Facial Nerve
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Direct answer
Motor nerve of facial expression though it is, the facial nerve carries four functional components — motor, parasympathetic, taste and a small sensory supply, all but motor travelling in the nervus intermedius. It leaves the brainstem at the cerebellopontine angle, crosses the internal acoustic meatus, and takes the longest bony course of any nerve: the facial canal, bending at the geniculate ganglion, crossing the middle ear's medial wall, and turning down behind it to the stylomastoid foramen. Within the canal it gives off the greater petrosal nerve, the nerve to stapedius and the chorda tympani; in the parotid gland it splits into five terminal branches.
What you must remember
- Functional components: motor to muscles of expression, stapedius, stylohyoid and posterior belly of digastric; secretomotor fibres from the superior salivatory nucleus; taste from the anterior two-thirds of the tongue; a small sensory field from the ear.
- Intratemporal course: internal acoustic meatus, first genu at the geniculate ganglion, tympanic segment above the oval window, second genu at the pyramidal eminence, mastoid segment to the stylomastoid foramen.
- Three intratemporal branches: greater petrosal nerve at the geniculate ganglion (secretomotor to the lacrimal gland and nasal and palatal mucosa via the pterygopalatine ganglion); nerve to stapedius; chorda tympani just above the stylomastoid foramen (taste plus secretomotor to the submandibular ganglion).
- Extracranial: posterior auricular nerve and branches to digastric and stylohyoid at the stylomastoid foramen, then the parotid plexus.
- Terminal branches: temporal, zygomatic, buccal, marginal mandibular and cervical.
- Localisation ladder: dry eye — at or above the geniculate ganglion; hyperacusis — above the nerve to stapedius; lost taste and salivation — above the chorda tympani; all preserved — at or below the stylomastoid foramen.
- UMN versus LMN: a lower motor neurone lesion paralyses the whole hemiface including the forehead (Bell's palsy); an upper motor neurone lesion spares the forehead because the upper facial nucleus receives corticobulbar fibres from both hemispheres.
- Ramsay Hunt syndrome: zoster of the geniculate ganglion — ear vesicles, facial palsy, often cochlear and vestibular features.
Localising a facial palsy, rung by rung
A 30-year-old wakes with a drooping right face. First decide the level. Ask about a dry eye: absent tears mean the greater petrosal branch was caught at the geniculate ganglion or above. Test hearing: hyperacusis means stapedius is paralysed, placing the lesion above its nerve. Test sugar and salt on the anterior two-thirds of the tongue: lost taste means the chorda tympani is involved above its origin, in the mastoid segment. A patient who keeps lacrimation, comfortable hearing and taste but has the whole hemiface weak has a lesion at or beyond the stylomastoid foramen — the common position in Bell's palsy. This ladder converts a vague "facial palsy" into an anatomical sentence.
Second, decide the type. Forehead and eye involved on one side means lower motor neurone — the whole nerve on that side is dead. Wrinkling preserved with the lower face paralysed means upper motor neurone — a cortical or capsular lesion sparing the bilaterally supplied upper nucleus. In the LMN case the eye matters most: lagophthalmos demands lubricating drops and nighttime taping, because an exposed cornea ulcerates.
Where students slip
Stapedius is credited to the trigeminal nerve — the reverse of giving tensor tympani to the facial. Stapedius is facial; tensor tympani is the mandibular division of V. The lacrimal gland is assigned to the chorda tympani; the chorda tympani carries taste and submandibular-sublingual secretomotor, while the greater petrosal serves the lacrimal gland. The nerve is said to "supply the parotid": it traverses it but supplies none of it. Finally, the tympanic and mastoid segments are confused; the nerve runs horizontally above the oval window, then turns 90 degrees downwards — which is why the chorda tympani, leaving the mastoid segment, re-enters the middle ear across the tympanic membrane.
Frequently asked questions
Which three branches arise within the facial canal?
The greater petrosal nerve at the geniculate ganglion (lacrimal and nasal secretomotor via the pterygopalatine ganglion), the nerve to stapedius, and the chorda tympani just above the stylomastoid foramen (taste from the anterior two-thirds plus secretomotor to the submandibular ganglion).
How do you localise the level of a facial nerve lesion clinically?
By the three intratemporal branches: dry eye places it at the geniculate ganglion or above, hyperacusis above the nerve to stapedius, lost taste above the chorda tympani; all three intact with pure weakness means at or below the stylomastoid foramen.
Why is the forehead spared in upper motor neurone facial palsy?
The upper facial nucleus receives corticobulbar fibres from both hemispheres, so a unilateral cortical lesion leaves frontalis working; a lower motor neurone lesion paralyses the entire hemiface.
What is Ramsay Hunt syndrome?
Herpes zoster of the geniculate ganglion — vesicles in the external acoustic meatus and pinna, ipsilateral lower motor neurone facial palsy, and often tinnitus, vertigo and hearing loss. The facial nerve traverses but does not supply the parotid; that secretion comes from the glossopharyngeal nerve via the otic ganglion.