Functional Neuroanatomy of CN III, IV, and VI Flashcards
This slide lists only some of the disease entities that may alter eye movements and pupillary function


This slide demonstrates the location of the four lower motor neuron nuclei involved in eye movement and pupil control. This is a dorsal view of the brainstem with the cerebral hemispheres and cerebellum removed.

Note that all four nuclei lie near the midline and are located dorsally (that is closer to the 4th ventricle) in the brainstem.
Note that CNs III and IV lie in the midbrain just ventral to what structures?

the superior and inferior colliculi respectively. Note that CN VI lies in the caudal half of the pons. I have added the Edinger Westphal nuclei (shown in red) to the drawing.

This slide presents a left lateral view of the brain stem shown on the right side of the slide with the relevant nuclei shown in color. The nuclei are enlarged to demonstrate the complex multinuclear structure of CN III. This multinuclear structure relates to the fact that CN III controls five separate eye muscles and accordingly has five separate nuclei.

The Edinger-Westphal nucleus is closely associated with CN III but is considered a separate nucleus.
What does the Edinger-Westphal nucleus do?
It is a parasympathetic pathway supplying the constrictor muscles of the iris.
The dorsal nuclei of CN III serve what muscle?
the inferior rectus muscle
The intermediate nucleus of CN III serves what muscle?
the inferior oblique muscle
The medial nucleus of CN III serves what muscle?
the superior rectus
The ventral nucleus of CN III serves what muscle?
the medial rectus.
The central caudal nucleus of CN IIII serves what?
the levator palpebrae superioris muscle.
This slide presents a dorsal view of the brainstem and nuclei that control eye movement and pupil constriction.
Note once again the midbrain location of CNs III and IV just ventral to what structure?

the quadrigeminal plate, i.e. the superior and inferior colliculi.
. Note the location of the abducens nucleus (CN VI) in the caudal half of the pons.
Recall that CN VII sends its axons over the top of CN VI creating what?
the facial colliculus. The facial colliculus is seen in the floor of the 4th ventricle as a raised bump and CN VI lies immediately below this bump.
CN III central nuclei, i.e. the Edinger-Westphal nucleus and the caudal nucleus provide bilateral input to their respective muscles. The remaining left and right paired CN III nuclei serve their respective ipsilateral muscles except for the medial nucleus that serves the contralateral superior rectus muscle.
However, the clinical significance of this ipsilateral, contralateral, and bilateral output of one or another of the CN III subnuclei is inconsequential since the CN III complex is small and near the midline and only in the rarest of conditions does it suffer a lesion localized to only the left or right half of the complex. Nevertheless, the fascicles, i.e. the axons from the subnuclei, join together and quickly lateralize. Thus lateralized lesions of the midbrain can produce unilateral eye movement and pupillary abnormalities as do lesions of CN III after it leaves the brainstem.
A ventral view of the brain stem showing the exit of CN III where?

at the junction of the midbrain and pons
CN IV exiting dorsally
Where does CN VI exit the midbrain?
at the pontomedullary junction.
Note that CN III exits ventrally at the level of the midbrain. CN __ is the only CN that exits the brainstem dorsally.
IV. Note it exits just caudal to the inferior colliculus.
How does CN IV act?
The trochlear nerve crosses over from its dorsal exit from the brainstem caudal to the inferior colliculus to innervate the contralateral super or oblique muscle; accordingly the left trochlear nucleus controls the right superior oblique and vis versa (see also Slide 14).
Which two cranial nerve nuclei controlling eye movement serve contralateral eye muscles?
the trochlear nucleus and the medial nucleus of CN III.
This slide presents a frontal view of the eyes and several brain stem sections depicting the location of CNs III, IV, and VI nuclei and their respective nerves.
Note the ipsilateral innervation of the lateral rectus muscle by the CN VI,
the ipsilateral innervation of the superior rectus, medial rectus, inferior rectus, and inferior oblique muscles by CN III,
and the contralateral innervation of the superior oblique by CN IV.
I36 takes you to C4
Note the error in this figure; the figure mistakenly shows the left CN III nuclei sending efferents fibers to the left superior rectus muscle when in fact the right medial nucleus of CN III is the source of this muscles innervation.

Be aware that the axons from the medial nucleus immediately cross over and join the contralateral fascicle such that there is no error in the figure regarding CN III fascicle within the brainstem.
T or F. A lesion of the oculomotor nucleus on one side would disrupt upgaze in both eyes.
T.
Loss of the CN III nucleus would remove what?
the source of fibers supplying the superior rectus on the opposite side. The lesion would also destroy the crossed fibers from the opposite intact oculomotor nucleus affecting the ipsilateral superior rectus. Neither superior rectus muscle would receive nerve impulses to contract the muscles. This is a primary characteristic of a dorsal midbrain stroke.
This slide presents a cross section through the midbrain. Note the locations of CNs III, IV, and VI and their proximity to the cerebral blood vessels.
Of particular importance is the route of CN III as it passes between what arteries?

the superior cerebellar artery and the posterior cerebral artery and then alongside the posterior communicating artery and underneath the internal carotid artery. The juxtaposition of CN III with these intracranial arteries subjects it to compression and injury from vascular outpouchings called aneurysms that not uncommonly develop in arterial branch points at the base of the brain.
To help you navigate future MTI and CT images, note the apparent Mickey Mouse face created by structures comprising the midbrain in the left figure. This region of the brain stem is seen on CT and MRI scans of the brain as shown on the right.

Familiarity with Mickey’s face allows for the approximation of these structures on brain scans. Mickey’s ears are created by the cerebral peduncles and the substantia nigra, his eyes by the red nuclei, his nose by the periaqueductal gray and cerebral aqueduct of Sylvius, and his chin by the superior colliculi.


















