CNn. III, IV, VI

Question 1

Where does the abducent nucleus lie?

Answer 1

Near midline just ventral to the 4th ventricle

Question 2

What is significant about the abducent nucleus’ proximity to the facial motor nucleus?

Answer 2

The axons of LMNs in the facial motor nucleus loop around the abducent nucleus creating an internal genu. Due to this relationship a lesion of the abducent nucleus can often present with deficits in the muscles of facial expression ipsilaterally.

Question 3

Describe the pathway of axons associated with LMNs in CN VI nucelus and the innervation this nucleus provides.

Answer 3

Axons will traverse the pons to exit ventrally as CN VI. They will provide innervation to the lateral rectus muscle ipsilaterally.

Question 4

What is the location of the trochlear nucleus and the pathway of its projections?

Answer 4

This nucleus lies at the level of middle midbrain at midline and posterior to the medial longitudinal fasciculus. Axons of this nucleus exit the brainstem dorsally inferior to the inferior colliculus. Axons DECUSSATE to innervate contralateral superior oblique muscles.

Question 5

Where is the occulomotor nucleus located?

Answer 5

Midlline of rostral midbrain

Question 6

What types of innervation does CN III supply?

Answer 6

Motor innervation to 5 out of 7 EOM and GVE-P to the ciliary and sphincter pupillae muscles

Question 7

Describe the visceral motor (GVE-P) innervation which is carried by CN III

Answer 7

Pre-gang para cell bodies lie in the edinger-westphal nucleus, which is superior and posterior to the CN III nucleus. Axons run with CN III to synapse at the ciliary ganglion. Post-gang fibers run with short ciliary nerves to the ciliary and sphincter pupillae muscles.

Question 8

Why does the visual system need to be highly accurate (3)?

Answer 8

1. Phototransduction is slow, utilizes second messenger system.
2. Fovea is a small area and object must be maintained here to accurately visualize them.
3. Binocular vision requires precise alignment.

Question 9

Describe how voluntary movements of the eyes are mediated from the cortex to the nuclei of CNn III, IV, and VI

Answer 9

UMNs project from the cerebral cortex to basal nuclei to determine appropriateness and cerebellum to coordinate smoothness.
UMNs descend through the genu of the internal capsule to the center for lateral gaze, which is a program pattern generator.
Center for lateral gaze will project to the nuclei for CNn. III, IV, and VI.

Note: there is no direct corticobulbar innervation of these nuclei

Question 10

What is the function of the center for lateral gaze?

Answer 10

Organizes conjugate and disconjugate movements

Question 11

What are the five eye movement systems?

Answer 11

Three are referred to as targeting systems:
1. Saccades
2. Smooth pursuit
3. Vergent
Two will stabilized images on the retina during head movements.
4. Vestibuloocular reflex
5. Optokinetic movements

Question 12

What two systems mediate conjugate movements?

Answer 12

Saccades and smooth pursuit

Question 13

Which phase of nystagmus is saccades responsible for?

Answer 13

Fast-phase

Question 14

What is significant about the visual input during a saccade?

Answer 14

During the rapid movements of a saccade the visual input would be a blur. Considering this the CNS ignores these visual inputs and constructs a visual perception from the two point of fixation on either end of a saccade.

Question 15

What are saccades?

Answer 15

Saccades are rapid movements which are responsible for moving the eyes between points of fixation when scanning an environment.

Question 16

What is the paramedian pontine reticular formation (PPRF)?

Answer 16

Center which coordinates rapid horizontal gaze.

Question 17

What is the rostral interstitial medial longitudinal fasciculus (riMLF)?

Answer 17

Center which coordinates rapid vertical gaze.

Question 18

Describe the process of a saccade from initiation to PPRF and riMLF. Include where information influencing initiation may come from.

Answer 18

Initiation of a saccade begins with UMNs located in the frontal eye fields (BA 6/8). The FEF will receive input from the supplementary eye field and parietal eye field. These UMNs project to the superior colliculus, the riMLF (bilaterally), and the PPRF (contralaterally). The superior colliculus will project to the PPRF and riMLF.

Question 19

What type of information does the supplementary eye field provide?

Answer 19

Planning for multiple sequences of saccades

Question 20

What type of information does the parietal eye field supply?

Answer 20

Automatic responses to things which appear in the periphery

Question 21

Describe the projections which allow the PPRF to control horizontal gaze.

Answer 21

PPRF projects to the ipsilateral abducens nucleus, which will project to the contralateral occulomotor nucleus.

Question 22

Describe the projections which allow the riMLF to control vertical gaze.

Answer 22

riMLF projects to all three motor nuclei.

Note: we don’t need to be more specific

Question 23

Briefly describe smooth pursuit and how it functions

Answer 23

Slower tracking movements which maintain moving objects on the fovea. This system requires an object to be moving. Functions through visual association cortices calculating vectors.

Question 24

Describe how the smooth pursuit system conducts impulses from the cerebral cortex to the motor nuclei of III, IV, and VI.

Answer 24

The FEF will receive input from parietal/supplementary eye fields and visual association cortices. UMNs project from the cortex to the pontine nuclei then cerebellum, specifically the flocculus. Here the smooth pursuit system takes advantage of the flucculus’ projections to the vestibular nuclei and controls eye movements through these nuclei and their projecitons through the ascending MLF.

Question 25

What is vergence?

Answer 25

This is the convergence of the eyes on a near target. This utilizes CN III alone.

Question 26

What is the stimulus for vergence? What two other events occur simultaneous with vergence?

Answer 26

Retinal disparity.

A close object will cause a loss of focus and lead to accommodation and pupillary constriction.

Note: convergence, accommodation, and pupillary constriction is referred to as the near triad

Question 27

Describe where initiation of vergence occurs, where input comes from, and where output travels.

Answer 27

Initiation - Frontal eye field
Input - parietal/supplementary eye fields
Output - riMLF (lies adjacent to the nucleus of CN III)

Question 28

What is the optokinetic system?

Answer 28

REFLEXIVE oscillatory movements (like nystagmus) in response to steady velocity.

Note: This differentiates this system from the vestibuloocular reflex which functions in response to acceleration

Question 29

Describe the process of the optokinetic system

Answer 29

Motion sensitive ganglion cell in the retina will project to the lateral geniculate nucleus, superior colliculus, and accessory optic nucleus.
Lateral geniculate nucleus will send information to primary visual cortex.
Superior colliculus collects all visual information
Accessory optic nucleus is located in the rostral midbrain adjacent to pretectal nuclei
Information from motion sensitive ganglion cells and visual association cortices travel through the accessory optic nucleus to the vestibular nuclei and eventually the motor nuclei of III, IV, and VI via the MLF.