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Facial Nerve (CN VII) Anatomy & Function
The facial nerve resembles a telephone
cable and contains
7,000 individual nerve fibers.
Each fiber carries electrical im-pulses
to a specific facial muscle. Information
passing along the fibers of this nerve
allows us to laugh, cry, smile, or frown,
hence the name, "the nerve of facial
expression."
When half or more of these individual nerve
fibers are interrupted, facial weakness
occurs. If these fibers are irritated,
then movements of the facial muscles
appear as spasms or twitching.
The facial nerve not only carries nerve impulses
to the muscles of the face, but also to the
tear glands, to the saliva glands, and to the
muscle of the stirrup bone in the middle ear
(the stapes). It also transmits taste from the
front of the tongue. Since the function of the
facial nerve is so complex, many symptoms may
occur when the fibers of the facial nerve are
disrupted. A disorder of the facial nerve may
result in twitching, weakness, or paralysis
of the face, in dryness of the eye or the mouth,
or in disturbance of taste.
Photograph of facial nerve during a facial nerve preserving
parotidectomy for benign parotid tumor (pleomorphic
adenoma). This surgery was performed by Drs.
Azizzadeh and Larian with the Facial Paralysis
Institute
The anatomy of the facial nerve is very complex. The facial
nerve passes through the base of the skull in
transit from the brain to the muscles of facial
expression. After leaving the brain, the facial
nerve enters the bone of the ear (temporal bone)
through a small bony tube (the internal auditory
canal) in very close association with the hearing
and balance nerves. Along its inch-and-a-half
course through a small canal within the temporal
bone, the facial nerve winds around the three
middle ear bones, in back of the eardrum, and
then through the mastoid (the bony area behind
the part of the ear that is visible). After
the facial nerve leaves the mastoid, it passes
through the salivary gland in the face (parotid
gland) and divides into many branches, which
supply the various facial muscles. The facial
nerve gives off many branches as it courses
through the temporal bone: to the tear gland,
to the stapes muscle, to the tongue (for taste
sensation), and to the saliva glands.
The facial nerve exits the skull behind the
ear at stylomastoid foramen and enters
the core of a salivary gland in front
of the ear (parotid gland). Within the
parotid gland divides into an upper and
lower division. It further divides into
5 main branches including the frontal,
zygomatic, buccal, marginal mandibular
and platysmal. There is extensive arborization
of the facial nerve branches and significant
variations exists in different individuals.
Major types and percentages of a facial
nerve branching and anastomoses.
In 13%, major divisions
(temporal and facial) are independent; in 11%,
anastomoses occur between rami of the temporal
division; in 22%, connections occur between
adjacent rami from the major divisions; in 21%,
anastomoses representing a composite of those
in the11% and 22% categories occur; in 12%,
proximal anastomoses occur within the temporal
component, as well as distal interconnection
between the latter and the cervical component;
in 9%, two anastomotic rami connect the buccal
divisin of the cervical to the zygomatic part
of the temporal; in 5%, a transverse ramus,
from the trunk of the nerve, contributes to
the buccal ramus formed by anastomosis between
the two major divisions; in 7% richly plexiform
communications occur, especially within the
temporal portion of the nerve. Variations of
the facial nerve include branches that pass
through clefts in superficial veins or the formation
nerve loops through which pass superficial veins.
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