Seventh Cranial Nerve (CN VII) – Anatomy, Blood Supply

• The main motor nucleus
• The parasympathetic nuclei
• The sensory nucleus

Structure of Seventh Cranial Nerve (CN VII)

There are four major functions of the facial nerve:

• General somatic efferent (motor supply to facial muscles)
• General visceral efferent (parasympathetic secretomotor supply to submandibular and sublingual salivary glands and the lacrimal gland)
• Special visceral afferent (taste sensation from anterior two-thirds of the tongue)
• General somatic afferent (cutaneous sensations from the pinna and the external auditory meatus).[rx][rx]

Motor Pathway

The upper motor neuron is located in the facial motor area of the precentral gyrus. The axons from the upper motor neuron travel along the ipsilateral corticobulbar tract to the lower pons, where most fibers cross to the other side and synapse with the lower motor neuron. The main motor nucleus (lower motor neuron) divides into four subnuclei; dorsal, intermediate, lateral, and medial. The dorsal subnucleus innervates the facial muscles of the ipsilateral upper quadrant and receives corticobulbar input from both hemispheres. Conversely, the lateral subnucleus is connected to contralateral corticobulbar fibers only, and it innervates muscles of the ipsilateral lower quadrant of the face.[rx][rx] Due to this difference in innervation, in an upper motor neuron facial palsy, only the contralateral lower quadrant of the face is paralyzed, while the ipsilateral half of the face suffers paralysis in a lower motor neuron facial palsy. The main motor nucleus is responsible for the voluntary control of facial muscles. Also, the motor nucleus supplies the auricular muscles, the posterior belly of the digastric muscle, the stapedius muscle, and the stylohyoid muscle. The emotional facial expression follows a different pathway and is under the influence of the limbic and extrapyramidal systems.[rx]

Parasympathetic Pathway

The superior salivatory and lacrimal nuclei make up the parasympathetic nuclei of the facial nerve. They are in the lower pons posterolateral to the facial motor nucleus. Efferent fibers from the hypothalamus supply the superior salivatory nucleus. In addition, the nucleus of the solitary tract delivers information regarding taste to the superior salivatory nucleus. The superior salivatory nucleus supplies the sublingual and submandibular salivary glands, as well as the palatine and nasal glands. The inputs to the lacrimal nucleus are from (a) hypothalamus (emotional response), and (b) sensory trigeminal nerve (reflex lacrimation secondary to eye irritation). The parasympathetic efferent pathway to the facial nerve from the brainstem is through the nervus intermedius.[rx]

Sensory Pathway

The sensory nucleus, located posterolateral to the motor nucleus and parasympathetic nuclei in the pons, receives taste information from the palate, floor of the mouth, and anterior two-thirds of the tongue. The first-order neurons of the taste fibers are in the geniculate ganglion, and the taste fibers synapse in the nucleus of the tractus solitarius (second-order neuron) in the brainstem. Axons of the second-order neurons cross to the contralateral side and ascend through the medial lemniscus to the thalamus, where they synapse with the third-order neuron. Efferents from the third-order neurons ascend through the internal capsule and the corona radiata, terminating in the tasting area of the sensory cortex in the postcentral gyrus and the insula. Some projections reach the hypothalamus.[rx]

General sensory fibers also have their first-order neurons located in the geniculate ganglion and the second-order neuron located in the spinal trigeminal nucleus. The sensory and parasympathetic fibers of the facial nerve are collectively bound in a fascial sheath; these structures together have the name ‘nervus intermedius.’[rx]

• Special visceral efferent (SVE) fibers – (branchiomotor) are a major component of the facial nerve. Their function is to innervate the muscles of facial expression, the stapedius muscle, the stylohyoid muscle, and the posterior belly of the digastric muscle. The neurons of these fibers are localized in the facial nucleus in the caudal pontine tegmentum.
• General visceral efferent (GVE) fibers – (parasympathetic preganglionic motor fibers) leave the facial nerve as the greater petrosal nerve and the chorda tympani nerve. After synapsing in the pterygopalatine ganglion, the greater petrosal nerve provides postganglionic parasympathetic innervation to nasal, oral, and palatine glands.
• The general visceral afferent (GVA) – component is very small. The primary sensory neurons are in the geniculate ganglion and gather sensory information via the greater petrosal nerve from the nasal cavity, part of the soft palate, and the sinus cavities. Fibers enter the brain stem with the intermediate nerve and synapse in the nucleus of the solitary tract.
• The special visceral afferent (SVA) – component (primarily sensory) carries information about taste from the anterior two-thirds of the tongue. Its fibers travel with the lingual nerve and the chorda tympani. The primary neurons are in the geniculate ganglion – at the genu of the facial nerve. The fibers enter the brain via the intermediate nerve and terminate at the ipsilateral nucleus of the solitary tract in the rostral medulla.
• The general somatic afferent (GSA) – component gathers sensory information from the pinna of the ear and the external acoustic meatus by forming the auricular nerve together with the vagal nerve. Primary sensory neurons are located in the geniculate ganglion. Their central processes enter the brain stem with the intermediate nerve and terminate in the spinal nucleus of cranial nerve V.[rx][rx][rx]

Blood Supply of Seventh Cranial Nerve (CN VII)

The upper motor neuron of the facial nerve located in the precentral gyrus receives its blood supply from the middle cerebral artery, whereas the facial nucleus containing the lower motor neuron in the pons is supplied by the anterior inferior cerebellar artery which is a branch of the basilar artery. After exiting the brainstem, up to the internal auditory meatus, the facial nerve receives its blood supply from the internal auditory artery, which often branches off from the anterior inferior cerebellar artery or occasionally originates from the basilar artery directly. Within the facial canal, the petrosal branch of the middle meningeal artery and the stylomastoid artery both supply blood to the facial nerve. After the stylomastoid foramen, the main blood supply to the facial nerve is from a branch of the stylomastoid artery. Within the parotid gland, the facial nerve receives blood from the transverse facial artery and the superficial temporal artery as well as one of either the occipital artery or the posterior auricular artery.[rx]

Nerves

The course of the facial nerve axons inside the brainstem

The facial nerve is comprised of a motor root (containing motor fibers) and the nervus intermedius (containing sensory and parasympathetic fibers). The axons from the motor nucleus curve posteriorly around the abducent nucleus; they then pass through the facial colliculus of the floor of the fourth ventricle, and finally, emerge from the anterolateral brainstem. The nervus intermedius of the facial nerve also emerges from the anterolateral brainstem (pontomedullary junction).[rx]

Segments of the facial nerve 

• The first segment of the facial nerve –  is the intracranial/cisternal segment. This segment comprises the two roots of the nerve which emerge from the pons. The motor root and the nervus intermedius then pass through the posterior cranial fossa along with the vestibulocochlear nerve (VIII cranial nerve) at the cerebellopontine angle, finally entering the internal acoustic meatus in the temporal bone.
• The second segment of the facial nerve – is the meatal (canalicular) segment. This part of the facial nerve appears in the superior quadrant of the internal acoustic meatus.
• The third part is the labyrinthine segment – This segment begins after the facial nerve has passed through the internal acoustic meatus. After the internal acoustic meatus, the motor root of the facial nerve, and the nervus intermedius enter the facial canal, and they then pass between the cochlea and vestibule before bending posteriorly at the geniculate ganglion, at which point the motor root and nervus intermedius join. The labyrinthine segment gives off three branches: the greater superficial petrosal nerve (containing parasympathetic fibers for the lacrimal gland and taste fibers from the palate), the lesser petrosal nerve, and the external petrosal nerve.
• The fourth segment of the facial nerve – is the tympanic segment. The tympanic segment begins as the facial nerve passes posteriorly at the geniculate ganglion. This segment is in the medial wall of the middle ear cavity, directly below the lateral semicircular canal.
• The fifth segment of the facial nerve – is the mastoid segment. This segment begins after the tympanic segment moves downwards, distal from the pyramidal eminence of the middle ear cavity. From here, the mastoid segment travels through the facial canal and up to the stylomastoid foramen. This segment gives off three branches: the nerve to stapedius muscle, the chords tympani (containing parasympathetic fibers to sublingual and submandibular salivary glands and taste fibers from the anterior two-thirds of the tongue), and the sensory branch which joins the auricular branch of the vagus nerve. The sensory branch carries general somatic afferent fibers from the pinna and the external auditory meatus.
• The final segment of the facial nerve – is the extratemporal segment. This segment begins as the facial nerve exits the temporal bone through the stylomastoid foramen. After exiting, the facial nerve gives off two branches; the posterior auricular nerve (supplying the posterior auricular muscle, the superior auricular muscle, and the occipital belly of the occipitofrontal muscle) and the digastric nerve (supplying the posterior belly of the digastric muscle and the stylohyoid muscle). Next, this segment of the facial nerve enters the parotid gland. Within the parotid gland, the facial nerve gives off two main trunks. These are the superior temporofacial and inferior cervicofacial trunks. These two trunks give rise to the parotid plexus, and from this plexus, five branches emerge. These are known as the temporal (or frontal), zygomatic, buccal, mandibular, and cervical branches, which carry motor fibers to the facial muscles.[rx][rx][rx][rx]

The facial nerve exits the brain stem from its ventrolateral surface at the cerebellopontine angle. It consists of two parts: a proper facial nerve and the intermediate nerve. The proper facial nerve contains only a motor component and very small general somatic afferent component, whereas the intermediate nerve carries sensory and parasympathetic visceromotor components.

Parts of the facial nerve

• The facial nerve anatomy can be divided based on its relation to the cranium and the temporal bone into intracranial, intratemporal, and extratemporal parts.[rx]

Intracranial Part

• The upper motor neuron (UMN) of the facial nerve is located in the primary motor cortex of the frontal lobe. UMN axons descend ipsilaterally as the corticobulbar tract via the genu of the internal capsule and reach the facial nucleus in the pontine tegmentum. The facial nucleus is divided into a dorsal and ventral region. It contains the cell bodies of the facial nerve lower motor neurons (LMN).
• The dorsal region supplies innervation of the muscles of the upper face, whereas neurons in the ventral region innervate muscles of the lower face. The dorsal aspect of the facial nucleus receives input from both the left and right cerebral hemispheres. This results in both hemispheres having control over the muscles of the upper face. The ventral aspect of the facial nucleus receives mainly contralateral inputs. As a result, the left hemisphere partially controls partially the upper left and right side of the face, and fully the lower right side of the face.

The intermediate nerve carries descending parasympathetic GVE fibers from the superior salivatory nucleus and ascending GVA, GSA, and SVA fibers from the geniculate ganglion.

The Intrtemporal Part

• The infratemporal part of the facial nerve – begins when the facial nerve, together with the intermediate nerve, passes through the internal auditory meatus of the temporal bone to enter the facial canal within the petrous part of the temporal bone. After synapsing on the geniculate ganglion, the facial nerve gives rise to the first branch; the greater petrosal nerve, which carries visceromotor parasympathetic fibers (GVE) to the lacrimal gland and GVA from the nasal cavity, paranasal sinuses, and part of the soft palate. The greater petrosal nerve joins the deep petrosal nerve which carries sympathetic postganglionic fibers from the superior cervical ganglion. Together they form the nerve of the pterygoid canal that innervates the pterygopalatine ganglion in the pterygopalatine fossa.
• The second branch of the facial nerve – running in the facial canal is the nerve to the stapedius muscle, which provides motor (SVE) innervation to the stapedius muscle of the inner ear.
• The chorda tympani nerve – is the last branch of the facial nerve within the facial canal and at the same time, the terminal extension of the intermediate nerve. It runs through the ossicles in the middle ear and exits the tympanic cavity at the petrotympanic fissure where it joins the lingual nerve, which is itself a branch of the trigeminal nerve. The chords tympani nerve carries two kinds of fibers: parasympathetic  GVE to the submandibular ganglion and SVA fibers from the anterior two-thirds of the tongue. Fibers from the submandibular ganglion later innervate the submandibular and sublingual glands.[rx][rx][rx][rx]

Intracranial branches

• The greater petrosal nerve arises at the superior salivatory nucleus of the pons and provides parasympathetic innervation to several glands, including the nasal glands, the palatine glands, the lacrimal gland, and the pharyngeal gland. It also provides parasympathetic innervation to the sphenoid sinus, frontal sinus, maxillary sinus, ethmoid sinus, and nasal cavity. This nerve also includes taste fibers for the palate via the lesser palatine nerve and greater palatine nerve.
• The communicating branch to the otic ganglion arises at the geniculate ganglion and joins the lesser petrosal nerve to reach the otic ganglion.^[rx] The nerve to the stapedius provides motor innervation for the stapedius muscle in the middle ear The chorda tympani provides parasympathetic innervation to the sublingual and submandibular glands, as well as special sensory taste fibers for the anterior two-thirds of the tongue.^[rx]

Extracranial branches

Distal to the stylomastoid foramen, the following nerves branch off the facial nerve:

• The posterior auricular nerve which controls movements of some of the scalp muscles around the ear
• Branch to the posterior belly of the digastric muscle as well as the stylohyoid muscle
• Five major facial branches (in the parotid gland) – from top to bottom:
  • Temporal branch
  • Zygomatic branch
  • Buccal branch
  • Marginal mandibular branch
  • Cervical branch

Intraoperatively the facial nerve is recognized at 3 constant landmarks

• At the tip of the tragus where the nerve is 1 cm deep and inferior
• At the posterior belly of the digastric by tracing this back to the tympanic plate, the nerve can be found between these two structures
• By locating the posterior facial vein at the inferior aspect of the gland where the marginal branch would be seen crossing it.
• Lateral semicircular canal
• Foot of incus

The Extratemporal Part

The extratemporal part of the facial nerve begins when the facial nerve leaves the cranium through the stylomastoid foramen. As the facial nerve exits, it gives GSA fibers to the pinna of the ear and external auditory meatus and SVE fibers to the posterior belly of digastric, stylohyoid, the superior and inferior auricular, and occipitalis muscles. Thereafter, the facial nerve divides at the end of the posterior edge of the parotid gland into the terminal branches. Usually, five branches can be identified:

• The temporal branch – innervating the frontalis and orbicularis oris muscles and the muscles in the upper part of the face.
• The zygomatic branch – innervating the middle part of the face
• The buccal branch – innervating the cheek muscles, including the buccinator muscle
• The mandibular branch – innervating muscles of the lower part of the face
• The cervical branch – innervating the muscles below the chin and, among others, the platysma muscle[rx][rx]

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 a 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.

Muscles of Seventh Cranial Nerve (CN VII)

Essentially, all muscles of facial expression receive their innervation via the facial nerve. [rx][rx]As stated, the facial nerve innervates the following:

• The muscles of facial expression – responsible for the expression of emotions by changing facial expression
• The stylohyoid muscle – draws the hyoid bone backward, which initiates a swallowing action and elevates the tongue
• The posterior belly of the digastric muscle – together with the anterior belly of the digastric muscle, elevates  the hyoid bone and is involved in any complex movements involving the jaw
• The stapedius muscle of the middle ear – stabilizes the stapes, preventing excessive movement in response to loud sounds.

Functions of The facial nerve

Facial expression

The main function of the facial nerve is motor control of all 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. The taste sensation is sent to the gustatory portion (superior part) of the solitary nucleus. The general sensation from the anterior two-thirds of the 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 parasympathetic fibers that travel in the facial nerve originate in the superior salivatory nucleus.

The facial nerve also functions as the efferent limb of the corneal reflex.

References