IX, efferent vagus X) — both of which probe whether the sensory guardianship of the airway is intact, the very function the larynx was built for before it ever learned to speak.