What are the structures involved in the visual pathway
ON> OC>OT>LGN>OR>VCU
Where are the 2 synapses in the visual pathway
Btwn bipolar and gang cells
At the LGN
What is the nerve fibre arrangement here?
Arrangement of NF ?
What happens at the chiasm
Decussation.
53% of fibres cross
Only nasal fibres cross
Inferior ret fibres ( IT & IN) cross anteriorly
Superior ret fibres (ST & SN) cross posteriorly
Chiasm associated structures
Circle of Willis- connecting ring of arteries supplying brain
Aneurysm of ICA affects
Pituitary gland - below chiasm
Pituitary adenoma affects
Arrangement of fibres in tract
Structure of LGN ( lateral geniculate nucleus)- cells and fibres
Horseshoe shape
6 layers
2,3,5 = uncrossed fibres ( ipsilateral eye)
1,4,6 = crossed fibres( contralateral eye)
Layer 1 & 2 = magnocellular, contrast, low light, info from rods
Layer 3,4,5,6 = parvocellular, colour, detail, info from cones
Retinotopic arrangement of fibres
Where do OR extend to? What is the name of the outermost bundle?
to the parietal and temporal lobes of brain
superior ret in pari lobe ( middle cerebral art)
inferior ret in temp lobe (post cerebral art)
Meyers loop = outside bundle
fibres from inferior ret/ sup VF
inside loop = sup ret fibres
VF defects and OR
lesions on outside or inside of Meyers loop= 1/4 of each VF= quadrantanopia
after Meyers loop, OR close together so both sup and if fribres affected =1/2 field defect= hemianopia
VF defects according to chaism and which eyes affected?
Pre-chiasmal- VF defect would be on same side as lesion on 1 eye
Chiasmal- only TEMPORAL VF affected of BOTH eyes
Post-chiasmal- BOTH eyes affected SAME side
Primary visual cortex defects and why this happens?
Macular fibres take up most space so defects are congruous and macular sparing- not all mac fibres damaged
Bld supply thru visual field pathway. Vessels affect VF defect
VF terminology
Explain the gang cell receptive fields
cells respond to dark or light
have an on / off centre
light shine in centre of ON CENTRE= cell firing increases
light shine in centre of OFF CENTRE= cell firing decreases
Structure of visual cortex
What happens in the visual cortex? What are the 2 streams of HLP
HLP= higher level processing
Dorsal stream to temporal lobe
actions, motion, psn
short term mem
magnocell
retina
Ventral stream to brainstem
object recognition/ colour/detail
long term mem
parvocell
fovea
Position of frontal eye fields
in pre motor cortex
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