Facial Nerve (VIIth Cranial Nerve) , A Detailed description

Facial Nerve (VIIth Cranial Nerve)

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1.    Origin

2.    Nuclei

3.    Course

4.    Distribution

5.    Branches

6.    Functional columns

7.    Ganglia involved

8.    Supranuclear connections

9.    Reflexes involving facial nerve

10.                       Applied Aspect

Introduction-

Facial nerve is 7th cranial nerve involved in both sensory and motor functions. Its major part is motor and small part is sensory. Apart from giving motor supply to muscles of facial expressions and muscles of second pharyngeal arches, it also gives motor supply to glands of head and face regions namely lacrimal, sublingual and submandibular glands. Small part also contribute for sensory supply to external ears.Special sensory afferent fibers for taste supply anterior 2/3rd of tongue except circumvallate papillae. Cranial Nerve VII is the most commonly paralysed of all peripheral nerves, owing to the great length of its canal in the temporal bone, where it is at risk of compression when swollen.

Origin-

Facial nerve originates from lower part of Pons. Fibers originate from nuclei of facial nerves namely; motor nucleus of facial, lacrimatory necleus, superior salivatory nucleus, nucleus of tractus solitarius and fibers from spinal nucleus of trigeminal. These fibers loop around the nucleus of abducens nerve in pons at the level of facial colliculli, elevation present in floor of fourth ventricle. This looping is internal genu of facial nerve and looping cause elevation termed as Facial colliculus. It comes out from brainstem at the level of pontomedullary junction inbetween pons and olive of medulla. Nerve fibers lie between abducens nerve medially and vestibullocochlear nerve laterally. It has two types of fibers. Motor fibers are present medially and sensory is laterally. Also, motor fibers are more thicker as compared to sensory fibers. Sensory fibers are known as Nervus intermedius.

Nuclei of Facial nerve-

1.    Special Visceral (Branchial) Efferent Nuclei

              Motor Nucleus of facial

-         Situated in the lower part of the pons

-         Fibres arising from the nucleus  - course dorsomedially towards the floor of fourth ventricle

-         Loop behind the motor nucleus of the abducent nerve loop (internal genu of facial nerve)  

-         Elevates the floor of fourth ventricle - forms the facial colliculus

2.    General Visceral Efferent Nuclei

      Superior salivatory and lacrimatory nuclei

-          Dorsal part of pons medial to the motor nucleus of facial nerve

-         Preganglionic fibres from superior salivatory nucleus pass to the submandibular ganglion through the nervus intermedius- geniculate ganglion, facial nerve - chorda tympani branch

-         Preganglionic fibres from lacrimatory nucleus reach the pterygopalatine ganglion through nervus intermedius and greater petrosal nerve

3.    Special Visceral Afferent Nuclei

       Nucleus of solitary tract

-         Rostral portion of the nucleus is concerned with taste sensations

-         Receives the special visceral afferent fibres from facial, glossopharyngeal and vagus nerves and is frequently referred to as Gustatory nucleus

4.    General somatic afferent (GSA)

     Cell bodies in the geniculate ganglion

-         peripheral processes of these cells innervate part of the skin of the external ear, while the central processes terminate in the Spinal nucleus of trigeminal nerve

Branches of Facial Nerve

1.    Greater petrosal nerve

         joined by deep petrosal nerve to form nerve of pterygoid canal

         provides secretomotor supply to lacrimal, nasal and palatal glands. 
2.  Nerve to stapedius.
3.  Chorda tympani nerve

          joins lingual nerve & carry taste fibres from anterior two-third of

          the tongue and provides preganglion fibres to the submandibular   

          ganglion

5.    Posterior auricular nerve  supply occipitalis and posterior   

auricular muscles
6.  Nerve to posterior belly of diagastric and stylohyoid muscles.
7. Terminal branches (temporal, zygomatic, buccal, mandibular  

          and cervical) to the muscles of facial expression.

Functional Column of Facial nerve

     

        

Course of Facial Nerve-

A) INTRACRANIAL COURSE

1.    In Brainstem:

Facial nerve originates from lower part of Pons. Fibers originate from nuclei of facial nerves namely; motor nucleus of facial, lacrimatory necleus, superior salivatory nucleus, nucleus of tractus solitarius and fibers from spinal nucleus of trigeminal. These fibers loop around the nucleus of abducens nerve in pons at the level of facial colliculli, elevation present in floor of fourth ventricle. This looping is internal genu of facial nerve and looping cause elevation termed as Facial colliculus. It comes out from brainstem at the level of pontomedullary junction inbetween pons and olive of medulla. Nerve fibers lie between abducens nerve medially and vestibullocochlear nerve laterally. It has two types of fibers. Motor fibers are present medially and sensory is laterally. Also, motor fibers are more thicker as compared to sensory fibers. Sensory fibers are known as Nervus intermedius.

2.    Pontomedullary junction to stylomastoid foramen (Intrapetrous part)-

Motor and sensory root of facial nerve pass along with vestibulocochlear nerve and labyrinthine vessels through internal acoustic meatus. At the bottom of meatus , both roots join together to form trunk of facial nerve to enter the bony facial canal. In facial canal, facial nerve passes above the vestibule to reach medial wall of tympanic cavity and bends backward. The bend is known as external genu of facial nerve, having swelling known as Geniculate ganglion, containing cell bodies of pseudo-unipolar neurons. It passes backward and downward above the promontory to reach medial wall of aditus to mastoid antrum. Then, it passes vertically downward along posterior wall of tympanic cavity and exits through the stylomastoid foramen.

At the point of external genu, it gives greater petrosal nerve as a branch consisting of parasympathetic fibers to supply lacrimal gland. Before exit to stylomastoid foramen, it gives nerve to stapedius, to supply stapedius muscle and chorda tympani nerve, having special sensory and parasympathetic fibers to supply anterior 2/3^rd of tongue except circumvallate papilla for taste sensation and sublingual and submandibular gland for secretions.

B) EXTRACRANIAL COURSE

     Facial nerve exits from stylomastoid foramen. It passes superficial to styloid process of temporal bone and pierces the postero-medial surface of parotid gland. Before that, it gives posterior auricular branch to supply occipital belly of occipito-frontalis and auricularis posterior and intrinsic muscles of auricle. It also gives branches named as nerve to posterior belly of diagastric and nerve to stylohyoid muscle. Within the parotid gland it is related superficial to retromandibular vein and external carotid artery. It divides into temporo-facial and cervico-facial trunks. Temporo-facial trunk gives out temporal and zygomatic branches. Cervico-facial trunk gives out buccal, marginal mandibular and cervical branches. Five terminal branches look like goose feet, known as pes anserinus.

Facial Nerve in Nut-Shell

           Motor

             Muscles of face, cheek, and scalp;

             Stapedius, muscle of middle ear;

             stylohyoid; and posterior belly of digastric

           Sensory

             Taste from anterior two thirds of tongue, floor

             of mouth, and palate, general sensation from external ear

           Secretomotor (Parasympathetic)

             Submandibular and sublingual salivary glands, lacrimal gland, and                glands of nose and Palate

          Passes through (Foramens)

             Internal acoustic meatus, facial canal, stylomastoid foramen

Facial Muscle Paralysis

Ø The facial muscles are innervated by the facial nerve. Damage to the facial nerve in the internal acoustic meatus (by a tumor), in the middle ear (by infection or operation), in the facial nerve canal (perineuritis, Bell’s palsy), or in the parotid gland (by a tumor) or caused by lacerations of the face will cause distortion of the face, with drooping of the lower eyelid, and the angle of the mouth will sag on the affected side. This is essentially a lower motor neuron lesion.

Ø An upper motor neuron lesion (involvement of the pyramidal tracts) will leave the upper part of the face normal because the neurons supplying this part of the face receive corticobulbar fibers from both cerebral cortices.

Testing the Integrity of the Facial Nerve

Ø If the 6th and 7th nerves are not functioning, this would suggest a lesion within the pons of the brain.

Ø If the 8th and 7th nerves are not functioning, this would suggest a lesion in the internal acoustic meatus.

Ø  If the patient is excessively sensitive to sound in one ear, the lesion probably involves the nerve to the stapedius.

Ø  Loss of taste over the anterior two thirds of the tongue implies that the seventh nerve is damaged proximal to the point where it gives off the chorda tympani.

Ø To test the facial nerve, the patient is asked to show the teeth by separating the lips with the teeth clenched, and then to close the eyes. Taste on each half of the anterior two thirds of the tongue can be tested with sugar, salt, vinegar, and quinine for the sweet, salt, sour, and bitter sensations, respectively.

Ø part of the facial nerve nucleus that controls the muscles of the upper part of the face receives corticobulbar fibers from both cerebral cortices. Therefore, in patients with an upper motor neuron lesion, only the muscles of the lower part of the face will be paralyzed.

Ø However, in patients with a lower motor neuron lesion, all the muscles on the affected side of the face will be paralyzed. The lower eyelid will droop, and the angle of the mouth will sag. Tears will flow over the lower eyelid, and saliva will dribble from the corner of the mouth. The patient will be unable to close the eye and cannot expose the teeth fully on the affected side.