It belongs to the cranial nerves in position VII. It is the nerve that is responsible for facial expression.
The facial nerve pathways are variable, and knowledge of the key in intratemporal and extratemporal milestones is essential for an accurate physical diagnosis and safe and effective surgical intervention on the head and neck.
The facial nerve is made up of approximately 10,000 neurons, 7,000 of which are myelinated and innervate the nerves of facial expression .
Three thousand of the nerve fibers make up the intermediate nerve and are somatosensory and secretomotor.
Facial nerve embryology
In the third week of gestation, the facioacoustic primordium gives rise to the VII and VIII cranial nerves. During the fourth week, the chorda tympani can be distinguished from the main branch.
The first goes ventrally to the first branchial arch and ends near a branch of the trigeminal nerve that eventually becomes the lingual nerve. The main trunk goes to the mesenchyme, approaching the epibranchial placode.
The geniculate ganglion, the medial nerve, and the greater petrosal nerve are visible by the fifth week. The second branchial arch gives rise to the muscles of facial expression in the seventh and eighth weeks.
To innervate these muscles, the facial nerve runs through the region and eventually becomes the middle ear. By the eleventh week, the facial nerve has become extensively arborized.
In the newborn, the anatomy of the facial nerve approximates that of an adult, except for its location in the mastoid, which is more superficial.
Cortex and inner capsule
Voluntary responses of the facial muscles (eg, Smile when taking a photograph) arise from efferent discharge from the motor face area of the cerebral cortex.
The motor face area is located in the precentral and postcentral convolutions. The facial motor nerves are represented in the homunculus diagram below with the forehead facing up and the eyelids, midface, nose, and lips located sequentially more inferiorly.
Discharges from the facial motor area are carried through the fascicles of the corticobulbar tract to the internal capsule, then through the upper midbrain to the lower brainstem, where they synapse on the nucleus of the pontine facial nerve.
The nucleus of the pontine facial nerve divides into an upper and a lower half, bilaterally.
The corticobulbar tracts of the superior face cross and cross again on the way to the pons; tracts to the underside intersect only once.
First studies of the facial nerve
In 1987, Jenny and Saper conducted an extensive study of proximal facial nerve organizations in a primate model and found evidence that in monkeys, upper facial movement is relatively preserved in upper motor neuron injury, because these neurons receive relatively little direct cortical input.
In contrast, the lower facial muscles are more severely affected, because their motor neurons depend on significant cortical innervation. The authors believe that these observations also explain similar findings in humans.
In their study, Jenny and Saper found that descending corticofacial fibers from monkeys innervated the lower facial nuclear motor region bilaterally, but with a contralateral predominance.
The upper nuclear regions of the facial motor received little direct cortical innervation on both sides of the brain.
The deficits seen with unilateral ablation of the corticobulbar fibers reflect the fact that the upper facial motor neurons do not receive significant cortical innervation.
Lower facial motor neurons contralateral to the lesion, which have functional loss, depend on direct contralateral cortical innervation, with the remaining ipsilateral cortical projections insufficient to conduct them.
These findings may explain why a focal lesion in the facial area on one side of the motor cortex in humans prevents closure of the eyelids and movement of the forehead, but results in paralysis of the lower face.
Patients may have preservation of forehead function with lesions in the nucleus of the pontine facial nerve, with selective lesions in the temporal bone, or with a lesion of the nerve in its distribution in the face.
An accurate neurological diagnosis is best accomplished by examining deficits in conjunction with “the company they keep.”
A cortical lesion that produces an inferior facial deficit is generally associated with a motor deficit of the tongue and weakness of the thumb, fingers, or hand on the ipsilateral side.
The nerve fibers that influence emotional facial expression are believed to arise in the thalamus and globus pallus.
Supranuclear pyramidal lesions generate the replacement movements of the face initiated as emotional responses and reflexes. With nuclear and infranuclear lesions, involuntary and voluntary facial movement loss occurs.
The nuclei of the facial nerve also receive attachments from other nuclei of the brain stem.
The entrance of the trigeminal nerve and the nucleus form the basis of the trigeminofacial reflexes; for example, the corneal reflex. The entrance of the acoustic nuclei to the nucleus of the facial nerve is part of the stapedial reflex response to loud noises.
Intratemporal course of the facial nerve
The facial nerve travels through the petrous temporal bone, in a bony canal called the facial or fallopian canal.
No other nerve in the body travels such a long distance through a bone canal. Due to this bony layer around the nerve, inflammatory processes involving the central nervous system and the facial nerve or traumatic injuries to the temporal bone can produce unique complications.
Laberintic segment (proximal)
The labyrinthine segment of the facial nerve lies below the middle cranial fossa and is the shortest segment in the facial canal (approximately 3.5-4 mm in length). In this segment, the nerve runs obliquely forward, perpendicular to the axis of the temporal bone.
The facial nerve and the medial nerve remain distinct entities at this level. The term labyrinthine segment is derived from the location of this segment of the nerve immediately posterior to the cochlea.
The nerve is posterolateral to the ampullary ends of the horizontal and superior semicircular canals and rests on the anterior part of the vestibule in this segment.
The labyrinthine segment is the narrowest part of the facial nerve and is susceptible to compression through edema.
This is the only segment of the facial nerve that lacks an arterial cascade anastomosis, rendering the area vulnerable to embolic phenomena, low-flow states, and vascular compression.
After traversing the labyrinthine segment, the facial nerve changes direction, marking the location of the geniculate ganglion.
The geniculate ganglion is formed by the union of the intermediate nerve and the facial nerve in a common trunk.
It is the sensory ganglion of the facial nerve, which supplies taste fibers from the anterior two thirds of the tongue through the tunica eardrum, as well as the taste fibers of the palate through the greater petrous nerve.
Extratemporal facial nerve
The facial nerve exits the facial canal through the stylomastoid foramen. The nerve travels between the digastric and stylohyoid muscles and enters the parotid gland .
A number of useful landmarks are used to locate the facial nerve. The topographic landmarks, shown in the image below, can serve as guides to locate the course of the facial nerve and its branches.
For example, a line drawn between the tip of the mastoid and the angle of the jaw can serve as a useful landmark for the upper limits of a neck dissection.
Removal of parotid tissue below this line can be performed with relative safety.
The topographic trajectory of the temporal and / or marginal mandibular rami should be identified during a rhytidoplasty, excision of the submandibular gland, and / or neck dissection.
The temporal branch can be located approximately along a line that extends from the lobe junction (approximately 5 mm below the tragus), anterior and superior to a point 1.5 cm above the lateral aspect of the ipsilateral eyebrow.
Surgical landmarks to the facial nerve include the tympanomastoid suture line, the tragal pointer, and the posterior belly of the digastric muscle.
The tympanomastoid suture line lies between the mastoid and tympanic segments of the temporal bone and is approximately 6-8 mm lateral to the stylomastoid foramen.
The main trunk of the nerve can also be found midway between (10 mm posteroinferior to) the blunt medial border of the tragal cartilage, the so-called “tragal pointer” and the posterior belly of the digastric muscle.
The nerve is generally inferior and medial to the pointer.
The facial nerve crosses the styloid process laterally and enters the parotid gland. The nerve lies in a fibrous plane that separates the deep and superficial lobes of the parotid gland.
In the parotid gland, the nerve divides into two main divisions in the so-called pes anserinus; that is, the superiorly directed cervicofacial divisions and the inferior ones of the facial nerve.
After the main point of division, there are 5 main branches of the facial nerve, as follows:
- Mandibular marginal.
The facial nerve is responsible for all the muscles of facial expression. Of these, the facial nerve innervates 14 of the 17 paired muscle groups of the face on its deep side.
The three muscles innervated at its superficial or lateral borders are the buccinator, the levator of the corner of the mouth, and the mental muscles.
There are frequent connections between the buccal and zygomatic branches. The temporal and marginal mandibular branches are at greatest risk during surgical procedures and are generally terminal connections without anastomotic connections.
Facial nerve palsy
The spectrum of facial motor dysfunction is broad and characterizing the degree of paralysis can be difficult. Various systems have been proposed, but since the mid-1980s, the House-Brackmann system has been widely used.
On this scale, grade I is assigned to normal function, and grade VI represents complete paralysis. The intermediate grades vary depending on the function at rest and with effort.