196 Neuroanatomy: Draw It to Know It
Know-It Points
Cranial Nerves 3, 4, & 6: Anatomy
■ Cranial nerve 3 emerges from the anterior, medial
midbrain and enters the interpeduncular cistern;
then, it passes through the prepontine cistern,
the lateral dural wall of the cavernous sinus, and
through the superior orbital fi ssure to enter the
orbit.
■ Cranial nerve 4 passes dorsally out of the posterior
aspect of the midbrain and courses around the
outside of the opposite cerebral peduncle through the
ambient cistern; then, it passes through the lateral
dural wall of the cavernous sinus and the superior
orbital fi ssure to enter the orbit to innervate the
superior oblique muscle on the side opposite its
nucleus of origin.
■ Cranial nerve 6 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.
■ Th e posterior communicating artery, which connects
the internal carotid artery and ipsilateral posterior
cerebral artery, passes just above the oculomotor
n e r v e.Extraocular Movements
■ Cranial nerve 3 innervates the majority of the
extraocular muscles: the medial rectus, superior
rectus, inferior rectus, and inferior oblique.
■ Cranial nerve 6 innervates the lateral rectus.
■ Cranial nerve 4 innervates the superior oblique.
■ All of the extraocular muscles except for the inferior
oblique attach at the orbital apex in a common
tendinous ring : the annulus of Zinn.
■ Cardinal positions of gaze: medial rectus—
adduction, lateral rectus—abduction, superior
rectus—elevation (with eye in abduction), inferior
rectus—depression (with eye in abduction), superior
oblique—depression (with eye in adduction), inferior
oblique—elevation (with eye in adduction).
■ Each muscle’s chief action in primary position
(ie, looking straight ahead) is its primary action.
■ Each muscle’s secondary and tertiary actions are its
additional rotational eff ects on the eye.
■ Th e mnemonic “SUPERIOR people do NOT extort”
indicates that the superior muscles are both intorters.
■ Th e mnemonic “OBLIQUE muscles rotate the eye
OUT” indicates that the oblique muscles are abductors.Oculomotor Nuclei ( Advanced )
■ Th e superior rectus subnuclei project contralaterally,
the single levator palpebrae subnucleus projects
bilaterally, and the remaining subnuclei project
ipsilaterally.
■ Superior rectus subnucleus lesions produce bilateral
palsies because when the superior rectus fi bers exit
their subnucleus, they immediately pass through the
contralateral subnucleus.
■ Th e accommodation refl ex (or near-response) is a
three-part refl ex that brings near objects into focus
through lens thickening, pupillary constriction, and
inward rotation of the eyes — eye convergence.
■ Th e preganglionic perioculomotor cell group lies
along the dorsal aspect of the oculomotor nucleus.
■ Th e motoneuron division of the perioculomotor cell
group lies along the medial aspect of the oculomotor
nucleus.