The Facial nerve, the 7th cranial nerve, has sensory, parasympathetic and motor fibres.
Its course can be divided into (3) or Six segments (depending on how specific you wish your divisions to be).
The external auditory canal and internal auditory canal lie in the same coronal plane.
1. Intracranial course - 30mm long
(a) cisternal segments (b)intracanalicular
2. Intratemporal - 30mm long
(c)labryinthine, (d) tympanic and (e) mastoid segments
3. (f) Extracranial
Intracranial Supranuclear Pathway
The precentral gyrus inputs for the facial nerve are from the lateral aspect.
Remember that the subnuclei for the forehead and eyebrow recieve bilateral corticobulbar tracts!
So an upper motor lesion will SPARE the forehead muscles, whilst a lower motor lesion will also affect them.
The Facial nerve actually has 4 nuclei, each of which is responsible for 1 of the nerves 4 main functions (2 of nuclei have efferent fibres and 2 have afferent fibres);
The facial nerve nuclus
This nucleus innervates all the muscles of the 2nd branchial arch - of which the majority are facial muscles - hence the nerves name, but there are 5 more non facial expression muscles that it also supplies ( posterior belly of digastric, stapedius and stylohyoid, transverse and oblique auricular muscles). Note that there are some muscles in the face that it doesn't supply, the muscles of mastication, and the levator palpebrae superioris (this is from the occulomotor nerve, so that when you look upwards your eyelids also retract too).
This nucleus is therefore said to have
efferent branchiomotor fibres (motor fibres to the muscles of a branchial arch). The cell bodies of those fibres are in the facial nerve nucleus.
In the embryo, the 6th cranial nerve nuclei ascends from alongside the facial nerve nucleus, and this drags the facial nerve nuclei fibres upwards, which is why in adults the fibres have an intrapontine course that runs backwards, dorsally, and can't exit the pons until they have looped around the 6th cranial nerve which sits dorsally almost abutting the 4th ventricle. The bend around the 6th cranial nerve is known as the internal genu of the facial nerve. As they loop around the fibres actually make an indent in the 4th ventricle known as the facial colliculus
Superior Salivatory Nuclus
"Someone knows your sad by frowning and crying, someone know's you are angry by scowling and spitting".
The superior salivatory nucleus has efferent parasymphathetic secrretomotor fibres to the pterygopalatine and submaxillary glanglions, and the post-ganglionic fibres for this go on to create tears and some saliva (parotid also makes saliva).
This nucleus is located in the pontine tegmentum, dorsolaterally to the facial nerve nucleus, the nerve cell bodies are located in this nucleus.
Nucleus of tractus solitarius
Taste fibres for the anterior 2 thirds of the Tongue.**
Spinal nucleus of the trigeminal - special
afferent fibres are recieved in this nucleus. The actual nerve cell bodies however lie in the geniculate ganglion, and their central projections extend back into this nucleus.
This nucleus forms a tract downwards, to the level of the medulla, and specifically to the level where the hypoglossal and pharyngeal nerves exit the medulla, because it receives their taste inputs as well (9 for posterior 1/3 and vagus for epiglottius)
Spinal part of the Trigeminal Nucleus
General Somatic Afferent Fibres.
This Nucleus is responsible for touch sensation for parts of the ear; for the concha of the auricle, for the posterior wall of the external auditory canal. The cell bodies of these afferent neurons (like the special afferent fibres) are in the geniculate ganglion.
This nucleus also extends downwards - to the cervical spinal cord (hence the name) - as it recieves multiple other inputs.
An easy way to remember the 4 fibre nuclei and the nerves functions is ramsay hunt syndrome - herpes zoster reactivation of the geniculate ganglion which is a ganglion early in the course of the facial nerve and so typically causes impairment to all 4 functions of the facial nerve ;
- facial droop (facial)
- lost of taste (nucleus of tractus solitarius),
- dry eyes and mouth (superior salivatory nucleus) and
- pain and vesicles to a small section of the ear and posterior surface of the ear canal which demonstrate the nerves somatic afferent fibres (spinal part of the trigeminal nucleus)
Otherwise you can remember that the nuclei are 4 S's:
- Seventh Motor Nuclei (facial motor nuclei)
- Superior Salivatory Nuclei
- Solitarius (tractus solitarius, nucleus of the solitary tract)
- Spinal Trigeminal Nucleus.
1.Labryinthine Branch of AICA until the fundus of the IAC.
2.Superficial Petrosal artery of the middle meningeal artery Distal to the IAC.
3.Stylomastoid Artery distal part of facial canal.
Intracranial cisternal segment
The facial nerve comes from the brainstem at the level of the pontomedullary junction, about 1.5mm anterior to the vestibulocochlear nerve which comes off at the same level (In the picture below, you can see the facial nerve in red, and the vestibulocochlear in yellow).
The facial nerve actually comes off as 2 seperate nerves.
The fibres of the facial nerve nuclei come off seperately to the other 3. Some textbooks describe the fibres from the facial nucleus as the "motor root" and the "sensory root" as the one containing the special sensory afferent (taste) general sensory afferent and autonomic fibres. With the facial nerve anterior, and the vestibulocochlear, the sensory root, travelling in between them is known as the nervus intermedius (or the nerve of wrisberg).
Coming off between the pons and cerebellum (when viewed through an axial slice across the brain), the nerves are said to course through the cerebellopontine angle and cerebellopontine cistern.
They travel there for around 15mm.
Through the cerebellopontine cistern the nerves are devoid of an epineurium, but do have a covering of dura and arachnoid mata.
Author: Patrick Lynch
Original file: http://commons.wikimedia.org/wiki/File:Brain_stem_sagittal_section.svg
License: Creative Commons Attribution 2.5 License 2006
Another important structure that travells through the cerebllopontine cistern (seen below) is the AICA, anterier inferior cerebellar artery. This is clinically significant in that it can in rare cases loop around these nerves here, causing a vascular loop, which leads to hemifacial spasm.
When approached from the middle cranial fossa - the internal auditory canal can be found by bisecting the 120° angle between the Greater superficial petrosal nerve and the arcuate eminence, and then by drilling in an inferior direction near the petrous ridge until the superior semicircular canal (SSCC) has been identified.
The superiomedial anterior aspect of the temporal bone can be considered a safe area - kawase's area - to drill -
it generally contains pneumatized spaces.
Intracranial canalicular * 1cm
After crossing the Cerebellopontine cistern, the 2 divisions of the facial nerve and vestibulocochlear nerve, as well as the labyrinthine artery - a branch from the AICA which is their blood supply - enter the internal acoustic meatus.
This canal is located within the posterior surface of the temporal bone. It is very short, only around 8mm to 1 cm. Its beginning is called the porous, and its end the fundus. The dural and arachnoid covering of the nerves always tapers out at or before the fundus of the Internal acoustic canal, which delineates the intracranial from the intratemporal segments.The 2 divisions of the facial nerve join before the fundus. The facial nerve lies anterior superior to the vestibulocochlear nerve in the internal auditory canal.
Relationships of the nerves in the fundus of the internal auditory Canal:
The fundus of the internal auditory canal is divided into 4 segments by 2 ridges of bone. The ridge running from top to bottom, spliting the centre is called Bill's Bar. The ridge running horizontally across the IAC is called the falciform crescent or horizontal crest.
At the brainstem, the cochlear nerve is most inferior. Facial nerve anterior. Vestibular nerve posterior. Then all the nerves rotate 90 degrees within the canal. At the brainstem, the vestibular nerve appears grey, the cochlear white.
As the cochlear is anterior, the cochlear nerve occupies an anterior quadrant at the fundus, whilst the two divisions of the vestibular nerve (superior and inferior) move posteriorly to innervate the more posteriorly positioned semicircular canals.
Remember 7up. Coke Down: Facial nerve occupies the anteriorsuperior position at the fundus. Cochlear Nerve occupies the anteriorinferior position.
Relations: the fundus of the internal acoutic meatus abuts the mediail wall of the vestibule and the base of the modiolus
Intratemporal Labryinthine Section
The facial nerve goes through the meatal forearm of the fundus of iac to enter the labryinthine section of the facial or fallopian canal - the meatal forearm is the narrowest portion of the entire canal just .68mm.
The labryinthine section is the shortest and thinnest.
The labryinthine section runs anteriolaterally - almost at right ankles to the petrous pyramid and paralleling the axis of the arcuate eminence of the superior semicircular canal - for 3-5mm, over the junction of the vestibule and the cochlear it lies in a sulcus between the cochlear and vestibular labyrinth, the intervestibulocochlear groove from the fundus of the Internal auditory canal to reach the medial wall of the epitympanic recess of the middle ear.
There it turns sharply posteriorly, anatomically termed the external genu of the facial nerve. Here the nerve also bulges, due to a ganlgion. This ganglion of the genu is the geniculate ganlgion.
Relations of the labryinthine section
Superior Semicircular Canal Posteriorly.
Vestibule Inferiorly (junction of vestibule and cochlear)
Superiorly a thin plate of bone seperating the nerve from the middle cranial fossa and dura.
Intratemporal Labryinthine Section: Geniculate Ganglion.
The Geniculate ganlgion is a bulge at the external genu of the facial nerve. It contains the bipolar ganglion cell bodies of all the sensory fibres - the general sensory afferent that gives us temperature/pain/touch from the external auditory canal and the special visceral afferent cell bodies that give us taste from the anterior 2/3 of the tongue.
The fossa is quadrangular in shape and measures about 2-3mm.
It has 2 branches.
1. Superficial Greater Petrosal Nerve
2. Sensory branch to the tympanic plexus.
The geniculate ganglion is not necessarily covered by bone (18%) of time in direct contact with the dura - putting it at risk during middle cranial fossa surgery.
Intratemporal Labryinthine Section: Geniculate Ganglion: Superficial Greater Petrosal Nerve
The SGPN arrises from the anterior aspect of the geniculate ganglion. The preganglionic parasympathetic fibres from the superior salivatory nucleus that travelled with the nervus intermedius go straight through the geniculate ganlgion without stopping and come out anteriorly as the superficial greater petrosal nerve.
The Superficial Greater Petrosal Nerve goes through the facial hiatus to course along the superior surface of the petrous temporal bone, (its extradural), It courses anteriormedially running parellel with the apex of the petrous bone. it courses underneath the trigeminal nerve and under the internal carotid artery, running above the fibrous floor of the foreamen lacerum to reach the forearm lacerum where its jointed by the deep petrosal nerve.
They then course as the vidian nerve or as the nerve of the pterygoid canal to synapse in the pterygopalatine plexus.
1. Preganglionic fibres to the sphenopalatine plexus, postganglionic innervate the lacrimal/palate and nasal glands.
2. Carries taste fibres from the soft palate.
The petrosal branch of the middle mengingeal artery runs with it (enters through the facial foramen to supply the facial nerve distal to the IAC fundus.
Intratemporal Tympanic Section
This is the most common site of nerve dehescience.
The tympanic segment runs from the geniculate ganglion to the posterior or 2nd genu.
The nerve bulges on the medial wall of the middle ear over the promontory/cog superiorly and the cochleaform process/ and then runs just posteriomedially to the incus.
It runs through the medial wall of the middle ear just above the oval window. Below the bulge of the lateral semi-circular canal.
It passes medial to the malleous and incus.
Underneath the nerve lie the bulge of the promontory and oval window. (its the cephalad margin of the oval window niche).
At the level of the oval window, it starts to slowly curve inferiorly.
At the pyramidal turn, the nerve protrudes
posteriolaterally to the lateral semicricular canal. At this level the vestible lies medial to the 2nd genu.
There are several important surgical landmarks for identifying the facial nerve in the middle ear:
1. Cochleaform Process and cog
2. Oval Window
4. Lateral SemiCircular canal indentation into middle ear
Cochleariform process: small bony protuberance (from which tensor tympani muscle turns 90 0 to insert into malleus) lies 1 mm inferior to geniculate ganglion at anterior end of tympanic segment. The Tensor Tympani tendon wraps around the cochleaform process to insert into the malleolus.
Cog: bony ridge hanging from tegmen tympani lies 1 mm above & posterior to cochleariform process.
Oval window/: nerve lies just superior
Intratemporal Mastoid Section
The processus pyradmialis demarcates the upper aspect of this section.
At the sinus tympani (posterior aspect of the middle ear).
The second genu lies beneath the posterior portion of the horizontal (lateral) semicircular canal.
The lateral semicircular canal is an extremely important landmark in mastoidectomy approaches.
Its seperated from the posterior fossa by a distance of about 4-5mm. The difference is covered by retrofacial air cells.
Two branches - the stapedius muscle and the corda tympani.
Stapedius comes off near the upper end.
Gives off the corda tympani branch approximately half way along its segment.
Intratemporal Mastoid Section: Chorda Tympani
Contains preganglionic parasympathetic fibres from the superior salivatory nucelus.
Also contains afferent taste fibres from the anterior 2/3'ds of the tongue destined for the nucleus of the solitary tract.
branch from mastoid segment of facial nerve: the branching site can be variable from either the proximal, mid- or distal mastoid segment of facial nerve; occasionally the chorda tympani can even branch from the facial nerve after exiting the mastoid styloid foramen
posterior canaliculus: the chorda tympani then courses superiorly to the level of the manubrium/neck of malleus; the distance of ascent is variable depending on the initial branching pattern from the mastoid segment of facial nerve
Traverses Middle Ear: tympanic segment is the segment of chorda tympani as it traverses through the middle ear cavity between the malleus and incus in a posteroanterior direction.
The chorda tympani arches across pars flaccida medial to the upper part of the handle of malleus and traverses above the insertion of tensor tympani
anterior canaliculus: the chorda tympani re-emerges on the anterior wall of the middle ear cavity.Also known as the canal of Hugier which is the medial portion of petrotympanic fissure and continues along the
petrotympanic fissure, which is medial to the temporomandibular joint.
Anterior tympanic branch of maxillary artery accompanies this nerve along this canal. Chorda exits the skull through a small foramen behind the base of spine of the sphenoid. At its exit it is closely related to the medial surface of temporomandibular joint.
the chorda tympani exits the petrotympanic fissure and joins the lingual nerve approximately 2 cm below the skull base.
In the infratemporal fossa the chorda tympani nerve descends medial to the spine of sphenoid and angles forward to join the lingual nerve about 2 cms below the skull base. This junction lies close to the lower border of lateral pterygoid muscle.
Emerges from the stylomastoid forearm at the anterior boarder of the digastric ridge. As it approaches the stylomastoid forearm, the nerve becomes encircled in the fibrous tendon of the digastric muscle, which forms part of its sheath, the posterior belly of digastric is behind and below it, it comes out of the forearm facing forward.
There are 4 landmarks for identifying the facial nerve in its extracranial course:
1. The tragal pointer: nerve located 1cm anterior, inferior and deep.
2. The styloid process
3. Posterior belly of digastric, superior aspect. Digastric Ridge (identifies the plane).
4. Tympanomastoid Suture*: Nerve is 6-8mm deep.
The posterior belly of the digastric, mastoid tip, tragal pointer, and the tympa- nomastoid suture line are key anatomical landmarks. The nerve lies 2 to 4 mm inferior to the tympanomastoid suture line.
The Mastoid Process lies lateral.
The posterior auricular artery lies just lateral to the nerve.
Leaving the stylomastoid forearm the nerve gives off 2 branches - Branch to the extrinsic muscles of the ear (and occipital belly of occipitofrontalis). Branch to posterior belly of digastric (and stylohyoid). Lateral Parapharyngeal Space.
Mnemonic "Style or Die" = stylohyoid, auricular branch + Digastric (posterior belly).
Nerve then continues antieriolaterally.
Crosses the lateral surface of the styloid process to enter the posterior surface of the parotid gland. .
Enters the posterior surface of the parotid gland on the posterior belly of the digastric.
Nearly 80% of gland is lateral and 20% parotid is medial.
Lies under the cover of the parotomessenteric fascia.
First divide into temporofacial and cervicofascial.
First lies DEEP to the parotomasseteric Fascia.
Then crosses the zygomatic arch, in its middle third.
About 2 cm's above the zygomatic arch, it changes layers! It comes to lie just deep to the superficial temporal fascia!
Landmarks for the temporal branch
The main nerve branch is delineated by Pitanguys line - .5cms below tragus to 1.5cm superiolateral to lateral canthus.
The Sentineal Vein is another important landmark - that's the medial zygomaticotemporal vein.
First lies deep to the parotmasseteric fascia.
Then changes layers at the zygomatic ligament - around 4 cms anterior to the tragus.
Mandibular branch may have between 1-4 branches but 2 is the most common.
Muscles of the lower lip are supplied by the marginal mandibular nerve of the facial nerve and those of the upper lip and orbicularis oris by the buccal branch of the facial nerve.
The facial nerve innervates facial muscles on their deep surface except the buccinator, levator anguli oris, and mentalis (level 4).
Most Common variations
1. Bony dehiscience. - temporal segment adjacent to the oval window. (Baxter of Maschuchus Eye and EAR) . Facial nerve protruded in 26%.
2. Anomalies of course.
3. A persisting embryonic artery or vein.