Reflex and voluntary saccades Flashcards
What are supra nuclear used for?
Higher centres
- Initial eye movements for different purpose
- send motor commands to the relevant brainstem generator
What happens generator do in supra nuclear?
Activates burst cells which fire particular patterns of activity sent via axons in the inter nuclear pathway
What happens in the inter nuclear pathway?
Medial longitudinal fasciculus to ocular motar neurones
What happens in ocular motor neurones?
Activating ocular motor neurones and causing similar firing patterns
-Required for contraction of their EOM
What are saccades?
Fast conjugate re directions of bi foveal gaze up to 1000 deg/s
How can saccades occur?
Reflexive (Visually cued)
Voluntary (internally generated)
What is reflexive saccades?
> Express to novel stimulus
>When scanning an object of interest
What is voluntary saccades?
Cognitively or memory guided
What brainstem generators are activated for saccades?
PPRF- paramedian pontine reticular formation
riMLF - rostral interstitual nucleus of medial longitudinal fasciculus
What 5 higher control centres are for fast saccadic eye movements?
> > Frontal eye field (FEF)+SEF( Supplimentary eye field)
Parietal eye field (PEF)
Midbrain superior colliculus (SC)
Midline Cerebellar cortex and nuclei
What do superficial layers receive?
Receive direct retinal inputs and contain maps of opposite visual hemifield?
What is the map of opposite hemiffield?
Centre to peripheral VF- anterior to posterior SC
Lower to upper VF, Lateral to medial SC
What are superficial neurones connected to ?
Deeper layers
What do the deeper layers contain?
Pre motor neurones and a map of saccade directions, in register with the visual map in the layers above
>Centre to peripheral gaze -anterior to posterior SC
>Down to up gaze - lateral to medial SC
What projections do deep layers do to the PPRF?
Crossed projections to PPRF - horizontal saccades generator
What projections do the deep layers make to riMLF ?
Bilateral projections to the riMLF - the vertical saccade generator
What does unilateral lesion to superior colliculus cause?
Abolish reflexive express saccades into the opposite hemi field
How many scanning saccades do we make in a day?
250,000 every day
?What is purpose of saccades?
To examine different regions of interest on objects by bo fixating them with our high acuity foveas
So acquiring more detailed knowledge about them
What does PEF recieve?
Direct inputs from Dorsal stream Extra staita areas
Where does PEF project ?
Sends descending projections to deep layers neurones in saccade map of ipsilateral SC
How does PEF send axons to deep layer neurones?>
Via the internal capsule of posterior limb
What does it allow PEF to do?>
Issue commands to these SC neurones and hotwire their direct access to the brainstem saccade generators
What does unilateral lesions to PEF cause?
Contralateral hemi- spatial neglect or improverished scanning in the opposite hemi field (Sticky fixation)
What is the descending projects from PEF?
PEF: Premotor neurone ell body in gray mattar of inferior parietal lobule
What is the descending projects from Internal capsule?
Posterior portion:Axon in white matter
What is the descending projects from Cerebral peduncle?
Axon in white matter will exit and travel to deep layers of SC
What does right lesion of PEF cause ?
Reduced scanning and neglect in the left hemi field
How do you get voluntary saccades ?
Because you decide to look at something different
Because you are instructed to look at this
What are voluntary saccades influenced by>?
Cognative factors
>You want to inspect something specific
What are volunatry saccades executed as?
Scanning across words on page while reading
What does frontal eye field mediate?
Initiates all self generated saccades
How are self generated saccades initiated?
Via direct projections to the brainstem saccade generators
How does self generated axons run?
Descending axons run in the internal capsule then cerebral peduncle
How does elf generated saccades project to PPRF ?
Cross over to the PPRf and go bilaterally to riMLF
What does each FEF contain?
A map of saccades directions, mainly towards the opposite side of hemifield
What does unilateral lesion to FEF cause?
Ablish voluntary saccades away from affected side (Paralysis of conta -versive gaze)
What is the descending projections from the FEF?
From FEF to PPRF axons cross in the upper pons for horizontal
To riMLF, bilateral for straight up or down
What does left FEF lesion cause ?
Right gaze palsy
How are PEF and SEF connected to each other and other ispilateral PEF?
Via dense inter cortical projections
How does FEF & SEF also have another connection ?
Direct descending projections to the deep layers of the superior colliculus
What does the descending projections to deep layers of superior colliculus do?
They inhibit the deep layer neurones , preventing the occurrence of unwanted express saccades during more purposeful voluntary visual search
How are cerebellar cortex and deep nuclei involved in saccades?
Gain control
How do the cerebellar cortex and deep nuclei function as ?
Common supervisor
What is activated during voluntary and reflex saccade pathway?
Activates brainstem generators PPRF and riMLF
