Function: mixed nerve responsible for motor innervation of muscles of facial expression + taste of anterior ⅔ of tongue + parasympathetic innervation of lacrimal and submandibular glands

1. Lacrimation (via greater superficial petrosal nerve)
2. Stapedius reflex: sound damping
3. Taste of anterior ⅔ of tongue (via chorda tympani nerve to lingual nerve)
4. Facial expression (platysma + orbicularis oculi)
5. Secretion of lacrimal + submandibular + sublingual glands (via nervus intermedius)

Nuclei: one motor nucleus + two sensory nuclei located within ventrolateral pons

1. Motor nucleus: ventrolateral deep in reticular formation of the caudal part of the pons
   Innervation to: stapedius m., stylohyoid m., posterior belly of digastric m., occipitalis m., buccinator, muscles of facial expression, platysma
2. Nucleus solitarius (sensory nucleus)
   • nervus intermedius: sensation from anterior ⅔ of tongue, skin on + adjacent to ear
3. Superior salivatory nucleus (parasympathetic secretomotor innervation)
   • greater petrosal n.: secretion of lacrimal glands, nasal cavity, paranasal sinuses
   • chorda tympani: submandibular + sublingual glands

Exit from brainstem: 2 separate nerve roots traversing cerebellopontine angle cistern

1. motor root anteriorly
2. sensory root posteriorly

Segments: circuitous course

1. intracranial segment from brainstem to porus acusticus internus:
   • pontine segment:
     • motor root fibers of facial n. run dorsomedially towards 4^th ventricle
     • curve anterolaterally to hook around upper pole of abducens nucleus (= geniculum)
     • form facial colliculus (= elevation in floor of 4^th ventricle)
     • nerve descends anterolaterally through reticular formation + continues lateral from corticospinal tract
   • cisternal segment:
     • facial n. exits from lateral aspect of brainstem at pontomedullary junction
     • courses anterolaterally in cerebellopontine angle cistern to internal auditory canal (IAC) above crista falciformis
2. intracanalicular (IAC) segment
3. labyrinthine segment emerges from anterosuperior aspect of IAC
   • 3–4 mm short segment of facial n. traveling anterolaterally within its own bony canal (= fallopian canal) and curving anteromedially over top of cochlea
   • terminates in anteromedial genu (geniculate ganglion) where greater superficial petrosal n. branches pass anteromedially to carry parasympathetic fibers to lacrimal gland
4. tympanic segment
   = 12 mm long segment from geniculate ganglion to posterior genu underneath lateral semicircular canal
   • horizontal segment: facial n. at first (anterior) genu makes a 130° turn posteriorly + horizontally along medial wall of mesotympanum lateral to vestibule between lateral semicircular canal (above) and oval window (below) to reach sinus tympani
     • “snake eyes” on COR CT at level of cochlea corresponding to proximal portion of tympanic + distal portion of labyrinthine segments
       
     • inferior to lateral semicircular canal + superolateral to oval window on COR CT at level of oval window
     • anterior genu superomedial to cochlear promontory
   • pyramidal segment: at sinus tympani facial n. turns gently posteroinferiorly to form second / posterior genu in pyramidal eminence; gives off the nerve for the stapedius muscle
5. mastoid segment (longest segment with 15–20 mm)
   facial n. assumes a more vertical position + descends just behind the posterior wall of the tympanic cavity from posterior genu through anterior mastoid (= medial wall of aditus ad antrum) + gives off chorda tympani just prior to exit from skull base through stylomastoid foramen
6. parotid / extracranial segment
   facial n. travels forward between superficial + deep lobes of parotid gland lateral to styloid process + external carotid a. + retromandibular v.

Branches:

1. Greater superficial petrosal nerve (parasympathetic + motor fibers) arises from geniculate ganglion, runs anteromedially, and exits at the facial hiatus on the anterior surface of the temporal bone + passes under Meckel cave near foramen lacerum
   • forms vidian nerve after receiving sympathetic fibers from deep petrosal nerve, which surrounds the internal carotid artery
2. Stapedial nerve (motor fibers) arises from proximal descending facial n.
3. Chorda tympani (sensory + parasympathetic fibers) leaves facial n. about 5–6 mm above stylomastoid foramen from the lateral aspect of the mastoid segment of the facial nerve
   • ascends in subtle curvature superoanteriorly in a bony canal (= canaliculus chorda tympani)
   • perforates posterior wall of tympanic cavity
   • proceeds anteriorly within tympanic cavity and crosses medial to handle of malleolus underneath mucosa of tympanic cavity
   • exits temporal bone through a minute canal (= anterior canaliculus) near petrotympanic fissure
   • joins lingual nerve (= branch of V₂) containing sensory taste fibers from anterior ⅔ of tongue + secretomotor fibers for submandibular + sublingual glands