Eye movement

Question 1

Summarize again which muscles control eyes.

Answer 1

Superior oblique = downward, inward
Inferior oblique = upward, outward

Question 2

Which nerves innervate them?

Answer 2

Abducens n. = lateral rectus, ipse
Trochlear n. = dorsal margin of brainstem, contra
Oculomotor n. = the rest, ipse
- ALSO constricter muscles of the lens + opening of the eyes

Question 3

What are conjugate eye movements? What 4 types can you recall?

Answer 3

Conjugate = eye moves in the same direction
1. Saccades = quick, ballistic
2. Smooth pursuit = slow, visually guided e.g. following a wasp
3. Optokinetic = smooth pursuit and saccades, visually guided, large scale visual stimulus
4. Vestibulo-ocular = slow, dependent on vestibular input

Question 4

How about Disconjugate eye movements?

Answer 4

Disconjugate = eyes move in opposite direction
- Vergence (Convergence and Divergence) - visually guided near (moving inward - one rotates to the right, the other left) and far (outward) fixations

Question 5

Elaborate on the graph below - saccade

Answer 5

Plus: If a new stimulus occured we would be able to make saccade towards it only after completing the first one

Question 6

With the graph explain smooth pursuit movement.

Answer 6

Impossible to do without a visual target (red line) -> we’d make multiple saccades

We make a saccade towards the target -> then we can smoothly follow it

Question 7

Explain Optokinetic nystagmus. Which system can help out and how?

Answer 7

E.g. reading, watching passing vehicles
= first we make a smooth pursuit (tracking one car) -> saccade movement (shifting to another car)

High when motion is slow - helps to stabilize our vision -> as movement fastens it cannot keep up -> switch to vestibulo-ocular reflex maintains fixation

Question 8

Recall how vestibulo-ocular reflex works.

Answer 8

Question 9

How do we fixate on near targets? (which muscles, nerves)

Answer 9

Near fixation - requires activation of oculomotor n. -> contraction of both medial rectus muscles (adduction - convergence)

Far fixation - activation of abducens n. -> contraction of both lateral rectus muscles (abduction - divergence)

Question 10

What encodes the amplitude of movement? What id we record activity of an appropriate neuron when moving in the lateral direction (small distance, greater distance, back to medial)?

Answer 10

Amplitude determined by rate and duration of activity in appropriate n.

• Experiment - moving eye to lateral direction
  
  • first big burst of abducens activation -> when the distance small, abducens n. can rest after the movement is done
  • BUT once it reaches certain level of deviation it will require sustained action of abducens n. for the eye to stay deviated to the lateral side
  • If we go back (adduction) - abducens fall silent (probably interneuron)

Question 11

What stands for PPRF - what is it or what is it doing?

Answer 11

Paramedian Pontine Reticular Formation (PPRF) = set of interneurons are integrating signals (sensory and command) in order to coordinate firing of abducens and oculomotor n.
= Horizontal gaze center

Question 12

What happens with PPRF when moving the eyes to the left?

Answer 12

1. I want to move my eyes to the left
2. PPRF on the left side integrates the commands
   3. Activates abducens n. on the left side
   -> contraction of the left lateral rectus
   3. Internuclear n. passes the midline ->
   activate n. of the oculomotor nucleus -> contraction of the right medial rectus

Question 13

What adds to PPRF?

Answer 13

Vertical gaze center = network of interneurons that coordinates neurons corresponding to muscles moving eyes down or up

Question 14

What are the two sites that send info to PPRF (+location, automatic/voluntary)?

Answer 14

1. Frontal Eye Fields - anterior part of the Premotor c., posterior to superior and middle frontal gyri (Brodmann’s area 8)
   
   • voluntary
2. Superior Colliculus
   
   • automatic

• Both begin to fire before the eye movement is made, thus likely control direction and amplitude of it
• Look at the path: FEF -> ipse SC -> contra PPRF (or ventral gaze center)

Question 15

How do they represent information? Explain in the picture.

Answer 15

• Include motor map informed by their sensory maps which represent contralateral visual fields
• Pay attention to the coloured axis
  • what happens if we stimulate the sites?

Question 16

Explain the experiment in the picture:

Answer 16

Procedure: monkey trained to fixate on a point -> than target presented -> monkey should shift its eye gaze towards the target
-> BUT experimenters stimulated SC which changed fixation

Results:
=> monkey makes compensatory saccade towards the actual target despite the shift
- showing SC doesn’t function exactly on the basis of visual map (more so calculating what saccade is needed to be made in order to reach the target)

Question 17

To what kind of info does SC react to?

Answer 17

Visual, auditory, somatosensory -> directs gaze towards a novel stimulus

Question 18

What is the clinical benefit of FEF and SC having the same path to create a saccade?

Answer 18

If upon certain damage a hypofunction occurs in one of the structures -> immediate: problem with saccades
-> after few months one structure may compensate for the other -> saccades restored

Question 19

What is the main difference between FEF and SC?

Answer 19

FEF = intentional saccades e.g. looking at the time
- contralateral control (patient may not make saccading eye movement contralateral to their damage)

SC = automatic/express saccade (unexpected sensory stimulus) e.g. sudden shriek when my mum finds a snake