198 Neuroanatomy: Draw It to Know It
Cranial Nerves 3, 4, & 6: Anatomy
Here, we will draw the course of cranial nerves 3, 4, and
6 in sagittal view. First, draw a midsagittal section
through the brainstem from the midbrain to the pon-
tomedullary sulcus; be sure to include a cerebral pedun-
cle. Next, label the cavernous sinus, the superior orbital
fi ssure, and the orbit. Note that cranial nerves 3 and 4
pass through the lateral dural wall of the cavernous sinus,
whereas cranial nerve 6 passes through the cavernous
sinus, itself. Also note that all of the extraocular muscles
except for the inferior oblique share a common tendi-
nous ring , called the annulus of Zinn; cranial nerves 3
and 6 pass through this ring to innervate their respective
muscles, whereas cranial nerve 4 passes above it to inner-
vate the superior oblique muscle.
Now, draw the oculomotor complex of cranial nerve
3 within the dorsal, center of the midbrain and indicate
that it lies at the level of the superior colliculus. In axial
view, show that the oculomotor fascicles emerge from
the anterior, medial midbrain and enter the interpedun-
cular cistern. Th en, in the sagittal diagram, show that the
oculomotor nerve passes through the prepontine cistern,
the lateral dural wall of the cavernous sinus, and the
superior orbital fi ssure to enter the orbit.
Next, draw the trochlear nucleus of cranial nerve 4 at
the level of the inferior colliculus. In axial view, indicate
that the fourth nerve passes dorsally out of the posterior
aspect of the midbrain and courses around the outside of
the opposite cerebral peduncle through the ambient cis-
tern. Th en, in sagittal view, complete its course through
the lateral dural wall of the cavernous sinus and through
the superior orbital fi ssure to enter the orbit to innervate
the superior oblique muscle on the side opposite its
nucleus of origin. Th e trochlear nerve is the only cranial
nerve to exit posteriorly from the brainstem and the only
cranial nerve to send all of its fi bers to the contralateral
side. When the long, thin trochlear nerve is injured, the
aff ected eye is elevated (aka hypertropic). Hold your fi sts
with your index fi ngers straight out to demonstrate the
direction of the eyes. To demonstrate a right fourth nerve
palsy, elevate your right hand. Now, tilt your head both
ways; tilting your head toward the elevated (aff ected)
side worsens the disconjugate lines of vision whereas tilt-
ing it the opposite way (towards the normal side) brings
the lines of vision into alignment: patients with a fourth
nerve palsy tilt their head away from the aff ected eye.
Now, draw the clivus; it is a fusion of the sphenoid
and occipital bones that sits directly across from the
brainstem. Th en, draw the abducens nucleus of cranial
nerve 6 in the dorsal, inferior pons. Show that the sixth
nerve exits the brainstem at the pontomedullary sulcus,
climbs the clivus, passes over the petrous apex, and then
passes through the cavernous sinus and superior orbital
fi ssure to enter the orbit. Next, also indicate that as part
of its path into the cavernous sinus (aft er crossing over
the petrous apex), the sixth nerve passes through
Dorello’s canal: a dural channel within a basilar venous
plexus. In this stretch, the abducens nerve is in close
proximity to the fi rst division of cranial nerve 5, and
injury to both nerves, here, is called Gradenigo’s syn-
drome. Note that cranial nerve 6 dysfunction is oft en a
harbinger of increased intracranial pressure because the
abducens nerve is fi xed where it pierces the dura and can
be stretched when there is downward herniation of the
brainstem.
Next, let’s show the arteries that can compress the
aforementioned cranial nerves. First, show that the pos-
terior cerebral artery (PCA) sits above both the third
and fourth cranial nerves, and then that the superior cer-
ebellar artery (SCA) sits below them. Next, show that
the anterior inferior cerebellar artery (AICA) lies in
close relation to the sixth cranial nerve as it exits the
pontomedullary sulcus. Th en, show that the internal
carotid artery (ICA) lies within the cavernous sinus, and
show that the posterior communicating artery (Pcomm)
passes just above the oculomotor nerve to connect the
ICA and PCA.^1 – 7