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.
This slide presents a coronal section through the cavernous sinus at the level of the pituitary gland. Recall that the cavernous sinus is a maze of venous sinusoids located lateral to what structures?
the pituitary gland and sphenoid sinus. The cavernous sinus drains blood from the eyes and cortical veins to empty eventually into the jugular vein.
What CNs pass through the cavernous sinus?
Note that CNs III, IV, VI (red boxes) and the V1, V2 (green boxes) divisions of the trigeminal nerve travel within the sinus.
Note also that the internal carotid artery travels within the sinus and begins a hairpin turn within the sinus, called the carotid siphon, before entering the subarachnoid space.
What else travels through the cavernous sinus?
Sympathetic fibers traveling with the internal carotid artery on their way to dilator muscles of the pupil also travel through the cavernous sinus.
It is important to recognize that hemorrhage from a ruptured internal carotid artery aneurysm, tumors, infections, and inflammatory diseases such as Tolosa-Hunt syndrome may affect the cavernous sinus to produce a syndrome called the cavernous sinus syndrome. Lesions of the cavernous sinus may produce symptoms and signs affecting all or just some of the structures passing through the sinus.
This slide presents a lateral view of the orbit, its contents, and the innervation of the eye muscles and eye lid. Its purpose is to allow you to trace the pathways from the Edinger-Westphal nucleus, CN III and the sympathetic fibers traveling initially with the internal carotid artery as they innervate their target organs.
Note the parasympathetic innervation of the pupillary constrictors and ciliary muscles.
The sympathetic innervation of the Tarsal muscle provides autonomic elevation of the eye lid that is not subject to voluntary control.
Note the sympathetic innervation of the pupil dilator muscles as well
What innervates of the levator palpebra superioris, which allows for voluntary elevation of the upper eye lid
CN III.
What does the ciliary muscle do?
produces changes in lens shape as the eyes converge on a target moving toward the eyes. The resulting pupillary constriction produces the accommodation reflex.
This slide presents the four recti muscles with their CN innervation and the primary direction that they move the eye. Note that the medial and lateral recti move the eye globe towards the nose (adduction) or away from the nose (abduction) respectively. Note that while the slide depicts pure vertical movement for the superior and inferior recti, in fact these muscle will produce a degree of torsional movement because of the manner in which they are anatomically connected to the eye.
For the purposes of this course and for the ease of clinical examination when you become practicing physicians (unless you specialize in ophthalmology) we will disregard all torsional movements for the superior and inferior recti muscles.
This slide presents the two oblique muscles with their CN innervation and the primary direction that they move the eye.
How do the superior obliques move the eye? Inferior obliques?
They cause intorsion of the eye and the inferior oblique causes extorsion.
To understand how you can disregard the torsional eye movements when examining patients, you need to comprehend the material in the next two slides.
First, notice the axis of the eye shown as a line drawn through the center of the lens back to the fovea of the retina. In fact this axis is the most direct path of light traveling through the lens to reach the fovea, the area of the retina with the most acute vision.
Now, contraction of the lateral or medial rectus muscles moves the eye globe in such a way that the axis of the eye does not rotate about itself, that is there is no intorsion or extorsion.
In contrast, the other four eye muscles connect to the globe in such a manner that when they contract the eye globe does rotate about its axis, that is the eye intorts or extorts. Importantly, it is possible to eliminate most of this rotational movement of the eye globe if the eyes are examined in specific positions. I will explain this on the next slide and then present the steps to examine eye movement
This slide attempts to explain the physics that permits examination of the eyes without dealing with the torsional movements. It presents a dorsal view of the eyes and extraocular muscles.
Note that when the eyes look to the right the superior rectus of the right eye becomes almost parallel with the axis of the right eye. Thus when the right superior rectus contracts, it will not cause the right eye to rotate around its (the right eye) axis, that is there will be no intorsion or extorsion.
Accordingly, with the eye in an abducted position, that is looking laterally, the superior rectus can be simply tested by asking the patient to look upward.
Similarly, when the left eye looks to the right, that is, it is placed in the adducted position, the superior oblique muscle becomes almost parallel with the axis of the left eye. When the left superior oblique contracts it cause the left eye to look downwards without accompanying intorsion or extorsion. The superior oblique causes the eye to look downward because it loops around the trochlear before inserting on the posterior portion of the eye. If these physical relationships are unclear, then simply review and remember the next two slides to be able to examine patients without remembering the torsional functions of the extraocular muscles.
This slide presents the primary fields of gaze for testing the patient’s right eye without having to consider torsional movements.
The slide is oriented with you as the examiner looking directly at the face and eyes of your patient. By focusing on the right eye and asking the patient to look to his right (abducted) or left (adducted) and then asking him to look up and then down you are able to isolate the movements of all six extraocular muscles of the right eye without having to consider torsional movements.
When either eye is in the abducted position, that is looking laterally, elevation of the eye is controlled primarily by the superior rectus muscle. Accordingly, weakness of elevation of an abducted eye means that the superior rectus is weak or the nerve innervating it is injured.
This slide simply presents the primary gaze field for testing eye movement in your patients. When you face your patient and ask them to look to their right, up, and then down you are testing in order the lateral rectus, superior rectus, and inferior rectus of their right eye and simultaneously testing the medial rectus, inferior oblique, and superior oblique muscles of their left eye.
This slide demonstrates the pupillary light reflex. This is a dorsal view of a silhouetted brain, brain stem, and eyes. Describe the initial parts of this pathway.
1) A bright light flashed in front of the left eye sends afferent signals via the optic nerve through the optic chiasm and optic tract to synapse in the left and right pretectal nuclei. The latter are located in the midbrain just under the superior colliculi.
Note the Edinger-Westphal nucleus is outlined as a single midline structure just above the nose of the upside down Mickey Mouse. The nose is the cerebral aqueduct as you may recall from slide #10.
This slide presents some of the clinically relevant nuances of the Edinger-Westphal parasympathetic path to the pupil constrictor muscles. Note this is a frontal view of the eyes and brain stem with the optic nerves (in yellow), brain stem (in tan), and an enlarged section of CN III (in orange).
Note once again that the figure presents two separate Edinger-Westphal nuclei rather than correctly presenting a single midline nucleus.
Focus your attention to the enlarged section of CN III. Note the location of the blood vessels serving the periphery of the nerve and also separate blood vessels serving the center of the nerve.
Note also the darker band lying along the dorsal medial surface of the nerve. This band represents the parasympathetic fibers from the Edinger-Westphal nucleus. Their position along the dorsal medial surface of CN III figures into the clinical presentation of two neurological syndromes that have markedly different causes and prognosis.
What mediates the accommodation reflex?
1) Contraction of the ciliary muscle causes the suspensory ligaments attached to the lens to relax and thus the lens increases its diameter.
The phasic firing of this conjugate link between the abducens nucleus and the contralateral ventral nucleus of CN III is regulated by what?
the paramedian pontine reticular formation or PPRF, also known as the horizontal gaze center and the parabducens nucleus. Thus, activation of the left PPRF triggers the left abducens nucleus to simultaneously activate contraction of the left lateral rectus muscle and the right oculomotor nucleus to activate the right medial rectus muscle.
The vestibular system is also an additional important regulator of extraocular movement. Head movement strongly influences eye movement, and this is mediated via the semicircular canals, the vestibular nerve, and vestibular nuclei. Note that this diagram is a significant over-simplification of the vestibular regulation of eye movement.
Describe the vestibulo-ocular reflex (VOR)
Rotation of the head to the right in the horizontal plane stimulates the right horizontal semicircular canal. This prompts the vestibular apparatus to signal the right medial vestibular nucleus via the right vestibular nerve.
Saccadic eye movement, shown in the left figure, is under the control of what?
the frontal eye fields and is either voluntarily triggered or reflexive. For example, if you hold your head still and look with only your eyes from one corner of the room to the other, you are voluntarily activating your saccadic system.
This slide presents an artists drawing of two men, one (A) with a lesion of the left frontal cortex causing right hemiparesis and eyes looking away from the paretic side and
a second man (B) with a lesion of the left brain stem at the pontine level causing the same right hemiparesis as A but now with eyes looking toward the paretic side.
Note that the diagram on the left of the frontal eye field pathway to the PPRF shows only the right PPRF and the lesion B corresponds to injury to the left PPRF.
