Physiology of saccades and fixation in peripheral gaze Flashcards
What 2 principle muscle fibres do the rectus and oblique have?
Fibrillenstrucktur fibres - 80% total - large active fast phasic contract ion
Felderstruktur fivres - 20% - smaller tonic contraction
What is the white stuff in the cytoplasm?
Myofilaments
What type of eye movement is needed for peripheral vision?
Large saccades
What type of eye movement is needed for central vision?
Small saccades
What do saccades require?
Fast muscle contractioninvolving fibrillenstrucktur fibres to rapidly move eyes
How are saccades mediated by?
Saccade generator nuclei (PPRF, riMLF)
What do peripheral fixation require?
sustained contraction by felderstrucktur fibres to prevent eyes from drifting and hold steady gaze for periods of fixation and object inspection
How are peripheral saccades mediated?
From brainstem by neural integrator nuclei
What are neural integrator nuclei for?
For horizontal and vertical gaze holding
What nuclei is needed for horizontal and vertical gaze holding?
Neural Intergrator nuclei
What neural intergrator nuclei is needed for Horizontal gaze holding?
PeriHypoglossal nucleus (PHN)
What is another name for PHN?
Nucleus Prepositus hypoglossi nucleus
What neural intergrator nuclei is needed for vertical gaze holding?
Intersitial nuclei of cajal (INC)
Where is the PHN found?
In medulla, below abducens nucleus
Where is INC found?
Upper midbrain, above oculomotor complex
How to EBC fire ?
High frequency up to 1000HZ for brief seconds 10-15msec before saccades begin
What does their firing duration determnine of EBC?
Amplitude of saccades
What does higher frequency and longer duration burst of EBC result in?
Faster and larger saccades
What does lower frequency and shorter duration burst of EBC result in?
Smaller saccades
How is eye drifting back to primary gaze a problem?
Elastic restoring forces of orbital connective tissue supporting the eye causes it to drift back to primary gaze position
What if you get a lesion in NPH or INC?
Failure to hold eccentric gaze
effected eye drifts back to primary position with new saccades to re fixate the target of interest
What manifest deviation can you get with lesions in NPH OR INC?
Nystagmus
How are saccades controlled?
Omni pause cells
Where are pause cells located?
Midline near PPRF and abducent nucleus in Nucleus Raphe interpositus
What do pause cells do?
Fire continuously to tonically inhibit excitatory burst cells in PPRF and riMLF
What can NRI lesions cause?
Opsoclonus -saccades in random direction with no pause cells for fixation
What can NRI cause ?
Blindness
What are higher saccades control centres for?
- Visually cured , reflexive saccades
- Midbrain : Superior colliculus
- Posterior cerebral cortex: Parietal eye field - Internally generated, voluntary saccades
- Frontal cerebral cortex : Frontal eye field and supplementary eye field
Where is the PEF located?
Parietal eye field - Inferior parietal lobule - below intra parietal sulcus within brodmann area 39 & 40
Where is FEF located?
Frontal eye field- Posterior end of middle frontal gyrus -infront of primary motor cortex within brodmann area 6
Where is SEF located?
Supplementary eye field - posterior end of superior frontal gyrus above FEF within brodmann areas 6 & 8
Why nuclei is responsible for horizontal saccades?
Peri hypoglossal nucleus
