Download Oculomotor System

Oculomotor System
Dr. G.R. Leichnetz
Why study eye movements?
Abnormalities of ocular motility are often a clue
to the localization of lesions and diagnosis of CNS
disease….
because the neural networks for different types of
eye movements involve structures throughout the
CNS, which, when lesioned, produce very
predictable and specific clinical deficits.
For example, if one observes oculomotor deficits in saccades
or smooth pursuit, the clinician knows what sub-set of
structures belong to that particular neural network which
could account for the deficits.
Since our eyes are embedded in our heads, the
disturbances that most likely affect our vision
are head perturbations. If we did not have eye
movements the visual image would “slip” on
the retina every time we moved our heads.
Some eye movements stabilize our gaze
(maintain fixation), while other eye movements
allow us to shift our gaze to new objects of
interest.
Types of Eye Movements
Conjugate
Disconjugate
Modified from R. J. Leigh & D. S. Zee, The Neurology of Eye Movements
All types are conjugate (both eyes move together in the same direction)
except vergence, which is disconjugate.
Different types of eye movements have different underlying
neural networks, but ultimately connect with specific groups of
motor neurons within the extraocular motor nuclei (III, IV, VI),
the “final common pathway” to the extraocular muscles, to
produce appropriate eye movements .
Different Pre-Oculomotor Neural Networks
?
Levator palpebrae
(elevates upper lid)
Sup. rectus
Lateral rectus
Oculomotor (III):
superior rectus
inferior rectus
medial rectus
inferior oblique
levator palpebrae
Trochlear (IV):
superior oblique
Abducens (VI):
lateral rectus
Inf. rectus
Inf. oblique
All the extraocular muscles are innervated
by C.N. III, except the lateral rectus (VI)
and superior oblique (IV).
LR6SO4
Paired muscles function together to produce movements
in three principal planes.
Horizontal eye movements- medial and lateral rectus
Upward eye movements- superior rectus and inferior oblique
Downward eye movements- inferior rectus & superior oblique
Extraocular Motor Nuclei
Oculomotor
complex
The oculomotor nucleus
contains separate motor neuron
cell groups that innervate
specific extraocular muscles:
superior and inferior rectus, medial
rectus, inferior oblique, and levator
palpebrae
Oculomotor
complex
Inf. rectus
Sup.
rectus
Inf.
oblique
Haines, Fundamental Neuroscience
Medial
rectus
Haines
Labelled motor
neuron cell groups
in the oculomotor
nucleus after
injection of HRP in
target extraocular
muscles.
The trochlear nucleus, located in the caudal midbrain
above the medial longitudinal fasciculus, innervates
the superior oblique muscle .
Trochlear
nucleus
MLF
MLF
Trochlear
nucleus
MLF
Haines
Labeled cells bodies of
neurons in the trochlear
nucleus after an HRP
injection of the superior
oblique muscle.
The abducens nucleus, located in the caudal pons below
the facial colliculus, innervates the lateral rectus muscle.
MLF
MLF
Abducens
nucleus
Haines
The abducens nucleus
after HRP injection into
the lateral rectus muscle.
The abducens nucleus contains two populations of
neurons:
1. Lateral rectus motor neurons- 70%
2. Internuclear neurons (project via the MLF to the
contralateral medial rectus motor neurons)- 30%
Neural Networks for
Different Types of Eye
Movement
Vestibulo-Ocular System
Second-order vestibular fibers that originate in the
vestibular complex ascend through the MLF to the
extraocular motor nuclei of III, IV, and VI.
These vestibulo-ocular projections are canalspecific, and orchestrate reflexive compensatory
eye movements to maintain fixation during head
movement, the vestibulo-ocular reflex (VOR).
Precise vestibulo-ocular projections to specific motor neuron
cell groups in the the oculomotor, trochlear, and abducens
nuclei facilitate reflexive eye movements that compensate for
head movements (to maintain fixation).
Vestibulo-ocular Reflex (VOR)
III
IV
Medial
longitudinal
fasciculus
VI
Vestibular
complex
Saccadic System
Saccades are rapid eye movements that shift gaze to
an object (or event) of interest.
Voluntary saccadic eye movements are initiated in the
frontal eye field.
Reflexive saccadic eye movements are initiated in the
superior colliculus.
Both the FEF and SC project to pre-oculomotor
centers in the brainstem reticular formation adjacent
to the extraocular motor nuclei.
Saccadic Eye Movements
Voluntary saccadic eye
movements are initiated in
the frontal eye field.
Reflexive saccadic
eye movements are
initiated in the
superior colliculus.
SC
SC
Superior Colliculus
The superior colliculus is a
laminated structure located in the
tectum of the dorsal midbrain,
concerned with the coordination of
reflex saccades, which can occur in
response to visual, auditory, or
somatosensory stimuli.
It receives direct retinal input
which establishes a visual map in
its superficial layer.
Courtesy, Dr. Robert F. Spencer
S
I
D
An injection of tritiated amino acid into
the eye labels the retinal input into the
superficial layer of the superior
colliculus. A map of the visual field is
represented in the superficial layer.
The intermediate and deep layers of the SC are in
registry with the visual map in the superficial layer.
There is a retinotopic map
of the visual world on the
surface of the SC
S
I
The deep layer is “motor”
and is in registry with the
visual map in the
superficial layer.
D
The intermediate layer is
integrative; receives input
from the FEF, cerebellum &
basal ganglia, affecting motor
output in deep layer.
When the SC initiates a reflexive saccade to look at an object,
it can precisely target the location of the object in the visual world,
so it can initiate an eye movement of appropriate amplitude and
direction to bring the image onto the fovea of the retina.
The deep layer of the superior colliculus gives rise to
projections to pre-oculomotor brainstem centers for
reflexive saccadic eye movements, and tectospinals to
cervical spinal cord to coordinate reflex head movements.
Deep layers project to
brainstem preoculomotor
centers and cervical spinal cord
Tectospinals descend to
cervical cord to terminate on
neck muscle motoneurons
Both the FEF and SC project to the pre-oculomotor
saccade centers in the brainstem reticular formation
near the extraocular motor nuclei.
For vertical saccades- rostral iMLF and INC (midbrain)
For horizontal saccades- PPRF (pons)
Vertical
Saccades
(riMLF, INC)
III
IV
VI
Horizontal
Saccades
(PPRF)
Lesions of either brainstem region are differentiated on the basis
of oculomotor deficits in vertical vs. horizontal eye movements.
Center for Vertical Downward Saccades:
Rostral Interstitial Nucleus of the Medial Longitudinal Fasciculus
For Vertical Saccades: the preoculomotor nuclei, the
rostral interstitial nucleus of the MLF (riMLF) and the
interstitial nucleus of Cajal (INC) are located in the
rostral midbrain reticular formation where it merges
with the subthalamic region.
riMLF
STN
SN
From Haines, Fundamental Neuroscience
riMLF
riMLF
Haines
The rostral iMLF (premotor center for vertical downward saccades) is
located in the rostralmost medial midbrain reticular formation where
its neurons are interspersed among the rostralmost fibers of the MLF.
Center for Vertical Upward Saccades:
Interstitial Nucleus of Cajal
From Haines, Fundamental
Neuroscience
Oculomotor
Complex
INC
Haines
The interstitial nucleus of Cajal contains premotor neurons that
project to cell groups in the oculomotor nuclei (superior rectus
and inferior oblique) for vertical upward eye movements. Like
the riMLF, it neurons are interspersed among the fibers of the
rostral MLF.
For Horizontal Saccades: the preoculomotor center is located in
the caudal medial pontine reticular formation, and is called the
paramedian pontine reticular formation (PPRF).
Abducens Nucleus
PPRF
Haines
VI
PPRF
Large neurons in the PPRF have
ascending branches to abducens
nucleus, and descending projections
to cervical spinal cord (for gaze)
Haines
The center for horizontal saccades (PPRF) contains premotor
neurons that project to the abducens nucleus (lateral rectus motor
neurons and internuclear neurons). It also projects to the cervical
spinal cord for “gaze” (eye movement + head movement).
Horizontal Saccade
Connections
Frontal
eye field
Superior
colliculus
Neurons in the PPRF
project to the abducens
nucleus (affecting ipsilateral
Oculomotor
nucleus
lateral rectus motor neurons and
internuclear neurons that project
to the contralateral medial rectus
motor neurons)
for conjugate horizontal
eye movements.
Abducens
nucleus
Modified from Haines
PPRF
To cervical spinal cord
Its large cells also
project to the cervical
spinal cord affecting
head movement, hence
“gaze.”
Saccadic System
Frontal Eye Field
Superior
Colliculus
The FEF (voluntary saccades) and SC (reflexive saccades)
both project to riMLF and INC (vertical saccades) and
PPRF (horizontal saccades)
Smooth Pursuit Eye Movements
Smooth pursuit eye movements are slow eye
movements used to follow (track) a moving
object across the visual scene.
Pursuit eye movements cannot be produced voluntarily.
To maintain fixation, the eyes must move at
the same speed as the visual target (eye
movement must match target velocity).
To do this, the pursuit system involves seeing
the object (visual cortex), analyzing visual
motion (pre-occipital cortex, area MT), and
calculating the appropriate size and velocity
of eye movement (cerebellum).
Smooth Pursuit Pathway
Visual Cortex to MT Visual Area
(V5) to dorsolateral basilar
pontine nucleus (DLPN) to
“oculomotor part” of the
posterior lobe vermis
Visual
Cortex
Lesion of these structures can
result in pursuit deficits
Oculomotor
Vermis
DLPN
Oculomotor
vermis
Dorsolateral
pontine nucleus
Posterior lobe
Optokinetic System
When the visual scene is moving (eg. riding on a
train) to maintain fixation on an object (following
telephone poles) produces oscillating eye movements
which consist of a slow component that follows the
object and then a fast component where eyes snap
back to look at the next pole, ie. eye movements
similar to nystagmus, called optokinetic nystagmus,
OKN.
The neural network for these eye movements involves
the pretectum (receives visual input from the retina),
which projects to the reticulotegmental nucleus
(NRTP) of the pons, which then relays the
information to the cerebellum (flocculonodular lobe).
Optokinetic Pathway
Visual cortex
Retina
Retina and/or visual cortex to
pretectal area to nucleus
reticularis tegmenti pontis (NRTP)
to flocculonodular cortex
Lesion of these structures can
produce OKN deficits.
Flocculus
Vergence Eye Movements
Vergence eye movements are the only
disjunctive type of eye movement.
In convergence, both medial rectus
muscles contract.
The pathway subserving convergence is not
well known. But what is known is that the
neural pathway reaches the two medial
rectus cell groups in the oculomotor nucleus
w/o going through the MLF (convergence is
preserved in an MLF lesion, ie. INO).
Convergence is part of the
“near response,” ie. focussing on a
near object, involves:
convergence
accommodation (lens thickening)
pupillary constriction
Oculomotor Lesions
Oculomotor (IIIrd) Nerve Palsy, left eye
From: Fix, High-Yield Neuroanatomy
right
left
Ptosis
Mydriasis
Exotropia
With lesion of the left
oculomotor nerve (C.N. III),
the left eye is abducted (due
to unopposed action of the
lateral rectus). There is
ipsilateral ptosis (paralysis of
levator palpebrae) and the
ipsilateral pupil is dilated
(mydriasis; disruption of
parasymp. fibers to
constrictor pupillae).
left
left
Abducens (Sixth) Nerve Palsy, right eye
right
Ipsilateral
eye does
not abduct
left
From: Fix, High-Yield Neuroanatomy
Lesion of the right
abducens nerve (C.N. VI)
results in paralysis of the
right lateral rectus muscle.
When looking to the right,
the right eye will not
abduct. Esotropia.
right
Abducens Nucleus Lesion
Right
Cannot abduct the
ipsilateral eye or adduct
the contralateral eye
Right
Left
Lesion of abducens nucleus produces ipsilateral paralysis of
conjugate horizontal eye movements (cannot abduct ipsilateral
eye due to lesion of lateral rectus motoneurons, and cannot
adduct contralateral eye due to lesion of internuclear neurons
that project to contralateral medial rectus motor neurons)
MLF Lesion: right MLF
Right
Ipsilateral eye will not
adduct.
Right
Left
Abducens
internuclear
neurons
project thru
MLF to
contralateral
medial rectus
motor neurons
Lesion of MLF produces
internuclear ophthalmoplegia, INO)
cannot adduct eye ipsilateral to
MLF lesion due to interruption of
internuclear axons
(but medial rectus will contract with
convergence)
MLF Lesion: Internuclear Ophthalmoplegia, right eye
right
left
Ipsilateral eye
does not adduct
Fix
Lesion of the right MLF disrupts axons of internuclear
neurons whose cell bodies are located in the abducens
nucleus and which project to the right medial rectus
cell group of the oculomotor nucleus.
On looking to the left, the ipsilateral eye (on the
lesioned side) will not adduct on attempted conjugate
eye movement to the opposite side..
But since the medial rectus cell group itself is intact,
the ophthalmologist can get adduction in convergence.
Lesion of the MLF also produces nystagmus (disrupts VOR).
PPRF Lesion
A lesion of the paramedian pontine reticular formation
produces an ipsilateral paralysis of conjugate
horizontal gaze.
Ipsilateral conjugate
horizontal gaze
paralysis
Frontal Eye Field Lesion on right
right
left
Fix
Lesion of the frontal eye field
(area 8) on the right results in
a paralysis of conjugate eye
movements to the left. The eyes
are deviated to the right
(toward side of lesion).
The FEF lesion disrupts projections
to contralateral PPRF, resulting in a
contralateral paralysis of conjugate
horizontal gaze