Centres for control of eye movements

Question 1

What is steady fixation?

Answer 1

Maintenance of gaze when head is still

Question 2

What does the vestibulo-ocular reflex do?

Answer 2

Maintains fixation to compensate for rapid head movements

Question 3

What does the optokinetic response do?

Answer 3

Slow compensatory movements to maintain fixation on large moving target (slow head movement)

Question 4

What is smooth pursuit?

Answer 4

Slow conjugate movements for steady tracking of a small moving target

Question 5

What are saccades?

Answer 5

Rapid conjugate movement to bring the eyes into a new position to enable viewing eccentrically located object

Question 6

What is vergence?

Answer 6

Generates equal but opposite eye movements to maintain visual target simultaneously on both foveas during change of depth of gaze

Question 7

Where are the frontal eye fields in the brain?

Answer 7

Premotor cortex in middle frontal gyrus
Contain cells exhibiting pre-saccadic activity for voluntary eye movements and accomodation
Maintains visual attention and reflex eye movements

Question 8

What does a lesion on one side of the premotor cortex cause optically?

Answer 8

Inability to saccade towards opposite side (eyes deviate to the side of the lesion)

Question 9

What is the fronto-mesencephalic pathway?

Answer 9

Fibres from frontal eye fields travel within anterior limb of internal capsule, passing through thalamus to midbrain where they decussate and reach superior colliculus

Question 10

What does the superior colliculus do?

Answer 10

Initiate and control saccades independent of frontal eye fields
Initiates reflex orientating saccades
If frontal eye field damaged, superior colliculi can compensate

Question 11

What are the paramedian pontine reticular formations?

Answer 11

Neurons in the pons lateral to CN VI nucleus
Horizontal gaze centre
Final common pathway horizontal movements; quick face of nystagmus, saccadic movements, smooth pursuit

Question 12

How does horizontal gaze occur?

Answer 12

Fibres from the PPRF liase with ipsilateral CN VI nucleus.
CN VI controls ipsilateral lateral rectus but also joins CN III via MSF to control contralateral medial rectus
Vestibular nucleus also contributes fibres
Memorise diagram

Question 13

Medial longitudinal fasciculus (MSF)?

Answer 13

Fibre tract running from thalamus to anterior horn cells of the spinal cord
Well developed between vestibular nuclei and CN III nucleus
Provides co-ordination of horizontal and vertical gaze
Commonly affected by demylination and ischaemia

Question 14

Describe the pathway involved in voluntary horizontal gaze to the left

Answer 14

Right frontal eye field
Superior colliculus
Left PPRF
Left CN VI-> left LR
Right CN III-> right MR

Question 15

How is smooth pursuit controlled>

Answer 15

Temporal eye field

Dorsolateral pontine nuclei determine direction and velocity of eye movements necessary to track target

Question 16

What is the most important structure in vertical gaze?

Answer 16

Medial longitudinal fasciculus in midbrain

Question 17

How does the cerebellum control eye movements?

Answer 17

Modulating activity of oculomotor system and vestibulocular reflexes
Spatiotemporal translator to tune amplitude of eye movements in response to stimulus
Dysfunction leads to impaired smooth pursuit, impaired fixation and nystagmus

Question 18

Where are the vestibular nuclei and what do they do?

Answer 18

Lower pons and upper medulla under 4th ventricle
Afference-from vestibular ganglion and spinocerebellar tracts
Efference- cerebellar connection, PPRF, vertical gaze centres and oculomotor nuclei

Question 19

How does the vestibulo-ocular reflex work?

Answer 19

Semicircular canals -> vestibular ganglion -> vestibular nucleus -> oculomotor nuclei

Question 20

How does the near response work?

Answer 20

Frontal eye fields-> pretectal convergence centre-> oculomotor nucleus (including Edinger Westphal nucleus)
Causes bilateral accomodation, miosis and convergence
Pathways passes ventrally
EWN effects pupil via ciliary muscle and sphincter pupillae

Question 21

How does convergence occur?

Answer 21

Activation of both medial rectus subnuclei in CN III nucleus of midbrain

Question 22

Why can one aspect of the near reflex be lost without affecting the others?

Answer 22

The 3 elements of the near reflex are not tied to each other

Question 23

What is a major difference between the near reflex and pupil reflexes?

Answer 23

The accomodation pathway reaches the EWN without passing through pretectal nucleus

Question 24

What is the oculocardiac reflex?

Answer 24

Pressure on eye/stretching of muscles causes nausea and bradycardia
Occurs via trigeminal nerve. Trigeminal spinal nucleus is in the area of visceral moto nuclei in vagus nerve. Vagus nerve is activated
Can occur under GA in strabismus surgery

Question 25

Which nerve is responsible for reflex lacrimation?

Answer 25

Sensory branches of trigeminal nerve
Synpase in pterygopalatine ganglion
Reflex-parasympathetic but basal tear secretion is sympathetic

Question 26

What can cause the blink reflex?

Answer 26

Tactile, visual or auditory stimuli

Question 27

Which nerves are involved in the blink reflex?

Answer 27

Facial nerve to orbicularis muscle closes the eye

Question 28

What is Bell’s reflex?

Answer 28

Upward movement of globe which occurs with forcible eyelid closure
Absent in 10% of individuals