The facial nerve is made of 10,000 fibers (7000 mylinated motor, 3000 unmylinated sensory & parasympathetic)

The facial nerve is made of:

1- facial nerve proper :input from facial motor nucleus

special visceral efferent fibers (motor): Post belly of digastrics muscle Stylohyoid muscle Muscles of facial expressions(including Masseter muscles)

Except Stapedius muscle which arise from outside the main nucleus which explain: 1. the normal stapideal reflex in case of congenital facial palsy 2. absent reflex in case of brain stem lesions

2- nervus intermedius (nerve of wrisberg)

General Visceral motor ( presynaptic secretory parasympathetic): input from Superior salivatory nucleus

a) Greater superficial petrosal :pterygopalatine ganglion: 1. Lacrimal gland 2. Nasal glands 3. Palatine glands

b) Chorda tympani :submandibular ganglion:

1. Submandibular gland 2. Sublingual gland 3. Minor salivary gland

General sensory (Proprioceptive & Cutaneous): input to nucleus solitarus 1. Auricular concha 2. External auditory canal 3. Tympanic membrane

Special Visceral sensory:( input into nucleus solitarus)

a) Greater superficial petrosal :Taste from Soft palate b) Chordi tympani: Taste from the ant 2/3 of the tongue

Note that genigulate ganglion is relay ganglion only for taste fibers Nervus Intermedius:

Exists the Brain stem adjacent to the motor branch of the facial nerve

It clings to the adjacent VIII nerve complex rather than the facial nerve

Joins the VII nerve as it approaches the Internal auditory canal to form the common Facial nerve

Facial Nerve

Location of facial nerve related

nucleus:

1- Facial Motor nucleus: in the caudal end of the pon

2- Superior salivatory N: dorsal to the nucleus

ambiguous

3- Nucleus solitarus: medulla

Note that the facial nerve emerge

at the ponto-medullary junction

Facial Nerve Pathway:

Central pathways; Supranuclear pathway: a- Branchial motor

b- Visceral motor Facial nucleus & brainstem Cerebellopontine angle

Intratemporal pathway: Peripheral Pathway:7 branches

Segments of facial nerve see essential p 200

Segment From To Size Branches

Intracranial part 30mm

1-CP < Pon Fundus of

IAC

24mm(3/4)

2-Meatal segment (IAC) Fundus of

IAC

Meatal

foramen

8 mm (1/4)

Intratemporal/intrepetrous part

( within the fallopian canal)

30mm

Periganglionic 3-labyrinthine

segment

Meatal

foramen

Genigulate

ganglion

4mm 3 superficial Petrosal nerves

Greater petrosal is the 1st branch

4-tympanic

segment

Genigulate

ganglion

Pyramidal

eminence

8-11 mm

5-mastoid /vertical segment Pyramidal

eminence

Styloid

foramen

8-14 mm 1-stapedis 2- Chordi Tympani

3- nerve from auricular branch

of vagus

6-Extra-cranial part(parotid) 15-20 mm 7 branches

Remember the rule of multiples of 4 to remember the average of intratemporal,4,8,12,16=parotid segment General Notes:

meatal segment: internal auditory canal extends from the meatal fundus to meatal foramen the facial nerve lies ant to the vestibular nerve & sup to the cochlear nerve

Fallopian canal: contains the intratrmporal segment of the facial nerve extends from the meatal foramen at the fundus of IAC into styloid foramen

labrynthin segment: fallobian canal is narrowest in this segment esp at the meatal foramen

tympanic segment: the majority of Facial Intratemporal injuries occurs in the tympanic segment

the most common site of dehiscence 40-50%

After emerging from stylomastoid foramen (post lateral to the styloid process), the nerve courses

ant & inferiorly lateral to styloid process & external carotid artery

to enter the post border of the parotid gland

at this stage the nerve lies on the post belly of digastrics muscle

once it enters the parotid gland substance ,it bifurcates into :

a) temporozygomatic division

b) lower cervicofacial division The functions of the Facial Nerve:

1. Efferent fibers: a. Motor fibers b. Presynaptic parasympathetic fibers supply:

the lacrimal gland,nasal,palate,sublingual,submandibular

2. Afferent fibers: a. General sensation b. Taste sensation

Fiber type Facial part Cranial nucleus Segment branches

Motor Facial nerve proper Facial nucleus in Pon Vertical segment

Stapedius

Extracranial -Tympanozygomatic division -Lower cervicofacial division

Presynaptic parasympathetic

Nervus intermedius

Superior salivatory nucleus

Labrynthin Greater petrosal Lesser petrosal

Vertical Chordi tympani

Taste Solitary nucleus Vertical Chordi tympani

General sensation Extracranial Post auricular

Motor part: see p99 netter

Posterior auricular Occipital frontalis muscle Post auricular muscle

Posterior belly of digastrics =

Stylohyoid muscle =

Tempofacial Temporal division occipitofrontalis ant & sup auricular branch Orbicularis oculi

Zygomatic division Orbicularis oculi Zygomatic muscles

Common between 2 trunk

Buccal Buccinators Around the nose & mouth

Lower cervicofacial Marginal Mandibular Orbicularis oris Depressor labii inferioris Depressor anguli oris

Cervical Platyzma

Central Neural Pathways

Supranuclear Pathways:

a- Branchial motor:

motor input from the precentral gyrus (main somatomotor cortex) corresponding to the Broad man 4 6 8

projections from this precentral gyrus making up: 1- cortcobullbar tract: which goes through the

internal capsule Then

2- pyramidal tract: which goes through within the basal pons

In the caudal pons: Most VII fibers cross the midbrain to reach to reach the contralateral facial neuclus Small number innervates the ipsilateral facial neuclus,

a majority which r destined for the temporal branch of

the facial nerve

This innervation pattern explains why central nervous system lesions spare the forehead muscles,since it receive

from both cerebral cortices,whereas peripheral lesions involves all branches of the facial nerve

b- Visceral motor (Preganglionic parasympathetic fibers):

o Cell bodies of Preganglionic parasympathetic fibers arise in the Superior salivary nucleus

Nucleus & Brainstem:

a- Facial nucleus & brain stem:

The efferent projections from the facial motor nucleus emerge dorsomedially to form compact bundle that loops

over the caudal end of the abducens nucleus beneath the facial colliculus or internal genu

The neurons then pass between facial nerve nuclus & trigeminal spinal nucleus

Emerging from the brain stem at the caudal border of the pons (pontomedullary Junction)

b- Salivatory nucleus & brainstem:

The cell bodies of the preganglionic parasympathetic neurons arise in Superior salivatory nucleus in the pon

It joins the facial nerve after it has passed the abducens nucleus

Cerebellopontine angle

VII emerges between the abducens N (VI) and the vestibuloacoustic N (VIII) in the cerebellopontine angle

This intimate relationship between the facial nerve & vestibulocochlear nerve takes on critical importance when

lesions such as a vestibular schwannoma arise in the region of cerebellopontine angle

In this location the facial nerve is placed in jeopardy both during the growth of tumor & during attempted surgical resection

It Lies above and slightly anterior to CN VIII.

During its lateral course through the cerebellopontine angle & the internal auditory canal, the relative positions of the VII& VIII changes by rotating 90 degree

The average distance between the point where the nerves exit the brain stem and the place where they enter into

the internal auditory canal (IAC) is approximately 15.8 mm.

In the CPA the VII is covered with pia & bathed in cerebrospinal fluid,& devoid of epineurium,leaving it susceptible to manipulation trauma during intracranial surgery

Intratemporal Nerve Pathways: Within the temporal bone, the facial nerve passes through 4 regions before its exists out of the stylomastoid foramen:

1-meatal segment 2-labyrinthine segment 3-Tympanic segment (horizontal) 4-Vertical/descending/mastoid segment

The intratemporal pathway of the facial nerve in the fallopian canal is 3cm long

This makes fallopian canal the longest human

osseous canal of a nerve Because of this bony shell around the nerve, inflammatory processes involving the CNS, facial nerve, and traumatic injuries to the temporal bone can produce unique complications.

The fallopian canal is Z shape

Note the beginning of the fallopian canal at the

lateral end of the IAC

1-Meatal segment:

For details see temporal bone: post surface: meatal foramen

2-Proximal or labyrinthine segment:

The labyrinthine segment of the facial nerve lies beneath the middle cranial fossa

The term labyrinthine segment: it is the closest segment of the facial nerve to the cochlea

At the lateral portion of the IAC, The facial nerve pierces the Meatal foramen (the beginning of the fallopian canal) to

enter the labyrinthine segment

Passing laterally between the cochlea ( anterior) &

ampullated ends of the horizontal and superior semicircular

canals (Posterior)

It then runs back posteriorly at the Geniculate ganglion where:

1. the nervus intermedius joins the facial nerve proper

2. fibers for taste synapse ).

It is the shortest segment in the fallopian canal (approximately 3.5-4 mm in length).

The labyrinthine segment is the narrowest part of the fallopian canal & the narrowest part of labyrinthin segment is

at the the enterance from the internal auditory meatus (0.62m compared with 1.6 mm at root entary zone) so as a

result ,it is believed that infections/inflammation causing edema of the facial nerve within the fallopian canal which can

lead to permenant/temporary paralysis of the nerve such as in Bell palsy

This is the only segment of the facial nerve that lacks anastomosing arterial cascades (contains only superior

petrosal artery), making the area vulnerable to embolic phenomena, low-flow states, or vascular compression.

In this segment, the nerve is directed obliquely forward, perpendicular to the

axis of the temporal bone.

Both the facial nerve and the nervus intermedius remain distinct entities at this

level.

After traversing the labyrinthine segment, the facial nerve changes direction to

form the first genu (ie, bend or knee), marking the location of the genigulate

ganglion.

So the genigulate ganglion is considered the end of the labyrinthine segment &

lie just sup to the nerve

The 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 2/3 of the tongue are added to the

geniculate ganglion from the chorda tympani.

3 nerves branch from the geniculate ganglion:

1. Superficial greater superficial petrosal nerve

2. Superficial external petrosal nerve.

3. Contribution to Superficial lesser petrosal nerve

Note: Petrosal nerve: is a nerve traveling through the petrous

portion of the temporal bone

1-The greater petrosal nerve:

It carries:

1- Preganglionic parasympathetic fibers (secretomotor)

to the lacrimal gland & the nasal & maxillary sinus &

palatine mucosal gland.

2- Some minor taste neurons that supply the soft palate

It emerges from the upper portion of the ganglion anteriorly

It exits the petrous temporal bone via the facial canal

hiatus to enter the middle cranial fossa.

The nerve passes deep to the trigeminal ganglion

(Gasserian ganglion) in a groove on the anterior surface of

the petrous bone to the foramen lacerum, through which it travels to the pterygoid canal.

In the pterygoid canal, the greater petrosal nerve joins the deep petrosal nerve to become the nerve of the pterygoid canal

(Vidian nerve).

Axons from this nerve synapse in the pterygopalatine ganglion (sphenopalatine 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.

2-The external petrosal nerve:

An inconstant branch that carries sympathetic fibers to the middle meningeal artery;

3-The lesser petrosal nerve carries:

See temporal bone:middle cranial surface: superior tympanic canaliculus

Tympanic or horizontal Segment:

So called because it runs in the middle ear cavity against the medial wall of the epitympanic

extends Posteriorly from the geniculate ganglion to the pyrimdal eminence

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

Land mark of the proximal end of the tympanic segment of the Facial Nerve is The cochleariform process

"cog," a small bony prominence projecting

The facial nerve then travels Posteriorly:

o sup to: the stapes & oval window

o Inf to: the lateral semicircular canal

The distal portion of the facial nerve emerges from the middle ear between the posterior wall of the middle ear

cavity and the horizontal semicircular canal.

This is just distal to the pyramidal eminence, where the facial nerve makes a second turn marking the 2nd genu.

a) The nerve then curves inferiorly at its 2nd

Genu post to:

The Ant end of the facial nerve canal is marked by processus cochleariformis.

It is: curved projection of bone over the medial wall

concave ant

it houses the tendon of the tensor tympani muscle as it turns laterally to the handle of the malleus.

1. oval window

2. pyramidal process

3. Stapedius tendon

b) Ant to;

lateral semicircular canal

c) Inf to:

Short process of the incus

Location of sensory fibers:

1. Tympanic segment: anteriolateral

2. Mastoid segment: posteriolateral

Bony dehiscence of the fallopian canal:

We have 2 kinds of dehiscence

1-Pathological dehiscence:

Like the one caused by cholesteatoma

2-Natural dehiscence:

The % of ppl who has dehiscence in tympanic & mastoid is 55%

Majority occurs in the tympanic segment,

80% of tympanic segment dehiscence involves the canal adjacent to the oval window

This is Even infections of the middle ear mucosa can cause facial N palsy in patients with an exposed facial nerve.

Always anticipate finding a dehiscent or prolapsed facial nerve in its tympanic segment, especially in patients with congenital ear deformities.

When the bone is thin or nerve is exposed by disease, there r 2-3 straight B.V clearly visible along this line of nerve, these r the only straight B.V in the middle ear & indicate that the facial nerve is very close by

Note:

Superior petrosal artery travels in the fallopian canal

Iatrogenic injury of the facial nerve:

The most common site of injury in the middle ear surgery is: tympanic segment

The most common site of injury during mastoid surgery is: at the pyramidal turn posterio-lateral to the horizontal SCC

Clues of aberrant facial nerve:

1. Congenital malformed auricle

2. Ossicular abnormalities

3. Caraniofacial anomalies

4. Conductive hearing loss

Vertical, Descending, or Mastoid Segment

The 2nd 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.

The mastoid segment is the longest part of the intratemporal course of the facial nerve, approximately 10-14 mm long.

During middle ear surgery, the facial nerve is most commonly injured at the pyramidal turn.

The 3 branches that exit from the mastoid segment of the facial nerve are:

1. Nerve to the stapedius muscle

2. chorda tympani nerve

3. Nerve from the auricular branch of the vagus.

The auricular branch of the vagus nerve arises from the jugular foramen and joins the facial nerve just distal to the point at

which the nerve to the stapedius muscle arises. Pain fibers to the posterior auditory canal may be carried with this nerve.

The chorda tympani:

See middle ear :lateral wall:3 chordi tympani

The facial nerve exits the fallopian canal via the stylomastoid foramen.

The nerve travels between the digastric and stylohyoid muscles and enters the parotid gland.

A sensory branch exits the nerve just below the stylomastoid foramen and innervates the posterior wall of the external

auditory canal and a portion of the tympanic membrane.

Bells Palsy is caused by an inflammation within a small bony tube called the fallopian canal. The canal is an extremely

narrow area. An inflammation within it is likely to exert pressure on the nerve, compressing it. Likewise, if the nerve itself

becomes inflamed within this small canal, it can encounter pressure, with the same result of compression.

The nerve has not yet exited the skull ( INTRATEMPORAL portion) and divided into its several branches, resulting in

impairment of all functions controlled by the 7th nerve. If only part of the face is affected, the condition is not Bell's palsy.

The nervus intermedius conveys

(1) afferent taste fibers from the chorda tympani nerve, which come from the anterior 2/3 of the tongue;

(2) taste fibers from the soft palate via the palatine and greater petrosal nerves

(3) preganglionic parasympathetic innervation to the submandibular, sublingual,& lacrimal glands.

The fibers for taste originate in the nucleus of the tractus solitarius (NTS), and the fibers to the lacrimal,

nasal, palatal mucus, and submandibular glands originate in the superior salivatory nucleus.

Fibers to the lacrimal gland are carried with the greater superficial petrosal nerve until it exits the skull,

where they branch off as the Vidian nerve, as shown below.

The most important landmarks for identifying the facial nerve in the mastoid are:

1. the horizontal semicircular canal

2. the fossa incudius

3. the digastric ridge.

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

relationship.

The digastric ridge points to the lateral and inferior aspect of the vertical course of the facial nerve in the temporal bone. In

poorly pneumatized temporal bones, the digastric ridge may be difficult to identify. The distal aspect of the tympanic

segment can be surgically located via a facial recess approach. The chorda tympani nerve and the fossa incudis can be

used to identify the nerve when performing a facial recess approach.

The long process of the incus points toward the facial recess. The chorda tympani nerve serves at the lateral margin of the

triangular facial recess. The chorda tympani nerve can be exposed along its length and can be followed inferiorly and

medially to its takeoff from the main trunk of the facial nerve.

Exposure of the facial nerve after a cortical mastoidectomy. The facial recess has been opened by

thinning of the posterior canal wall. The recess is identified using the incus, chorda tympani, and

horizontal semicircular canal as landmarks.

Summary to the course of the facial nerve in the temporal bone:

a) Labryinthin segment

The facial nerve enters the meatal foramen

Meatal foramen located at:

the internal auditory canal fundus/lateral end

anterior to bill's bar

superior to flaciform crest

this foramen is the beginning of fallobian canal

the nerve direct in the labyrinthine segment in lateral direction

b) genigulate ganglion & 1st genue (40-8 degree turn):

Located at the lateral end of the labyrinthine segment

Occurs superior to the basal turn of the cochlea (promontory), anterior to the ampullated ends of the superior & lateral semicircular canal

located:

superior-posteriomedial to the cochleariformis process (landmark of genigulate ganglion)

c) Horizantal/tympanic segment:

The nerve directs posteriorly with inferiolateral inclination which makes the facial canal prominence

located:

inferior to the lateral semicircular canal

Superior to the oval window

Deep to the short process of the incus

d) 2nd

genue (95-120 degree) & vertical segment:

Located:

Post to:

oval window

Stapedius tendon

pyramidal process

Ant to;

lateral semicircular canal

Inf to:

Short process of the incus

lateral and posterior to the pyramidal process

Above & behind the pyramid

The nerve directs inferiorly & slightly laterally

One of the most important landmarks for identifying the second genu in the mastoid is the lateral semicircular

canaL The second genu hugs the inferior aspect of the lateral semicircular canal and this relationship is

extremely constant. The pyramidal eminence is another useful landmark for the second genu where the facial

nerve makes a sharp turn downwards, marking the beginning of the mastoid segment. This can be located

surgically by the interval between the short process of the incus laterally and the lower border of the horizontal

canal medially. The nerve is lateral and posterior to the pyramidal process which creates two recesses in the

mesotympanum, the facial recess laterally and the sinus tympani medially (Figure 241c.2). The posterior semicircular

canal is located just posterior to the second genuand also marks the superior end of the retrofacial air cells,

which are helpful in delineating the medial aspect of the facial canaL

source scott brown

Extra-temporal segment of the facial nerve:

o Exit the skull base through stylomastoid foramen

Posteriolateral to styloid process

Anteriomedial to mastoid process

Between the stylohyoid and posterior belly of digastrics muscle

Note that the stylomastoid foramen is the only constant land mark of the facial nerve

extracranial Land Mark: see netter p223

Identification of the nerve depends on marking the position of the posterior belly

of the digastric muscle, the external meatal cartilage, the tympanomastoid suture

line, and the styloid process.

1. 1.5 cm below the external auditory canal

2. Tympanomastoid suture

The most reliable land mark for the extra-cranial trunk

Arise from the styloid foramen which is post to the

tympanomastoid suture

directed 6-8mm anterio-inferior (between the suture

line & styloid process)

3. Posterior belly of digastrics muscle is 1cm below the facial

nerve

4. Styloid process: lies lateral to the process

o Before it enters the parotid gland it gives:

the post auricular

branch to stylohyoid

branch to the post belly of digastrics muscle

o Facial nerve branches as it enters the parotid forming Pes anserinus

o The 1st major subdivision of the extracranial facial nerve is usually situated within the parotid gland.

o The main trunk of the nerve divides into 2 major divisions:

a) upper temporofacial

b) lower cervicofacial.

o Within the substance of the parotid gland, each divides, some rejoining and then dividing again to emerge finally from

the parotid gland in 5 main groups of branch (see netter p201)

1. Temporofacial:

a) temporal branch:

parallel with the temporal vessels & auriculotemporal nerve

becomes superficial at junction between the hair bearing & non hair bearing skin

innervates the frontalis muscle inferiorly

b) zygomatic branch

2. cervicofacial division:

a) marginal mandibular branch:

superficial to the post facial vein

b) cervical branch:

supply the platyzma

superficial to the post belly of digastrics muscle

note that cervical branch & mandibular branch are in close association with the parotid gland and lies directly under the platysma in the plane of deep cervical fascia

3. buccal branch from both divisions:

parallel to the parotid duct ( sup/inferior)

o pes anserinus:

plexiform arrangement of branching (the site of division between the upper & lower part)

lies in the parotid gland

below the stylomastoid foramen 1.3cm

superficial to the retromandibular vein and external carotid artery

truly plexiform can sustain surgical injury better than those that are not.

Sacrifice of one small branch in a plexiform nerve is rarely accompanied by a significant or noticeable facial weakness.

Post auricular incision:

In adults, the incision is placed 8 to10 mm posterior to the

postauricular sulcus & extends inf 2 mastoid tip

In children younger than 2 years, the inferior portion of this incision

must be placed more posteriorly than in adults,not extending below

the level of the EUC because:

1. tympanic ring is underdeveloped

2. mastoid pneumatization is incomplete,

3. stylomastoid foramen is quite shallow.

Therefore, the facial nerve is vulnerable to injury because it will be closer & more superficial to the mastoid cortex

as it exits the stylomastoid foramen.

With maturation the facial nerve is placed more medially & inferiorly

Note: any developmental abnormality of the ear is an alert 4 the possibility of facial nerve anomalies

The facial nerve differs from peripheral nerves in that it traverses a long bony canal.

The canal limits the normal supply of regional nutrient vessels, increasing the reliance on a longitudinal blood supply.

only two nutrient vessels provide the arterial supply of the facial nerve within the Fallopian canal:

The stylomastoid artery:

branch of the post-auricular artery

enter the stylomastoid foramen

divides into two or more branches

run superficially within the epineurium to the second genu.

These overlap with the larger descending branch of superior petrosal artery

Superior petrosal artery:

branch of the middle meningeal artery

divides into two or more longitudinal vessels

reaching as far as the stylomastoid foramen.

Its smaller ascending branch passes proximally to the entrance of the bony Fallopian canal

Blood supply of the facial nerve

The labyrinthine branch of the anterior inferior cerebellar artery anastomoses with the ascending branch of the petrosal artery at the entrance to the Fallopian canaL

There is no overlap within the labyrinthine portion of the Fallopian canal and therefore the labyrinthine portion of the facial nerve receives its vascular supply from only one

nutrient vessel, the petrosal artery.

Facial nerve injuries:

Neuropraxia: compression of axon ( distrubtion of the nerve)

Axonotmesis: cut of axon but preservation of endoneurium

Neurotmesis: cut of the axon & endoneurium

.