FACIAL NERVE:

It is the 7th Cranial Nerve.
It has a medial motor root and a lateral sensory root, the nervous intermedius.

FUNCTIONAL COMPONENTS

Special Visceral (brachial) Efferent

· Nucleus of the facial nerve lies in the lower part of Pons supplies various muscles innervated by facial nerve. The part of nucleus that supplies muscles of the upper part of the face receives corticonuclear fibers from the motor cortex of both the right and left sides. In contrast the part of nucleus that supplies muscles of the lower part of the face receive corticonuclear fibers only from the opposite side of cerebral hemisphere.

General Visceral efferent (parasympathetic)

· Superior salivatory nucleus lies in lower part of pons. It sends fibers through the facial nerve and its chorda tympani branch to the Submandibular ganglion for the supply of Submandibular and sublingual salivary glands.
· Lacrimatory nucleus lies near the salivatory nuclei gives fibers that pass through facial nerve and its branches relay in the pterygopalatine ganglion and supply the lacrimal gland.

Special Visceral afferent

· Nucleus of tractus solitarius (gustatory) carries taste sensation from anterior 2/3rd of the tongue and from the palate.

General Somatic Afferent fibers

They probably innervate a part of skin over the ear. The nerve does not give any direct branches to the ear but some fibers may reach it through communication with vagus nerve. Proprioceptive impulses from muscles of the face are believed to travel through branches of trigeminal nerve to reach the mesencephalic nucleus of the nerve

MOTOR ROOT
Supplies Muscles of face, scalp, auricle, buccinator, platysma, Stapedius, stylohyoid & posterior belly of diagastric

SENSORY ROOT
It carries the taste fibers from anterior two thirds of tongue, floor of mouth and palate.
It also conveys parasympathetic secretromotor fibers to Submandibular and sublingual salivary glands, lacrimal gland and glands of nose and palate.

ORIGIN
The two roots of facial nerve emerge from the anterior surface of brain between the pons and medulla oblongata. They pass laterally forward in the posterior cranial fossa with the Vestibulocochlear nerve to the opening of internal acoustic meatus. At the bottom of the meatus the nerve enters the facial canal runs laterally above the vestibule of labyrinth until it reaches the medial wall of tympanic cavity. (Middle ear)Here the nerve expands to form sensory Geniculate ganglion. The nerve then bends sharply backwards above the promontory, on arriving at the posterior wall of the middle ear, it curves downward. On the medial side of the aditus of the mastoid antrum. It descends in the posterior wall of middle ear, behind the pyramid and finally emerges through the stylomastoid foramen.

IMPORTANT BRANCHES OF INTRAPETROUS PART OF FACIAL NERVE:

GREATER PETORSAL NERVE

It arises from facial nerve at the geniculate ganglion. It contains preganglionic parasympathetic fibers that pass to the pterygopalatine ganglion and are there relayed through the zygomatic & lacrimal gland; other post ganglionic fibers pass through nasal & palatine nerves to the glands of mucous membrane of nose and palate. It also contains taste fibers from the nose and palate. It also contains many taste fibers from the mucous membrane of the palate.

The nerve emerges on the superior surface of petrous part of temporal bone and runs forward in a groove. It runs below the trigeminal ganglion and enters the foramen lacerum.

It is here joined by the deep petorsal nerve from sympathetic plexus on the Internal Carotid Artery and forms nerve of pterygoid canal. This passes forward and enters the pterygopalatine fossa where it ends in the pterygopalatine ganglion.

THE NERVE TO STAPEDIUS
It arises from the facial nerve as it descends in the facial canal behind the pyramid. It supplies the muscle with in the pyramid.

CHORDA TYMPANI
It arises from the facial nerve above the stylomastoid foramen. It enters the middle ear close to posterior border of tympanic membrane. It then runs forward over the tympanic membrane and crosses the root of the handle of the malleus. It lies in the interval between the mucous membrane and the fibrous layer of tympanic membrane.
Taste fibers
The nerve leaves the middle ear through petrotympanic fissure and enters infratemporal fossa where it joins the lingual nerve. Chorda tympani contain many taste fibers from the mucous membrane covering the anterior two thirds of tongue. (not the vallate papillae)
Taste fibers are the peripheral processes of the cells in the Geniculate ganglion.
Parasympathetic secretomotor fibers
The nerve also contains pre ganglionic parasympathetic secretomotor fibers that reach the Submandibular ganglion and are there relayed to the Submandibular and sublingual salivary gland.
As the nerve runs forward in the substance of parotid gland. It divides into 5 terminal branches

BRANCHES AT ITS EXIT FROM THE STYLOMASTOID FORAMEN

THE POSTERIOR AURICULAR NERVE:
It arises just below the stylomastoid foramen. It ascends between the mastoid process and the external acoustic meatus and supplies the
· Auricularis posterior
· The Occipitalis
· Intrinsic muscles on the back of the auricle

THE DIAGASTRIC BRANCH
It arises close to the previous nerve. It is short and supplies the posterior belly of diagastric muscle.

THE STYLOHYOID BRANCH
It may arise with the diagastric branch. It is long and supplies the stylohyoid muscle.

TERMINAL BRANCHES WITH IN THE PAROTID GLAND

THE TEMPORAL BRANCH
It emerges from the upper border of the gland and supplies
The anterior and superior auricular muscles
The frontal belly of occipitofrontalis
The orbicularis occuli
The corrugator supercilli

THE ZYGOMATIC BRANCH
It emerges from the anterior border of the gland and supplies the orbicularis occuli

THE BUCCAL BRANCH
It emerges from the anterior border of gland below the parotid duct and supplies the buccinator muscle and muscles of upper lip and nostril.

THE MANDIBULAR BRANCH
It emerges from the anterior border of the gland and supplies the muscles of lower lip.

THE CERVICAL BRANCH
It emerges from the lower border of the gland and passes forward in the neck below the mandible to supply depressor anguli oris muscle.

The facial nerve is the nerve of second arch and supplies all muscles of facial expression. It does not supply skin but its branches communicate with branches of trigeminal nerve.

EXAMINATION
Routinely only the motor function of the 7th nerve is tested.

MOTOR FUNCTION
· When 7th nerve is paralyzed, the patient may complain of inability to close the eyelid, collection of food in the mouth and dribbling of saliva on the affected side of the mouth and deviation of the angle of the mouth towards the opposite side.


On inspection, palpebral fissure may be wide and nasolabial fold may be flattened on theparalyzed side.
· Ask the patient to frown or wrinkle the forehead.
There would be no wrinkling on the affected side.

· Ask the patient to close the eyes; the affected side will remain open and there will be brisk upward rolling of the eye ball (Bell’s phenomenon). To test the power of orbicularis occuli ask the patient to close the eyes as strongly as possible while you try to open the upper eyelids. The affected side will be weak.
· Ask the patient to inflate the cheek and tap on both sides with finger. The weak side will be deflated easily.
· Ask the patient to show the teeth. The angle of mouth will be deviated towards the healthy side.
· The patient cannot whistle as air escapes from the paralyzed side.
· The patient will complain of unusually loud sounds on paralyzed if nerve to Stapedius is involved.

TASTE
Test taste of the anterior two third of the tongue by following technique.
· Get solutions of four common tastes- sweet, salt, sour and bitter.
· Instruct the patient to identify the taste, either by writing or raising fingers, e.g. one finger if taste is sweet, two fingers if salty and so on.
· Ask the patient to protrude the tongue. Hold it with a gauze, dry it and test each side separately.
· Put a drop of each solution one by one and ask for response.
· Test bitter at the end.

SECRETOMOTOR FUNCTION
Lacrimation and salivation can be tested by various tests but it is not done routinely.

INTERPRETATION

The facial nerve is the most commonly affected cranial nerve by lesion of both upper motor neuron and lower motor neuron.

UPPER MOTOR NEURON LESION
Manifestations are on the opposite side. Upper half of the face (wrinkling of the forehead, closure of the eyelid) is less severely affected because the part of the facial nerve nucleus which supplies muscles of the upper half of the face is connected with both cerebral hemispheres; the part of the facial nerve nucleus which supplies muscles of lower half of the face is connected only with the contra lateral cerebral hemispheres. Smiling and other emotional movements are usually preserved in UMN lesion because there is a separate path for these movements.

LOWER MOTOR NEURON LESION
Whole of the ipsilateral half of the face is affected. Bell’s palsy is the most common cause of isolated lower motor neuron facial palsy. Etiology is unknown. The lesion is in the facial canal.

As facial nerve has a long route and gives off branches at various sites, the site of lesion can be localized with considerable precision.

· If the lesion is after the nerve exits from the skull, there is only weakness of the facial muscles.
· If the lesion is in the facial canal, between the chorda tympani and branch to Stapedius, in addition to motor weakness, there is loss of taste as well.
· If the lesion is between the branch to Stapedius and internal auditory meatus, there is hyperacusis on the affected side, in addition to motor weakness and loss of taste.
· Geniculate ganglion can be affected by herpes zoster(Ramsay Hunt syndrome). In addition to other features of facial palsy, there are vesicles in that part of external auditory meatus which gets sensory supply from the facial nerve.
· If the lesion is in the internal auditory meatus, in addition to the features of facial nerve palsy, 8th nerve is also paralyzed.
· If the lesion is in the cerebello-pontine angle, there are signs of cerebellar dysfunction and 5th, 6th, 7th, and 8th nerves are also involved.
· In pontine lesion, along with the facial nerve, the 6th nerve is also paralyzed and there is upper motor neuron hemiplagia on the opposite side.