Nuclei and fibre types
CN VII carries four fibre types from nuclei in the pons and upper medulla. It leaves the brainstem as two roots: the large motor root and the smaller nervus intermedius, which carries the sensory and parasympathetic fibres.
| Fibre type | Nucleus | Function |
|---|---|---|
| Special visceral efferent (SVE): second pharyngeal arch motor | Facial motor nucleus | Facial expression, stapedius, stylohyoid, posterior belly of digastric |
| General visceral efferent (GVE): preganglionic parasympathetic | Superior salivatory nucleus | Lacrimal, nasal and palatine glands (via pterygopalatine ganglion); submandibular and sublingual glands (via submandibular ganglion) |
| Special visceral afferent (SVA): taste | Nucleus of the solitary tract | Anterior two-thirds of the tongue and the palate |
| General somatic afferent (GSA) | Spinal trigeminal nucleus | Skin of part of the concha and external acoustic meatus |
The sensory cell bodies lie in the geniculate ganglion in the temporal bone. Motor fibres loop around the abducens nucleus (the internal genu) before leaving the pons.
Emergence and skull foramen
CN VII emerges at the cerebellopontine angle, at the lower border of the pons, lateral to CN VI and medial to CN VIII. With CN VIII it enters the internal acoustic meatus in the petrous temporal bone, then travels in the facial canal and leaves the skull through the stylomastoid foramen, between the styloid and mastoid processes.
Course and branches
The facial nerve has an intratemporal course that gives off its parasympathetic, taste and stapedial branches, and an extracranial course that gives the motor branches to the face.
In the temporal bone
- Labyrinthine segment: from the internal acoustic meatus to the geniculate ganglion. This is the narrowest part of the facial canal.
- Geniculum: the nerve turns sharply backward at the geniculate ganglion, where the greater petrosal nerve arises. It carries parasympathetic fibres forward to foramen lacerum, joins the deep petrosal nerve to form the nerve of the pterygoid canal, and relays in the pterygopalatine ganglion to the lacrimal gland and the glands of the nose and palate.
- Tympanic segment: runs backward along the medial wall of the middle ear, above the oval window.
- Mastoid segment: descends behind the middle ear to the stylomastoid foramen, giving the nerve to stapedius and the chorda tympani. The chorda crosses the inner surface of the tympanic membrane, leaves through the petrotympanic fissure and joins the lingual nerve, carrying taste from the anterior two-thirds of the tongue and parasympathetic fibres to the submandibular ganglion.
Outside the skull
Just below the stylomastoid foramen it gives the posterior auricular nerve (auricular muscles and occipital belly of occipitofrontalis) and branches to the posterior belly of digastric and stylohyoid. It then enters the parotid gland, crosses superficial to the retromandibular vein and external carotid artery, and divides into five terminal branches.
| Branch | Main muscles |
|---|---|
| Temporal | Frontal belly of occipitofrontalis, upper orbicularis oculi |
| Zygomatic | Orbicularis oculi |
| Buccal | Buccinator, muscles of the upper lip and nose |
| Marginal mandibular | Muscles of the lower lip and chin |
| Cervical | Platysma |
CN VII passes through the parotid but does not supply it. Parotid secretomotor fibres come from CN IX via the otic ganglion.
Clinical testing
CN VII is tested mainly through the muscles of facial expression, comparing upper and lower face.
- Upper face: raise the eyebrows, then screw the eyes shut against the examiner's attempt to open them.
- Lower face: puff out the cheeks, show the teeth, whistle.
- Corneal reflex: CN VII is the efferent limb, closing the eye.
- Other functions when needed: taste on the anterior two-thirds of the tongue, tear production (Schirmer test) and hyperacusis (sensitivity to loud sound), which indicates loss of stapedius.
The forehead distinguishes the two types of palsy. The part of the facial nucleus supplying the upper face receives input from both cerebral hemispheres, while the part for the lower face receives mostly contralateral input. An upper motor neurone (UMN) lesion, such as a stroke, therefore spares the forehead; a lower motor neurone (LMN) lesion weakens the whole of one side of the face.
Lesions and palsies
A lower motor neurone facial palsy causes weakness of the whole of one side of the face: the brow drops, the eye does not close, the nasolabial fold flattens and the mouth droops. On attempted eye closure the eyeball rolls upward (Bell's phenomenon).
- Bell's palsy: an idiopathic LMN palsy, probably from swelling of the nerve in the narrow labyrinthine segment of the canal. Most patients recover; early oral steroids help.
- Ramsay Hunt syndrome: herpes zoster reactivation in the geniculate ganglion, with vesicles in the ear and sometimes vertigo and hearing loss.
- Parotid disease and surgery: malignant parotid tumours can invade the nerve; parotidectomy puts the branches at risk.
- Temporal bone fracture: transverse fractures injure the nerve more often than longitudinal ones.
- Cerebellopontine angle tumour: vestibular schwannoma stretches CN VII, though weakness is usually late.
- Marginal mandibular branch injury: this branch can loop below the lower border of the mandible, deep to platysma, and is at risk in submandibular gland excision.
The level of an intratemporal lesion can be deduced from the branches lost. A lesion proximal to the geniculate ganglion reduces tear production; one above the nerve to stapedius adds hyperacusis; one above the chorda tympani adds taste loss; a lesion at the stylomastoid foramen affects facial muscles only.