CN VII: Facial nerve

CN VII: Facial nerveAnatomy and function of the facial nerve
~ 9 min

In this note, we are going to discuss the anatomy and function of the facial nerve (n. facialis), which is the CN VII.

Facial nerve (n. facialis)
Facial nerve (n. facialis)

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).

Intermediate nerve (n. intermedius)
Intermediate nerve (n. intermedius)

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.

Motor nucleus of the facial nerve (nucleus nervi facialis)
Motor nucleus of the facial nerve (nucleus nervi facialis)
Motor nucleus of the facial nerve (nucleus nervi facialis)
Motor nucleus of the facial nerve (nucleus nervi facialis)

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.

Facial nerve (n. facialis)
Facial nerve (n. facialis)
Genu of the facial nerve (genu nervi facialis)
Genu of the facial nerve (genu nervi facialis)

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.

Facial colliculus (colliculus facialis)
Facial colliculus (colliculus facialis)

The source of preganglionic parasympathetic fibers is the superior salivatory nucleus (nucl. salivalorius superior), which is located in the tegmentum of the pons.

Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)

The geniculate ganglion (ganglion geniculi),

Geniculate ganglion (ganglion geniculi)
Geniculate ganglion (ganglion geniculi)

which is located in the facial canal of the temporal bone, contains the cell bodies of the sensory neurons.

Facial canal (canalis facialis)
Facial canal (canalis facialis)

These fibers travel towards the medulla to reach the solitary nucleus and the spinal trigeminal nucleus.

Spinal trigeminal nucleus (n. spinalis nervi trigemini)
Spinal trigeminal nucleus (n. spinalis nervi trigemini)
Spinal trigeminal nucleus (n. spinalis nervi trigemini)
Spinal trigeminal nucleus (n. spinalis nervi trigemini)
Solitary nucleus (nucleus tractus solitarii)
Solitary nucleus (nucleus tractus solitarii)
Solitary nucleus (nucleus tractus solitarii)
Solitary nucleus (nucleus tractus solitarii)

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.

Facial nerve (n. facialis) exits the brainstem
Facial nerve (n. facialis) exits the brainstem

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.

Internal acoustic meatus (meatus acusticus internus)
Internal acoustic meatus (meatus acusticus internus)
Nerve
Nerve

The motor fibers, repeating all the curvatures of the facial canal,

Facial canal (canalis facialis)
Facial canal (canalis facialis)
Nerve in the facial canal
Nerve in the facial canal

exit the petrous part of the temporal bone through the stylomastoid foramen and pierce the parotid gland.

Stylomastoid foramen (foramen stylomastoideum)
Stylomastoid foramen (foramen stylomastoideum)

Here the nerve forms the parotid plexus, from which five terminal motor branches arise.

Parotid salivary gland (glandula parotidea)
Parotid salivary gland (glandula parotidea)
Parotid plexus (plexus intraparotideus)
Parotid plexus (plexus intraparotideus)
Pes anserinus major (pes anserinus major)
Pes anserinus major (pes anserinus major)

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)

Greater petrosal nerve (n. petrosus major)
Greater petrosal nerve (n. petrosus major)
Greater petrosal nerve (n. petrosus major)
Greater petrosal nerve (n. petrosus major)

and the chorda tympani (chorda tympani).

Chorda tympani (chorda tympani)
Chorda tympani (chorda tympani)

The greater petrosal nerve emerges directly from the geniculate ganglion. It contains preganglionic parasympathetic fibers from the superior salivatory nucleus.

Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)
Superior salivatory nucleus (nucleus salivatorius superior)

Exiting the facial canal, it passes through the following structures: the canal, hiatus, and groove for the greater petrosal nerve,

Hiatus for the greater petrosal nerve (hiatus canalis nervi petrosi majoris)
Hiatus for the greater petrosal nerve (hiatus canalis nervi petrosi majoris)
Hiatus for the greater petrosal nerve (hiatus canalis nervi petrosi majoris)
Hiatus for the greater petrosal nerve (hiatus canalis nervi petrosi majoris)
Groove for the greater petrosal nerve (sulcus nervi petrosi majoris)
Groove for the greater petrosal nerve (sulcus nervi petrosi majoris)
Groove for the greater petrosal nerve (sulcus nervi petrosi majoris)
Groove for the greater petrosal nerve (sulcus nervi petrosi majoris)

the foramen lacerum,

Foramen lacerum (foramen lacerum)
Foramen lacerum (foramen lacerum)

and then enters the pterygopalatine fossa via the pterygoid canal.

Pterygoid canal (canalis pterygoideus)
Pterygoid canal (canalis pterygoideus)
Pterygopalatine fossa (fossa pterygopalatina)
Pterygopalatine fossa (fossa pterygopalatina)

The chorda tympani is formed directly above the stylomastoid foramen.

Stylomastoid foramen (foramen stylomastoideum)
Stylomastoid foramen (foramen stylomastoideum)

It contains sensory fibers, which are the dendrites of the neurons of the geniculate ganglion,

Geniculate ganglion (ganglion geniculi)
Geniculate ganglion (ganglion geniculi)

and preganglionic parasympathetic fibers from the superior salivatory nucleus. The nerve enters the tympanic cavity near the posterior border of the tympanic membrane.

Tympanic cavity (cavitas tympani)
Tympanic cavity (cavitas tympani)

It exits the tympanic cavity through the petrotympanic fissure and enters the infratemporal fossa,

Petrotympanic fissure (fissura petrotympanica)
Petrotympanic fissure (fissura petrotympanica)
Infratemporal fossa (fossa infratemporalis)
Infratemporal fossa (fossa infratemporalis)

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.

Facial muscles
Facial muscles
Platysma (platysma)
Platysma (platysma)
Stylohyoid muscle (m. stylohyoideus)
Stylohyoid muscle (m. stylohyoideus)
Posterior belly of the digastric muscle (venter posterior m. digastrici)
Posterior belly of the digastric muscle (venter posterior m. digastrici)

The following branches arise toward the area of innervation:

1. The nerve to stapedius muscle (n. stapedius), which is a small intratemporal branch,

Nerve to stapedius muscle (n. stapedius)
Nerve to stapedius muscle (n. stapedius)

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.

Posterior auricular nerve (n. auricularis posterior)
Posterior auricular nerve (n. auricularis posterior)

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.

Stylohyoid branch (r. stylohyoideus)
Stylohyoid branch (r. stylohyoideus)

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.

Temporal branch (r. temporalis)
Temporal branch (r. temporalis)

2. The zygomatic branch (r. zygomaticus) emerges at the anterior border of the parotid gland and innervates the orbicularis oculi muscle.

Zygomatic branch (r. zygomaticus)
Zygomatic branch (r. zygomaticus)

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.

Buccal branch (r. buccalis)
Buccal branch (r. buccalis)

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.

Marginal mandibular branch (r. marginalis mandibulae)
Marginal mandibular branch (r. marginalis mandibulae)

5. The cervical branch (r. colli) emerges at the inferior border of the gland and innervates the platysma muscle.

Cervical branch (r. colli)
Cervical branch (r. colli)

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.

Pterygopalatine ganglion (ganglion pterygopalatinum)
Pterygopalatine ganglion (ganglion pterygopalatinum)
Pterygopalatine ganglion (ganglion pterygopalatinum)
Pterygopalatine ganglion (ganglion pterygopalatinum)

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).
Inferior orbital fissure (fissura orbitalis inferior)
Inferior orbital fissure (fissura orbitalis inferior)
Zygomatic nerve (n. zygomaticus)
Zygomatic nerve (n. zygomaticus)
Lacrimal nerve (n. lacrimalis)
Lacrimal nerve (n. lacrimalis)
  • via the sphenopalatine foramen to the glands of the nasal mucosa,
Sphenopalatine foramen (foramen sphenopalatinum)
Sphenopalatine foramen (foramen sphenopalatinum)

joining the posterior nasal nerves (nn. nasales posteriores).

Posterior nasal nerves (nn. nasales posteriores)
Posterior nasal nerves (nn. nasales posteriores)
Posterior nasal nerves (canalis palatinus major)
Posterior nasal nerves (canalis palatinus major)

via the greater palatine canal to the glands of the oral mucosa, joining the palatine nerves (nn. palatini).

Palatine nerves (nn. palatini)
Palatine nerves (nn. palatini)

Chorda tympani

The chorda tympani splits off from the main trunk of the facial nerve before exiting the stylomastoid foramen.

Chorda tympani (chorda tympani)
Chorda tympani (chorda tympani)

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.

Lingual nerve (n. lingualis)
Lingual nerve (n. lingualis)

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.

Submandibular ganglion (ganglion submandibulare)
Submandibular ganglion (ganglion submandibulare)
Submandibular ganglion (ganglion submandibulare)
Submandibular ganglion (ganglion submandibulare)
Submandibular salivary gland (glandula submandibularis)
Submandibular salivary gland (glandula submandibularis)
Sublingual salivary gland (glandula sublingualis)
Sublingual salivary gland (glandula sublingualis)
Dictionary

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
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