In this note, we are going to discuss the anatomy and function of the facial nerve (n. facialis), which is the CN VII.
It is responsible for several functions, including facial expressions, taste sensation, salivation, and lacrimation.
According to the generally accepted classification of functional components, it contains several fiber types.
- General somatic afferent fibers (GSA)
- Special visceral afferent fibers (SVA)
- General visceral efferent fibers (GVE)
- And special visceral efferent fibers (SVE)
Sometimes the sensory and parasympathetic fibers in the facial nerve are distinguished separately as the so-called intermediate nerve (n. intermedius) (CNXIII, or the nerve of Wrisberg).
The source of motor fibers is the motor nucleus of the facial nerve (nucleus nervi facialis), located in the dorsal part of the pons, anteriorly to the abducens nucleus.
This nucleus is characterized by a certain somatic and topical organization. It consists of separate cell groups, each of which innervates a specific muscle or muscle group. Motor root axons pass dorsally towards the floor of the IV ventricle, looping around the nucleus of the abducens nerve, forming the so-called internal genu of the facial nerve, and then go ventrolaterally.
There is a small elevated area on the surface of the rhomboid fossa called the facial colliculus (colliculus facialis) at the place where the facial nerve loops around the nucleus of the abducens nerve.
The source of preganglionic parasympathetic fibers is the superior salivatory nucleus (nucl. salivalorius superior), which is located in the tegmentum of the pons.
The geniculate ganglion (ganglion geniculi),
which is located in the facial canal of the temporal bone, contains the cell bodies of the sensory neurons.
These fibers travel towards the medulla to reach the solitary nucleus and the spinal trigeminal nucleus.
It is believed that the facial nerve transmits general sensation only from the tympanic membrane and a small part of the external acoustic meatus.
The facial nerve exits the brain in the area of the cerebellopontine angle.
Piercing the dura mater, the nerve goes to the internal acoustic meatus, at the bottom of which there is an opening leading to the facial canal.
The motor fibers, repeating all the curvatures of the facial canal,
exit the petrous part of the temporal bone through the stylomastoid foramen and pierce the parotid gland.
Here the nerve forms the parotid plexus, from which five terminal motor branches arise.
But now let’s get back to the intratemporal branches that arise from the facial nerve within the facial canal: the greater petrosal nerve (n. petrosus major)
and the chorda tympani (chorda tympani).
The greater petrosal nerve emerges directly from the geniculate ganglion. It contains preganglionic parasympathetic fibers from the superior salivatory nucleus.
Exiting the facial canal, it passes through the following structures: the canal, hiatus, and groove for the greater petrosal nerve,
the foramen lacerum,
and then enters the pterygopalatine fossa via the pterygoid canal.
The chorda tympani is formed directly above the stylomastoid foramen.
It contains sensory fibers, which are the dendrites of the neurons of the geniculate ganglion,
and preganglionic parasympathetic fibers from the superior salivatory nucleus. The nerve enters the tympanic cavity near the posterior border of the tympanic membrane.
It exits the tympanic cavity through the petrotympanic fissure and enters the infratemporal fossa,
where it merges with the lingual nerve (a branch of the mandibular nerve) to carry special visceral afferent/taste sensation (SVA fibers) from the anterior two-thirds of the tongue.
We will discuss these intratemporal branches in greater detail a bit later.
Now let’s focus on the motor branches.
The motor fibers of the facial nerve innervate the structures originated from the second pharyngeal arch. These include the facial muscles, the platysma, the stapedius, the stylohyoid muscle, and the posterior belly of the digastric muscle.
The following branches arise toward the area of innervation:
1. The nerve to stapedius muscle (n. stapedius), which is a small intratemporal branch,
arises inside the facial canal and supplies SVE or branchiomotor fibers to the stapedius muscle located in the tympanic cavity.
2. The posterior auricular nerve (n. auricularis posterior) is the first extracranial branch.
It arises directly below the stylomastoid foramen, goes posteriorly and superiorly, innervating the intrinsic auricular muscles and occipital belly of the occipitofrontalis muscle.
It arises directly below the stylomastoid foramen, goes posteriorly and superiorly, innervating the intrinsic auricular muscles and occipital belly of the occipitofrontalis muscle.
3. The stylohyoid branch (r. stylohyoideus), which innervates the respective muscle and sometimes gives off a branch to the posterior belly of the digastric muscle.
Finally, the facial nerve pierces the parotid gland and gives rise to its five terminal branches:
1. The temporal branch (r. temporalis) emerges at the superior border of the parotid gland and innervates the frontal belly of the occipitofrontalis muscle, the orbicularis oculi muscle, and the corrugator supercillii.
2. The zygomatic branch (r. zygomaticus) emerges at the anterior border of the parotid gland and innervates the orbicularis oculi muscle.
3. The buccal branch (r. buccalis) emerges at the anterior border of the gland, inferiorly to the parotid duct and innervates the orbicularis oris, buccinator and zygomaticus muscles.
4. The marginal mandibular branch (r. marginalis mandibulae) emerges at the anterior border of the gland and supplies the depressor labii inferioris, depressor anguli oris and mentalis muscles.
5. The cervical branch (r. colli) emerges at the inferior border of the gland and innervates the platysma muscle.
As already mentioned, the motor nucleus of the facial nerve is not uniform. Its dorsal part innervates the muscles of the superior half of the face and receives fibers from the corticonuclear pathway of both ipsi- and contralateral side.
The ventral part of the nucleus innervates muscles of the lower half of the face and receives fibers only from the contralateral corticonuclear tract.
Therefore, a supranuclear lesion of the facial nerve will not result in a motor impairment of the upper half of the face (since they have double innervation). And the muscles of the lower half of the face will be affected on the side opposite to the side of the lesion.
Infranuclear lesions of the facial nerve are a bit different. The fibers exiting the nucleus innervate all the facial muscles of the ipsilateral side. Accordingly, lesions of these fibers result in facial paralysis on the affected side.
Now let’s return to the greater petrosal nerve.
Greater petrosal nerve
The preganglionic parasympathetic fibers of the greater petrosal nerve enter the pterygopalatine ganglion, the axons of which form postganglionic fibers.
The latter, as part of the branches of the trigeminal nerve, pass to the areas of innervation:
- via the inferior orbital fissure to the lacrimal gland, joining the zygomatic nerve (n. zygomaticus), then the lacrimal nerve (n. lacrimalis).
- via the sphenopalatine foramen to the glands of the nasal mucosa,
joining the posterior nasal nerves (nn. nasales posteriores).
via the greater palatine canal to the glands of the oral mucosa, joining the palatine nerves (nn. palatini).
Chorda tympani
The chorda tympani splits off from the main trunk of the facial nerve before exiting the stylomastoid foramen.
Its sensory fibers provide gustatory innervation of the anterior two-thirds of the tongue.
The gustatory fibers, which are the peripheral processes of the sensory neurons of the geniculate ganglion, go as part of the lingual nerve and end in the taste buds of the mucous membrane of the tongue.
In addition, preganglionic parasympathetic fibers (GVE fibers) from the chorda tympani travel to the submandibular ganglion, innervating the submandibular and sublingual glands which stimulate salivary secretions.
CN VII: Facial nerve
- Peripheral nervous system
- systema nervosum periphericum
- Abducens nerve
- n. abducens
- Nucleus of the abducens nerve
- nucleus nervi abducentis
- Facial nerve
- n. facialis
- Motor nucleus of the facial nerve
- nucleus nervi facialis
- Geniculate ganglion
- ganglion geniculi
- Nucleus of the solitary tract
- nucleus solitarius
- Superior salivatory nucleus
- nucleus salivatorius superior
- Greater petrosal nerve
- n. petrosus major
- Chorda tympani
- chorda tympani
- Nerve to the stapedius muscle
- n. stapedius
- Deep petrosal nerve
- n. petrosus profundus
- Nerve of the pterygoid canal
- n. canalis pterygoidei
- Communicating branch with the tympanic plexus
- r. communicans cum plexo tympanico
- Chorda tympani
- chorda tympani
- Posterior auricular nerve
- n. auricularis posterior
- Digastric branch
- r. digastricus
- Stylohyoid branch
- r. stylohyoideus
- Parotid plexus
- plexus parotideus
- Temporal branches
- rr. temporales
- Zygomatic branches
- rr. zygomatici
- Buccal branches
- rr. buccales
- Marginal mandibular branch
- ramus marginalis mandibulae
- Cervical branch
- ramus colli
