Facial nerve (cranial nerve VII)
Verbal and nonverbal communication are integral parts of daily life. While verbal communication is based on information exchange via spoken word, nonverbal communication is achieved through silent, nonverbal cues such as gestures, body language and facial expressions.
Facial expressions, or microexpressions, are generated by the facial muscles, a group of mimetic muscles found underneath the skin of the face and scalp.
The seventh cranial nerve (CN VII), the facial nerve, is responsible for providing motor innervation to these facial muscles, enabling you to smile or frown. In addition to motor fibers, this multitasking nerve also contains sensory and parasympathetic components.
This article will allow you to explore the anatomy and variety of functions of the facial nerve while also reviewing related clinical conditions.
|Structure and features||
Fibers: General somatic afferent (GSA), special visceral afferent (SVA), general visceral efferent (GVE), special visceral efferent (SVE, branchiomotor)
Origin: Cerebellopontine angle (Motor and sensory roots)
Exits skull: Stylomastoid foramen
Associated nuclei: Motor nucleus of facial nerve (SVE), superior salivatory nucleus (GVE), nucleus of solitary tract (SVA), principal sensory nucleus of trigeminal nerve (GSA)
Associated ganglia: Geniculate ganglion (pterygopalatine ganglion, otic ganglion, submandibular ganglion)
|Parts and branches||
Intracranial: Origin → internal acoustic meatus
Intratemporal: Internal acoustic meatus → stylomastoid foramen
Greater petrosal nerve, nerve to stapedius muscle, chorda tympani
Extracranial: After stylomastoid foramen
Posterior auricular nerve, digastric branch, stylohyoid branch
Parotid plexus: temporal branches, zygomatic branches, buccal branches, marginal mandibular branch, cervical branch
Main: Motor innervation to muscles of facial expression (SVE)
Others: Taste innervation of anterior two-thirds of tongue and palate (SVA), parasympathetic innervation of lacrimal, nasal, palatine and salivary glands (except parotid) (GVE), sensation to parts of auricle and retroauricular region (GSA)
- Origin and course
- Branches and innervation
- Functions of the facial nerve
- Clinical relations
Origin and course
The facial nerve (CN VII) arises from two divisions: a motor root and a smaller sensory root, commonly referred to as the intermediate nerve.
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.
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.
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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Branches and innervation
As the facial nerve gives off multiple branches along its course within the cranial cavity and once it exits the skull, dividing its branches according to their anatomical location will facilitate learning of the structures.
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.
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.
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.
Functions of the facial nerve
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:
Test yourself and consolidate what you've learned about the facial nerve with our quiz below!
Facial nerve injury
The facial nerve (CN VII) has a wide range of functions. Thus, damage to CN VII will produce variable symptoms, depending on the site of lesion and affected branch(es).
Similar to the divisions of its branches, lesions can occur during the intracranial, intratemporal or extracranial course of the nerve which result in different clinical manifestations.
Intratemporal facial nerve lesions
Damage to the nerve near its origin, the geniculate ganglion or within the facial canal will lead to loss of motor, taste as well as autonomic functions, in some cases. The potential affected branches would include:
- Greater petrosal nerve: impaired lacrimal fluid production ipsilaterally,
- Nerve to stapedius: hypersensitivity to sound
- Chorda tympani: reduced salivary secretion of submandibular/sublingual glands and loss of gustatory function of the anterior two thirds of the tongue ipsilaterally.
Extracranial facial nerve lesions
Once the nerve has exited the skull, the facial nerve can be damaged along its extracranial course to the face. The supply of motor fibers will thus be disrupted, resulting in paralysis or weakness of the facial expression muscles (facial nerve palsy).
Extracranial lesions are associated with a variety of etiologies:
- Idiopathic (formerly known as Bell’s palsy): sudden, ipsilateral facial paralysis with no detectable cause.
- Infection: viral infection, typically due to herpes simplex virus type 1 (HSV-1), which can lead to inflammation of the nerve (viral neuritis).
- Iatrogenic: damage to terminal five branches of the facial nerve during surgical removal of parotid gland.
- Compression: during forceps delivery leading to neonatal facial palsy.
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