CN VII
The facial nerve is the seventh cranial nerve, or simply cranial nerve VII. It emerges from the
brainstem between the pons and the medulla, and controls the muscles of facial expression,
and functions in the conveyance of taste sensations from the anterior two-thirds of the
tongue and oral cavity. It also supplies preganglionic parasymphatetic fibers to several head
and neck ganglia.
Cranial nerve VII
Structure
The motor part of the facial nerve arises from the facial nerve nucleus in the pons while the
sensory and parasympathetic parts of the facial nerve arise from the nervus intermedius.
Upon reaching the temporal bone, the nerve's path can be divided into the internal auditory
canal, labyrinthine segment, intratympanic segment, and descending or vertical segment.
The labyrinthine segment is the narrowest portion of this pathway and is described to be
approximately 0.7mm in diameter. The descending segment is the area where the branches
of the chorda tympani and nerve to the stapedius branch from the facial nerve. The facial
nerve eventually exits via the stylomastoid foramen to enter into the parotid where it
branches into its peripheral segments.
The motor part and sensory part of the facial nerve enters the petrous temporal bone via
the internal auditory meatus (intimately close to the inner ear) then runs a tortuous course
(including two tight turns) through the facial canal, emerges from the stylomastoid
foramen and passes through the parotid gland, where it divides into five major branches.
Though it passes through the parotid gland, it does not innervate the gland (This is the
responsibility of cranial nerve IX, the glossopharyngeal nerve).
The facial nerve forms the geniculate ganglion within the facial canal at the genu, the first
bend in the canal.
The nerves of the scalp, face, and side of neck.
Intracranial branches

Greater petrosal nerve - provides parasympathetic innervation to several glands,
including the nasal gland, palatine gland, lacrimal gland, and pharyngeal gland. It also
provides parasympathetic innervation to the sphenoid sinus, frontal sinus, maxillary
sinus, ethmoid sinus and nasal cavity.

Nerve to stapedius - provides motor innervation for stapedius muscle in middle ear

Chorda tympani

Submandibular gland

Sublingual gland

Special sensory taste fibers for the anterior 2/3 of the tongue.
Extracranial branches
Distal to stylomastoid foramen, the following nerves branch off the facial nerve:

Posterior auricular nerve - controls movements of some of the scalp muscles around the
ear

Branch to Posterior belly of Digastric muscle as well as the Stylohyoid muscle

Five
major
facial
branches
(in
parotid
gland)
-
from
top
to
bottom
(a
helpful mnemonic being Tolong Zip Baju Makcik Cepat):

Temporal branch of the facial nerve

Zygomatic branch of the facial nerve

Buccal branch of the facial nerve

Marginal mandibular branch of the facial nerve

Cervical branch of the facial nerve
Intra operatively the facial nerve is recognized at 3 constant land marks:
1. At the tip of tragal cartilage where the nerve is 1cm deep and inferior
2. At the posterior belly of digastric by tracing this backwards to the tympanic plate the
nerve can be found between these two structures
3. By locating the posterior facial vein at the inferior aspect of the gland where the
marginal branch would be seen crossing it.
Nucleus
The cell bodies for the facial nerve are grouped in anatomical areas called nuclei or ganglia.
The cell bodies for the afferent nerves are found in the geniculate ganglion for taste
sensation. The cell bodies for muscular efferent nerves are found in the facial motor
nucleus whereas the cell bodies for the parasympathetic efferent nerves are found in
the superior salivatory nucleus.
Embryology
The
facial
nerve
is
developmentally
derived
from
the hyoid
arch (second
pharyngeal branchial arch). The motor division of the facial nerve is derived from the basal
plate of the embryonic pons, while the sensory division originates from the cranial neural
crest.
Function
Facial expression
The main function of the facial nerve is motor control of most of the muscles of facial
expression.
It
also
innervates
the
posterior
belly
of
the digastric muscle,
the
stylohyoid muscle, and the stapedius muscle of the middle ear. All of these muscles are
striated muscles of branchiomeric origin developing from the 2nd pharyngeal arch.
Facial sensation
In addition, the facial nerve receives taste sensations from the anterior two-thirds of
the tongue via the chorda tympani; taste sensation is sent to the gustatory portion of
the solitary nucleus. General sensation from the anterior two-thirds of tongue are supplied by
afferent fibers of the third division of the fifth cranial nerve (V-3). These sensory (V-3) and
taste (VII) fibers travel together as the lingual nerve briefly before the chorda tympani leaves
the lingual nerve to enter the tympanic cavity (middle ear) via the petrotympanic fissure. It
joins the rest of the facial nerve via the canaliculus for chorda tympani. The facial nerve then
forms the geniculate ganglion, which contains the cell bodies of the taste fibers of chorda
tympani and other taste and sensory pathways. From the geniculate ganglion the taste fibers
continue as the intermediate nerve which goes to the upper anterior quadrant of the fundus
of the internal acoustic meatus along with the motor root of the facial nerve. The intermediate
nerve reaches the posterior cranial fossa via the internal acoustic meatus before synapsing
in the solitary nucleus.
The facial nerve also supplies a small amount of afferent innervation to the oropharynx below
the palatine tonsil. There is also a small amount of cutaneous sensation carried by
the nervus intermedius from the skin in and around the auricle (outer ear).
Other
The
facial
nerve
also
supplies parasympathetic fibers
to
the submandibular
gland and sublingual glands via chorda tympani. Parasympathetic innervation serves to
increase the flow of saliva from these glands. It also supplies parasympathetic innervation to
the nasal mucosa and the lacrimal gland via the pterygopalatine ganglion.
The facial nerve also functions as the efferent limb of the corneal reflex.
Functional components
The facial nerve carries axons of type GSA, general somatic afferent, to skin of the posterior
ear.
The facial nerve also carries axons of type GVE, general visceral efferent, which innervate
the sublingual, submandibular ("spit"), and lacrimal glands ("cry"), also mucosa of nasal
cavity ("snot").
The facial nerve also carries axons of type SVE, special branchial-motor efferent, which
innervate muscles of facial expression, stapedius, the posterior belly of digastric, and the
stylohyoid.
The facial nerve also carries axons of type SVA, special visceral afferent, which provide taste
to anterior two-thirds of tongue via chorda tympani
The facial nerve also carries axons of type GVA, general visceral afferent, which provide
sensation to the soft palate and parts of the nasal cavity.
Examination
Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the
eyes tightly (inability to do so is called lagophthalmos)[1] , pursing the lips and puffing out the
cheeks, all test the facial nerve. There should be no noticeable asymmetry.
In an UMN lesion, called central seven, only the lower part of the face on the contralateral
side will be affected, due to the bilateral control to the upper facial muscles (frontalis and
orbicularis oculi).
Lower motor neuron lesions can result in a CNVII palsy (Bell's palsy is the idiopathic form of
facial nerve palsy), manifested as both upper and lower facial weakness on the same side of
the lesion.
Taste can be tested on the anterior 2/3 of the tongue. This can be tested with a swab dipped
in a flavoured solution, or with electronic stimulation (similar to putting your tongue on a
battery).
Corneal reflex. The afferent arc is mediated by the General Sensory afferents of
the Trigeminal Nerve. The efferent arc occurs via the Facial Nerve. The reflex involves
consensual blinking of both eyes in response to stimulation of one eye. This is due to the
Facial Nerve's innervation of the muscles of facial expression, namely Orbicularis oculi,
responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of
both Cranial Nerves V and VII.