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THE TOP TEN THINGS YOU
SHOULD KNOW ABOUT
THE OCULOMOTOR
SYSTEM
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 Duane’s
Video of Opsoclonus
• Additionally, accommodation and
pupillary responses are produced by
intraocular muscles
Meet the muscles
Muscle
Primary action
Example
Medial rectus
Adduction
Towards the
midline/nose
Lateral rectus
Abduction
Away from the
midline/nose
Superior rectus
Elevation
Inferior rectus
Depression
Superior oblique
Intorsion
Inferior oblique
Extorsion
Meet the muscles (Cont.)
Muscle
Primary action
Ciliary muscle
Positive accommodation:
acts against suspensory
ligaments
Sphincter pupillae
iris muscle
Pupilloconstriction
Dilator pupillae
iris muscle
Pupillodilation
9. The stretch reflex is absent. Gently
press on your eye and you’ll see the
world move.
• Proprioceptive feedback from the extraocular 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 extraocular muscles. This is known as
efference copy or corollary discharge.
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.
MVN - Medial vestibular nucleus
NPH - Nucleus prepositus hypoglossi
EBN - Excitatory burst neuron
IBN - Inhibitory burst neuron
Excitatory
Inhibitory
VIDEO SHOWING
INTERNUCLEAR
OPHTHALMOPLEGIA
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
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Extra-ocular muscles
Class of Eye
Movement
Main Function
Vestibular
Holds images of the seen world steady on the
retina during brief head rotations
Optokinetic
Holds images of the seen world steady on the
retina during sustained head rotation
Visual fixation
Holds the image of a stationary object on the
fovea
Smooth pursuit
Holds the image of a small moving target on the
fovea; with optokinetic responses, aids gaze
stabilization during sustained head rotation
Nystagmus quick
phases
Reset the eyes during prolonged rotation and
direct gaze toward the oncoming visual scene
Saccades
Bring images of objects of interest onto the
fovea
Vergence
Moves the eyes in opposite directions so that
images of a single object are placed or held
simultaneously on both foveas
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Intra-ocular muscles
Class of Eye
Movement
Main Function
Accommodation
Focuses images on fovea
Pupillary Light Reflex
Controls illumination levels of retina
6. Vestibular responses. You
can’t read without them.
Your head turns in one direction with a
certain velocity, and because of the
vestibular ocular reflex (VOR), your eyes
turn with an equal velocity (if the VOR
gain is 1.0) in the opposite direction. This
reflex has a latency of less than 10
millseconds.
Once the transient head rotation ceases,
your eyes have turned to a new position.
They need to remain at that position and
not drift back to primary position.
To achieve this, a tonic signal
proportional to the integral of the eye
velocity signal is generated and sent to
the extraocular motoneurons to maintain
the new eye position.
VOR gain is low at low frequencies
Vestibulo-ocular reflex
MVN - Medial vestibular nucleus
NPH - Nucleus prepositus hypoglossi
EBN - Excitatory burst neuron
IBN - Inhibitory burst neuron
Excitatory
Inhibitory
Increased firing rate
with rightward head turns
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Extra-ocular muscles
Class of Eye
Movement
Main Function
Vestibular
Holds images of the seen world steady on the
retina during brief head rotations
Optokinetic
Holds images of the seen world steady on the
retina during sustained head rotation
Visual fixation
Holds the image of a stationary object on the
fovea
Smooth pursuit
Holds the image of a small moving target on the
fovea; with optokinetic responses, aids gaze
stabilization during sustained head rotation
Nystagmus quick
phases
Reset the eyes during prolonged rotation and
direct gaze toward the oncoming visual scene
Saccades
Bring images of objects of interest onto the
fovea
Vergence
Moves the eyes in opposite directions so that
images of a single object are placed or held
simultaneously on both foveas
5. Optokinetic responses. The
world drifts without them.
When large-field stimuli move, your eyes tend to
track the overall movement. This is an adaptive
response to the slip of the image of the outside
world on the retina that occurs when VOR gain is
not 1.0.
This response is mediated by neurons in the
pretectum and the medial superior temporal (MST)
region of cortex. These neurons indirectly
modulate vestibular neurons.
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)
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 slowphase 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 afternystagmus (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.
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Extra-ocular muscles
Class of Eye
Movement
Main Function
Vestibular
Holds images of the seen world steady
on the retina during brief head
rotations
Optokinetic
Holds images of the seen world steady
on the retina during sustained head
rotation
Visual fixation
Holds the image of a stationary object
on the fovea
Smooth pursuit
Holds the image of a small moving
target on the fovea; with optokinetic
responses, aids gaze stabilization
during sustained head rotation
VOLUNTARY
Saccades
Bring images of objects of interest
onto the fovea
VOLUNTARY
Vergence
Moves the eyes in opposite directions
so that images of a single object are
placed or held simultaneously on both
foveas
VOLUNTARY
4. Saccadic eye movements. You can’t look
at anything interesting without them.
• FAST - 40-90 MS IN
TOTAL DURATION
•BALLISTIC
Pulse of firing rate is required
to produce the transient force
needed to move the eye
rapidly despite viscous drag
Sustained firing rate is
required to hold the eye in a
new postion despite the
elastic forces that try to return
it to primary position
Right Medial Rectus Motoneuron - Saccades
Horizontal saccades are generated in the
paramedian pontine reticular formation (PPRF)
VERTICAL
SACCADES ARE
GENERATED
HERE
Vertical saccades are generated in the rostral interstitial nucleus
of the medial longitudinal fasciculus (riMLF)
MVN - Medial vestibular nucleus
NPH - Nucleus prepositus hypoglossi
EBN - Excitatory burst neuron
IBN - Inhibitory burst neuron
Excitatory
Inhibitory
Increased firing rate
with rightward head turns
OMNIPAUSE NEURON
(OPN)
EBN
Burst size proportional
to saccade size
NI - Neural Integrator
Excitatory burst neuron- small saccade
Excitatory burst neuron- medium saccade
Excitatory burst neuron - large saccade
Omnipause neuron - various saccades
OMNIPAUSE NEURON
(OPN)
EBN
Burst size proportional
to saccade size
NI - Neural Integrator
THE SUPERIOR COLLICULUS
PROJECTS TO THE PPRF
2-D map of
contralateral saccades
SUPERIOR COLLICULUS
MOTOR MAP
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).
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.
Experimental 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.
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Extra-ocular muscles
Class of Eye
Movement
Main Function
Vestibular
Holds images of the seen world steady on the
retina during brief head rotations
Optokinetic
Holds images of the seen world steady on the
retina during sustained head rotation
Visual fixation
Holds the image of a stationary object on the
fovea
Smooth pursuit
Holds the image of a small moving target on the
fovea; with optokinetic responses, aids gaze
stabilization during sustained head rotation
Nystagmus quick
phases
Reset the eyes during prolonged rotation and
direct gaze toward the oncoming visual scene
Saccades
Bring images of objects of interest onto the
fovea
Vergence
Moves the eyes in opposite directions so that
images of a single object are placed or held
simultaneously on both foveas
• Smooth pursuit: Tracking eye movements conjugate. Velocity of visual target
• Visual cue: retinal slip velocity of visual target.
3.
Smooth pursuit eye movements. You can’t
track anything interesting without them
• Smooth pursuit: Tracking eye movements conjugate. Velocity of visual target. Slow.
• Visual cue: retinal slip velocity of visual target.
SMOOTH PURSUIT PATHWAYS
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Extra-ocular muscles
Class of Eye
Movement
Main Function
Vestibular
Holds images of the seen world steady on the
retina during brief head rotations
Optokinetic
Holds images of the seen world steady on the
retina during sustained head rotation
Visual fixation
Holds the image of a stationary object on the
fovea
Smooth pursuit
Holds the image of a small moving target on the
fovea; with optokinetic responses, aids gaze
stabilization during sustained head rotation
Nystagmus quick
phases
Reset the eyes during prolonged rotation and
direct gaze toward the oncoming visual scene
Saccades
Bring images of objects of interest onto the
fovea
Vergence
Moves the eyes in opposite directions so that
images of a single object are placed or held
simultaneously on both foveas
2.
Vergence. Without it, you can’t get
a closer look.
• Vergence: Eye
movements in depth.
Disconjugate - left and
right eyes move in
opposite directions.
Far viewing
F
F
Near target
F
F
Blurred images fall on
non-corresponding
retinal locations blur and disparity signals
Accommodation
and convergence
F
F
Eyes converge and lenses
focus - reduced
blur and disparity signals
Far viewing
F
F
Near target
F
F
Blurred images fall on
retina - blur signal
Accommodation
and
accommodative
convergence
F
F
Eyes change focus
- reduced blur signal.
Also accommodative
convergence
2-D map
of saccades
MVN - Medial vestibular nucleus
NPH - Nucleus prepositus hypoglossi
EBN - Excitatory burst neuron
IBN - Inhibitory burst neuron
Excitatory
Inhibitory
Increased firing rate
with rightward head turns
Internuclear ophthalmoplegia - adduction
during convergence is not reduced
FUNCTIONAL CLASSES OF EYE MOVEMENTS
Intra-ocular muscles
Class of Eye
Movement
Main Function
Accommodation
Focuses images on fovea
Pupillary Light Reflex
Controls illumination levels of retina
MODULATE ROOM LIGHTS
Pupillary light reflex
Pupillary light reflex
Direct
Consensual
Pupillary light reflex
Direct
Consensual
Pupillary light reflex
Direct
Consensual
Pupillary light reflex
Direct
Consensual
1. Pupillary light reflex. If it’s absent,
there’s 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.
Pupillary light reflex: Afferent deficit
Pupillary light reflex: Afferent deficit
Neutral Density
Filter (0.5 log unit)
0.5 log unit
Relative Afferent
Pupillary Deficit
(RAPD)
Pupillary light reflex: Efferent deficit
TOP TEN LIST
1. Pupillary light reflex. If it’s absent, there’s a problem.
2. Vergence. Without it, you can’t get a closer look.
3. Smooth pursuit eye movements. You can’t track anything
interesting without them
4. Saccadic eye movements. You can’t look at anything interesting
without them.
5. Optokinetic responses. The world drifts without them.
6. Vestibular responses. You can’t 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 you’ll
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.