Facial Nerve

Original Editor - Wendy Walker.

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Introduction

Facial nerve

The Facial Nerve is the seventh Cranial Nerve.

It is composed of approximately 10,000 neurons which comprise 2 roots:

1. 7,000 myelinated, innervating the muscles of facial expression (motor root).
2. 3,000 fibres, being mixed sensory, taste and parasympathetic root, known as the Nervus Intermedius.

This nerve has an extremely complicated course, and the description below is a simplified overview which provides the main details which physiotherapists are required to be aware of when treating patients with damage to the Facial Nerve.

Smile

For a full description of the complexities of this nerve, please see the video lower down on the page.

The facial nerve is responsible for all movements of facial expression, including:

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

General Course of Nerve

In anatomical terms the facial nerve can be divided into 2 main parts :

1. Intracranial - inside the brain and the skull
2. Extracranial - in the face and neck

Intracranial segment

Facial nerve and brainstem nuclei

Both the motor root and nervus intermedius emerge from the pontomedullary sulcus and pass into the internal acoustic meatus merging at the geniculate ganglion.

The geniculate ganglion, located in the facial canal, is a sensory ganglion of the facial nerve. It contains the cell bodies of the fibers responsible for conducting taste sensation from the anterior two-thirds of the tongue. Also, neurons located at the ganglion contribute to the sensory innervation of other sites, such as the palate, the pinna of the ear and ear canal.

Geniculate-ganglion

From here it performs a number of turns as it passes through the temporal bone closely related to the inner ear and middle ear, giving off a number of branches, before exiting the skull via the stylomastoid foramen.

Extracranial segment

The first extracranial branch is Posterior Auricular Nerve which is the motor supply to some of the muscles around the ear, then immediately after this another small branch provides motor fibres to the posterior belly of the Digastric Muscle and to the Stylohyoid muscle.

It then passes between the stylohyoid and digastric muscle 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 parotid gland, 5 branches emerge:

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

These branches have many and varied connections/pathways running between them; in addition, there are communicating pathways from other cranial nerves including (but not limited to) the Trigeminal Nerve, Vestibular/Auditory Nerve and Hypoglossal Nerve.

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

The details of the exact connections between the five terminal branches can vary hugely in different individuals. This should be no surprise to us, as it is easy to see the wide variety of different smile shapes, for instance. One recent study on cadavers, in 2018, looked at the length, diameter of divisions, number and course of terminal branches and the connections between them, and identified 12 different patterns.

Five Distal Branches

Schematic drawing of the facial nerve

The 5 branches are:

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

For details of the muscles listed below, please see the anatomy pages Facial Muscles - Upper Group and Facial Muscles - Lower Group.

The temporal or frontal trunk innervates the following muscles:

• Frontalis
• Orbicularis oculi (superior portion and palpebral region/eyelid)
• Corrugator supercilii
• Procerus (AKA Pyramidalis Nasi)

The zygomatic division innervates the following muscles:

• Orbicularis oculi (lower/inferior portion)
• Zygomaticus major (also supplied by buccal branch)
• Elevator ala nasi
• Levator labii superioris

The buccal division gives off fibres to innervate:

• Buccinator
• Orbicularis oris, superior portion
• Zygomaticus major (also supplied by zygomatic branch)
• Zygomaticus minor
• Risorius
• Levator Anguli Oris
• Compressor (AKA Transverse) nasi
• Dilatator naris muscles
• Depressor septi nasi (AKA Depressor Alae Nasi)

Mandibular division innervations are found in the following muscles:

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

The cervical division provides platysma innervation.

Here is a video describing the course of the Facial Nerve, both intracranial and on the face:

Embryology

The facial nerve (along with the acoustic nerve, CN 8) arises from the fascioacoustic primordium which forms by the 3rd week of gestation.

The geniculate ganglion, greater superficial petrosal nerve and nervus intermedius are all visible by the 5th week of gestation.

The 2nd branchial arch gives rise to the muscles of facial expression in the 7th & 8th weeks. By the 11th week the facial nerve has developed its branches.

In the newborn baby the facial nerve anatomy is the same as that of an adult, with the exception of its location in the mastoid, which is more superficial in the baby.

Imaging

CT scanning and 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 oedema formation. Under certain circumstances, the facial nerve can be viewed, and swelling or disruption may be seen.