208 Neuroanatomy: Draw It to Know It
Cranial Nerve 12
Here we will draw the hypoglossal nerve (cranial nerve 12)
course in sagittal view and then we will create a diagram
to understand the actions of the extrinsic tongue muscles
that it innervates. We begin with the hypoglossal nerve
path from the medulla to the tongue. First, draw an axial
view of the medulla and label the hypoglossal nucleus
within the posterior medulla; it spans from the upper to
the lower regions of the medulla. Next, label a few key
structures along the hypoglossal nerve course: the infe-
rior olive, the vertebral artery, and the hypoglossal canal.
Now, show that the hypoglossal nerve passes anteriorly
through the medulla and exits between the medullary
pyramid and the inferior olive at the ventrolateral (aka
preolivary) sulcus. Th en, show that the hypoglossal nerve
passes through the premedullary cistern, lateral to the
vertebral artery, to exit the skull base through the hypo-
glossal canal.
Now, show that aft er the hypoglossal nerve exits the
hypoglossal canal, it passes through the medial nasopha-
ryngeal carotid space, where it descends in close proxim-
ity to the carotid and internal jugular vasculature before
approaching the hyoid bone and terminating in the
tongue musculature. Th e proximity of cranial nerve 12 to
the internal carotid artery makes it susceptible to injury
from carotid dissection, because clot formed from dissec-
tion can expand the carotid wall enough to compress the
adjacent nerves. Cranial nerves 9, 10, and 11 also run
within this space, so a combined cranial nerve 9, 10, 11,
and 12 injury suggests a possible carotid dissection.
Next, let’s show the termination of the hypoglossal
nerve under the tongue, or “glossus” (hence the name
“hypoglossal nerve”). Th e hypoglossal nerve innervates
the large intrinsic tongue muscle mass, which comprises
the superior and inferior longitudinal muscles and the
transverse and vertical muscles, and it innervates three
of the four extrinsic tongue muscles: styloglossus, hyo-
glossus, and genioglossus. Palatoglossus is the only extrin-
sic tongue muscle not innervated by cranial nerve 12;
its innervation comes from cranial nerve 10, the vagus
nerve. Genioglossus is the most clinically important of
these three extrinsic tongue muscles because whereas the
other tongue muscles receive bilateral cortical innerva-
tion, genioglossus receives predominately contralateral
cortical innervation; therefore, it is the most helpful for
clinical localization, as we discuss at the end.^13
To understand the action of all of these muscles, we
will draw a sagittal schematic of their innervation. Show
that styloglossus attaches the tongue to the styloid pro-
cess; indicate that it pulls the tongue up and back. Th en,
show that hyoglossus attaches the tongue to the hyoid
bone; indicate that it depresses the tongue. Finally, show
that genioglossus attaches the tongue to the anterior
mandible; indicate that it provides tongue protrusion.
Now, in axial section, show that both genioglossi
point towards the center of the anterior mandible. Th eir
opposing horizontal angles cancel, and so the tongue is
directed forward when it is activated (you can also feel
the tongue curl when you protrude it because the genio-
glossi pull the center of the tongue downward, as well).
Th us, when one side of the genioglossus is impaired
(either from genioglossus atrophy or from hypoglossal
nerve or nucleus injury), the tongue moves forward and
toward the side of the lesion (away from the intact side).
Demonstrate this with your index fi ngers. Hold your
fi sts in front of you and point your index fi ngers toward
midline. Now drop one of your fi sts: the tongue pro-
trudes toward the side of the lesion (away from the intact
side). Corticonuclear innervation to the portion of each
hypoglossal nucleus that innervates the genioglossus is
predominantly contralateral, however. Th erefore, with
lesions proximal to the hypoglossal nucleus, tongue pro-
trusion is either normal or it points away from the side of
the lesion: for instance, in a right-side hemispheric lesion,
the left genioglossus will be weak, so the tongue will
deviate toward the left (away from the injured side of the
brain).^1 – 4 , 6 , 7