224 Neuroanatomy: Draw It to Know It
Cranial Nerve 7: Anatomy (Cont.)
Now, we will show the nerve path for each facial nerve
fi ber type. Begin with the SVE fi ber component; show
that it originates from the facial nucleus, sweeps over the
abducens nucleus in the internal genu, exits the brainstem
at the pontomedullary junction as the motor root, passes
through the cerebellopontine angle cistern, enters the
petrous bone through the internal acoustic meatus, and
then passes through the meatal, labyrinthine, and hori-
zontal segments. Show that it gives off the stapedius nerve
branch at the beginning of the mastoid segment and then
drops straight down and exits the skull through the stylo-
mastoid foramen and divides into several nerve branches,
which innervate the muscles of facial expression.
Next, let’s draw the GSA fi bers; they originate from
small portions of the external ear, which comprises the
auricle (aka pinna) and external acoustic meatus (aka
external ear canal). Th e peripheral process of the GSA
component enters the skull through the stylomastoid
foramen, ascends the mastoid segment, and traverses the
horizontal segment, and its cell bodies reside in the genic-
ulate ganglion. Th e central process of the GSA compo-
nent emerges from the geniculate ganglion; passes through
the labyrinthine segment, through the internal acoustic
meatus, and through the cerebellopontine angle cistern
within the nervus intermedius; enters the brainstem at
the pontomedullary junction; and descends the spinal
trigeminal tract to synapse in the inferior portion of the
pars caudalis portion of the spinal trigeminal nucleus.
Now, let’s illustrate the SVA fi bers, which originate
from the anterior two thirds of the tongue. Indicate that
the peripheral process of the SVA component joins the
chorda tympani, ascends the mastoid segment, and tra-
verses the horizontal segment, and its cell bodies lie within
the geniculate ganglion. From the geniculate ganglion,
show that its central process traverses the labyrinthine
and meatal segments, crosses the cerebellopontine angle
cistern within the nervus intermedius, enters the brain-
stem at the pontomedullary junction, and synapses in
the superior portion of the solitary tract nucleus.
Next, let’s draw the GVE fi bers, which carry pregan-
glionic parasympathetic fi bers. Th e GVE component
originates from the superior salivatory nucleus; it exits
the brainstem at the pontomedullary junction, traverses
the cerebellopontine angle cistern within the nervus
intermedius, passes through the meatal segment, and
divides at the end of the labyrinthine segment (ie, at the
geniculate ganglion). Th e upper division exits the petrous
bone through a hiatus along the anterior petrous surface
as the greater petrosal nerve; it passes through foramen
lacerum and then the pterygoid canal to innervate the
pterygopalatine ganglion, which provides parasympa-
thetic postganglionic innervation to the nasal, palatine,
and lacrimal glands. Th e lower division continues with
the rest of the facial nerve through the horizontal seg-
ment and then through the superior portion of the mas-
toid segment, where it joins the chorda tympani to
innervate the submandibular ganglion, which provides
postganglionic parasympathetic innervation to the sub-
mandibular and sublingual glands.
Finally, let’s include some associated nerve structures
that either join or run alongside the facial nerve. First,
show that the vestibulocochlear nerve also passes through
the internal acoustic meatus along with the facial nerve.
Th en, show that the chorda tympani merges with the
lingual nerve, a branch of the mandibular division of the
trigeminal nerve, which carries sensory aff erent informa-
tion from the fl oor of the mouth. Lastly, show that the
sympathetic deep petrosal nerve joins the greater petrosal
nerve in the pterygoid canal; together they form the nerve
of the pterygoid canal (aka the vidian nerve).^1 – 8 , 10 – 12