Facial Nerve Anatomy



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Facial Nerve Anatomy
The facial nerve is the nerve of facial expression. The pathways of the facial nerve are variable, and knowledge of the key intratemporal and extratemporal landmarks is essential for accurate physical diagnosis and safe and effective surgical intervention in the head and neck.
The facial nerve is composed of approximately 10,000 neurons, 7,000 of which are myelinated and innervate the nerves of facial expression. Three thousand of the nerve fibers are somatosensory and secretomotor and comprise the nervus intermedius. The course of the facial nerve and its central connections can be roughly divided into 6 segments:


Segment

Location

 Length in mm

Supranuclear

Cerebral Cortex

N.A.

Brain stem

Motor nucleus of facial nerve, superior salivatory nucleus of tractus solitarius

N.A.

Meatal segment

Brain stem to Internal Auditory Canal (IAC)

Thirteen to Fifteen

Labyrinthine segment

Fundus of IAC to facial hiatus

Three to four

Tympanic segment

Geniculate ganglion to pyramidal eminence

Eight to Eleven

Mastoid segment

Pyramidal process to stylomastoid foramen

Ten to Fourteen

Extratemporal segment

Stylomastoid foramen to pes anserinus

Fifteen to Twenty



Anomalous Courses of Facial Nerve

Dehiscence of facial canal. Most common anomaly

Common sites: oval window and geniculate ganglion. Exposed nerve is more susceptible to injury during otologic surgery.

Most course anomalies are within temporal bone

1.Prolapse of nerve against stapes

2.Bifurcation around stapes

3.Deviation across promontory

4.Knuckle at the pyramidal (second) turn

5.

Anomalies are more common in malformations of the ear.

The facial nerve has four components with distinct functions:




  • Branchial motor (special visceral efferent) Supplies the muscles of facial expression; posterior belly of digastric muscle; stylohyoid, and stapedius. Branchial motor fibers constitute largest portion of the facial nerve.




  • Visceral motor (general visceral efferent) Parasympathetic innervation of the lacrimal, submandibular, and sublingual glands, as well as mucous membranes of nasopharynx, hard and soft palate which originates from Superior salivatory nucleus.




  • Special sensory (special afferent) consists of afferent fibers which convey taste information from the anterior 2/3 of the tongue and the hard and soft palates.



  • General sensory (general somatic afferent) is a minor component of facial nerve and consists of afferent fibers which convey general sensory information from the skin of the concha of the external ear and from a small area of skin behind the ear. It may also supplement the mandibular division of CN V in providing sensation from the wall of the acoustic meatus and the outer surface of the tympanic membrane.

The last three components are bound in a distinct fascial sheath from the branchial motor fibers. Collectively these three components are referred to as the nervus intermedius.



Course of Facial Nerve: The motor nucleus of the facial nerve is located in the ventrolateral part of the reticular formation of the pons near its caudal border. A large number of fine bundles of fibers emerge from it to form first part of root of this nerve. These fibers then form a compact group in the floor of fourth ventricle usually referred to as ascending part. This part then turns laterally over dorsal surface of the nucleus of abducens to form facial colliculus. This bend around the abducens nucleus is known as the genu.
The facial nerve enters the internal auditory meatus with the auditory nerve and lies in a groove along the upper and anterior part of the auditory nerve and the pars intermedia is placed between the two, and joins the inner angle of the geniculate ganglion. Beyond the ganglion its fibres are generally regarded as forming the chorda tympani.
At the bottom of the meatus, the facial nerve enters the aquaeductus Fallopii, and follows the course of that canal through the petrous portion of the temporal bone, from its commencement at the internal meatus, to its termination at the stylo-mastoid foramen. It is at first directed outward between the cochlea and vestibule toward the inner wall of the tympanum; it then bends suddenly backward and arches downward behind the tympanum to the stylo-mastoid foramen. At the point where it changes its direction, it presents a reddish gangliform swelling (geniculate ganglion). On emerging from the stylo-mastoid foramen it runs forward in the substance of the parotid gland, crosses the external carotid artery, and divides behind the ramus of the lower jaw into two primary branches, temporo-facial and cervico-facial from which numerous offsets are distributed over the side of the head, face, and upper part of the neck, supplying the superficial muscles in these regions.

Branches:

Inside the facial canal: (1) Greater petrosal nerve - provides parasympathetic innervation to lacrimal gland, as well as special taste sensory fibers to the palate via the nerve of pterygoid canal. (2) Nerve to stapedius - provides motor innervation for stapedius muscle in middle ear. (3) Chorda tympani - provides parasympathetic innervation to submandibular and sublingual glands and special sensory taste fibers for the anterior 2/3 of the tongue.

Distal to stylomastoid foramen: Posterior auricular nerve - controls movements of some of the scalp muscles around the ear

Five major facial branches (in parotid gland) - from top to bottom: (1) Temporal branch of the facial nerve, (2) Zygomatic branch of the facial nerve, (3) Buccal branch of the facial nerve, (4) Marginal mandibular branch of the facial nerve , (5) Cervical branch of the facial nerve
A helpful mnemonic device for remembering the major branches are the phrases: "To Zanzibar By Motor Car",


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