Control of Eye Movements - UBC

Question 1

What is meant by visual gaze?

Answer 1

The coorindated movements of the right and left eyes to a visual target
Requires ipsilaterally and contralteral communication of the cranial nerves 3,4,6.
This communication occurs via the ascending medial longitudinal fasiculus.

Question 2

What is the function of the ascending medial longitudinal fasiculus?

Answer 2

Criticial for coordinated eye movements both ipsilateraly and contralteraly via CN3,4,6.

Question 3

What are the horizontal gaze centres in the brain?
What are they responsible for?

Answer 3

Horizontal gaze centre is located in the pons.
Specifically the paramedian pontine reticular formation.
Moves eyes from left to right

Question 4

What is the vertical gaze centre?
What is it responsible for?

Answer 4

Located in the midbrain
Specifically midbrain reticular formation and pretectal areas
Resonsible for looking on a vertical plane (up or down)

Question 5

What is responsible for coordinated horizontal gaze to the right?

Answer 5

Horizontal gaze centre gives input to ipsilateral abducens nucleus to direct gaze to the ipsilateral side.
Synapse with CNVI projections to the ipsilateral lateral rectus (right eye looks to the right)

For the left eye horizontal gaze centre projects to the right abducens nucleus, then synapses with a branch that projects to the contralateral oculomotor nucleus via the medial longitudinal fasiculus.
Synapse with projects of CN3 to the left medial recturs (left eye looks to the right)

Coordinated movement of both eyes to look in the same direction.

Question 6

What is meant by saccadic eye movement?

Answer 6

Rapid simultaneous movements of the eyes that redirect the gaze between different fixation points.
Allows us to scan our environment by directing our gaze to different objects of interest to ensure projected on fovea.

Question 7

What is reflexive saccades?

Answer 7

Reflexive saccades - visually guided saccades in responsive to external cues (pro-saccade), e.g looking at trees whilst in moving car

Question 8

What is volitional saccades?

Answer 8

Volantary eye movements independent of any visual stimulus
- anti-sccades - looking away from a stimuli
- memory saccades (looking wo where a stimulus use to be but no longer is)
- predictive saccades (to where a stimulus will be)

Question 9

What structures tend to initiate saccadic movement of the eye?

Answer 9

Volitional saccades - tend to have a cortical origin in the frontal eye fields
Reflexive saccades - tend to have a subcortical initiation in the superior collicluli in the posterior midbrain.

Question 10

What is the pathway of a reflexive sacade to a stimuli on the right?

Answer 10

Stimuli on right visual field
Is detect in left side of brain in primary visual cortex.
Activates left frontal eye fields which projects to the the left superior colliculus
Within the colliculus visual layers are activated and project to corresponding motor layers.
Projection from the superior colliculus synapse in the contrlateral (right) horizontal gaze centre.
Which then activates horizontal gaze circuitry for coordinated eye movement to the right.

Question 11

What is the responsibility of the descending medial longitudinal fasiculus?

Answer 11

Enables communication between the brainstem nuclei, comprises the medial and lateral vestibulospinal tracts.
Originates from the vestibular nuclei and influences muscle tone for balance and posture it is also critical for positioning of the head and neck.

Question 12

How do the ascending medial longitudinal fasiculus and the descending longitudinal fasiculus communicate?

Answer 12

Converge on the vestibular nuclei.

Question 13

What is smooth pursuit eye movement?
How is this different to saccades?

Answer 13

Smooth pursuit - track slowly moving objects, co-ordinates so that visual target is stabilised in the fovea of the retina, must integrate the cortex, cerebellum and vestibular nuclei.

Contrast to saccades - which are rapid and change the stimuli in the centre of the fovea.

Question 14

Describe smooth pursuit circuitry as if moving gaze to track to the left.

Answer 14

1. The primary visual cortex and the frontal eye fields send projections to the ipsilateral pontine nuclei and synapse
2. Fibres from the pontine nuclei project to the contralateral cerebellum and synapse
3. Fibres then project to the ipsilateral vestibular nuclei and synapse
4. Then projects to the contralteral abducens nuclei
5. Produces co-ordinated movement of both eyes yo look in the same direction.

Question 15

What is the vestibulo-ocular reflex?

Answer 15

Rapid adjustment of the eye movement in response to movement of the head.
This stabilises the image on the retina and prevents it from becoming blurry.
Eye movements tend to be complete opposites of the head movement

Question 16

What is the vestibulo-colular refelx in response to left head movement?

Answer 16

1. Head movement is detected by the vestibular organ in the inner ear, due to movement of endolymph distorting stereocilia.
2. Information projects to the ipsilateral vestibular nuclei and synapse
3. Projects across the midline to the contralateral abducens nucleus
4. Results in co-ordinated eye movement to stabilise image on the retina (abducens nucleus acts on contralateral oculomotor nucleus)