Nuclei and fibre types
CN X carries five fibre types from four medullary nuclei. Its sensory cell bodies lie in the superior (jugular) ganglion and the larger inferior (nodose) ganglion, just below the skull.
| Fibre type | Nucleus | Function |
|---|---|---|
| General visceral efferent (GVE): preganglionic parasympathetic | Dorsal motor nucleus of the vagus | Heart, airways, and gut from oesophagus to about two-thirds along the transverse colon |
| Special visceral efferent (SVE): fourth and sixth arch motor | Nucleus ambiguus | Muscles of the pharynx, larynx and soft palate, and palatoglossus |
| General visceral afferent (GVA) | Nucleus of the solitary tract | Thoracic and abdominal viscera, larynx, aortic arch baroreceptors and aortic bodies |
| Special visceral afferent (SVA): taste | Nucleus of the solitary tract | Epiglottis and the root of the tongue |
| General somatic afferent (GSA) | Spinal trigeminal nucleus | Part of the auricle, external acoustic meatus and outer eardrum; dura of the posterior cranial fossa |
Cardioinhibitory neurons, which slow the heart, lie both in the dorsal motor nucleus and close to the nucleus ambiguus.
Emergence and skull foramen
CN X emerges from the medulla as a row of rootlets in the postolivary sulcus, below those of the glossopharyngeal nerve. It leaves the skull through the jugular foramen, in its middle part, sharing a dural sheath with the accessory nerve.
Course and branches
The vagus descends through the neck and thorax to the abdomen, and the two sides take different paths around the great vessels.
In the neck
The vagus runs within the carotid sheath, behind and between the internal jugular vein and the internal, then common, carotid artery. Its neck branches are:
- Auricular branch (Arnold's nerve): to the external acoustic meatus and outer eardrum.
- Pharyngeal branch: the main motor supply to the pharyngeal plexus, supplying the constrictors, the soft palate muscles except tensor veli palatini, and palatoglossus.
- Superior laryngeal nerve: divides into an internal branch, which pierces the thyrohyoid membrane with the superior laryngeal artery to supply sensation above the vocal folds, and an external branch, which runs with the superior thyroid artery to cricothyroid.
- Cardiac branches: to the cardiac plexus.
In the thorax
- Right vagus: crosses in front of the right subclavian artery, where it gives the right recurrent laryngeal nerve, which hooks under the artery. It then runs down beside the trachea, passes behind the root of the right lung and joins the oesophageal plexus.
- Left vagus: descends between the left common carotid and left subclavian arteries and crosses the left side of the aortic arch, where it gives the left recurrent laryngeal nerve. This hooks under the arch beside the ligamentum arteriosum. The vagus then passes behind the root of the left lung to the oesophageal plexus.
Both recurrent laryngeal nerves ascend in the groove between the trachea and oesophagus. They supply all intrinsic muscles of the larynx except cricothyroid, and sensation below the vocal folds.
In the abdomen
The oesophageal plexus reforms as the anterior vagal trunk, mainly from the left vagus, and the posterior vagal trunk, mainly from the right. Both pass through the oesophageal hiatus at T10. The anterior trunk gives gastric and hepatic branches; the posterior trunk gives gastric branches and a large coeliac branch that supplies the foregut and midgut.
Clinical testing
CN X is tested through the palate, voice, cough and swallow.
- Palate: ask the patient to say 'aah' and watch the uvula. In a unilateral lesion, the intact side of the palate lifts and the uvula is pulled away from the side of the lesion.
- Gag reflex: CN X is the efferent limb; CN IX is the afferent limb.
- Voice and cough: listen for hoarseness and a weak, breathy 'bovine' cough, which suggests a vocal fold palsy.
- Swallow: observe a sip of water for coughing or nasal regurgitation.
- Laryngoscopy: confirms vocal fold movement, and is routine before and after thyroid surgery.
Lesions and palsies
The effects of a vagal lesion depend on its level: high lesions affect the palate, pharynx and larynx, while low lesions affect only the recurrent laryngeal nerve.
| Lesion | Main causes | Features |
|---|---|---|
| Recurrent laryngeal nerve, one side | Thyroidectomy; on the left, lung cancer, aortic arch aneurysm, mediastinal nodes, enlarged left atrium | Hoarse voice, weak cough; vocal fold lies near the midline |
| Recurrent laryngeal nerves, both sides | Thyroid surgery, thyroid cancer | Stridor and airway obstruction, as both folds lie near the midline |
| External laryngeal nerve | Ligation of the superior thyroid artery at thyroidectomy | Loss of high pitch and voice fatigue |
| High vagal lesion | Jugular foramen tumour, skull base fracture, lateral medullary syndrome | Dysphagia, nasal regurgitation, hoarseness, uvula deviation |
The left recurrent laryngeal nerve is injured more often by disease because its course is longer and loops into the chest. On the right, a non-recurrent laryngeal nerve occurs when the right subclavian artery arises abnormally from the arch; the nerve then runs straight to the larynx and is at risk at thyroidectomy. Stimulating Arnold's nerve during ear cleaning can trigger a cough or, rarely, fainting.