Laryngeal Innervation in Detail
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Direct answer
Every intrinsic laryngeal muscle except cricothyroid obeys the recurrent laryngeal nerve, which also carries sensation from the mucosa below the vocal folds; cricothyroid belongs to the external laryngeal nerve, while the internal laryngeal nerve supplies sensation above the folds and pierces the thyrohyoid membrane alongside the superior laryngeal artery. The recurrent nerves differ in course — the right loops under the right subclavian artery, the left under the aortic arch at the ligamentum arteriosum, both ascending in the tracheo-oesophageal groove — and their injury produces recognisable cord positions: a complete recurrent nerve lesion parks the cord paramedian, a bilateral one threatens the airway with stridor, and an external laryngeal lesion steals the high voice. Semon's law — abductor fibres fail before adductors — explains why partial lesions first silence the posterior cricoarytenoid.
What you must remember
- Motor map: recurrent laryngeal nerve to all intrinsic muscles except cricothyroid; external laryngeal nerve (of the superior laryngeal) to cricothyroid and part of inferior pharyngeal constrictor.
- Sensory map: internal laryngeal nerve above the vocal folds (including vallecula and epiglottis — its injury risks silent aspiration); recurrent laryngeal nerve below the folds, including the subglottis.
- Course asymmetry: right recurrent nerve loops around the right subclavian artery, left around the aortic arch via the ligamentum arteriosum — hence the left is longer and more exposed to mediastinal and thoracic disease.
- The only abductor: posterior cricoarytenoid, bilaterally supplied by the recurrent nerves, is the sole muscle that opens the glottis — its failure is the airway problem.
- Cord positions: median (adducted, as in phonation), paramedian (near adducted — complete recurrent nerve section), intermediate (cadaveric position — all muscles relaxed, as in combined superior plus recurrent loss), and fully abducted.
- Semon's law: in a developing lesion, abductors are paralysed before adductors, so an irritated or partially compressed nerve first loses glottis opening — the clinical basis of stridor preceding hoarseness in some postoperative patients.
- Non-recurrent laryngeal nerve: a right nerve that runs straight from the vagus to the larynx, associated with an aberrant right subclavian artery — a surgical surprise in well under one per cent.
Reading cord palsies off the nerve
The patterns follow the muscle left unopposed. Unilateral complete recurrent laryngeal palsy paralyses all muscles except cricothyroid on that side: the cord lies paramedian (the intact cricothyroid plus unopposed midline pull keep it near the midline), the voice is breathy and hoarse, and with time the opposite cord compensates by crossing the midline — surgery to medialise the cord is reserved for failures of compensation. Bilateral recurrent palsy leaves both cords paramedian with a slit-like glottis: the voice may be deceptively strong, but inspiration becomes stridulous, and exertion desaturates — this is the post-thyroidectomy airway emergency, sometimes needing reintubation or tracheostomy first.
Isolated external laryngeal nerve injury reads differently: cricothyroid cannot lengthen the fold, so the voice loses its upper register and power — the "weak, monotonous voice" of a high superior-pole ligature. Bilateral internal laryngeal anaesthesia is the quiet danger: a larynx that cannot feel saliva aspirates silently, which is why superior laryngeal nerve blocks and high vagal injuries carry aspiration risk more than hoarseness. The examiner's sequence — muscle, nerve, cord position — is answered from this paragraph.
Where candidates slip
The recurring error is granting the recurrent laryngeal nerve "all the muscles": cricothyroid is the exception, and forgetting it turns the external laryngeal nerve question into a zero. The second is naming an adductor when asked for the abductor — only posterior cricoarytenoid opens the glottis, a one-muscle answer worth marks every year. The third is the cadaveric position: it appears when both recurrent and superior laryngeal nerves are lost (complete vagal palsy above the nucleus ambiguus contribution), not in an isolated recurrent lesion, which stays paramedian. And when asked why the left nerve is vulnerable in thoracic disease, the answer is the aortic arch loop — the anatomy of the ligamentum arteriosum is the reason a mediastinal tumour presents with a hoarse voice.
Frequently asked questions
Which intrinsic laryngeal muscles are supplied by the external laryngeal nerve?
Only cricothyroid (with a contribution to the inferior pharyngeal constrictor); every other intrinsic muscle belongs to the recurrent laryngeal nerve.
Which muscle abducts the vocal cord and what happens if it is paralysed?
Posterior cricoarytenoid, the sole abductor; bilateral paralysis leaves the cords paramedian, producing inspiratory stridor with a threatened airway.
What is Semon's law?
In a slowly progressing recurrent laryngeal lesion, abductor fibres fail before adductor fibres, so glottis opening is lost first — stridor may precede obvious hoarseness.
In which position does the cord lie after complete recurrent laryngeal nerve section?
Paramedian, because unopposed adductor tone and intact cricothyroid hold it near the midline; the intermediate or cadaveric position needs additional superior laryngeal loss.
Why does a left mediastinal tumour cause hoarseness?
The left recurrent laryngeal nerve loops under the aortic arch at the ligamentum arteriosum, so it traverses the thorax and is susceptible to left-sided mediastinal and apical lung disease.