THE TOP TEN THINGS YOU SHOULD KNOW ABOUT THE OCULOMOTOR SYSTEM

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THE TOP TEN THINGS YOU SHOULD KNOW ABOUT THE
OCULOMOTOR SYSTEM
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10. Movements of the eyes are produced by six
extra-ocular muscles. If they, or the neural
pathways controlling them, are not functioning
normally, eye movements are abnormal.
Video OF Duanes
Video of Opsoclonus

• Additionally, accommodation and pupillary
  responses are produced by intraocular muscles

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Meet the muscles
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Meet the muscles (Cont.)
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9. The stretch reflex is absent. Gently press
on your eye and youll see the world move.

• Proprioceptive feedback from the extra-ocular
  muscles is not used to keep track of eye
  position.
• The brain keeps track of eye position by keeping
  track of the signals sent to the motoneurons that
  innervate the extra-ocular muscles. This is
  known as efference copy or corollary discharge.

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8. Except for changes in viewing distance,
normal eye movements are yoked.

• Yoking the eyes move the same amount in the same
  direction.
• Vertical eye movements are normally always yoked.
  
• Projections from the abducens nucleus to medial
  rectus motoneurons by way of the medial
  longitudinal fasciculus provides the basis for
  horizontal yoking. During convergence, the eyes
  move equal amounts in opposite directions.

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MVN - Medial vestibular nucleus NPH - Nucleus
prepositus hypoglossi EBN - Excitatory burst
neuron IBN - Inhibitory burst neuron
VIDEO SHOWING INTERNUCLEAR OPHTHALMOPLEGIA
Excitatory
Inhibitory
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7. Eye movements are controlled by distinct
neurological subsystems.

• Eye movements stabilize the image of the external
  world on the retina
• Eye movements bring images of objects of interest
  onto the fovea

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FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular
muscles
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FUNCTIONAL CLASSES OF EYE MOVEMENTS
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6. Vestibular responses. You cant read without
them.
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VOR gain is low at low frequencies
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Vestibulo-ocular reflex
MVN - Medial vestibular nucleus NPH - Nucleus
prepositus hypoglossi EBN - Excitatory burst
neuron IBN - Inhibitory burst neuron
Excitatory
Increased firing rate with rightward head turns
Inhibitory
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FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular
muscles
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5. Optokinetic responses. The world drifts
without them.
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VESTIBULAR NUCLEUS NEURON
A. ROTATION IN DARKNESS (Vestibular but no
Optokinetic) B. ROTATION IN LIGHT
(Vestibular and Optokinetic) C. NO ROTATION.
OPTIC FLOW. (Optokinetic but no Vestibular)
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Vestibular-optokinetic interactions
Schematic summary of vestibular-optokinetic
interaction occurring in response to
velocity-step rotations. Graphs on the left show
characteristics of the stimulus (head velocity
during rotation or drum velocity during
optokinetic stimulation) graphs on the right
show the responses (slow-phase eye velocity,
quick phases having been removed). R, right L,
left t, time. In the top panel,
constant-velocity rotation to the left in the
dark produces slow-phase movements to the right
(per-rotatory nystagmus, RN) with initial eye
velocities equal to head velocity (VOR gain
1.0).
When rotation stops, nystagmus starts in the
opposite direction (postrotatory nystagmus, PRN).
In the middle panel, an optokinetic stimulus
(drum rotation to the right) causes a sustained
optokinetic nystagmus (OKN), with slow phases to
the right during the entire period of
stimulation. When the lights are turned off
during stimulation, eye movements do not stop
immediately but persist as optokinetic
after-nystagmus (OKAN). In the lower panel, the
subject is rotated in the light (natural
situation of self-rotation). This gives a
combined vestibular and optokinetic stimulus. The
response is a sustained nystagmus. When the chair
stops rotating, eye movements stop nearly
completely postrotatory nystagmus is suppressed
by the opposite-directed optokinetic
after-nystagmus and by visual fixation of the
stationary world.
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FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular
muscles
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4. Saccadic eye movements. You cant look at
anything interesting without them.

• FAST - 40-90 MS IN TOTAL DURATION
• BALLISTIC

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Right Medial Rectus Motoneuron - Saccades
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Horizontal saccades are generated in the
paramedian pontine reticular formation (PPRF)
Vertical saccades are generated in the rostral
interstitial nucleus of the medial longitudinal
fasciculus (riMLF)
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EBN
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Excitatory burst neuron- small saccade
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Excitatory burst neuron- medium saccade
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Excitatory burst neuron - large saccade
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Omnipause neuron - various saccades
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EBN
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LOCAL FEEDBACK MODEL
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THE SUPERIOR COLLICULUS PROJECTS TO THE PPRF
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SUPERIOR COLLICULUS MOTOR MAP
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A block diagram of the major structures that
project to the brain stem saccade generator
(premotor burst neurons in PPRF and riMLF). Also
shown are projections from cortical eye fields to
superior colliculus. FEF, frontal eye fields
SEF, supplementary eye fields DLPC, dorsolateral
prefrontal cortex IML, intramedullary lamina of
thalamus PEF, parietal eye fields (LIP) PPC,
posterior parietal cortex SNpr, substantia
nigra, pars reticulata. Not shown are the
pulvinar, which has connections with the superior
colliculus and both the frontal and parietal
lobes, and certain projections, such as that from
the superior colliculus to nucleus reticularis
tegmenti pontis (NRTP).
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Disorders of the saccadic pulse and step.
Innervation patterns are shown on the left, eye
movements on the right. Dashed lines indicate the
normal response. (A) Normal saccade. (B)
Hypometric saccade pulse amplitude (width
height) is too small but pulse and step are
matched appropriately. (C) Slow saccade
decreased pulse height with normal pulse
amplitude and normal pulse-step match. (D)
Gaze-evoked nystagmus normal pulse, poorly
sustained step. (E) Pulse-step mismatch
(glissade) step is relatively smaller than
pulse. (F) Pulse-step mismatch due to
internuclear ophthalmoplegia (INO) the step is
larger than the pulse, and so the eye drifts
onward after the initial rapid movement. Experimen
tal cerebellectomy completely abolishes the
adaptive capability-for both the pulse size and
the pulse-step match.296 Monkeys with lesions
restricted to the dorsal cerebellar vermis cannot
adapt the size of the saccadic pulse they have
pulse-size dysmetria .416,416a On the other hand,
monkeys with floccular lesions cannot match the
saccadic step to the pulse to eliminate
pulse-step mismatch dysmetria.298 This evidence
suggests that the repair of conjugate saccadic
dysmetria is mediated by two different cerebellar
structures the dorsal cerebellar vermis and the
fastigial nuclei control pulse size, and the
flocculus and paraflocculus control the
pulse-step match.
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FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular
muscles
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• Smooth pursuit Tracking eye movements -
  conjugate. Velocity of visual target
• Visual cue retinal slip velocity of visual
  target.

A
B
B
A
B
A
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3. Smooth pursuit eye movements. You cant
track anything interesting without them

• Smooth pursuit Tracking eye movements -
  conjugate. Velocity of visual target. Slow.
• Visual cue retinal slip velocity of visual
  target.

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Right Medial Rectus Motoneuron - Smooth pursuit
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SMOOTH PURSUIT PATHWAYS
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FUNCTIONAL CLASSES OF EYE MOVEMENTSExtra-ocular
muscles
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2. Vergence. Without it, you cant get a
closer look.

• Vergence Eye movements in depth. Disconjugate -
  left and right eyes move in opposite directions.

A
B
B
B
A
A
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Accommodation and convergence
Far viewing
Near target
F
F
F
F
F
F
Blurred images fall on non-corresponding
retinal locations - blur and disparity signals
Eyes converge and lenses focus - reduced blur
and disparity signals
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Far viewing
Near target
Accommodation and accommodative convergence
F
F
F
F
F
F
Eyes change focus - reduced blur signal. Also
accommodative convergence
Blurred images fall on retina - blur signal
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Right Medial Rectus Motoneuron - Vergence
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2-D map of saccades
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MVN - Medial vestibular nucleus NPH - Nucleus
prepositus hypoglossi EBN - Excitatory burst
neuron IBN - Inhibitory burst neuron
Excitatory
Increased firing rate with rightward head turns
Inhibitory
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NEAR RESPONSE NEURON - VERGENCE
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NEAR RESPONSE NEURON - SACCADES
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Internuclear ophthalmoplegia - adduction during
convergence is not reduced
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FUNCTIONAL CLASSES OF EYE MOVEMENTS
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EW
SOA

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Edinger-Westphal neuron
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Pupillary light reflex
Direct
Consensual
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Pupillary light reflex
Direct
Consensual
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Pupillary light reflex
Direct
Consensual
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Pupillary light reflex
Direct
Consensual
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1. Pupillary light reflex. If its absent,
theres a problem.
AFFERENT DEFECTS PUPILS APPROX. EQUAL IN SIZE.
BUT RESPONSE TO LIGHT IN ONE EYE IS LESS THAN THE
RESPONSE TO LIGHT IN THE OTHER EYE. EFFERENT
DEFECTS PUPILS MAY BE OF DIFFERENT SIZES
(ANISOCORIA). PUPIL OF ONE EYE REACTS MORE TO
LIGHT IN EITHER EYE THAN THE PUPIL OF THE OTHER
EYE TO LIGHT IN EITHER EYE.
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Pupillary light reflex Afferent deficit
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Pupillary light reflex Afferent deficit
Neutral Density Filter (0.5 log unit) 0.5 log
unit Relative Afferent Pupillary Deficit (RAPD)
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Pupillary light reflex Efferent deficit
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TOP TEN LIST
1. Pupillary light reflex. If its absent,
theres a problem. 2. Vergence. Without it,
you cant get a closer look. 3. Smooth pursuit
eye movements. You cant track anything
interesting without them 4. Saccadic eye
movements. You cant look at anything
interesting without them. 5. Optokinetic
responses. The world drifts without them. 6.
Vestibular responses. You cant read without
them. 7. Eye movements are controlled by
distinct neurological subsystems. 8. Except for
changes in viewing distance, normal eye movements
are yoked. 9. The stretch reflex is absent.
Gently press on your eye and youll see the world
move. 10. Movements of the eyes are produced by
six extra-ocular muscles. If they, or the neural
pathways controlling them, are not functioning
normally, eye movements are abnormal.