Eye Movements I-II

Question 1

Types of eye movements

Answer 1

1. smooth pursuit: tracking (keeps object on fovea)
2. saccades: rapid, balistic (brings object onto fovea)
3. vestibular ocular reflex (VOR) and optokinetic nystagmus (OKN): combination of pursuit and saccades
4. vergence: moving the fovea to an object closer (convergence) or farther (divergence)

Question 2

Characteristics of pursuit eye movemetns

Answer 2

• elicited by a large moving visual stimulus
• slow movements used to track a moving object on fovea

Question 3

Characteristics of saccades eye movemetns

Answer 3

• = rapid movement that brings eyes to a predetermined target or position @ rate of up to 700 degrees/second
• initiated by a high frequency burst by the extraocular muscles ==> calculated steady rate

Question 4

Characteristics of vestibulo-ocular reflex

Answer 4

• rotation of head signaled by semi-circular canals ==> contraction of appropriate set of extraocular to oppose head rotation and fix gaze
  
  • contraction = “conjugate contraction”

Question 5

Characteristics of optokinetic nystagmus

Answer 5

• = rhythmic pattern of saccades and tracking movements
• often elicited by a passing scene

Question 6

Characteristics of Nystagmus

Answer 6

• rapid, “sawtooth” movement of eyes = rapid saccade to center of eye position
• elicited by continually turning head:
  
  • head turns right, eyes turn left until edge of vision is reached ==>
  • quickly snap to new fixation point
• inappropriate nystagmus = indication of pathology

Question 7

Conjugate vs. Vergence movements

Answer 7

• conjugate = both eyes moving in the same direction
  
  • fast or slow
  • initiated by a variety of sensory input; mostly visual and vestibular stimuli
• vergence = eyes moving in opposite directions
  
  • e.g. both eyes turn nasally
  • “near reflex” =
    
    • medial recti contract
    • • pupils constrict
    • • ciliary muscles contract

Question 8

Control of smooth pursuit eye movements

Answer 8

1. analysis of position, direction of movement, and speed @ visual cortex ==>
2. descending commands to brainstem conjugate movement generators (oculomotr and abducens nuclei)

• can only maintain foveation @ rate = 50 degrees/second
  
  • stimuli moving faster use smooth + saccades

Question 9

Main pattern-generating centers for eye movements

Answer 9

• w/in or near CN nuclei @ brainstem:
• oculomotor nucleus
• abducens nucleus

Question 10

Control of saccades

Answer 10

• vertical saccade pattern generator = near oculomotor nucleus
• horizontal saccade pattern generator = paramedian pontine reticular formation (PPRF), near the abducens nucleus
• control centers = cortex + superior colliculus
  
  • e.g. frontal lobes can accomplish voluntary drive saccades:
    
    • frontal eye field ==> reticular formation OR
    • fronal eye field ==> superior colliculus ==> reticular formation

Question 11

Mechanism/control of VOR

Answer 11

1. Head rotates to right ==> deflection of cupula @ horizontal canals
2. Right horizontal canal ==> depolarization of hair cellls
3. ==> excitation @ right vestibular nuclei ==> project via medial longitudinal fasciculus (MLF)
4. ==> left lateral rectus motor neurons @ abducens nucleus + activation of internuclear interneurons ==>
5. contraction of left lateral rectus & right medial rectus

Question 12

Definitions of internuclear opthalmoplegia

Answer 12

• damage to MLF ==>
• disrupt the coordination fo medial and lateral recti during horizontal gaze movements

Question 13

Horizontal gaze problems: MLF vs. Medial Rectus muscle/motor neurons

Answer 13

• MLF = medial rectus not actively participating in pursuit or horizontal saccades, but DO participate in vergence movements
• MR muscle/motor neurons = NO pursuit/saccades and NO vergence movements