anatomy facial nerve

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Facial nerve anatomy

By

MOHAMMED HUSSIEN

EmbryologyThe rhombencephalon (or hindbrain) is

divided into the myelencephalon (caudal), which becomes the medulla oblongata, and the metencephalon (cranial), which becomes the pons and cerebellum.

EmbryologyThe facioacoustic (acousticofacial)

primordium appears during the third week of life

It is attached to the metencephalon just cranial to the otic vesicle.

The facial part of the acousticofacial primordium migrates cranial and ventral to end adjacent to the epibranchial placode, which is located on the dorsal and caudal aspect of the first branchial cleft

Embryology By the end of the fourth week of gestation

the facial nerve splits into 2 parts, caudal and rostral trunks.

The chorda tympani nerve exits rostrally and courses ventrally to the first pharyngeal pouch to enter the mandibular arch.

Shortly thereafter, the nerve approaches the epibranchial placode, inducing the appearance of the large dark nuclei of neuroblasts that represent the future geniculate ganglion.

Fascioacoustic ganglionFascioacoustic ganglion

Embryology 5th week Mesenchymal concentrations that form the

cephalic muscles are seen in association with their nerves, while the epibranchial placode disappears and the geniculate ganglion is identifiable.

The greater superficial petrosal nerve (GSPN) is present.

The chorda tympani nerve enters the mandibular arch and terminates just proximal to the submandibular ganglion, near a branch of the trigeminal nerve that will become the lingual nerve.

The posterior auricular nerve appears near the chorda tympani.

Embryology 5th weeka discrete nervus intermedius develops, making

this an important temporal reference point for gestational disorders.

The GSPN courses to the lateral aspect of the developing internal carotid artery (ICA), where it joins the deep petrosal nerve and continues as the nerve of the pterygoid canal. It terminates in a group of cells that will become the pterygopalatine ganglion.

At this point, the most distal branches of the facial nerve are a loose network or interconnecting twigs

Embryology 7th weekThe nervus intermedius is now smaller than

the motor root and enters the brain stem between the vestibulocochlear nerve and the motor root of the facial nerve.

The chorda tympani and lingual nerve unite proximal to the submandibular gland.

The posterior auricular nerve now divides into cranial and caudal branches.

Embryology 7th weekSeveral branches are visible in the peripheral

portion of the seventh nerve. All of the peripheral branches lie deep to the myoblastic laminae that will form the facial muscles.

At the end of the seventh week, the separations between the terminal branches continue to increase to the extent that all peripheral divisions can be identified

Embryology 7th weekThe parotid gland is beginning to develop

from the parotid bud at this stage. The temporal, zygomatic, and upper buccal

branches are superficial to the parotid primordium, while the lower buccal, mandibular, and cervical branches are deeper.

Multiple facial muscles appear at this time as well, including the zygomaticus major and minor, depressor anguli oris, buccinators, and frontalis.

EmbryologyWeek 8 A sulcus develops around the facial nerve that is

the beginning of the fallopian canal. The orbicularis oris, levator anguli oris, and orbicularis oculi muscles appear.

Week 9 Auricularis anterior, corrugator supercilii,

occipital and mandibular platysma, and levator labii superioris alaeque nasi muscles appear. All the cranial nerves more closely resemble their adult relationships.

Embryology11th week the facial nerve has branches

extensively and the majority of anastomosis have occurred .

At term the anatomy of the facial approximates that of the adult with the exception of its superficial location within a poorly formed mastoid.

Anatomy The facial nerve (FN), like most of the

cranial nerves, is a mixed nerve containing motor, sensory and parasympathetic fibers.

Nuclei of the facial nerveMotor nucleus: in the Pons fibers from it

will surround the abducent nerve nucleus to form the facial colliculus.

Nucleus solitarious: in the medulla oblongata and receives the taste fibers

Superior salivatory nucleus: in the Pons and gives rise to the parasympathetic fibers

Nervous intermedius

Functional components of the FNThe facial nerve can be divided into four

functional components: two efferents and two afferents:

Motor fibers (efferent): The upper part of the facial nucleus receives pyramidal fibers from both sides, while the lower part receives pyramidal fibers only from the contra lateral side.

Parasympathetic fibers (efferent).Taste fibers (afferent).Sensory fibers (afferent).

Functional components of the FNFunctional components of the FN

Course of the FNThe facial motor nucleus lies in the lower

part of pons, close to the abducent nerve nucleus.

The fibers of the facial nerve wind around the abducent nerve nucleus and emerge from the lower border of the pons.

Course of the FNThe course of the FN is divided into three

portions:

1)Intracranial portion (23-24 mm long). 2)Intratemporal portion (28-30 mm): It is

divided into: a-meatal segment (8-11). b-Labyrinthine segment (3-5 mm): Narrowest

part. c-Tympanic segment (8-11 mm). d-Mastoid segment (10-14 mm).3)Extratemporal portion.

Facial Nerve Anatomy – Intracranial Segment

The portion of the nerve from the brainstem to the internal auditory canal

Made up of two components1. Motor root2. Nervus intermedius – carries preganglionic

parasympathetic fibers and special afferent sensory fibers

- Both join at the CPA/IAC to form the common facial nerve

Facial Nerve Anatomy – Intratemporal Segments

• Meatal (8-10mm)– Portion of the facial nerve

traveling from porus acusticus to the meatal foramen of IAC

– Travels in the anterior superior portion of the IAC (7-UP, 8-Down)» Posterior superior – superior vestibular nerve

» Posterior inferior – inferior vestibular nerve

» Anterior inferior – cochlear nerve

IAC

The nerves of the internal auditory canal

Labyrinthine

Labyrinthine(3-5mm)– From fundus to the geniculate ganglion

– Runs in the narrowest portion of the IAC (0.68mm in diameter)

– Greater superficial petrosal nerve comes off at this point

Labyrinthine

The term labyrinthine segment is derived from the location of this segment of the nerve immediately posterior to the cochlea.

The nerve is posterolateral to the ampullated ends of the horizontal and superior semicircular canals and rests on the anterior part of the vestibule in this segment.

The transtemporal course of the facial nerve is shown. Note the vascular arcades

feeding the facial nerve throughout its course in the bony fallopian canal.

Facial Nerve Anatomy – Intratemporal Segments

• Tympanic (8-11mm)– At the geniculate ganglion the nerve makes 40-80 degree turn to proceed posteriorly across the tympanic cavity to the pyramidal eminence.

– Highest incidence of dehiscence here (40-50% of population)

Tympanic segmentThe tympanic segment extends from the geniculate

ganglion to the horizontal semicircular canal

The nerve passes behind the cochleariform process and the tensor tympani.

The cochleariform process is a useful landmark for finding the facial nerve. The nerve lies against the medial wall of the cavum tympani, above and posterior to the oval window.

Intratemporal course of FN

Tympanic segmentThe distal portion of the facial emerges from

the middle ear between the posterior wall of the external auditory canal and the horizontal semicircular canal.

This is just distal to the pyramidal eminence, where the facial nerve makes a second turn marking the second genu.

Tympanic segmentThe most important landmarks for identifying

the facial nerve in the mastoid are the horizontal semicircular canal, the fossa incudis, and the digastric ridge.

The second genu of the facial nerve runs inferolateral to the lateral semicircular canal. This is a relatively constant relationship.

Mastoid• Mastoid (10-14mm)

– The second genu marks the beginning of the mastoid segment.

– The second genu is lateral and posterior to the pyramidal process.

– The nerve continues vertically down the anterior wall of the mastoid process to the stylomastoid foramen.

– Gives off branches to the stapedius muscle and the chorda tympani

Intratemporal course of FN

Facial Nerve Anatomy – Extratemporal Segments

• Nerve exits stylomastoid foramen– Postauricular nerve - external auricular and

occipitofrontalis muscles– Branches to the posterior belly of the digastric and

stylohyoid muscles• Enters parotid gland splitting it into a

superficial and deep lobe• It enters the posteromedial surface of the

parotid gland superficial to ECA and retromandibular vein

• Pes Anserinus– Branching point of the extratemporal segments in the parotid– To Zanzibar By Motor Car

» Temporal» Zygomatic» Buccal» Marginal mandibular» Cervical

Extratemporal part

Extratemporal part

Facial Nerve Components– Motor

• Supplies muscles of facial expression• Stylohyoid muscle• Posterior belly of digastric• Stapedius muscle• Buccinator

– Sensory• Taste to anterior 2/3 of the tongue• Sensation to part of the TM, the wall of the EAC,

postauricular skin, and concha– Parasympathetic

• Supplies secretory control to lacrimal gland and some of the seromucinous glands of the nasal and oral cavities

• Chorda tympani carries parasympathetics to the submandibular and sublingual glands

The geniculate ganglionThe geniculate ganglion is formed by the

juncture of the nervus intermedius and the facial nerve into a common trunk.

Additional afferent fibers from the anterior two thirds of the tongue are added to the GG from the chorda tympani.

Three nerves branch from the geniculate ganglion: the greater superficial petrosal nerve, the lesser petrosal nerve, and the external petrosal nerve.

The greater petrosal nerveemerges from the upper portion of the

ganglion and carries secretomotor fibers to the lacrimal gland.

The greater petrosal nerve exits the petrous temporal bone via the greater petrosal foramen to enter the middle cranial fossa.

The nerve passes deep to the Gasserian ganglion (ie, trigeminal ganglion) to the foramen lacerum, through which it travels to the pterygoid canal.

The greater petrosal nerveIn the pterygoid canal, the greater petrosal

nerve joins the deep petrosal nerve (sympathatic nerve from plexsus around ICA) to become the nerve of the pterygoid canal.

Axons from this nerve synapse in the pterygopalatine ganglion; postganglionic parasympathetic fibers, which are carried via branches of the maxillary (V2) divisions of the trigeminal nerve (CN V), innervate the lacrimal gland and mucus glands of the nasal and oral cavities.

Chorda tympaniThe chorda tympani is the terminal branch of

the nervus intermedius. The chorda runs laterally in the middle ear, between the incus and the handle of the malleus.

The nerve crosses the middle ear cavity and exits through the petrotympanic fissure (ie, canal of Huguier) to join the lingual nerve.

Chorda tympaniThe chorda tympani nerve carries

preganglionic secretomotor fibers to the submaxillary and sublingual glands.

The chorda also carries special sensory afferent fibers (ie, taste fibers) from the anterior two thirds of the tongue and fibers from the posterior wall of the external auditory canal responsible for pain, temperature, and touch sensations.

Other branchesNerve to stapedious Posterior auricular nerveNerve to posterior belly of diagastric

Surgical anatomy of the FNA) Identification of intratemporal part:During middle ear and mastoid surgery: the short process of

the incus (fossa incudis), cochleariform process, horizontal and posterior canals, pyramid, and digastric ridge mark the course of the FN.

The second genu hugs the inferior aspect of the lateral SCC. The pyramid is a useful landmark for the second genu.

B) Identification of extratemporal part:During parotid surgery, the main trunk of the nerve is

identified by the tragal pointer. The nerve is usually located 1 cm medial and inferior to the pointer.

Another useful landmark is the tympanomastoid fissure, the nerve can be identified 6-8 mm below the inferior drop off of the fissure.

A peripheral branch can be identified and followed proximally towards the main trunk of the FN.

Hitselberger` sign involving decrease sensitivity in the posterosuperior aspect of the concha corresponding to sensory disturbance of the facial suggests a space occupying lesion in IAC

findings that alert the physician to possible facial abnormalities: ossicular anomalies, craniofacial anomalies

Why might a tumor of the middle ear

cause abnormal taste sensation???A patient with hyperacusis and unusual

taste,facial weakness, hearing loss???What may result from the relative small

size of mastoid process???Dropped lower lip with submandibular

growth???Hyperacusis is a very acute sense of

hearing caused by damage of stapedious

Leveling

Blood supply to facial nerve – clinical relevanceCourses between the epineurium and periosteum

– making the blood supply at risk when mobilizing at the first genu

External carotid arteryStylomastoid artery (branch of the

postauricular artery of external carotid artery)Greater petrosal artery (branch of middle

meningeal artery)Internal auditory artery (branch of the AICA)

Labyrinthe segment - lacks anastomosing arterial cascades thereby making the area vulnerable to ischemia

Components of a Nerve• Endonerium

Surrounds each nerve fiber Provides endoneural tube for regeneration Much poorer prognosis if disrupted

• Perinerium Surrounds a group of nerve fibers Provides tensile strength Protects nerve from infection Pressure regulation

• Epinerium Surrounds the entire nerve Provides nutrition to nerve

Components of a NerveComponents of a Nerve

Radiology facial nerve Coronal

let’s follow the facial nerve back out, from internal to external, on coronal CT images

Radiology facial nerve axial

On axial CT images, an easy landmark to start with the styloid process. From there, the course of the facial nerve can be traced from inferior to superior, external to internal, as follows: