Facial Nerve

 FACIAL  NERVE

• Facial nerve consists of motor root and sensor root (nervous intermedius).

NUCLEI  &  FUNCTIONAL  COMPONENTS

Motor  Nucleus :-

• It is situated in the caudal part of pons in front of the abducent nucleus.
• It represents special visceral efferent column.
• They supply muscles of facial expression.

Fibres from the motor nucleus pass dorso-medially towards the caudal end of abducent nucleus

then they run rostrally behind the nucleus forming facial colliculus in the floor of fourth ventricle

these fibres bend abruptly downwards and forwards forming internal genu

they emerge at the lower border of pons through the motor root.

Superior  Salivatory  Nucleus :-

• It is situated dorso-lateral to the caudal part of motor nucleus.
• It represents the general visceral efferent column.
• It gives origin to the preganglionic parasympathetic secreto-motor fibres which emerge through the sensory root.

Upper part of Nucleus Tractus Solitarius :-

• It represents special visceral afferent and general visceral afferent column.
• It receives taste sensation from the anterior two-thirds of tongue and soft palate.

Upper part of Spinal Nucleus of Trigeminal Nerve :-

• It receives cutaneous sensations from the auricle through auricular branch of vagus.
• Cell bodies of these fibres are located in the genicular ganglion of facial nerve.

CENTRAL  CONNECTIONS

• The cortico-nuclear fibres of opposite side control the activities of part of motor nucleus which supplies muscles of lower part of face.
• The cortico-nuclear fibres of both sides control the activities of part of motor nucleus which supplies muscles of upper part of face.

ORIGIN

• Both the roots of facial nerve are attached to the lower border or pons between the olive and inferior cerebellar peduncle.
• Motor root is large and lies on the medial side of sensor root.

COURSE  &  RELATIONS

Intracranial  Course :-

From the brain stem both roots of facial nerve pass laterally forwards and enter the internal acoustic meatus

in the meatus the motor root lodges in a groove over the vestibulo-cochlear nerve separated from it by sensory root

at the bottom of meatus both the roots unite to form trunk of facial nerve

trunk of facial nerve enters the bony facial canal, where the nerve passes above the vestibule of internal ear

then it reaches the medial wall of tympanic cavity where it bends backward to form external genu

external genu presents a swelling known as genicular ganglion which contains cell bodies of pseudo-unipolar sensory neurons

from the genu the nerve passes backward and downward above the promontory lodging in a bony canal

then it reaches the medial wall of aditus to mastoid antrum

passes vertically downward along the posterior wall of tympanic cavity

Finally, the nerve leaves the temporal bone through stylomastoid foramen.

Extracranial  Course :-

From the stylomastoid foramen the neve passes forward superficial to the styloid process of temporal bone

 

then enters the parotid gland by piercing its posterior-medial surface

 

within the gland the nerve runs forward for 1 cm superficial to the retro-mandibular vein and external carotid artery

 

then the nerve divides into temporo-facial and cervico-facial trunks.

• Temporo-facial trunk abruptly turns upward and subdivides into temporal and zygomatic branches.
• Cervico-facial trunk asses downward forward and subdivides into buccal, marginal mandibular and cervical branches.
• The five terminal branches radiate like goose’s foot from anterior border of the gland termed as pes anserinus.

BRANCHES

Communicating  Branches :-

In the internal acoustic meatus-

• to vestibulo-cochlear nerve.

At the genicular ganglion-

• to pterygo-palatine ganglion through greater petrosal nerve which arises from the genicular ganglion.
• It conveys preganglionic secretomotor fibres to lacrimal gland, glands of nasal mucous membrane and soft palate.
• It also conveys taste fibres from soft palate.
• to otic ganglion through a branch which joins the lesser petrosal nerve.
• to sympathetic plexus around the middle meningeal artery through external petrosal nerve.

In the facial canal-

• to auricular branch of vagus nerve.

Below the stylomastoid foramen-

• to glossopharyngeal, vagus, auriculo-temporal & great auricular nerves.

Behind the ear-

• to lesser occipital nerve.

In the face-

• to the branches of trigeminal nerve.

In the neck-

• to the transverse cervical cutaneous nerve.

Distributing  Branches :-

In the facial canal-

Nerve to the stapedius

• It arises opposite the pyramidal eminence.
• It supplies stapedius muscle.

Chorda tympani nerve

• It arises 6 mm above the stylomastoid foramen.
• It passes forward and upward in a separate bony canal appears in the posterior wall of tympanic cavity.
• It runs forward between the fibrous and mucous layers of tympanic membrane.
• It leaves the tympanic cavity through anterior canaliculus.
• Then it runs downward and forward in the infratemporal fossa where it provides a communicating branch to otic ganglion.
• It crosses medial side of spine of sphenoid bone and joins the posterior border of lingual nerve at an acute angle.
• It conveys taste fibres from anterior two-thirds of tongue except vallate papillae.
• It also carries preganglionic secreto-motor fibres for submandibular and sublingual glands.

Below the stylomastoid foramen-

Posterior auricular nerve

• It passes upward and backward behind the auricle.
• It supplies intrinsic muscles of superior surface of auricle, auricular posterior and occipital belly of occipito-frontalis muscles.

Nerve to the posterior belly of digastric muscle
Nerve to the stylohyoid muscle

In the face-

Temporal branch

• It passes upward and forward in front of the auricle crosses the zygomatic arch.
• It supplies intrinsic muscles of lateral surface of auricle, auricularis anterior and superior muscles, upper part of orbicularis oculi, frontalis and corrugator supercilii muscles.

Zygomatic branch

• It runs along the zygomatic arch.
• It supplies lower part of orbicularis oculi muscle.

Buccal branch

• It consists of superficial and deep parts.
• Superficial branches supply procerus muscle.
• Deep branches subdivide into upper buccal and lower buccal branches.
• Upper buccal branch passes above the parotid duct and supplies zygomaticus major and minor, levator anguli oris, levator labii superioris, levator labii superioris alaeque nasi and muscles of nose.
• Lower buccal branch passes below the parotid duct and supplies buccinator, orbicularis oris muscles.

Marginal mandibular branch

• It curves upward and forward across the lower border of mandible at the angle of masseter.
• It appears in the face after crossing superficial to the facial artery and vein.
• It supplies the risorius, depressor anguli oris, depressor labii inferioris and mentalis muscles.

Cervical branch

• It appears in the anterior triangle of neck through apex of parotid gland.
• It supplies platysma muscle.

APPLIED  ANATOMY

Supranuclear  Paralysis :-

• It involves the upper motor neurons of cortico-bulbar and cortico-reticular fibres to facial nucleus.
• It is caused by occlusion of blood vessel supplying internal capsule or motor cortex.
• This results in loss of movements of lower facial muscles of contralateral side.
• Upper facial muscles are escaped due to bilateral control of motor cortex on to the motor nuclei which supply these muscles.
• Volitional movements of lower part of the face are more affected than emotional movements.

Nuclear  Paralysis :-

• A lesion in the pons may involve the motor nucleus of facial nerve along with the abducent nucleus.
• This results in lower motor neuron paralysis producing loss of movements of all facial muscles on the affected side.
• It is associated with internal strabismus due to involvement of lateral rectus muscle of eye ball.
• Sometimes a pontine lesion affects pyramidal tract and facial nucleus, known as Millard-Gublar syndrome.

Infranuclear  Paralysis :-

• A lesion interrupting peripheral part of facial nerve known as Bell’s palsy.
• It produces lower motor neuron paralysis of all facial muscles on the affected side.
• Bell’s palsy may be caused by sudden exposure to cold, middle ear infections, fractures, tumours.

Sites of lesion producing Bell’s palsy –

• Lesion in the internal acoustic meatus produces Bell’s palsy and deafness.
• Lesion at the genu produces diminished lacrimation and submandibular salivary secretion, reduced taste sensation on the anterior two thirds of tongue, hyperacusis and signs of Bell’s palsy.
• Lesion between the genu and pyramidal eminence produces diminished submandibular salivary secretion, reduced taste sensation on the anterior two thirds of tongue, hyperacusis and signs of Bell’s palsy.
• Lesion below the stylomastoid foramen produces Bell’s palsy.

Manifestations of Bell’s palsy –

• Transverse wrinkles of fore-head disappear and the eye brow droops.
• Palpebral fissure is wider due to unopposed action of levator palpebrae superioris.
• Unable to close the eyelids and tears roll over the cheek.
• Corneal reflex is disturbed.
• While trying to close eye lids eye ball is seen to be rolled upward – Bell’s phenomenon.
• Nasolabial fold disappears.
• During smiling the angle of mouth remains motionless.
• Food accumulates in the vestibule of mouth due to paralysis of buccinator.
• Pursing of the whistle is disturbed due to paralysis of orbicularis oris.