220 Neuroanatomy: Draw It to Know It
Cranial Nerve 7: Innervation
Here, we will address the corticonuclear innervation of
cranial nerve 7, the facial nerve. We focus on this subject
in particular because testing facial weakness can help us
diff erentiate an upper motor neuron injury (eg , cortical
stroke) from a lower motor neuron injury (eg, Bell’s
palsy). First, draw the left half of a face; label its upper
and lower divisions. At the bedside, we test the upper
face with forehead wrinkle and the lower face with forced
smile. Note that an involuntary smile (aka mimetic or
emotional smile) has a diff erent innervation pattern than
what we will draw, here, so be careful not to make your
patients laugh at this point in the exam.
Next, draw the bilateral cerebral hemispheres. Th en,
draw the left facial nucleus and divide it into its upper
and lower divisions. First, show that the upper division
receives bilateral corticonuclear projections and then
show that the lower division receives contralateral pro-
jections, only. Now, draw fi bers from the upper division
to the upper face and then show fi bers from the lower
division to the lower face. Th e fact that the upper face
receives bilateral corticonuclear innervation and the
lower face receives contralateral corticonuclear innerva-
tion allows us to distinguish upper motor neuron from
lower motor neuron lesions, as we will see when we walk
through the two clinical vignettes, next.^2
Vignette one involves a right cerebral cortical stroke
that aff ects the facial fi bers. Redraw the hemispheres,
facial nucleus, and face. Th en encircle the cortex of the
right hemisphere to indicate damage from the stroke. In
this situation, contralateral facial nuclear innervation is
lost. However, show that ipsilateral upper division facial
nuclear innervation is preserved. As a result, show that
the left upper face is strong and the left lower face is
weak. Th us, in cerebral cortical injury, there is contralat-
eral lower facial weakness with preserved upper facial
strength. As a small clinical pearl, when there is facial
weakness due to upper motor neuron injury, the palpe-
bral fi ssure on the paretic side of the face will oft en be
wider than on the normal side of the face because the
facial droop will pull down the lower eyelid.
Vignette two involves a left Bell’s palsy. Again, redraw
the hemispheres, facial nucleus, and face. Here, encircle
the facial nucleus, itself, to indicate damage from the
Bell’s palsy. Show that in this clinical circumstance,
innervation to the facial nucleus is preserved bilaterally,
but peripheral innervation to the face is lost. As a
result, show that both the left upper face and left
lower face are weak. In Bell’s palsy, there is paralysis of
the complete ipsilateral side of the face. Note that Bell’s
palsy can occur anywhere along the seventh cranial
nerve; therefore, the full clinical presentation of a
Bell’s palsy depends on where along its course the facial
nerve is aff ected. Th e next diagram will help us to under-
stand why there are such a wide variety of presentations
for Bell’s palsy and what those potential presentations
can be.^1 – 6 , 8 , 10