Facial Nerve Anatomy and management of bells palsy

Moderarator: Dr Upendra Sharma

Presenters: Dr Meghna

Dr Shamendra

• Nuclei of origin

• Roots: Sensory root (nervous intermedius)

Motor root

• Course – Intracranial/Intrapetrouspart,Extracranial part.

• Branches – of Communication

of Distribution

Nuclei of Origin :

• Motor Nucleus :

Site: situated in reticular formation in caudal part of pons below and in front of abducent nucleus.

At the cranial end of the abducent nucleus,the fibresbend abruptly downwards and forwards forming an internal genu (First genu)

And finally emerge at the lower border of the ponsthrough the motor root.

• Superior salivatorynucleus:

situated dorsolateral to caudal part of motor nucleus

Gives origin to pre ganglionic secretomotor fibres(parasympathetic)

which emerge through the sensory root

Sensory Root =misnomer (conveys secretomotorfibres also)

• Upper part of the nucleus of tractus solitarius :

acts as a afferent column

Recieves taste sensation from ant 2/3 of the tongue via corda tympani nerve

from soft palate through greater petrosal nerve.

• Course :

Divided by stylomastoid foramen into intracranial –intrapetrous part and extracranialpart.

Intracranial-intrapetrous part :

From the lower border of the pons , both roots of facial nerve along with vestibulococlear nerve, labyrinthine artery enter the internal acoustic meatus .

Within the Internal acoustic meatus, motor root lodges in a groove on the anterio superior surface of vestibulo coclear nerve and sensory root intervenes between them.

• At the bottom of the internal acoustic meatus, both roots combine to form trunk and enter the bony facial canal.

In the bony facial canal, nerve passes above the vestibule of the inner ear , and reaches the medial wall of epitympanic part of tympanic cavity.

In epitympanic part of tympanic cavity, facial nerve bends backwards forming a 2nd genu (external genu).

At the genu, facial nerve presents an asymetricalswelling known as genicular ganglion(contains cell bodies of pseudo unipolar sensory neurons).

• From the genu,facial nerve passses into another bony canal above the promontary and fenestravestibuli and reaches the medial wall of the aditusof mastoid antrum.Here nerve is related to bulging of the lateral semicircular canal.

finally nerve passes vertically downwards along the posterior wall of tympanic cavity and leaves the temporal bone through stylomastoid foramen.

• Extracranial part :

After exit from stylomastoid foramen, it changes its direction and passes forwards superficial to the styloidprocess of the temporal bone and pierces the posteriomedial surface of the parotid gland.

With in the gland,nerve runs forwards for about 1 cm superficial to retromandibular vein and ext carotid artery and then divides into temporo facial and cervicofacial trunks.Temporo facial trunk turns abruptly upwards and subdivides into Temporal

Zygomatic branches.

• The Cervico facial branch passes downward and forward and subdivides into Buccal ,

marginal mandibular

Cervical branches.

The five terminal branches radiate like goose feet through the anterior border of the gland and supply the facial muscles. Such branching pattern is known as the pes anerinus.

Points of special importance :• The stylomastoid foramen lies about 2 cm deep to

the middle of the anterior border of the mastoid process.In the new born, stylomastoid foramen lies at the surface of the mastoid bone ,since the mastoid process grows about the 2nd year after birth.Therefore , incisions behind the auricle are taken carefully in a child in order to avoid injury to facial nerve.

• Branches:

1) In the facial canal :

a) Nerve to stapedius : Hyperacusis

b) Chorda tympani nerve : Taste sensations,

Preganglionic secretomotor fibres to submandibular and sublingual gland.

2) Below the stylomastoid foramen :

a)Posterior auricular nerve : passes behind the auricle. Supplies intrinsic muscles on the cranial surface of the auricle , auricularisposterior and occipital belly of occipitofrontalis.

b) Nerve to post belly of digastric

c) Nerve to the stylohyoid muscle

3) In the face:

5 terminal branches:

a) Temporal branch :consists of 3 or 4 branches. They pass upwards and forwards in front of the auricle and crosses over the zygomatic arch 3 to 5 cm from lateral orbital margin. After that,they run deep to temporoparietal fascia(sup temporal fascia).

Supplies ,

Auricularis anterior

Auricularis superior

Upper part of orbicularis occuli

Frontalis

Corrugator supercilli

b) Zygomatic branch :

consists of 5 to 8 branches with significant functional overlap. Runs along the zygomatic arch and then deep to parotid messeteric fascia in the same plane as that of parotid duct.

Supplies lower part of orbicularis occuli.

c) Buccal branch :

Consists of superficial and deep parts.

Superficial branches : supply procerus

Deep branches : subdivide into upper and lower sets.

upper buccal : passes forwards above the parotid duct and supply Zygomaticus major

Zygomaticus minor

Levator anguli oris

Levator labi superioris

Levator labi superioris alaeque nasi

Muscles of the nose –dialator naris,

Dialator septi

Lower buccal : passes below the parotid duct Supplies Buccinator

Orbicularis oris .d) Marginal mandibular branch : consists of 1 to 3 branches .appears first in the neck(2 cm below the ramus) ,then curves upwards and forwards across the lower border of the mandible at the anterioinferior angle of the masseter

and appears in the face after crossing superficial to facial artery and vein.Supplies Risorius

Depressor angulli orisDepressor labi inferiorisMentalis

e) Cervical branch : consists of single branch.

comes out of apex of parotid gland and

appears in anterior triangle of neck and

Supplies Platysma by entering the muscle at

its cranial and middle 1/3rds.point of entry is

2-3 cm caudal to platysmal muscle branch of

facial artery.

Facial muscles responsible for facial expression:• Frowning : Corrugator supercilli forms vertical

wrinkles on forehead.Procerus forms transverse wrinkles across the root of the nose.

• Surprise,horror, fright : Frontalis by elevation of eyebrows and transverse wrinkles of the forehead.

• Anger : Dialator nares , Depressor septi.Theydialate the anterior nasal apertures and depressing the mobile part of the septum.

• Sadness : Levator anguli oris , Levator labisuperioris , zygomaticus minor by accentuating the naso labial furrow.

Depressor anguli oris by drawing the angle of the mouth downwards and laterally.

• Laughing : zygomaticus major which draws angle of the mouth upwards and laterally.

• Irony : Depressor labi inferioris by drawing the angle of mouth inferiorly and some what laterallly.

.

• Grinning : Risorius by retracting the angle of mouth laterally

• Disdain : Mentalis , by puckering the chin and protruding the lower lip.

Salient features about extra cranial facial

nerve :• The five divisions of facial nerve are to some extent

artificial .

• There is no distinct seperation between zygomatic and buccal branches

• As the branches exit from the parotid gland , there may be 8 to 15 branches making up to five divisions.

• Distal to parotid , there is significant arborization and interconnection of these branches.

• There is significant functional overlap and frequent duplication of branches providing same function.

• Each of the divisions consist of multiple branches

,except for cervical division which consists of single

branch.

• There are fewer crossover communications between

the frontal branches and marginal

mandibular branches. Hence,injuries to these nerves

are less likely to recover function in their respective

muscles

The branches of the facial nerve can be seen where they

exit from the anterior margin of the parotid. They have been separated

from the surrounding tissue in preparation for stimulation and functional

identification

Facial musculature:

• There are 18 paired muscles of the face, including

the orbicularis oris.

• Facial muscles are arranged in layers about the

mouth .

• The most superficial are the depressor anguli oris,

zygomaticus minor, and the orbicularis

oris.

The deepest layer consists of the buccinator,

mentalis,and the depressor anguli oris.

SMAS :• Five layers of the face: Skin;

Subcutaneous fat

Superficial musculoaponeurotic system (SMAS)–muscle layer;

Transparent fascia

Branches of the facial nerve.

• The muscles of facial expression are part of the SMAS layer.

• In the temporal region, this layer is not muscular but is fascial in quality and is represented by the

superficial temporal fascia (or temporoparietal) fascia.

• The fourth layer = layer of areolar tissue.

• In the temporal area, this layer is known as the

innominate or subgaleal fascia.

In the cheek-parotid–masseteric fascia; and

Neck-superficial cervical fascia.

Once under the SMAS, the facial nerve branches can be seen through this fourth layer.

• Though not totally accurate, galea–frontalis–temporoparietal fascia–SMAS– orbicularis oculi–platysma as a single layer,

• Similiarly , subgaleal fascia–innominate fascia parotid/masseteric fascia–superficial cervical fascia as a single layer.

The anatomic layers of the face.

Although the quality of the layers differs in various

areas of the face, the arrangement of layers

is identical. The facial nerve (cranial nerve [CN]

VII) branches innervate their respective muscles via

their deep surfaces.

• Facial nerve branches travel deep to the SMAS layer, but at some point these branches turn superficially to innervate the overlying muscles .

• Ex : Any dissection in the sub-SMAS plane in the cheek necessitates a change of surgical planes at the zygomaticus major muscle to avoid transection of the branch to this muscle. The dissection plane changes from sub-SMAS to subcutaneous by passing over the superficial surface of the zygomaticus major ,thereby preserving its innervation.