Eye Movements I-II Flashcards Preview

Nervous System: Unit III > Eye Movements I-II > Flashcards

Flashcards in Eye Movements I-II Deck (13):
1

Types of eye movements

  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)

2

Characteristics of pursuit eye movemetns

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

3

Characteristics of saccades eye movemetns

  • = 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

4

Characteristics of vestibulo-ocular reflex

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

5

Characteristics of optokinetic nystagmus

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

6

Characteristics of Nystagmus

  • 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

7

Conjugate vs. Vergence movements

  • 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

8

Control of smooth pursuit eye movements

  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

9

Main pattern-generating centers for eye movements

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

10

Control of saccades

  • 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

11

Mechanism/control of VOR

  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

12

Definitions of internuclear opthalmoplegia

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

13

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

  • 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