Week 4 Flashcards
Oculomotor system
what is the purpose of the oculomotor system?
to position the eyes so that targets of interest fall on the fovea. The foveal region of the eye provides high-resolution vision
what is the fovea?
a small pit in the retina that allows for high resolution vision
what are the three movements holding images steady on the retina?
- visual fixation
- vestibular
- optokinetic
what are the four movements directing the fovea to
- saccades
- nystagmus quick phase
- smooth pursuit
- vergence
visual fixation
holds the image of a stationary object when the head is still
vestibular eye movement
holds images steady on the retina during brief head rotations or translations
optokinetic
holds images steady on the retina during sustained head rotation
optokinetic
holds images steady on the retina during sustained head rotation
saccades
bring images of objects of interest rapidly onto the fovea
nystagmus quick phases
reset the eyes during prolonged rotation and direct gaze toward the oncoming visual scene
smooth pursuit
hold the image of a small moving target on the fovea; aids optokinetic responses to stabilize gaze during sustained head rotation
vergence
move both eyes in opposite direction so that images of a single object are simultaneously placed on the fovea of each eye
level 1 of the oculomotor system (low level)
- orbit and globe
- extraocular muscles
- cranial nerve nuclei
level II of the oculomotor system (middle level)
- version circuits
- vergence circuits
- neural integrator
level III of oculomotor system (high level)
- sacaddes
- pursuit and OKN
- fixation
- vestibular
level IV of oculomotor system (repair)
cerebellum
what is the pair of muscles for horizontal eye movements?
medial recti
lateral recti
what is the pair of muscles for vertical eye movements?
superior recti
inferior recto
what is the pair of muscles for vertical and torsional eye movements?
superior oblique
inferior oblique
what eye muscles go with the horizontal (lateral) canals
- ipsilateral medial recto (excited)
- contralateral lateral recti (excited)
- contralateral medial recti (inhibited)
- ipsilateral lateral recti (inhibited)
what eye muscles go with the posterior (inferior) canals
- ipsilateral superior oblique (excited)
- contralateral inferior recti (excited)
- ipsilateral inferior oblique (inhibited)
- contralateral superior rectus (inhibited)
what eye muscles go with the anterior (superior) canals
- ipsilateral superior rectus (excited)
- contralateral inferior oblique (excited)
- ipsilateral inferior recti (inhibited)
- contralateral superior oblique (inhibited)
cranial nerve III name and what it does
- oculomoter nerve
- innervates the superior and inferior recti, medial rectus, and inferior oblique
- responsible for pupilomotor and eyelid muscles
what happens with complete III nerve palsy
the eye is unable to move with the other eye, basically it has very little movement and the eyelid cannot function as it should
cranial nerve IV name and what it does
- trochlear nerve
* innervates only the superior oblique muscle
what happens with IV nerve palsy
vertical diplopia (usually the result of head trauma)
cranial nerve VI name and what it does
- abducens nerve
* innervates only the lateral rectus
what happens with VI nerve palsy
*horizontal diplopia that occurs when the pt looks in the direction that requires the lateral muscles to be contracted (toward the affected side) (usually from head injury, diabetes, or idiopathic)
version
the eyes moving together (where the right eye is looking, so is the left)
Hering’s law
(of equal innervation)
*each eye must undergo the same amount and direction of movement
vergence
the ability to move both eyes inward or outward at the same time
divergence
eyes move outward (looking from near to far)
convergence
eyes move inward (looking from far to near)
what type of issue would abnormal vergence indicate
a midbrain issue (central)
what do you need to have when testing vergence?
- you need a two channel recording system
- –if you can only view one eye you cannot tell if there is abnormal vergence
gaze holding
- the ability to keep the eye centered in the orbit which allows us to keep focus on an object
- –neural integrator is essential for this
what will happen if there is a problem with gaze holding?
the eye will drift off and then a snapping back bringing the eye back to center
nystagmus
the action of the eye drifting off and then snapping back to center
saccades
high level oculomotor control
*are rapid eye movements meant to bring a point of focus onto the fovea (very fast movements)
where in the brain are saccades produced and controlled
the occipitoparietal cortex, the frontal lobes, the basal ganglia, the superior colliculus, the cerebellum, and the brainstem
if saccades are abnormal, where is the lesion
- usually a brainstem or cerebellar lesion so central
- –but are not the only reason they could be abnormal (poor vision)
dysmetria
saccadic inaccuracu
hypermetria
overshoot (hallmark of cerebellar disease)
hypometria
undershoot
opsoclonus
- ocular flutter
- eyes jump wildly or flutter
- –some people (a/b 8% normal pop) can cause this to happen and is called voluntary nystagmus
pursuit eye movements
the ability to have the eyes follow a target at the same speed at which the target is traveling, keeping the image steady on the retina
what controls the pursuit eye movements
central pathway
- pursuit is not robust like saccade system however
- pursuits can easily be disturbed and therefore abnormalities on pursuit are generally not considered clinically significant
optokinetic nystagmus
a tracking response, closely related to the smooth pursuit system, however is tracking a field not an object
- the purpose is the stabilize an entire visual field
- OKN does not require good vision and it is much more robust than smooth pursuit but they are produced by similar cortical and brainstem structures
fixation
fixation is the process by which the brain suppresses internally generated eye movements
—during testing with or without a target available, unintended saccadic eye movement can occur called “square jerk waves” this is generally not clinically sig, but can make it hard to have a clear VNG tracing to analyze
what is the sight of lesion if there is nystagmus with fixation?
central
what is the sight of lesion if there is nystagmus without fixation
peripheral
congenital nystagmus
uncontrolled spontaneous nystagmus, is thought to be a miswired fixation system
pendular nystagmus
often occurring with oscillation of the soft palate and eyes after a brainstem or cerebellum lesion. listen for a “clicking” sound
