Facial Nerve Branches
On this page
Direct answer
Roughly 3 cm of canal carved in the temporal bone carries the facial nerve from the internal acoustic meatus to the stylomastoid foramen, and its branches leave the canal in a fixed order that lets you localise any lesion. In the labyrinthine segment the nerve presents the geniculate ganglion and gives off the greater petrosal nerve (lacrimal and nasal glands via the nerve of the pterygoid canal); in the tympanic-mastoid part it gives the nerve to stapedius and then the chorda tympani (taste from the anterior two-thirds of the tongue and parasympathetic supply to the submandibular and sublingual glands); at the stylomastoid foramen it exits to supply the posterior auricular nerve, digastric and stylohyoid branches, and finally fans out as the temporal, zygomatic, buccal, marginal mandibular and cervical branches on the face.
What you must remember
- Two roots: a large motor root and the nervus intermedius (taste, parasympathetic and sensory fibres); both enter the internal acoustic meatus together with the eighth nerve.
- Geniculate ganglion: the bend of the nerve at the first turn; here the greater petrosal nerve leaves, carrying preganglionic parasympathetic fibres to the pterygopalatine ganglion through the nerve of the pterygoid canal (Vidian), after being joined by the deep petrosal (sympathetic) nerve.
- Nerve to stapedius: given off in the descending (mastoid) segment; its paralysis produces hyperacusis — sounds abnormally loud because the stapedius cannot damp the stapes.
- Chorda tympani: crosses the medial surface of the tympanic membrane between the malleus and incus, exits the petrotympanic fissure, joins the lingual nerve; carries taste (anterior two-thirds) and preganglionic fibres to the submandibular ganglion.
- At the stylomastoid foramen: posterior auricular nerve, nerve to posterior belly of digastric and nerve to stylohyoid — all given off just below the foramen before the nerve enters the parotid gland.
- Terminal branches within the parotid: pes anserinus — temporal, zygomatic, buccal, marginal mandibular and cervical; supply all facial expression muscles; the gland's two lobes are connected around the nerve (retromandibular vein and external carotid deeper).
- Localisation by features: lacrimation lost + hyperacusis + taste lost = lesion at or above the geniculate ganglion; taste and hyperacusis intact = lesion below the chorda tympani or at the stylomastoid foramen; isolated facial weakness with no secretomotor or taste loss = distal, within the parotid.
- Aberrant regeneration: parasympathetic fibres regrowing to the submandibular or lacrimal glands produce crocodile tears (tears while eating) or gustatory sweating; the Bell's palsy that regrows wrongly explains them.
Localising a facial palsy by its lost branches
A young woman wakes with a right facial droop; three bedside tests place the lesion within millimetres. Check taste on the anterior right tongue with sugar: lost taste plus hyperacusis plus a dry right eye means the lesion sits at or above the geniculate ganglion, before the greater petrosal and chorda tympani left the trunk. Dry eye with preserved taste places it between the geniculate ganglion and the nerve to stapedius. Preserved taste, normal loudness tolerance and normal tears — the commonest picture of Bell's palsy — place the lesion in the mastoid segment below the chorda tympani or at the stylomastoid foramen, where oedema of the nerve in the tight bony canal does its damage; the absence of the correlation with severity is why electrical testing is reserved for uncertain cases. Ask next whether the forehead wrinkles: a lower motor neuron lesion weakens the whole ipsilateral face including the frontalis (both cerebral hemispheres supply its upper facial nucleus), whereas a cortical stroke spares the forehead — the single most exam-tested distinction in neuroanatomy. Finally, a parotid tumour with a slowly progressive facial palsy and a visible mass places the lesion distal to the stylomastoid foramen, inside the gland, where the nerve divides around the retromandibular vein.
Where students slip
Students assign taste to the facial nerve at the tongue without naming its vehicle — the chorda tympani joining the lingual nerve — and lose the mark to "lingual nerve carries taste", which is wrong; the lingual nerve is only the taxi. The second slip is the greater petrosal nerve's ganglion: the pterygopalatine, reached through the Vidian canal; candidates who say submandibular have confused it with the chorda tympani. Third, remember Bell's palsy is a diagnosis of exclusion — its cause idiopathic (herpes simplex reactivation commonly implicated), while Ramsay Hunt syndrome, herpetic vesicles in the ear with facial palsy, is the vesicular rival to know by name.
Frequently asked questions
Which branches arise within the facial canal?
The greater petrosal nerve at the geniculate ganglion, the nerve to stapedius, and the chorda tympani. Their involvement or sparing localises the level of a facial nerve lesion.
What does the chorda tympani carry and where does it join the lingual nerve?
Taste from the anterior two-thirds of the tongue and preganglionic parasympathetic fibres for the submandibular and sublingual glands. It joins the lingual nerve in the infratemporal fossa, about two centimetres below the skull base.
Why does a facial nerve lesion cause hyperacusis?
Loss of the nerve to stapedius paralyses the stapedius muscle, which normally dampens stapes movement. Undamped ossicular transmission makes ordinary sounds uncomfortably loud, particularly low-frequency noise.
How is the course of the facial nerve related to the parotid gland?
After leaving the stylomastoid foramen it enters the parotid gland and divides into its five terminal branches, which pass between the superficial and deep lobes. Surgery of the parotid is facial nerve surgery, and the nerve lies superficial to the retromandibular vein and external carotid artery.
Why is the forehead spared in an upper motor neuron facial lesion?
The upper part of the facial motor nucleus receives corticobulbar input from both hemispheres, so a unilateral cortical lesion leaves frontalis functioning. In a lower motor neuron lesion the entire ipsilateral half of the face, forehead included, is paralysed.