NV - Eye Movement Review - Week 6 Flashcards
What is a disadvantage of having front-facing eyes with high acuity?
Slight misalignment will result in diplopia
-the eyes need to be precisely controlled
Describe the action of the medial rectus.
Pure adductor
-toward the midline
Describe the action of the lateral rectus.
Pure abductor
-away from the midline
Describe the three actions of the superior rectus.
Elevation
Intorsion
Adduction
-elevation with increasing abduction
Describe the three actions of the inferior rectus.
Depression
Extorsion
Adduction
-depression increases as the eye abducts
-nil when the eye is adducted
What muscle is the only depressor in the abducted position?
Inferior rectus
Describe the three actions of the superior oblique.
Intorsion
Depression
Abduction
-depression increases as the eye adducts
-nil in abduction
What muscle is the only depressor in the adducted position?
Superior oblique
Describe the three actions of the inferior oblique.
Extorsion
Elevation
Abduction
-elevation increases with adduction
-nil in abduction
What muscle is the only elevator in the adducted position?
Inferior oblique
What are the three forms of slow eye movements?
Smooth pursuit
Vestibulo-ocular reflex
Optokinetic nystagmus
Why do saccades need to be fast?
Because vision is poor during saccades
-our clearly seen visual world is an illusion
-what are you hiding
Briefly describe how a leftward saccade is made, including what is happening before the saccade is made, where impulses are generated, integrated, and what each does.
Omnipause neurons in the PPRF normally inhibit excitatory burst neurons
When no longer dirven sufficiently by fixation cells in the superior colliculus, omnipause neurons become less active
This allows excitatory burst neurons to fire a burst of innervation to the abducens nerve nucleus, vestibular nucleus, and cerebellum
The burst is integrated into a tonic step of innervation
The burst and tonic activity are combined in the abducens nucleus producing a pulse-step
Pulse gets the eye where it needs to quickly
Step keeps it there
Keeping in mind how saccades are generated for one eye, how are both eyes moved together? What region of the brain is da?
The saccadic burst originates ipsilaterally to the direction of thr saccade (step produced here as well)
The abducens has interneurons which relay the signal to the contralateral medial rectus muscle via a fibre tract
-medial longitudinal fasiculus
For horizontal saccades, where are most of the brain structures responsible located (4)?
Pons and medulla
Midbrain
-3rd nerve nucleus
-superior colliculus
What brain structure is additionally involed in downward saccades?
Interstitial nucleus of cajal
What is the centre of saccadic action?
Superior colliculus
Where do commands to make a saccade come from (2)? What about inhibitory inputs (2)?
Various cortical areas and the retina itself sometimes
Inhibitory from frontal cortex
-striatum
-substantia niagra
What part of the brain is responsible for intentional saccade triggering?
FEF
-frontal eye field
What part of the brain is responsible for reflexive saccade triggering?
PPC
-posterior parietal complex
What part of the brain is responsible for reflexive saccade inhibition?
DLPFC
-dorsolateral prefrontal cortex
Is smooth pursuit under voluntary control?
Yes
Is the vestibulo-ocular reflex new or ancient? Explain.
Ancient reflex
-3 neuron arc
-has voluntary overlay
What does the vestibulo-ocular reflex compensate for?
Transient head movement
What is optokinetic nystagmus? What reflex is it complementary to and why?
Reflexive response to large visual field motion
Continues to operate for constant-velocity stimulus motion, thus complementary to VOR
What is the purpose of eye movements during fixation (2)? What is their magnitude?
Tiny subclinical eye movements that serve to prevent image fading and also appear to reflect shifts in attention
What are the two roles of the vestibular system?
Maintains balance
Maintains stable gaze as we move in the environment
What do semicircular canals sencse? What about the utricle and saccule?
Semicircular - angular acceleration
Utricle/saccule - linear acceleration (including gravity)
Describe the three neuron arc of the vestibulo-ocular reflex.
Ears innervated by 8th cranial nerve, projecting to the vesibular nucleus
Vestibular nucleus neurons project to the ocular motor nuclei
Ocular motor nuclei neurons drive the eye
What is the dolls head test?
Evaluates the VOR by thrusting the head suddenly and assessing how well the eyes maintain fixation
Interaction between what two neuronal pathways can lead to motion sickness and via what?
OKN and vestibular system via the vestibular nuclei
What is linearvection/cirularvection?
Feeling of motion despite being stationary
-linear/circular
List two stimuli for the vergence system.
Disparity
Blur
What is OKN generally integrated with?
Smooth pursuit
