Facial nerve (cranial nerve VII)

Intracranial course

The motor root of the facial nerve originates in the motor nucleus of facial nerve at the lower pontine tegmentum. Motor root axons merge with fibers from the superior salivatory nucleus and then pass dorsally towards the floor of the fourth ventricle, looping around the nucleus of abducens nerve before exiting the brainstem at the cerebellopontine angle.

The fibers of the sensory root travel towards the medulla to reach the nucleus of solitary tract, the visceral sensory part of the brainstem involved in relaying signals of satiety. Further fibers are projected to the principal sensory nucleus of the trigeminal nerve, which receives sensory input from the oral cavity.

Intratemporal course

The facial nerve roots leave the cranial cavity via the internal acoustic meatus to enter the facial canal in the petrous part of the temporal bone, where they fuse to form the facial nerve proper. The nerve makes a sharp anterior-to-posterior turn at a point known as the geniculum of facial nerve. It also enlarges at this point as the geniculate ganglion, which contains the cell bodies of the sensory neurons in the facial nerve.

The greater petrosal nerve arises directly from the geniculate ganglion. The facial nerve then gives off two further intratemporal branches (nerve to stapedius muscle and the chorda tympani) which arise before the facial nerve exits the temporal bone.

Extracranial course

The facial nerve leaves the facial canal through the stylomastoid foramen. It then gives off the posterior auricular nerve before continuing to enter the parotid gland located in the parotidomasseteric region of the face. It concludes its course by piercing the parotid gland, of which five terminal branches arise to form the parotid plexus.

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Intratemporal branches

It is important to note that from its origin until it enters the internal acoustic meatus, the facial nerve does not give off any branches. Having covered a short distance within the temporal bone, the facial nerve travels along the medial wall of the tympanic cavity.

• The greater petrosal nerve is the first branch to emerge directly from the geniculate ganglion. On its course towards the foramen lacerum, it merges with the deep petrosal nerve carrying sympathetic fibers to form the nerve of the pterygoid canal. It travels to the pterygopalatine ganglion to provide preganglionic parasympathetic innervation to the lacrimal gland as well as mucous glands of the nasal cavity, maxillary sinus and palate.

• The second intratemporal branch of the facial nerve is the nerve to stapedius muscle, supplying SVE/branchiomotor fibers to the muscle (responsible for dampening vibrations and protecting the hearing apparatus when exposed to loud sounds).

• The final intratemporal branch is the chorda tympani which traverses the middle ear and exits via the petrotympanic fissure. The chorda tympani merges with the posterior aspect of the lingual nerve (a branch of the mandibular nerve (CN V3)) to carry special visceral afferent/taste sensation (SVA fibers) from the anterior two-thirds of the tongue. In addition, general visceral efferent/preganglionic parasympathetic fibers (GVE fibers) from the chorda tympani are carried to the submandibular ganglion, innervating the submandibular and sublingual glands which stimulates salivary secretions.

Extracranial branches

The facial nerve exits the skull via the stylomastoid foramen, after which it gives off the following branches:

• The posterior auricular nerve is the first extracranial branch to emerge which continues to provide motor innervation to the occipital belly of the occipitofrontalis muscle (occipital branch) and intrinsic auricular muscles (auricular branch). It equally provides innervation of the skin around the external acoustic meatus and the retroauricular region.

• Additionally, the digastric and stylohyoid branches of the facial nerve are given off which carry motor fibers to the respective muscles.

Finally, the facial nerve pierces the parotid gland (but does not innervate it) and bifurcates into superior (temporofacial) and inferior (cervicofacial) trunks, which further give rise to its five terminal branches:

• Temporal branches: supply the frontalis, orbicularis oculi and corrugator supercilii muscles.
• Zygomatic branches: supply the orbicularis oculi muscle.
• Buccal branches: supply the orbicularis oris, buccinator and zygomaticus muscles.
• Marginal mandibular branch: supplies the depressor labii inferioris, depressor anguli oris and mentalis muscles.
• Cervical branch: supplies the platysma muscle in the neck.

Mnemonic

The following mnemonic can be used to memorize the terminal branches of the facial nerve: "To Zanzibar By Motor Car", standing for Temporal, Zygomatic, Buccal, Marginal mandibular and Cervical branches.

Special visceral efferent (SVE) fibers

Special visceral efferent, or branchiomotor fibers in the facial nerve supply the flat skeletal muscles of the face and scalp, the stapedius muscle of the middle ear, the posterior belly of the digastric muscle, and the stylohyoid muscle. They belong to neurons whose cell bodies are located in the motor neuron of facial nerve.

General visceral efferent (GVE) fibers

General visceral efferent fibers in the facial nerve are involved in parasympathetic component of the autonomic nervous system and play an important role in the innervation of the lacrimal gland, nasal and palatine glands as well as the submandibular and sublingual glands. They belong to neurons whose cell bodies are located in the superior salivatory nucleus.

General somatic afferent (GSA) fibers

General somatic afferent (i.e. sensory) fibers belonging to neurons whose cell bodies which are located in the geniculate ganglion provide innervation of the skin around the external acoustic meatus and the retroauricular region. They synapse with second order neurons in the principal sensory nucleus of trigeminal nerve.

Special visceral afferent (GVA) fibers

Last but not least, the facial nerve (chorda tympani branch) has a special sensory function and is responsible for carrying special visceral afferent fibers to convey taste sensation from the anterior two thirds of the tongue and soft palate. They belong to neurons whose cells bodies are located in the geniculate ganglion, and synapse in the nucleus of solitary tract.

You can appreciate each structure discussed in this article in further detail below:

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