Intrinsic Muscles of the Larynx
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Direct answer
Only one intrinsic muscle of the larynx abducts the vocal cords — the posterior cricoarytenoid — while the lateral cricoarytenoid, transverse arytenoid and thyroarytenoid adduct them; the cricothyroid tenses the cord and the thyroarytenoid relaxes it. The nerve supply is the examination's centre of gravity: the cricothyroid alone is supplied by the external laryngeal branch of the superior laryngeal nerve, and every other intrinsic muscle is supplied by the recurrent laryngeal nerve. The cricoarytenoid joint rocks and glides, so these muscles move the vocal processes apart or together, setting the rima glottidis for breathing and phonation. Cord positions after nerve injury — median, paramedian, intermediate — follow from which muscles still work; bilateral recurrent laryngeal palsy is an airway emergency.
What you must remember
- The only abductor: posterior cricoarytenoid, rotating the arytenoid so the vocal processes separate — the muscle whose bilateral loss causes stridor.
- Chief adductors: lateral cricoarytenoid, transverse arytenoid (the only unpaired intrinsic muscle) and the thyroarytenoid, which also slackens the cord through its vocalis fibres.
- The only tensor: cricothyroid, rocking the thyroid cartilage forward on the cricoid to lengthen and tense the cord — supplied by the external laryngeal nerve.
- Nerve rule: external laryngeal nerve for cricothyroid; recurrent laryngeal nerve for all the rest — the single most examined sentence in laryngeal anatomy.
- Cord positions: median (cadaveric position, complete bilateral lesion of both nerves), paramedian (recurrent laryngeal palsy — the intact cricothyroid pulls the cord toward the midline), intermediate or abducted (external laryngeal palsy with cricothyroid tone lost).
- Semon's law: in a progressive nerve lesion the abductor fibres (of the recurrent laryngeal nerve) are affected before the adductors, so the cord first lies adducted — classical theory, quoted with the caveat that modern understanding questions it.
- Clinical anchors: post-thyroidectomy stridor from bilateral recurrent laryngeal injury, hoarseness with loss of high-pitch from external laryngeal injury, and the long-standing dictum that any hoarseness persisting weeks after surgery demands laryngoscopy.
Managing stridor after thyroidectomy
The clock starts when a patient returns from theatre with noisy breathing. The anaesthetist is told first, and the surgeon thinks in anatomy. If one recurrent laryngeal nerve has been sacrificed, the cord lies paramedian and the patient is usually stable — hoarse, compensating, safe. If both are injured, both cords sit near the midline with a slit-like glottis, and stridor with a poor airway demands immediate intervention: humidified oxygen and steroids, and if the glottis is inadequate, reintubation or tracheostomy. The emergency is pure muscle anatomy: no posterior cricoarytenoids means no inspiratory widening.
Differentiate the quieter injury. The external laryngeal nerve, running with the superior thyroid vessels on the cricothyroid's surface, is injured when the superior thyroid artery is ligated far from the gland; the patient is not stridulous but loses the ability to tense the cord, so the voice fatigues and high notes vanish — the singer who cannot sing after thyroid surgery. Laryngoscopy shows a slightly bowed cord at rest. The lesson for the operation is vascular and muscular at once: ligate the superior thyroid artery close to the gland, and identify the recurrent laryngeal nerve in the tracheo-oesophageal groove before clamping anything near Berry's ligament.
How the examiner frames the muscles
The first question is the one-abductor fact, expected with the muscle's mechanism — posterior cricoarytenoid rotating the vocal process outward — because the mechanism explains the stridor. The second is the nerve supply rule with its exception, and the candidate who reverses it surrenders the easiest mark in the viva. The third is the cord-position ladder, and here precision decides grades: paramedian for recurrent laryngeal palsy, intermediate or abducted when cricothyroid tone is lost as well, and the cadaveric median position when both nerves are completely divided. Semon's law is then offered as the theory question, and examiners respect the candidate who states it and adds that modern clinical experience finds the reality more nuanced. Closing with the thyroid-surgery landmarks — the nerve's relation to the inferior thyroid artery and Berry's ligament — turns the whole topic into the applied answer boards reward.
Frequently asked questions
Which is the only abductor of the vocal cords?
The posterior cricoarytenoid, which rotates the arytenoid cartilages so the vocal processes separate; its bilateral paralysis causes inspiratory stridor.
Which nerve supplies the cricothyroid muscle?
The external laryngeal branch of the superior laryngeal nerve — every other intrinsic laryngeal muscle is supplied by the recurrent laryngeal nerve.
What cord position results from unilateral recurrent laryngeal nerve palsy?
The paramedian position, because the intact cricothyroid (external laryngeal nerve) adducts and tenses the paralysed cord toward the midline.
What is Semon's law?
The classical observation that in progressive recurrent laryngeal lesions, abductor fibres fail before adductor fibres, so the cord adducts first — quoted today with reservations.
Why does bilateral recurrent laryngeal nerve injury cause stridor?
Both cords lie paramedian with only a slit-like glottis, and the loss of posterior cricoarytenoid action means no inspiratory widening, creating an airway emergency.