week 35 visual pathway Flashcards
what is the visual pathway?
the route by which retinal stimuli is transferred to the occipital cortex of the brain from the retina
at which point will damage affect vision?
at any point along the pathway
what are the key structures of the pathway?
optic nerve
optic chiasm
optic tract
LGN
optic radiations
visual cortex
what are the associated structures?
pituitary gland
edinger-westphal nucleus
circle of wilis
how many neurons are in the pathway?
3
how many synapses?
2
where are the synapses located?
between bipolar and ganglion cells in the retina
at the LGN
what is the arrangement of nerve fibres in the optic nerve?
at the start, they arranged based on the retinal location
rearranged towards the end so macula fibres in the centre
which side of the visual field is each eye responsible for?
each eye forms an image of both the right and left side of the visual field
visual field therefore duplicated on the retina of both the right and left eye
what happens at the optic chiasm?
all info is sorted so all fibres transmitting info from the right visual field is together and same for the left
how do the nasal and temporal retinas work together?
nasal retina from one eye joins with info from the temporal retina of the other
what is the crossing of info from temporal to nasal called?
decussation
what happens to retinal fibres at the optical chiasm?
inferior retinal fibres cross anteriorly
superior fibres cross posteriorly
what are the groups of visual defects called?
pre-chiasmal
chiasmal
post-chiasmal
what happens with a pre-chiasmal lesion?
affects the visual field on the affected side
what would a RE pre-chiasmal lesion affect?
RE visual field
what happens with post-chiasmal lesions?
visual field of both eyes are affected
what happens with chiasmal lesions?
bi-temporal hemianopia
loss of peripheral vision
what is the circle of Willis?
connecting ring of arteries that supply the brain
an aneurysm in the internal carotid artery can press agaonst the chiasm
where is the pituitary gland located?
sits below the chiasm
what happens with a pituitary adenoma?
can press on the chiasm from below
where does the optic tract extend from and to?
from posterior chiasm to the LGN
what happens at the optic tract?
nerve fibres representing the right side visual field now together on the left of the pathway and vice-versa
right nasal and left temporal retina together
rearrangement of fibres so macula fibres now at the top
what happens if there is damage on the optic tract?
blindness in ipsilateral half of the visual field
what are the features of the LGN?
pair of nuclei so a site of a synapse
folded over horse shoe shape
six layers
where is the dorsal aspect of the LGN?
outside of the curve
where is the ventral aspect of the LGN?
inside of the curve
what is the nerve fibre arrangement in the LGN?
nerve fibre arrangement of optic tract reflected in LGN
retinotopic arrangement
macula fibres centrally
layers 2,3&5 have uncrossed fibres from the ipsilateral eye
layers 1,4&6 have crossed fibres from contralateral eye
what does ipsilateral eye mean?
eye is located on the same side of the body as the object or structure
what does contralateral eye mean?
eye is located on the opposite side of the body as the object or structure
what are the features of layers 1 and 2 of the LGN?
large cells
magnocellular
contrast, low light, visual search
what are the features of layers 3,4,5&6 of the LGN?
small cells
parvocellular
colour and detail
features of optic radiations?
nerve fibres extend to parietal and temporal lobes
potential for some parts to be affected by lesions some remain intact
where is Meyers loop and what fibres does it contain?
outside bundle of the radiating fibres
contains fibres from the inferior retina/superior visual field
inside bundle has fibres from superior retina/inferior visual field
what happens to the eye if there is a lesion on the inside or outside of the loop?
will affect a quarter of each visual field
what does quadrantanopia refer to?
loss of vision in a quarter of the visual field?
where is the superior retina/inferior visual field in relation to the brain?
the parietal lobe
where is the inferior retina/superior visual field in relation to the brain?
the temporal lobe
what happens if there is a lesion on the posterior part of optic radiations?
half-field defects
hemianopia
features of primary visual cortex?
fibres project to different areas of the cortex depending on origin and purpose
macular fibres take up the most space
what is a congruous defect?
similar defect in both eyes
what is a macular sparring defect?
hemianopia defect but the area around the macula is unaffected
what artery supplies blood to retina and extra-cranial optic nerve?
