Larynx — MRCS Part B Anatomy

The larynx dissected and seen from above, as at laryngoscopy. The cartilages, the view down the throat and the nerve supply are the three questions.

Cartilages and their tissue

The thyroid, cricoid and arytenoid cartilages are hyaline cartilage, and calcify with age, which is why they show on radiographs of the elderly. The epiglottis, corniculate and cuneiform cartilages, and the vocal processes of the arytenoids, are elastic cartilage and do not.

• Thyroid — two laminae meeting in front at the laryngeal prominence, with superior and inferior horns; the thyrohyoid membrane above it is pierced by the internal laryngeal nerve and superior laryngeal vessels.
• Cricoid — the only complete ring of the airway, a signet with its plate behind, at C6. The cricothyroid membrane in front is where an emergency airway goes.
• Epiglottis — a leaf fixed to the back of the thyroid cartilage by its stalk, which tips back over the inlet in swallowing.
• Arytenoids — pyramids on the back of the cricoid plate; the vocal process in front takes the vocal ligament, the muscular process behind takes the posterior and lateral crico-arytenoid muscles. They rotate and slide to open and close the glottis.
• Corniculate and cuneiform — small nodules in the aryepiglottic folds.

The view at laryngoscopy

The base of the tongue and the valleculae in front, the epiglottis, the aryepiglottic folds running back to the arytenoid swellings with the cuneiform and corniculate tubercles in them, the piriform fossae either side, and through the inlet the pink vestibular folds above and the white vocal folds below, meeting in front at the anterior commissure with the glottis between them. Below the cords the trachea rings are seen when the cords abduct.

Nerve supply

All from the vagus. The superior laryngeal nerve divides into the internal laryngeal nerve, which is sensory to the mucosa above the vocal folds, and the external laryngeal nerve, which is motor to cricothyroid, the muscle that tenses the cords, and runs with the superior thyroid artery. The recurrent laryngeal nerve supplies every other intrinsic muscle and is sensory below the folds; it runs in the tracheo-oesophageal groove with the inferior thyroid artery, the right having looped under the subclavian artery and the left under the aortic arch.

Nerve injury

An external laryngeal palsy, from ligating the superior thyroid pedicle too far from the gland, weakens the voice and loses its upper range, which a singer notices and most patients do not. A unilateral recurrent laryngeal palsy leaves the cord near the midline and gives a hoarse, breathy voice with a weak cough. A bilateral palsy leaves both cords near the midline: the voice can be almost normal but the airway is narrow, and stridor after a thyroidectomy is an emergency. Posterior crico-arytenoid is the only abductor, so it is the muscle whose loss closes the airway.