Eye Movements Flashcards
what is the purpose of eye movements?
achieve and maintain fixation
move the fovea to a target position and keep it there
what are the components of the gaze system?
oculomotor system (eyes moving in orbits) and head movement (orbits moving in space
what is a saccade
rapidly shifting the fovea to a visual target in the periphery
what is smooth pursuit
keep the image of a moving target on the fovea (need a target to follow)
what is vergence
movement of the eyes toward or away from the midline to adjust for distance
normal disconjugate movement
what is vestibulo-ocular
hold images still on the retina during quick head movements
what is optokinetic
maintain fixation on new objects during steady head movement or moving visual stimuli (like watching light poles in a moving car)
saccades and smooth pursuit
what is visual fixation
hold the eyes still during intent gaze, suppression of eye movement
what is conjugate eye movement
both eyes move in the same direction at the same speed
what muscles are involved with eye adduction
medial, superior, and inferior rectus
what muscles are involved with eye abduction
lateral rectus, superior and inferior oblique
what muscles are involved with eye depression
inferior rectus, superior oblique
what muscles does each direction of the H test check
what are the two passive forces that must be overcome to move the eye?
viscous force that opposes rapid movement and elastic force that restores the eye to a central position
what is saccadic pulse
high frequency neural burst to overcome orbit viscosity; achieves movement
what is saccadic step?
steady frequency neural activity to overcome tissue elasticity; maintains steady position
what are the pathway steps for eye movement
cortical eye fields - planning
basal ganglia - modulation
brainstem nuclei - pulse and step generators
CN motor nuclei
CNs
extra ocular muscles
why is the superior rectus subnuclei important?
the nuclei innervate the contralateral SR, so due to the fiber crossing a lesion of the nucleus would have a bilateral affect
what is the medial longitudinal fasciculus
a myelinated fiber tract that interconnects the oculomotor, trochlear, and abducens nuclei
what does the MLF do?
serves to facilitate eye movements and conveys info about head movement
where does the MLF ascend to?
the interstitial nucleus of Cajal
what does the paramedian pontinereticular formation do
processes cortical signals and generates a pulse command that projects to the abducens nucleus
what are the two neurons that come from the abducens nucleus during horizontal saccades?
motor neuron to the lateral rectus
internuclear neuron to contralateral oculomotor nucleus via MLF
horizontal saccades steps
control signals from frontal eye fields and superior colliculus synapse on the contralateral PPRF
from PPRF to ipsilateral abducens nucleus
motor neuron to LR and internuclear neuron to contralateral CN III nucleus via MLF
motor neuron to MR
where are omnipause neurons located? what do they do?
in the nucleus of the dorsal raphe
they prevent unwanted saccades by arresting them, providing stability to the system
what must be inhibited to generate a saccade?
omnipause neurons
what do burst neurons do?
excite inhibitory neurons that project to the contralateral PPRF and abducens nucleus
(contralateral from where the signal was generated)
what integrates the velocity signal to a tonic position?
medial vestibular nucleus, nucleus prepositus hypoglossi, flocculus of cerebellum
what would cause the eyes to drift back to the middle position after a saccade?
lack of integration of the velocity signal to a tonic position signal
where do the frontal eye fields project to for vertical saccades?
rostral interstitial nucleus of the MLF (riMLF) (pulse command)
interstitial nucleus of Cajal (step command)
what do vertical saccades require? where does this occur?
requires communication on both sides of the midbrain reticular formation which occurs via the posterior commissure
vertical saccades elevation pathway
cerebral cortex and superior colliculus send signals to both irMLF and interstitial nucleus of Cajal
both of these nuclei send signals to both ipsilateral and contralateral oculomotor nuclei to synapse on motor neurons for the contralateral SR and ipsilateral IO
vertical saccades depression pathway
CC and SC send signals to irMLF and interstitial nucleus
these nuclei send signals to the ipsilateral trochlear nucleus and contralateral oculomotor nucleus
motor neurons then synapse on the contralateral IR and SO
where do excitatory projections from the frontal eye field go to
descend to the superior colliculus, riMLF, and PPRF
where do the frontal eye fields receive input from?
supplementary eye fields
where does the posterior parietal cortex project to?
superior colliculus
what is the superior colliculus? what does it do?
a major visuomotor integration region
works to initiate eye movements toward stimuli
what strongly inhibits the superior colliculus?
substantia nigra pars reticulata
how is inhibition of the superior colliculus suppressed?
by the caudate nucleus, just before saccades
where does the superior colliculus project to?
PPRF or riMLF
what does smooth pursuit initiation?
frontal eye field
what calculates the velocity of a moving target?
middle temporal and medial superior temporal areas
oculomotor nerve palsy presentation
down and out eye position
decreased tone in all but LR and SO
impaired convergence
ptosis
trochlear nerve palsy presentation
impaired torsion and depression
head tilts away from affected eye to compensate for hypertrophic and extorsion
abducens nerve palsy presentation
impaired abduction of affected eye
adduction of affected eye at rest
