Facial Nerve

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Description

The Facial Nerve is the seventh Cranial Nerve.

It is composed of approximately 10,000 neurons, 7,000 of which are myelinated and innervate the nerves of facial expression.

The remaining 3,000 fibres are somatosensory and secretomotor, and are known as the Nervus Intermedius.

Movements Produced

All movements of facial expression, including:

Smile, close eyes, pucker lips, wrinkle nose, raise eyebrows, frown.

General Course of Nerve

The facial nerve has six named segments :

1. Intracranial
2. Meatal
3. Labyrinthine
4. Tympanic
5. Mastoid
6. Extratemporal

Intracranial (cisternal) segment

The nerve lies below the pons, lateral to the abducens nerve, medial to the vesibulococholear nerve.

The facial nerve and the nervus intermedius pass through the cerebellopontine angle to the internal acoustic meatus.

Meatal segment

The facial nerve and nervos intermedius lie in the upper quadrant, and pass through the internal acoustic meatus.

Labyrinthine segment

The facial and nervus intermedius both enter the Fallopian canal, then travel posteriorly at the geniculate ganglion; at this point the nervus intermdius joins the facial nerve, and 3 branches originate:

• greater superficial petrosal nerve
• lesser petrosal nerve
• external petrosal nerve

Tympanic segment

Here the  facial nerve  lies immediately below the lateral semicircular canal in the medial wall of the middle ear. It passes behind the cocholeariform orocess and oval window. 

Mastoid segment

Just distal to the pyramidal eminence thee nerve turns and passes vertically downwards heading to the sylomastoid foramen, through the fallopian canal. It gives off  3 branches:

• nerve to stapedius
• to chorda tympani (branch of nervus intermedius provides secretomotor fibres to the submandibular & sublingual glands, as well as taste sensation to the anterior two thrids of the tongue
• nerve from the auricular branch of the Vagus Nerve [CN 10] provides pain fibres to the posteriorpart of the external acoustic meatus

Extratemporal segment

As the nerve exits the stylomastoid foramen it gives off a sensory branch to part of the external acoustic meatus & tympanic  membrane.

It then passes between the stylohyoid and digastric muscles and enters the parotid gland, where it lies between the deep and superficial lobes of the gland. Here it divides into two main branches (at the pes asnerinus): superior temporofacial and inferior cervicofacial branches.

From the anterior border of the gland, 5 branches emerge:

1. temporal
2. zygomatic
3. buccal
4. mandibular
5. cervical

There is another detailed diagram of the course of the Facial Nerve on the Facial Palsy page.

5 Distal Branches

The facial nerve innervates 14 of the 17 paired muscle groups of the face on their deep side. The 3 facial muscles are innervated from other sources: buccinator, levator anguli oris, and mentalis.

1. Temporal (Frontal)
2. Zygomatic
3. Buccal
4. Marginal mandibular
5. Cervical
   

The temporal trunk innervates the following muscles:

• Frontalis
• Orbicularis oculi
• Corrugator supercilii
• Pyramidalis

The zygomatic division innervates the following muscles:

• Zygomaticus major
• Zygomaticus minor
• Elevator ala nasi
• Levator labii superioris
• Caninus
  Depressor septi
• Compressor nasi
• Dilatator naris muscles

The buccal division gives off fibers to innervate the buccinator and superior part of the orbicularis oris muscle.

Mandibular division innervations are found in the following muscles:

• Risorius
• Quadratus labii inferioris
• Triangularis
• Mentalis
• Lower parts of the orbicularis oris

The cervical division provides platysma innervation.

Embryology

By the third week of gestation, the fascioacoustic primordium gives rise to CN VII and VIII. During the fourth week, the chorda tympani can be discerned from the main branch. The former courses ventrally into the first branchial arch and terminates near a branch of the trigeminal nerve that eventually becomes the lingual nerve. The main trunk courses into the mesenchyme, approaching the epibranchial placode.
The geniculate ganglion, nervus intermedius, and greater superficial petrosal nerve are visible by the fifth week. The second branchial arch gives rise to the muscles of facial expression in the seventh and eighth week. To innervate these muscles, the facial nerve courses across the region that eventually becomes the middle ear. By the eleventh week, the facial nerve has arborized extensively. In the newborn, the facial nerve anatomy approximates that of an adult, except for its location in the mastoid, which is more superficial.

Imaging

Computed tomography (CT) scanning and magnetic resonance imaging (MRI) are useful in the diagnosis of injury to intratemporal and/or intracranial affections of the facial nerve, as they may reveal temporal fracture patterns (vertical, transversal, mixed) and edema formation. Under certain circumstances, the facial nerve can be viewed, and swelling or disruption may be seen.

Recent Related Research (from Pubmed)

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