Nuclei and fibre types
CN IX carries five fibre types from four nuclei in the medulla. Its sensory cell bodies lie in two small ganglia, the superior and inferior ganglia, at the jugular foramen.
| Fibre type | Nucleus | Function |
|---|---|---|
| Special visceral efferent (SVE): third pharyngeal arch motor | Nucleus ambiguus | Stylopharyngeus |
| General visceral efferent (GVE): preganglionic parasympathetic | Inferior salivatory nucleus | Parotid gland, via the otic ganglion |
| Special visceral afferent (SVA): taste | Nucleus of the solitary tract | Posterior third of the tongue, including the vallate papillae |
| General visceral afferent (GVA) | Nucleus of the solitary tract | Carotid sinus (baroreceptors), carotid body (chemoreceptors), pharynx and posterior tongue |
| General somatic afferent (GSA) | Spinal trigeminal nucleus | Middle ear and a small area around the external ear |
Emergence and skull foramen
CN IX emerges from the medulla as several rootlets in the postolivary sulcus, the groove between the olive and the inferior cerebellar peduncle, just above the rootlets of the vagus. It leaves the skull through the jugular foramen, in its anterior part, in a dural sheath separate from CN X and XI, with the inferior petrosal sinus nearby.
Course and branches
Below the skull, CN IX curves forward around stylopharyngeus to reach the pharynx and the back of the tongue.
- It descends between the internal jugular vein and internal carotid artery.
- It passes forward between the internal and external carotid arteries.
- It winds around the posterior border and then the lateral side of stylopharyngeus.
- It enters the pharynx through the gap between the superior and middle constrictors, passes deep to hyoglossus, and ends in the tonsil and posterior third of the tongue.
| Branch | Supply |
|---|---|
| Tympanic nerve (Jacobson's nerve) | Enters the middle ear, forms the tympanic plexus on the promontory for sensation to the middle ear and auditory tube, and continues as the lesser petrosal nerve |
| Carotid sinus nerve | Baroreceptors of the carotid sinus and chemoreceptors of the carotid body |
| Nerve to stylopharyngeus | Its only muscle |
| Pharyngeal branches | Join the pharyngeal plexus; sensory to the oropharynx |
| Tonsillar branches | Palatine tonsil and soft palate nearby |
| Lingual branches | Taste and general sensation to the posterior third of the tongue |
Parasympathetic route to the parotid
Preganglionic fibres run in the tympanic nerve, through the tympanic plexus, and leave the middle ear as the lesser petrosal nerve. This passes through the foramen ovale (or a small nearby canal) to the otic ganglion below the skull base. Postganglionic fibres then travel with the auriculotemporal nerve (CN V3) to the parotid gland.
Clinical testing
CN IX is tested mainly as the sensory limb of the gag reflex.
- Gag reflex: touching the posterior pharyngeal wall or tonsillar area with a tongue depressor causes the pharynx to contract. CN IX is the afferent limb and CN X the efferent limb. Test each side and ask whether the touch feels the same.
- Taste: taste on the posterior third of the tongue is rarely tested at the bedside.
- Palate: elevation of the soft palate on saying 'aah' tests CN X, not CN IX, but is examined at the same time.
An isolated CN IX lesion is rare and produces subtle signs, so the nerve is usually assessed together with CN X.
Lesions and palsies
A CN IX lesion causes loss of sensation and gag on one side of the pharynx and loss of taste on the posterior third of the tongue; weakness of stylopharyngeus is not detectable clinically.
- Jugular foramen syndrome (Vernet syndrome): CN IX, X and XI are damaged together at the foramen, by a glomus jugulare tumour, metastasis, schwannoma or skull base fracture. Adding CN XII gives Collet–Sicard syndrome; adding CN XII and the cervical sympathetic chain gives Villaret syndrome.
- Glossopharyngeal neuralgia: brief severe pain in the throat, tonsil and ear, triggered by swallowing or talking. Rarely, it causes bradycardia and fainting through the carotid sinus branch.
- Tonsillectomy: the nerve lies close to the tonsillar bed; injury can cause taste disturbance.
- Neck surgery: carotid endarterectomy and parapharyngeal surgery can injure the nerve high in the neck.
- Lateral medullary syndrome: infarction damages the nucleus ambiguus and solitary nucleus, affecting CN IX and X together.
Ear pain from throat disease is a clinical consequence of the tympanic branch: pain from the tonsil or a throat cancer is felt in the ear because both regions share CN IX.