CN 7

• Facial motor area of precentral gyrus → ipsilateral corticobulbar tract to the lower pons → Most fibres cross to the other side and synapse with the lower motor neuron

• Main motor nucleus (LMN) divides into four subnuclei (dorsal, intermediate, lateral and medial)
• Dorsal subnucleus innervates facial muscles of ipsilateral upper quadrant and receives corticobulbar input from both hemispheres.
• Lateral subnucleus connected to contralateral corticobulbar fibres only and innervates ipsilateral lower quadrant of the face

• Course
• Originates in the facial motor nucleus, situated in the caudal pontine tegmentum → Efferent fibers project dorsomedially toward the floor of the fourth ventricle → loop around the abducens nucleus (forming the genu of the facial nerve and the facial colliculus) → project ventrolaterally to exit the brainstem between the pons and medulla. → Enter the IAM accompanied by the intermediate nerve and the vestibulocochlear nerve → continue in the facial canal and exit the skull via the stylomastoid foramen. → penetrate the parotid gland → distribute peripherally to supply the muscles of facial expression, the stylohyoid, the posterior digastric muscles, and the platysma.

• Branches of extracanalicular facial nerve

Branch of Facial Nerve	Muscle Supplied
Posterior auricular nerve	Posterior auricular, superior auricular, occipital belly of occipitofrontalis
Digastric nerve	Posterior belly of digastric, stylohyoid
Temporal branches	Anterior auricular, superior auricular, part of frontalis, upper part of orbicularis oculi
Zygomatic branches	Orbicularis oculi
Buccal branches	Buccinator, upper lip muscles (levator labii superioris, levator anguli oris, zygomaticus major & minor, risorius), orbicularis oris, nostril muscles (dilator naris, compressor naris)
Marginal mandibular branch	Muscles of lower lip (depressor anguli oris, depressor labii inferioris), orbicularis oris, mentalis
Cervical branch	Platysma

• The anterior belly of the digastric is innervated by CN V, whereas the posterior belly of the digastric muscle is innervated by CN VII.

• During its course in the facial canal, the intermediate nerve carries fibres that distribute peripherally

• Two branches: (bound together in a facial sheath)
• Greater superficial petrosal nerve
• Preganglionic parasympathetic fibers that originate in the hypothalamus → superior salivatory nucleus → GSPN → pterygopalatine (sphenopalatine) ganglion → lacrimal gland
• Lacrimal nucleus has inputs from
• Hypothalamus (emotional response)
• Sensory trigeminal nerve (reflex lacrimation due to eye irritation)
• Chorda tympani nerve
• Preganglionic parasympathetic fibers that originate in the hypothalamus → superior salivatory nucleus → submandibular ganglion → sublingual and submandibular salivary glands
• Chorda tympani also carries pseudounipolar gustatory neurons whose cell bodies are located in the geniculate ganglion.
• 1st order neurons:
• Taste buds in the anterior two thirds of the tongue (the dorsal third is supplied by the glossopharyngeal nerve) → Geniculate ganglion
• 2nd order neurons:
• Geniculate ganglion → nucleus of the solitary tract.
• 3rd order neurons:
• Crosses midline via medial lemniscus to the thalamus
• 4th order neurons
• Thalamus → anterior limb of internal capsule → corona radiata → sensory cortex in poster central gyrus and insula

• Arises in the pons. Begins as two roots:
• Motor root with axons from motor nucleus curve posteriorly around the abducent nucleus then pass through the facial colliculus of the floor of the fourth ventricle and then emerge from anterolateral brainstem
• The nervus intermedius (sensory and parasympathetic fibres) root emerges from the anterolateral brainstem (pontomedullary junction)

• The two roots pass through the post fossa along with the CN8 at the CPA to enter the IAM

• Enter IAM in temporal bone.

• Appears in the superior quadrant of the internal acoustic meatus
• A bony conduit that transmits VII & VIII cranial nerves from pontomedullary junction to inner ear.
• Divided by a bony lamina (falciform crest) into :
• A. Smaller superior part: Superior vestibular N. and Facial Nerve
• B. Larger Inferior part: Inferior vestibular N. and Cochlear nerve.

• Thinnest part and frequently breaks in skull base fracture

• Starts after FN passes through the internal acoustic meatus

• The motor root and nervus intermedius enter the facial canal (z-shaped structure)

• Pass between the cochlea and vestibule before bending posteriorly at the geniculate ganglion, where they join to form the facial nerve.

• Horizontal

• Begins as FN passes posteriorly at the geniculate ganglion

• Is in the medial wall off the middle ear cavity

• Directly below the lateral semicircular canal

• Vertical

• From pyramidal eminence of middle ear cavity, travels through the facial canal and up to the styloid foramen

• Gives off 3 branches
• Nerve to stapedius muscle
• The chorda tympani
• Parasympathetic fibres to sublingual and submandibular salivary glands
• Taste fibres from anterior two- thirds of the tongue
• Sensory branch
• Which joins the auricular branch of the vagus nerve (carries general somatic afferent fibres from the pinna and external auditory meatus

• Begins as FN exits the temporal bone through the stylomastoid foramen

• Gives off two branches:
• Posterior auricular nerve (supplying posterior auricular muscle, superior auricular muscle and occipital belly of digastric muscle)
• Digastric nerve (supplying posterior belly of digastric muscle and stylohyoid muscle)

• Then enters the parotid gland

• Two main trunks given off in the parotid gland:
• Superior temporofacial trunk & Inferior cervicofacial trunks
• Two trunks give rise to parotid plexus which gives rise to 5 branches:
Frontalis branch
• Aka Temporal branch
• Reappraisal of the anatomy of the frontotemporal branches of the facial nerve
• The frontal branches of the facial nerve course in a fat pad in the anterior temple area between the superficial layer of temporalis fascia and the galea, where they could be damaged during in the subgaleal elevation of the scalp flap.
 

• Pitanguy line facial nerve protection during pterional approaches
• 0.5 cm below tragus and 2 cm above eye brow, along zygoma to lateral orbital rim location of frontalis branch of facial nerve
 

• Ways to protect the nerve
Interfascial
• If only the superficial and not the deep fascia is elevated
• The superficial temporalis fascia covers the temporalis muscle and attaches it to the superior temporal line and to the zygomatic arch.
• Before attaching to the zygomatic arch the superficial temporalis fascia splits in two layers
• Superficial
• Deep
• The fronto-temporal branch of the facial nerve runs in a fat pad within the two layers of the superficial temporalis fascia.
• With the skin flap elevation, the superficial layer of the temporalis fascia and the superficial temporal fat pad are exposed.
• The superficial layer of the superficial temporalis fascia is incised in an arching fashion and elevated together with the underlying fat pad that contains the facial nerve.
• If required and if retraction on the skin flap will be prolonged, the dissection can continue down until the arch of the zygoma.
• Preserving the fat pad will prevent injuring the nerve from retraction pressure. Finally, the fat pad is carefully retracted anteriorly and the temporalis muscle covered by the deep layer of the superficial temporalis fascia is exposed.
• Mr Laraway: Stay on the deep layer of the temporal fascia: i.e. stay as close as to the muscle surface will keep the frontotemporal branches safe.
Subfascial/submuscular dissection technique
• If both layers of the temporal fascia are elevated and reflected downward.
• The temporalis muscle is incised and elevated together with the superficial temporalis fascia without proceeding with the interfascial dissection.
• Compared with the interfascial technique, the submuscular dissection lowers the risk of facial nerve injury but restricts the working space.
• In both interfascial and submuscular dissections, the deep temporal artery — branch from the internal maxillary artery — may be preserved as the main blood supply for the temporalis muscle.
 


• Zygomatic branch
• Buccal branch
• Mandibular branch
• Cervical branche

Components	Ganglia	Nuclei	Exit through Skull	Target Organ	Function
Somatic motor	ㅤ	Facial motor	Internal auditory meatus	Muscles of facial expression; stylohyoid, posterior digastric muscles; platysma	Multiple actions in the face and neck
Visceral motor	Pterygopalatine	Superior salivatory	ㅤ	Lacrimal gland	Gland secretion
ㅤ	Submandibular	ㅤ	ㅤ	Submandibular and sublingual glands	ㅤ
Special sense	Geniculate	Solitary tract	ㅤ	Taste buds in the anterior two thirds of the tongue	Taste

• 4 Major functions:
• 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 2/3 of tongue)
• General somatic afferent (cutaneous sensations from the pinna and the external auditory meatus)
• Altered sensation in the posterior aspect of the external auditory canal (Hitzelberger’s sign) is secondary to compression of sensory fibers in the nervus intermedius branch of VII.

graph TD
    A[Facial nerve palsy] --> B("Lower motor neuron<br>(forehead muscles affected)");
    A --> C("Upper motor neuron <br> (forehead muscles not <br> affected) for example, <br> cortical or brainstem infarct,<br> intracranial tumour");

    B --> D["Idiopathic<br>(59-70%)<br>Bell's palsy"];
    B --> E["Traumatic<br>(10-23%)<br>Temporal bone fracture<br>Iatrogenic (post-surgical)<br>Sharp/blunt facial trauma<br>Birth canal trauma"];
    B --> F["Viral<br>(4.5-7%)<br>Herpes zoster virus<br>(Ramsay Hunt syndrome)"];
    B --> G["Neoplastic<br>(2.2-5%)<br>Acoustic neuroma<br>Parotid malignancy"];
    B --> H["Other<br>(3-5%)<br>Acute or chronic otitis media<br>Malignant otitis externa<br>Lyme disease<br>Misdiagnosed"];

• UMN lesion
• Can be in precentral gyrus or along the course of the corticobulbar tract up to 2nd order neuron in pons
• Causes:
• MCA stroke
• SOL along the pathway
• Lacunar stroke most common
• Presentation:
• Paralysis of contralateral lower quadrant of face (SPARING the contralateral upper quadrant) .

• LMN lesion in the pons
• Causes:
• Stroke
• Neoplasia
• Inflammation
• Presentation: Ipsilateral facial weakness of entire half of face.

• LMN lesion after the nerve exits the brainstem
• Depend on which branches affected.
• Motor — ipsilateral facial weakness.
• Nerve to stapedius — hyperacusis.
• Greater petrosal branch — loss of lacrimation.
• Chorda tympani — loss taste anterior 2/3 tongue, reduced salivary function.
• Causes:
• Infections (HSV, varicella-zoster, HIV),
• Inflammatory conditions (sarcoidosis, GBS),
• Vascular conditions
• Trauma
• Neoplasms

• Sign differentiating supranuclear from infranuclear lesions

Supranuclear	Infranuclear
Forehead intact bilaterally	Total facial palsy
FND, Hemiplegia on side of facial palsy	No FND
Ataxia	No hemiplegia
Reflexes intact	No ataxia
Tone maintained	No reflexes
Drooping corner of mouth	Flaccid
Slight flattening of nasolabial fold.	Not an isolated finding
No muscle atrophy/fasciculation	Muscle atrophy / fasciculations present.

• Vestibular schwannoma
• Close proximity to vestibulocochlear nerve.
• Risk of damage intra-op.
• Use facial nerve monitoring to protect

• Parotid gland surgery
• Nerve branches off within the parotid gland — at risk intra-op.
• 50% develop temporary weakness.
• 7% permanent facial palsy

• Inferior alveolar nerve block during dental procedures
• Can cause facial palsy if injection malplaced

• Cervicofacial rhytidectomy "facelift"
• Deep plane technique infers most risk.
• Buccal branch most commonly affected.

• Herpes zoster oticus

• Third most common cause of seventh cranial nerve palsy

• CN VII dysfunction

• Sign and symptoms
• Altered sensation of taste (Chordae tympany nerve)
• Facial weakness
• Vesicular eruptions on the pinna and in the external auditory canal (Only general Sensory afferent areas of CN7)

• This test distinguishes facial nerve injuries proximal and distal to the geniculate ganglion.

• The test involves placing a narrow strip of thin paper on the conjunctiva to assess for lacrimation.
• Injuries proximal to the geniculate ganglion tend to produce a dry eye,
• Injuries distal to the ganglion do not interfere with lacrimation.

• Whether the location of the facial nerve injury is proximal or distal to the geniculate ganglion is important because the choice of surgical approach differs with different sites of injury.

• Involuntary, spontaneous, localised quivering of a few muscles, or bundles within a muscle, but which are insufficient to move a joint

• Numbers
• Most common peripheral paralysis of the seventh cranial nerve
• Incident 15 to 20 per 100,000
• 1/60 life time risk
• Recurrence rate is 8% to 12%.
• Even without treatment, 70% of patients will have complete resolution.

• Idiopathic condition

• Pathophysiology
• Compression of the seventh cranial nerve at the geniculate ganglion.
• The first portion of the facial canal, the labyrinthine segment, is the narrowest; most cases of compression occur in the labyrinthine segment.
• Due to the narrow opening of the facial canal, inflammation causes compression and ischemia of the nerve.

• Sign and symptoms
• Rapid onset
• Unilateral.
• Causing partial or complete weakness of one-half of the face
• Changes in taste
• Sensitivity to sound
• Alteration in lacrimation and salivation

• Clinical case:
• A 34-year old female who recently suffered Bells palsy after contracting the flu has noticed her eyes starting to water at lunch during work and at dinner. She takes no medication, and her past medical history is only remarkable for seasonal allergies. Her physical examination is unremarkable. What anatomical structure most likely houses the pathological disruption seen in this case?
• This is “crocodile tears”. The chorda tympani fibres that normally supply salivation have regrown from the ganglion down the GSPN, to supply instead lacrimation. The ganglion is in the wall of the middle ear within the petrous temporal bone.
  The correct answer is: Petrous temporal bone

• Aka palpebral-oculogyric reflex

• Refers to the movement of the eyeballs in an upward direction when the eyelids are forcefully closed

• Aim to protect the cornea

• Reflex arc
• Afferent
• CN 7
• Efferent
• CN 3 → Superior rectus

• Grade 4 requires protection of cornea

• Melkersson-Rosenthal syndrome
• Triad of
• Recurrent orofacial edema,
• Recurrent seventh cranial nerve palsy,
• Lingua plicata
• A chronic granulomatous inflammation affecting (intermittently or persistently) mucocutaneous tissues and orofacial innervation

• Lyme disease causes bilateral seventh cranial nerve palsy

• The area of the facial nerve around the geniculate ganglion is the most commonly damaged by blunt trauma.

• Heerfrodt syndrome
• Uveoparotid fever
• Seventh cranial nerve palsy in sarcoidosis

• Corneal Reflex
• Stimulate cornea — closure both eyelids.
• Afferent = trigeminal nerve (V). Efferent = facial nerve

• Orbicularis oculi reflex
• Various stimuli (light, sound etc). Menace. Bilateral eye blink.
• Afferent = trigeminal, optic, vestibulocochlear.
• Efferent = facial nerve
• It is the reflex blinking that occurs in response to the rapid approach of an object.
• The reflex comprises blinking of the eyelids, in order to protect the eyes from potential damage, but may also include turning of the head, neck, or even the trunk away from the optical stimulus that triggers the reflex
• The neural pathway of the menace reflex comprises the optic (II) and facial (VII) nerves. It is mediated by tectobulbar fibres in the rostral colliculi of the midbrain passing from the optic tract to accessory nuclei, and thence to the spinal cord and lower motor neurones that innervate the head, neck, and body muscles affected by the reflex. The facial nerve is mediated through a corticotectopontocerebellar pathway

• Orbicularis orris reflex (snout reflex)
• Percussion of the upper lip/side of nose results in ipsilateral elevation of angle of mouth
• Afferent = trigeminal nerve. Efferent = facial nerve