Path of the Facial Nerve - Top: Innervation of superficial muscles by the facial nerve (CN VII) (green), and also some branches of the trigeminal nerve (sensory nerve in this view); 2nd: facial, trigeminal, and other nerves with some muscle tissues removed; 3rd: schematic course of facial nerve through brainstem and through skull; Bottom: Course of facial nerve through the temporal bone of skull.
The facial nerve (or seventh cranial nerve, CN VII) carries the signals that control the movements
of the facial muscles, which are the most significant of the muscles that produce facial expressions.
Some anatomists identify the "muscles of facial expression" as those innervated by the facial nerve,
but psychologically, eye muscles innervated by third, fourth, fifth, and sixth cranial nerves,
orbital muscles partly innervated by CN III,
jaw muscles innervated by the trigeminal nerve (fifth cranial nerve), and some other relatively
minor motor connections of cervical nerves to muscles affecting
facial appearance also play a role in production of facial expressions.
The origin of the facial nerve is a nucleus in the brainstem.
According to an anatomy textbook:
The motor nucleus of the facial nerve is located in the ventrolateral part of the reticular formation of the pons near its caudal border.
Its constituent cells are arranged so as to form a varying number of sub-groups which may possibly
be concerned with the innervation of individual facial muscles.
From the dorsal aspect of this nucleus there
emerge a large number of fine bundles of fibers, directed dorsomedially through the reticular formation.
These rather widely separated bundles constitute the first part of the root of the facial nerve.
Beneath the floor of the fourth ventricle the fibers turn sharply rostrad and are assembled
into a compact strand of longitudinal fibers, often called the ascending part of the facial nerve.
This ascends along the medial longitudinal bundle for a considerable distance (5 mm).
The nerve then turns sharpy lateralward over the dorsal surface of the nucleus of the abducens nerve,
and helps to form the elevation in the rhomboid fossa, known as the facial colliculus.
This bend around the abducens nucleus, including the asceding part of the facial nerve,
is known as the genu. The second part of the root of the facial nerve is directed ventrolaterally
and at the same time somewhat caudally, passing close to the lateral side of its own nucleus,
to make its exit from the lateral part of the caudal border of the pons.
The facial nerve courses its way from the brainstem through some problematic human tissues, which can cause trouble for the functioning of this nerve.
According to Gray's:
The facial nerve, firmer, rounder, and smaller than the auditory, passes forward and outward upon the middle
peduncle of the cerebellum, and enters the internal auditory meatus with the auditory nerve.
Within the meatus the facial nerve 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.
Occasionally a few of its fibres pass into the auditory nerve. Beyond he 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 (intumescentia ganglioformis, or 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.
Disease organisms attacking tissues through which the facial nerve runs can disorder its funtion. Bell's Palsy is a fairly common facial hemiparalysis caused by damage to this peripheral nerve,
usually by a disease in the internal meatus or foramen that affects the nerve tract.
It usually strikes overnight and gravely frightens its victim who wakes with one side of the face unable to move. Prognosis is for some recovery over several months, and often recovery is complete. A very rare congenital condition is absence of a functioning facial nerve, which results in a lack of facial expressions and has catastrophic social consequences. Many other named clinical abnormalities of the facial nerve are known, all of which, though interesting to the neurologist, have an impact on the life of the victim that is out of proportion to the small physical damage done, due to the importance of facial expression in normal interpersonal activities.