222 Neuroanatomy: Draw It to Know It
Cranial Nerve 7: Anatomy
Here, we will draw the four components of cranial nerve
7, the facial nerve. To begin, divide the nerve into four
segments: an extracranial segment — where the nerve
emerges from the petrous portion of the temporal bone
(the petrous bone); an intratemporal segment — where it
passes through the petrous bone; a cisternal segment —
where it passes through the cerebellopontine angle cis-
tern; and an intra-axial segment — its brainstem course.
Now, let’s establish the relevant cranial nerve 7 nuclei
and their end targets, and then we’ll illustrate their
paths.
First, for reasons that will become clear later, draw the
abducens nucleus of cranial nerve 6, which spans from
the mid to lower pons. Th en, draw the facial nucleus,
which spans from the lower pons to the pontomedullary
junction and houses special visceral eff erent (SVE) cells.
Indicate that this component innervates the muscles of
facial expression. Next, draw the superior salivatory
nucleus, which lies just above the pontomedullary junc-
tion and houses general visceral eff erent cells (GVE).
Indicate that this component provides preganglionic
parasympathetic innervation to the pterygopalatine and
submandibular ganglia. Now, draw the solitary tract
nucleus, which spans the height of the medulla and
receives the special visceral aff erent (SVA) fi bers of cra-
nial nerve 7. Indicate that these fi bers carry taste sensa-
tion from the anterior two thirds of the tongue. Next,
draw the pars caudalis portion of the spinal trigeminal
nucleus, which spans from the inferior medulla to the
upper cervical spinal cord and receives the general sen-
sory aff erent (GSA) fi bers of cranial nerve 7. Indicate
that these fi bers carry sensory information from select
portions of the external ear.
Now, let’s establish several key anatomic features of
the facial nerve course. First, label the internal genu; this
is the sweeping path the facial nerve takes over the top of
the abducens nucleus — it generates a small bump in the
fl oor of the fourth ventricle, called the facial colliculus.
Next, in the cisternal segment, at the level of the pon-
tomedullary junction, label the motor root, which con-
sists of special visceral eff erent (SVE) fi bers, only, and
then label the nervus intermedius (of Wrisberg ), which
consists of the remaining three fi ber types. Now, label
the internal acoustic meatus (aka internal auditory
canal), which is where the facial nerve enters the petrous
bone. Th e motor root and nervus intermedius merge
together within the internal acoustic meatus. Next, label
the geniculate ganglion, which houses the cell bodies for
the pseudo-unipolar facial sensory aff erents. Th en, label
the nerve to the stapedius muscle (an SVE branch). Th is
small muscle contracts the neck of the stapes to prevent
the transmission of high-energ y sounds through the
middle ear ossicles; note that it may also serve to extract
meaningful sound from background noise. Now, label
the greater petrosal nerve, which carries the GVE (upper
division) fi bers. Next, label the chorda tympani, which is
the nerve bundle of the combined GVE (lower division)
and the SVA fi bers. And fi nally, draw the stylomastoid
foramen, which is the opening through which the SVE
and GSA fi bers exit the petrous bone.
Before we draw the nerve paths, themselves, let’s sub-
divide the intratemporal segment of the facial nerve into
four descriptive segments. First, label the meatal seg-
ment, which is the segment wherein the facial nerve
passes through the internal acoustic meatus. Next, label
the labyrinthine segment, which is the segment wherein
the facial nerve exits the meatal segment and enters the
facial canal. Th is segment terminates at the geniculate
ganglion, which is identifi ed by its abrupt bend, the
external genu. Next, label the horizontal (aka tympanic)
segment, wherein the facial nerve runs posteriorly (hori-
zontally). Lastly, label the mastoid segment. In this seg-
ment, the facial nerve drops straight down and exits the
skull through the stylomastoid foramen.