Lecture 14: CN's and Eye Movement

Question 1

What are the four nuclei involved in eye movement?

Answer 1

1. Edinger- Westphal Nucleus
2. Oculomotor Nucleus
3. Trochlear Nucleus
4. Abducens Nucleus

**All are located in the midline of the dorsal (closer to the 4th ventricle) side of the brain.

Question 2

Where do CNIII and IV lie in the brainstem?

Answer 2

in the midbrain just ventral to the superior and inferior colliculi, respectively.

Question 3

Where does CNVI lie?

Answer 3

in the caudal half of the pons.

Question 4

What are the different nuclei of the Oculomotor nucleus and what eye muscles do they serve?

Answer 4

1. Dorsal Nucleus-> IR
2. Intermediate Nucleus-> IO
3. Medial Nuclei-> SR
4. Ventral Nucleus-> MR

Question 5

What other nuclei are closely associated with the oculomotor nucleus and what does they control?

Answer 5

• The Edinger- Westphal Nucleus, which serves as parasympathetic supply to the constrictor muscles of the iris.
• The Central Caudal Nucleus serves the levator palpebral superioris muscle.

Question 6

Where does CNVII send its axons?

Answer 6

Over the top of CNVI creating the facial colliculus, which is seen in the floor of the 4th ventricle as a raised bump.
**CNVI lies directly below this bump.

Question 7

What two nuclei provide bilateral input to their respective muscles?

Answer 7

Edinger-Westphal Nucleus and Caudal Nucleus

Question 8

Do the remaining CNII nuclei serve their respective muscles on the ipsilateral or contralateral side?

Answer 8

They serve their ipsilateral muscles with the exception of the medial nucleus that serves the contralateral SR muscle.

Question 9

Where does CNIV exit the brainstem?

Answer 9

It exits dorsally just caudal to the inferior colliculs.

**This is the only CN that exits the brainstem dorsally.

Question 10

Where does CNIII exit?

Answer 10

Ventrally at the junction of the midbrain and the pons.

Question 11

Where does CNVI exit?

Answer 11

Ventrally at the pontomedullary junction.

Question 12

Describe the path of the trochlear nerve.

Answer 12

It crosses over to innervate the contralateral SO muscle; accordingly, the left trochlear nucleus controls the right SO and vice versa.

Question 13

Which two cranial nerve nuclei controlling eye movement serve contralateral eye muscles?

Answer 13

Trochlear (SO) and the Medial Nucleus of CNIII (SR)

Question 14

Name the arteries and nerves that run through the cavernous sinus and explain the clinical significance of knowing this.

Answer 14

CNIII, IV, VI, V1, V2, the ICA, and sympathetic fibers traveling with the ICA on their way to the dilator muscles of the pupil.
**This is important because hemorrhage from a ruptured ICA aneurysm, tumors, infections and inflammatory disease such as Tolosa Hunt syndrome may affect these, producing Cavernous Sinus Syndrome.

Question 15

Describe the innervation of the elevator palpebral superioris muscle.

Answer 15

It is innervated by CNIII to allow for voluntary innervation of the upper eyelid.

Question 16

Describe the innervation of the tarsal muscle.

Answer 16

It is under sympathetic control so as to allow autonomis elevation of the upper eyelid.

Question 17

What kind of innervation are the pupil dilators under?

Answer 17

sympathetic

Question 18

What kind of innervation are the pupil constrictors and ciliary muscles under?

Answer 18

parasympathetic

Question 19

What function do the ciliary muscles serve?

Answer 19

They produce change in the lens shape as the eyes are converged, i.e. the accommodation reflex.

Question 20

Contraction of which two eye muscles never results in intorsion or extorsion?

Answer 20

LR and MR

Question 21

Can damage to CNIII be completely or partially damages?

Answer 21

Both.

Question 22

How would a completely damaged CNIII present?

Answer 22

Complete paralysis of all CNIII muscles, including the levator palpebrae superioris with resultant eye closure.

Question 23

How would a partially damaged CNIII present?

Answer 23

Paresis or weakened/ incomplete eye movement of 3rd nerve muscles and the eyelid will only be partially closed.

Question 24

Describe the sympathetic pathway of the pupil dilator muscle.

Answer 24

1. The primary neurons lie in the hypothalamus, travel through the lateral brainstem to eventually synapse with secondary neurons located in the intermediolateral gray horn of the spinal cord at the level of C8-T2.
2. The secondary neurons send their axons out through the ventral root to synapse in the paravertebral sympathetic chain ganglia onto tertiary neurons.
3. The tertiary axons send their axons to travel along with the ICA within the carotid sheath to enter the skull.
4. Intracranially, they join with the nasociliary nerve to innervate the superior tarsal muscle and the long and short ciliary nerves to innervate the dilator muscle of the pupil.

Question 25

Describe the parasympathetic innervation of the pupil (light reflex).

Answer 25

1. A bright light flashed in from of the left eye sends afferent signals via the optic nerve through the optic chasm and optic tract to synapse at the LEFT AND RIGHT PRERECTAL NUCLEI (located in the midbrain just under the superior colliculi).
2. Secondary neurons in the prerectal nuclei send bilateral axons to the EDINGER-WESTPHAL NUCLEI, which due to its bilateral nature will send signals to both left and right pupillary constrictor muscles.

Question 26

What are you testing by performing the light reflex test?

Answer 26

You are testing for the “indirect or consensual light reflex” response to examine the integrity of the afferent pathway.

Question 27

What arteries are CNIII in close association with?

Answer 27

It passes between the posterior cerebral artery and superior cerebellar artery and adjacent to the posterior communicating artery and the internal carotid artery.

Question 28

How will an aneurysm of one of the arteries first present?

Answer 28

Due to the peripheral location of the parasympathetic fibers on the ICA, one of the first signs of an aneurysm will be unilateral pupillary enlargement dues to damage tp the constrictor fibers. It can progress to unilateral paresis of all the extra ocular muscles innervated by this nerve.
***The acute onset of this should be considered a neurological emergency.

Question 29

How will occlusion of the vessels serving the center of CNIII as a result of DM present?

Answer 29

Since DM affects small vessels, it will affect those at the center of CNIII. However since the parasympathetic fibers run along the surface with the superficial blood vessels, these will likely not be affected. Therefore, it will present as unilateral paresis of the extra ocular muscles without unilateral pupillary enlargement.

Question 30

Describe the accommodation reflex.

Answer 30

-Convergence of the eyes along with parasympathetic thickening of the lens to allow for near vision.

Question 31

What does contraction of the ciliary muscles do?

Answer 31

It causes the suspensory ligaments attached to the lens to relax and thus the lens increases in diameter.

Question 32

What initiates the accommodation reflex?

Answer 32

It’s a combination of the cortical areas including the prefrontal eye fields and the occipital-parietal fields which cause pretectal activation that triggers the parasympathetic output from the EW nucleus to produce lens thickening and pupil constriction in conjunction with the activation of CNIII bilaterally to produce eye convergence.

Question 33

Is the accommodation dependent on the MLF pathway?

Answer 33

No. Thus the eyes can still be converged if there is a lesion in the MLF pathway.

Question 34

Describe the steps in the Accommodation Reflex.

Answer 34

1. The supraocularmotor area (SOA) coordinates the accommodation reflex through input from the visual cortex.
2. The SOA excites projections to the ciliary muscle, which increases curvature of the lens. Projections to the sphincter pupillae are also excited, which leads to pupillary constriction.
3. The SOA sends excitatory projections to both the oculomotor nuclei and causes contraction of both medial rectus muscles, allowing the eyes to converge.

Question 35

What is the MLF?

Answer 35

It is the connection between CNVI nuclei and CNIII nuclei.

Question 36

What will a failure in conjugate movement cause?

Answer 36

diplopia (double vision) since the cortex will be presented with two slightly different images.

Question 37

What is the purpose of the vestibulo-ocular reflex?

Answer 37

It is an important regulator of eye movement coordinating with head movement.

Question 38

What structures mediate the vestibulo-ocular reflex?

Answer 38

semicircular canals, the vestibular nerve and the vestibular nuclei.

Question 39

What are the steps of the vestibule-ocular reflex?

Answer 39

1. The medial vestibular nucleus sends its excitatory connection to the contralateral PPRF which in turn connects with the abducens nucleus.
2. Rotation of the head to the right will activate the lateral semicircular canal to signal the right medial vestibular nucleus and the right vestibular nucleus of CNIII.
   * **Initially shifting the head to the right will cause momentary movement of the eyes to the left.

Question 40

What are saccadic eye movements?

Answer 40

They are eye movements that are under control of the frontal eye field that can be either voluntarily triggered or reflexive.
**These can be inhibited by fixing your eye on an object.

Question 41

Describe the pathway of reflexive saccadic eye movements.

Answer 41

It begins in the frontal eye field and travels through corticobulbar fibers through the internal capsule tot he ipsilateral superior colliculus and on to the CONTRALATERAL PPRF.
**Thus activation of the right FEF activates the left PPRF which in turn activates the left CNVI and right CNIII.

Question 42

Describe slow pursuit eye movements.

Answer 42

• Are under control by the POT (parieto-occipito-temporal) area.
• Direct eye movement in the ipsilateral direction.
• “Just recall that the contralateral cerebellum and vestibular nuclei participate in this control mechanism.”
• Tested using the OKN test.

Question 43

If there is a lesion causing paralysis in the FEF (frontal cortex lesion), which direction will the eyes look?

Answer 43

Lesion of of left frontal lobe will produce a right hemiparesis and loss of input from the right PPRF, allowing the combo if the right FEF and left PPRF to drive the eyes towards the right.
“away from the paresis”

Question 44

If there is a lesion causing paralysis in the Left PPRF, which direction will the eyes look?

Answer 44

Lesion of the left pons produces a right hemiparesis and loss of function of the left PPRF allowing the right PPRF to drive the eyes towards the right.
“towards the paresis”