Transcript Eye Movements – the basics

Eye Movements
Normal Eye Movements
• Objective: Foveation
• Extraocular muscles
• Muscle innervation – CNs III, IV, and VI
• Cranial nerve nuclei
• Three primary types of movements
• Pathologic eye movements
Objective: foveation
• fovea – most sensitive portion of retina
• we need to orient, either voluntarily or
reflexively, to important stimuli
• accomplished by directing our eyes to a target
so that it is projected onto the fovea
• to maintain foveation, we depend on visual
feedback as the primary source of
information on eye movement accuracy
Extraocular muscles
• Superior rectus – moves eye up
• Inferior rectus – moves eye down
• Medial rectus – moves eye in (a-d-duction)
left
• Lateral rectus – moves eye out (a-b-duction)
• Inferior oblique – moves eye up when it is in an adducted position;
also extorts the eye.
• Superior oblique – moves eye down when it is adducted; also intorts
the eye.
Isolating Extraocular Muscles
CN III
CN III
• Superior division
- Superior rectus muscle
- Levator palpebrae superioris
muscle
• Inferior division
-
Medial rectus muscle
Inferior rectus muscle
Inferior oblique muscle
Parasympathetic innervation to
pupil
CN IV
CN VI
CN VI
Coordination of Eye Movements
• Separate systems exist to control each different
subtype of eye movement: saccades, smooth
pursuit, and vergence
• May be nuclear or supranuclear control
• May be reflexive or voluntary
• Separate systems exist to govern vertical and
horizontal eye movements
Targeting Eye Movements
1. Saccades: Quick, darting conjugate movements
which direct the eyes to a new target.
2. Smooth pursuit: A slower conjugate movement
which allows for tracking of a moving object, or
of a stationary object while we are moving.
3. Convergence: A dysconjugate movement of
both eyes toward the midline to allow for
focusing on a near object by adjusting the
angle between the eyes.
Variety of pathways contribute to saccadic
control and smooth pursuit
Saccades
• Under the control of three different areas
in the brain:
– voluntary saccades - frontal eye fields
(Brodmann’s area 8)
– reflexive saccades to complex stimuli - parietal
lobes (Brodmann’s area 7)
– reflexive saccades to elementary stimuli superior colliculi
Voluntary Saccades (horizontal)
results in
saccade to
contralateral
space
Voluntary Horizontal Saccades
FEF
FEF
CEREBRAL
HEMISPHERE
III
MIDBRAIN
III
PONS
VI
VI
PPRF
PPRF
Yoking Mechanism
LR
MR
MR
III
LR
MIDBRAIN
III
PONS
VI
VI
Reflexive Saccades
- to complex stimuli
originates in area 7 of
the parietal lobe
- to elementary stimuli
originates in superior
colliculi
dorsolateral prefrontal
cortex involved in
planning of eye mvts
Smooth Pursuit
Two types:
1. Voluntary (actually termed “smooth pursuit”)
movements - originate in the temporoparietal lobe
2. Reflexive - which are under vestibular
nuclear control alone and constitute what is
called the vestibulo-ocular reflex (VOR).
Voluntary
Smooth Pursuit
• originates near the angular
gyrus - Area 39 at the
temporal parietal occipital
junction
• cells in this region are able to
compute the speed and
direction of a moving object
• results in ipsilateral smooth
pursuit
IPSI
Optokinetic Reflex
• Combination of saccades and
smooth pursuit that allow tracking
of targets in turn (e.g. counting
sheep as they jump over a fence).
• smoothly pursue one target, then
saccade in the opposite direction
to pick up the next target
• parieto-temporal junction (smooth
pursuit area) projects down to
ipsilateral vestibular nucleus,
inhibits it allowing ipsilateral
smooth pursuit
• then, the FEF of the same
hemisphere generates a saccade
back (contralateral) to the next
target
Reflexive Smooth Pursuit - VOR
• maintains gaze on a
target despite head
movement
• reflex arc –
semicircular canal
opposite the head turn
detects motion and
activates the ipsi
vestibular n. which
deactivates its
inhibitory input on the
ipsilateral VI
• results in eyes turning
opposite to the head
turn
deactivates (-) input
VIII
p339 Medical Neuroscience, Nadeau et al
Convergence
• When areas of the occipital cortex detect a
discrepancy in the retinal projection from each
eye and amount of blur, a signal is sent to
initiate convergence.
• To bring a near object into focus actually
involves convergence, accomodation (lens
curvature increases) and pupillary constriction.
Together, these 3 movements are called the near
triad.
Pathologic eye movements
• Muscle
– Trauma, entrapment, inflammation, infiltrating diseases
• Neuromuscular Junction
– myasthenia gravis, botulism, organophosphate poisoning
• Cranial nuclei or nerve
– Brainstem: stroke, hemorrhage, multiple sclerosis, tumor, trauma
– Subarachnoid space: Increased intracranial pressure, aneurysm,
meningitis, sarcoidosis, autoimmune
– Cavernous sinus: Tumor, sinus thrombosis, pituitary apoplexy,
sphenoid sinusitis, carotid-cavernous fistula, Tolosa-Hunt
syndrome
– Orbit: Trauma, tumor, infection
right cranial nerve III palsy
right cranial nerve VI palsy
Gaze Palsy
• inability to look in a particular direction (ie.
neither eye can look right)
• lesion in the FEF, the PPRF, or the CN VI nucleus
– Lesion in the FEF – unable to look contralaterally,
eyes deviate toward the lesion, can be overcome with
VOR
– Lesion in PPRF or CN VI nucleus – inability to look
ipsilaterally with either eye
Voluntary Horizontal Saccades
FEF
FEF
CEREBRAL
HEMISPHERE
III
MIDBRAIN
III
PONS
VI
VI
PPRF
PPRF
Left MLF lesion – intranuclear ophthalmoplegia
Yoking Mechanism
LR
MR
MR
III
LR
MIDBRAIN
III
PONS
VI
VI
Acknowlegdements
•
•
•
Dr. Tariq Bhatti and his patients for clinical images
Dr. Angela McSwain, Dr. Nadeau’s text, Peter Duus ( “Topical Diagnosis in
Neurology”), and Frank H. Netter (Ciba Collection of Medical Illustrations, Vol 1)
Dr. Nancy Newman and Dr. Valerie Biousse, Neuro-ophthalmology Emory
University